The underwater acoustic sensor network (UWASN) is a system that exchanges data between numerous sensor nodes deployed in the sea. The UWASN uses an underwater acoustic communication technique to exchange data. Therefore, it is important to design a robust system that will function even in severely fluctuating underwater communication conditions, along with variations in the ocean environment. In this paper, a new algorithm to find the optimal deployment positions of underwater sensor nodes is proposed. The algorithm uses the communication performance surface, which is a map showing the underwater acoustic communication performance of a targeted area. A virtual force-particle swarm optimization algorithm is then used as an optimization technique to find the optimal deployment positions of the sensor nodes, using the performance surface information to estimate the communication radii of the sensor nodes in each generation. The algorithm is evaluated by comparing simulation results between two different seasons (summer and winter) for an area located off the eastern coast of Korea as the selected targeted area. The underwater acoustic sensor network (UWASN) has recently attracted considerable attention because of its potential for use in a variety of military and civilian applications, such as ocean environmental monitoring, ocean exploration, target detection, surveillance systems, and communication among underwater sensor nodes ,2,3,4.In the UWASN, a number of sensor nodes are deployed in the ocean to collect data, which must be successfully exchanged among adjacent sensor nodes. For this reason, each sensor node must be positioned to be able to perform collaborative communication tasks over a targeted area. The deployment efficiency of sensor nodes in the UWASN is evaluated in terms of their connectivity and the area covered by the limited number of sensor nodes in the targeted area, because underwater sensor devices are more expensive and more difficult to deploy at precise positions compared to sensor nodes on land .Most attempts to find an optimal deployment scheme for sensor nodes have assumed that every sensor node has the same sensing range ,7,8,9, wThis multipath channel causes significant delay in spreading, often covering hundreds of symbols, thereby causing inter-symbol interference (ISI). This happens because the underwater sound propagates with relatively low sound speed  . The ISIIt is necessary to develop an optimal deployment algorithm for underwater sensor nodes, which must be able to reflect performance variations in the sensor network due to environmental fluctuations. One of the methods to improve the performance of the UWASN is to use array processing. Array processing techniques using an array of spatially separated receivers have been reported to decrease communication error rates by eliminating the ISI caused by multipath channels in the ocean and to have better communication performance than single receivers that only use the equalizer technique ,16,17. IThe paper is organized as follows. The algorithm to obtain the optimal deployment scheme is composed of four sub-categories. First, underwater acoustic channel modeling is carried out using a ray-based acoustic model for the targeted area using the ocean environment database including bathymetry, sound speed profiles, and sediment types. Second, the communication performance is estimated using the array processing technique of a single-input multiple-output (SIMO) system. The signal received at each array channel is simulated via a convolution of the communication source signal with the simulated channel impulse response. Third, the PS for the targeted area is derived using the estimated communication performance. Lastly, the optimal deployment scheme for the sensor nodes is determined using the VFPSO method based on the estimated PS. The underwater communication channel is characterized by multipaths caused by the acoustic interactions with the sea surface and bottom interfaces, as well as the water medium itself, which causes the ISI, and resulting in serious distortion of communication signals. The ISI makes demodulation of the communication signal complex and difficult. Therefore, it is necessary to determine the optimal deployment scheme based on the communication PS estimated via communication channel modeling. The model input parameters associated with the ocean environmental information, including sound speed profiles, bathymetry, and sediment properties, are needed for accurate underwater acoustic channel modeling. Here, the bathymetry in the targeted area is extracted from ETOPO1, which is a global relief model of Earth’s surface integrating land topography and ocean bathymetry supplied by NOAA  ,25, withsolution . The valsolution .on model . The BELReal-time communication with a high data transfer rate and low bit error rate (BER) is required for a robust UWASN system. There is a limit to using a single sensor node in the UWASN because underwater communication is severely affected by the time-varying multipaths in the ocean environment. Recently, in a number of studies on underwater acoustic communication, sensor array systems have been used to achieve the high data rate with low BER ,16,17. IThe communication sequence used for the communication performance simulation is a binary phase-shift keying (BPSK) sequence with a center frequency of 10 kHz and a bit rate of 1 kbps. To simulate the communication signals received after propagating through the multipath ocean channel, the BPSK communication sequence is convolved with the channel impulse responses simulated using the method described in er (DFE)  to compeer (DFE) ,17. In ter (DFE) ,31. FinaAs described in the previous section, the process to estimate the communication performance is complex and time-consuming. If the process is repeated on candidate positions in the targeted area in the computation loop in order to search for optimal sensor positions, it will take too long to find the optimal solution. Here, the communication PS for the targeted area is constructed based on the BER performance estimated in The BER estimates as a function of the source-receiver range for the eight azimuthal angles of each grid point were made to obtain the communication PS of the targeted area. Now, the communication ranges for the eight azimuthal angles are averaged to represent the communication radius at the grid point. The minimum or maximum communication range can also be chosen optionally by a user as a representation of the communication radius at the grid point. The communication radii estimated for every grid point are interpolated to create the communication PS of the targeted area. In general, the performance of the algorithm for optimal deployment of the sensor nodes is evaluated in terms of maximizing the communication coverage rate and maintaining connectivity among the sensor nodes with a given number of nodes in the targeted area. Here, a hybrid method combining the advantages of the virtual force algorithm (VFA) ,20 and tThe VFA is a self-organizing search algorithm to determine the optimal distances among sensor nodes, which attempts to maximize the communication coverage using a combination of attractive and repulsive forces. As an initial step, the sensors are randomly placed in the targeted area. The distances between one sensor node and its neighboring nodes are compared to a communication threshold. In this work, the communication threshold was defined to be the smaller of the communication radii at two sensor node positions for two-way communication. The communication radius at a certain node position can be extracted from the communication PS map. If the distance is smaller than the threshold, a repulsive force arises between the two sensor nodes. In contrast, if the distance is too far apart, an attractive force arises. This process is performed for all neighboring nodes based on one sensor node, and a weighted sum is calculated to obtain the total force exerted on the node. This is repeated for each node. As the iteration of the algorithm continues, all sensors nodes are positioned, maintaining optimal distances between the sensor nodes.The PSO is a stochastic search algorithm that mimics social behavior of animals moving in flocks in order to find optimal positions. It has been widely used because it is an efficient optimization algorithm for solving dynamic optimization problems having fast convergence and robustness ,22. The d-th dimension at iteration n, respectively. The VFA has outstanding performance in adjusting the distance among the sensor nodes. However, there may be a limit because the strong attractive and repulsive forces among the sensor nodes may hinder the nodes from settling at optimal positions. In contrast, the PSO has the ability to find a global optimal solution. However, it does not have the ability to adjust the distances among the sensor nodes, which hinders the maximization of the communication coverage rate. Here, the VFPSO algorithm is applied to find the optimal positions of the sensor nodes in the UWASN, which is expressed by .(1)Fi→ The optimal deployment performance of the sensor nodes based on the communication PS was simulated for two different seasons, winter (February) and summer (August), in the targeted area located off the eastern coast of Korea. Optimal deployment in our simulation means the sensor deployment having a maximized communication coverage while maintaining connectivity of the sensor nodes. Here, the communication coverage rate is defined as the ratio of the area of communication coverage by the sensor nodes to that of the targeted area. Another important point to be examined is the connectivity among the sensor nodes, which is crucial for effective data communication of the UWASN system. The connectivity among the sensor nodes was estimated for the final deployment scheme to verify the performance of the VFPSO algorithm. The distance between each sensor node and its neighboring sensor nodes was compared to the communication threshold defined in A new method for the optimal deployment of underwater sensor nodes, which can be applied to the UWASN, has been proposed in this paper. The algorithm includes four sub-algorithms, which are a routine for underwater acoustic channel modeling in temporally and spatially varying ocean environments, a routine for estimating the communication performance using the simulated underwater acoustic channels, a routine for constructing the communication PS, and a routine for searching for the optimal deployment of the sensor nodes on the PS. Most previous studies for the optimal deployment of sensor nodes have been performed under the assumption that the sensing range of every sensor node is the same without consideration of the spatial and temporal variations of the ocean environment. In contrast, the proposed method is an algorithm finding the optimal positions of sensor nodes using the communication radii extracted from the communication PS, which is the main difference from the previous algorithms. Moreover, it uses the VFPSO algorithm, which is a hybrid optimization method combining the advantages of the VFA and the PSO. Therefore, the sensor nodes are not placed in equidistant intervals. The optimal deployment based on the PS has been simulated for two different seasons for an area located off the eastern coast of Korea as the selected targeted area. As a result, the communication coverage rate in winter (February) was somewhat better than that in summer (August), implying that the optimal deployment scheme may vary with variations of the ocean environment. The algorithm does not propose the optimal number of sensor node, but it finds the optimal positions of sensor nodes for a given number of sensor nodes.In this paper, it was assumed that the communication channel was time invariant. If the system is used for a short time period using portable sensors such as sonobuoy-type devices, the ocean parameters corresponding to that time should be used as input parameters. However, if it is for the long-term ocean surveillance monitoring for a certain area, it is highly recommended to use the ocean environmental parameters producing the worst communication radii to obtain the robust deployment scheme and, in this case, the sensor nodes would be closer to each other. Also, it was assumed that the BPSK signal with a center frequency of 10 kHz was transmitted as a communication sequence and each sensor node was composed of a three-receiver array with an element spacing of 1.5 m. Other communication sequences [such as OFDM ] or a different type of receiver system, which might be more robust to (frequency/time-selective)] fading channels, could provide better communication performance and produce a different deployment scheme; however, these are beyond the scope of this paper. Nevertheless, our algorithm can be applied to other types of communication sequences and receivers. In this paper, it was assumed that all sensor nodes were positioned near the seafloor. The depth change of the sensor nodes may yield a better communication performance, and in addition the combinations of different types of sensors, including AUV (Autonomous Underwater Vehicle), may improve the performance. The algorithm suggested herein can be extended to these cases, which will be our future work.


Formal techniques have been devoted to analyzing whether network protocol specifications violate security policies; however, these methods cannot detect vulnerabilities in the implementations of the network protocols themselves. Symbolic execution can be used to analyze the paths of the network protocol implementations, but for stateful network protocols, it is difficult to reach the deep states of the protocol. This paper proposes a novel model-guided approach to detect vulnerabilities in network protocol implementations. Our method first abstracts a finite state machine (FSM) model, then utilizes the model to guide the symbolic execution. This approach achieves high coverage of both the code and the protocol states. The proposed method is implemented and applied to test numerous real-world network protocol implementations. The experimental results indicate that the proposed method is more effective than traditional fuzzing methods such as SPIKE at detecting vulnerabilities in the deep states of network protocol implementations. Network protocol implementations are often prone to vulnerabilities, and formal verification techniques cannot address the problems in the implementations. Fuzz testing and symbolic execution are widely applied to detect vulnerabilities in network protocol implementations. However, it is difficult to reach the deep states of the stateful network protocols given the complex interactions and state transitions of these methods, because they do not fully exploit the packet interaction and state transition information.In this article, we propose a novel approach that uses an FSM model to guide the symbolic execution. More precisely, we utilize the L*  online lWe first build a prototype model-guided symbolic execution system to explore the protocol states and detect vulnerabilities in the deep states of the protocol. Then, we use the prototype system to test several real-world network protocol implementations and compare this system with the traditional fuzzing tool SPIKE.In this section, we briefly present research works related to the current study , 3. FuzzSymbolic execution is a powerful technique for analyzing program behavior, identifying bugs, and generating tests . The maiTraditional fuzzing and symbolic execution methods do not make full use of the protocol state information; thus, they have difficulty reaching the deep states of network protocol implementations. Jingling Zhao  proposedA state machine model is the standard way to model a protocol. To infer the protocol, state machine inferences are used to extract a state machine model for each network protocol implementation. This approach learns the state machine by sending network protocol packets and observing the response packets. There are two types of FSMs. The first type is called the Moore machine  and the Inference algorithm: L* was the first learning automata algorithm to use an active approach [approach , and it approach  that invJames proposed symbolic execution in 1976 , which tx < 17) ∧ (x > 10 ∧ x < 20)—the same as when the path is executed with the concrete values x = 11, 12, 13, 14, 15, 16.Symbolic execution is an enhanced testing technique because it is more efficient than normal testing. It requires testing each path only once due to the symbolic value, which achieves the same effect as testing a path using all the concrete values that match the path constraints. An example of symbolic execution is shown in Figs This study presents a new model-guided symbolic execution approach to detect flaws in network protocol implementations. The basic concept is to link the program paths and states of the protocol using an FSM to guide the symbolic execution. This approach helps in exploring the deep states of network protocol implementations.The method proceeds as follows: first, we extract the message formats from the protocol specification of the target network protocol implementation. Second, we automatically infer an abstract FSM of the network protocol implementation. Third, after acquiring the message formats and the protocol model, we use the message formats to construct symbolic packets, which are used as the input of symbolic execution, and we leverage the protocol model to guide the symbolic execution to improve the coverage of both program paths and protocol states. Finally, we monitor the symbolic execution procedure; when exceptions occur, we report the crashes and record the test cases.A Request for Comments (RFC) is a document that provides a description of a protocol specification. The document contains descriptions of the services, types and formats of messages exchanged and the rules governing the reactions of the entities involved. Network developers have implemented many different network protocol software applications based on these standard protocol specifications. The relationship between the protocol description, its specification, and the implementation of the ISAKMP protocol is shown in We analyze the RFC documents of the target network protocol implementations by focusing on the description of the message formats. In our work, the message formats are manually extracted from the RFC documents.We leverage the L* inference algorithm to infer the FSM model for the network protocol implementation. L* probes a black box with a sequence of messages, listens to the responses, and builds an FSM from the responses . The comTo leverage symbolic execution for network protocol implementations, we need to inject the input packet into memory by intercepting the network-related functions and their arguments, as shown in After injecting the input packet into memory, the packets are marked as symbolic packets by replacing some of the bytes of concrete input packets with symbolic values. To reduce the path explosion problem, specific interesting bytes can be selected for replacement. The model-guided symbolic execution leverages the FSM to guide the symbolic execution to concentrate on exploring the interesting states. With FSM, we can easily obtain an input packet sequence to reach any state of the FSM to build the relationships between program paths and protocol states. This link is shown in Figs The exception monitor component tracks the test cases and produces a crash report. This module leverages dynamic kernel instrumentation to catch unhandled exceptions. It also tracks CPU states and the execution context, which are useful for analysis in the later stages. Default exception handling functions can be called for unhandled exceptions by the network protocol implementations. In the proposed work, these functions are used to track test cases and produce the crash report. For WINDOWS system, we intercept the default exception handler function: UnhandledExceptionFilter. For *nix system, we intercept the exit function of the process, whose address can be acquired from System.map file.The proposed method is implemented based on S2E, and it is a full-system selective symbolic execution platform . S2E enaThis paper proposes an extension to S2E to leverage model-guided symbolic execution and find flaws in network protocol implementations. To infer the state machines for the network protocol implementations, we adopt LearnLib , which uWe evaluate the effectiveness of our proposed method by testing several known vulnerabilities in real-world File Transfer Protocol (FTP) implementations. We compThe experiments were conducted by hosting the QEMU virtual platform on two machines, namely, a host machine and a guest machine. The host machine includes an Intel Core i7-7700K 4.2 GHz CPU with 32 GB memory and was running Ubuntu 14.04. The guest machine was running Debian Linux 7 and Windows XP SP3. Our prototype system and the traditional fuzzing tool SPIKE were running on the host machine.Comparisons of the results of the prototype system and SPIKE for vulnerability detection are shown in This study presented a new method based on model-guided symbolic execution. The proposed method associates program paths with protocol states. It uses an FSM to guide the symbolic execution, allowing it to analyze interesting deep states of network protocol implementations. We built a prototype system and used it to test for several known vulnerabilities that exist in real-world implementations of various network protocols. The empirical results indicate that the proposed method is effective at detecting vulnerabilities.In possible future work, we plan to use our method to test several other network protocols and to seek to improve the efficiency of our method.S1 File(ZIP)Click here for additional data file.S2 File(ZIP)Click here for additional data file.


Dear Editor, Brunelin et al.  recentlyThe review is reported according to the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) format, but the inclusion criteria are ambiguous. The authors do not sufficiently follow the PICO format . The authors state the inclusion criteria as: “(i) full length original articles published in English language in peer-reviewed journals; (ii) patients with OCD according to DSM or ICD-10 criteria; (iii) detailed description of the stimulation method; and (iv) the use of repeated sessions of tDCS”.Additionally, the authors state the review aims to: “provide a comprehensive overview of existing literature on the effects of tDCS applied as a therapeutic tool to reduce OC symptoms in patients with treatment-resistant OCD and to discuss future applications of tDCS in OCD”. Surprisingly, the review did not clearly report the other components of inclusion criteria like comparison and outcome measures. Running a systematic review without full knowledge about the inclusion criteria can lead to problems with assessing the validity, applicability, and comprehensiveness of the systematic review .The authors non-systematically employed articles with wide and heterogeneous methodological designs such as randomized control trials and observational studies but there was no clear selection criteria regarding the methodological designs of the studies included. Furthermore, the qualities of evidence, risk of bias and heterogeneity of the included studies have not been reported. The authors have ignored other major components of a full systematic review. The aim of the systematic review is to assess the quality of included articles and heterogeneity to disclose the risk of bias and conclude the level of evidence. The concluded level of evidence is a major part to provide the research and clinical recommendations. Moreover, the systematic reviews have to provide the effectiveness of interventions with the level or quality of included evidence ,3. Thus,The systematic review is different from other types of literature reviews. It must provide an explicit, reproducible methodology and include a systematic search that attempts to identify all studies that would meet the eligibility criteria ,4. This


This paper presents a novel framework for probabilistic crack size quantification using fiber Bragg grating (FBG) sensors. The key idea is to use a high-order extended finite element method (XFEM) together with a transfer (T)-matrix method to analyze the reflection intensity spectra of FBG sensors, for various crack sizes. Compared with the standard FEM, the XFEM offers two superior capabilities: (i) a more accurate representation of fields in the vicinity of the crack tip singularity and (ii) alleviation of the need for costly re-meshing as the crack size changes. Apart from the classical four-term asymptotic enrichment functions in XFEM, we also propose to incorporate higher-order functions, aiming to further improve the accuracy of strain fields upon which the reflection intensity spectra are based. The wavelength of the reflection intensity spectra is extracted as a damage sensitive quantity, and a baseline model with five parameters is established to quantify its correlation with the crack size. In order to test the feasibility of the predictive model, we design FBG sensor-based experiments to detect fatigue crack growth in structures. Furthermore, a Bayesian method is proposed to update the parameters of the baseline model using only a few available experimental data points (wavelength versus crack size) measured by one of the FBG sensors and an optical microscope, respectively. Given the remaining data points of wavelengths, even measured by FBG sensors at different positions, the updated model is shown to give crack size predictions that match well with the experimental observations. Fatigue cracks emanating from the edge of holes are common dangerous defects in the aircraft industry . Under cThe optical fiber Bragg grating (FBG) sensor has been widely applied to structural health monitoring applications due to several advantages over conventional sensors, such as a small physical size, insensitivity to electromagnetic interference, lightweight, capability of sensing at high temperature and environmentally-unfavorable conditions and multiplexing ability . In ordeDespite the number of approaches proposed in the literature for crack detection using FBG sensors, quantitative evaluation of the crack size using direct FBG sensors measurements is not common. One of the major difficulties is that the correlation of the physical model and the reflection intensity spectrum for varying crack sizes changes with structure geometries and working conditions. Hence, a model obtained for a specific target is not generic and cannot be directly employed for other geometry or loading configurations, as it may lead to unreliable results. Therefore, parametric crack detection experiments using FBG sensors must first be done in order to establish the crack size quantification method for each specific target system. However, it may not be practical to conduct such testing for a wide range of applications due to time and economic constraints for real engineering applications.To address this issue, efforts have been made to calculate the strain distribution along the grating using the finite element method (FEM), rather than experimentally measuring the strain field using sensor techniques, such as strain gage and FBG sensors ,19,20. FHowever, obtaining accurate fracture mechanics solutions by a conventional FEM is computationally expensive since the mesh must conform to the crack geometry, and a high level of refinement is generally required in the vicinity of the crack tip. For simulation-based crack size quantification methods, the inherent limitation of the standard FEM becomes even severer since numerous FE analyses are needed to extract strain profiles along gratings and reflection intensity spectra corresponding to different crack sizes. To alleviate the computational burden of multiple reflection intensity spectrum simulations, the extended finite element method (XFEM), originally proposed by Belytschko and his coworkers ,23, is cAnother difficulty of crack size quantification is the uncertainty in the parameters of real structures, which are typically not captured by deterministic models, such as a standard FEM analysis. For realistic engineering applications, uncertainty in crack characterization may arise due to manufacturing conditions, sensor installation variability, specimen geometry, working conditions, environmental noise, operational errors, etc. Hence, direct correlation of the crack size FEM model and real structures will yield unreliable results. This is a challenging practical problem that requires the capability of model parameter tuning with an appropriate uncertainty quantification.To resolve the above difficulties in modeling and sizing of the hole-edge crack using FBG sensors, three major efforts are made in this paper: (1) a high-order XFEM is proposed to construct the strain profile ahead of the crack tip. Compared with the conventional XFEM method, the high-order XFEM has been shown to give more accurate displacement/strain/stress fields, especially when the observation point (FBG sensor) is close to the crack tip. (2) An appropriate model is proposed to describe the correlation between the FBG measurements and crack size; and (3) a Bayesian updating method is proposed to incorporate the experimental data for probabilistic crack quantification. In this paper, the high-order XFEM and Bayesian method are combined to form a general procedure for probabilistic hole-edge crack size quantification.This paper is organized as follows. First, the framework of the proposed probabilistic crack size quantification method is introduced. Next, the FBG reflection intensity spectra simulation method is introduced on the basis of the XFEM and the T-matrix method. Following this, the simulation results are used to establish the baseline crack size quantification model and to construct the prior distribution of model parameters. The experimental measurement data from the fatigue testing on an aluminum alloy component are employed to update the baseline model parameters using the Bayesian method and to obtain their posterior estimates. The influence of measurement location is also investigated, and conclusions summarizing this research are drawn.The objective of this study is to develop an overall methodology for reliable, efficient and accurate hole-edge crack quantification using FBG sensors. We consider a solid The Galerkin approximation of the proposed problem is to seek a kinematically-admissible displacement field In the XFEM, the standard FE polynomial approximation space is augmented with a set of enrichment functions according to the physics of the problem at hand. For linear elastic fracture problems, the XFEM displacement approximation The crack-tip branch functions Williams  for the For a detailed derivation of these enrichment functions, the interested reader is referred to our previous work .L = 10 mm with an edge crack of An illustrative example is presented in the following to demonstrate that the higher-order enrichment terms can serve as an alternative to strain/stress recovery techniques ,41,42 toThe normal stress L is subdivided into M segments, each segment with an effective, constant period, i-th segment is represented as T-matrix that relates the input, transmission and reflection field amplitudes, i-th segment, respectively. The elements of i-th section are defined as:The transfer matrix method (T-matrix method) is used to construct the reflection intensity spectrum of FBG from the strain profile along the grating obtained through XFEM. The T-matrix formulation approximates the applied strain as a piecewise constant function and calculates the average period in each grating segment due to the applied strain. Substituting this local period back into the coupled equations leads to the output fields along the entire grating ,44. Assue factor , in whicpression :(10)Ti=i-th segment. Thus, with all of the T-matrices for the individual segment, the overall amplitudes are calculated as:The coupling coefficients To calculate the reflectivity, the physical boundary conditions ion (15) . The refIt is well known that the FBG refection intensity spectrum obtained from the T-matrix method is accompanied by a series of side-lobes adjacent to the Bragg wavelength. FBG with the apodization profile of the refractive index is reported to efficiently decrease undesirable side-lobes . The raiThe T-matrix method can be adjusted to accommodate the apodization of an FBG sensor by replacing the The target system studied in this paper is a plate made of aluminum alloy 7075-T6, with the dimensions of 300 mm × 100 mm × 2 mm, as shown in The domain is discretized using a mesh consisting of 49,225 linear triangular elements, as shown in The T-matrix method is used to construct the reflection intensity spectrum of FBG from the strain profile along the grating obtained through XFEM. Direct usage of the measured reflection intensity spectrum of FBG sensors for estimating crack size is difficult, and data interpretation is generally required when extracting features that are sensitive to crack sizes. The cross-correlation coefficient, which is a quantitative measure of ‘overlap’ between the measured FBG reflection intensity spectrum and a reference spectrum for the same FBG was proposed by Park . The corTypically, the maximum detection algorithm is used to accomplish this task by searching for the wavelength corresponding to the maximum power in the reflection intensity spectrum . This alation in . Before r minus the distance L between each FBG sensor and the hole-edge, as shown in A detection model using the response surface method is introduced in order to quantify the relationship between the crack size and its corresponding wavelength shift. The model is given in Equation (20):.The simulation data from XFEM and the T-matrix method are used to estimate the model parameters, where the coefficients Given a value of wavelength shift, solving the parameterized model gives the corresponding crack size. It should be noted that this quantification model is an idealized model that is based on the numerical model. Nonetheless, realistic engineering applications introduce discrepancies due to uncertainties in crack characterization, manufacturing conditions, sensor installation variability, specimen geometry, working conditions, environmental noise, operational errors, etc., which are not captured in this idealized model. The effectiveness and accuracy of this model for real structures will be verified by the following experimental data.To validate our methodology, a fatigue crack damage detection experiment platform is developed, and FBG sensors are employed to extract the damage index. The overall experimental setup of the hole-edge crack detection consists of three major parts: the optical sensing system, a fatigue crack measurement system and a fatigue load-cycling system, shown in In the simulation, the crack is assumed to propagate along a straight path, whereas the crack propagation in the experiment is curved. Hence, in real applications, there is additional uncertainty related with crack propagation, which justifies the Bayesian updating procedure proposed in the current work. Before the fatigue testing, the reflection intensity spectra of the FBG sensors under free loading state are measured as a reference. These spectra are used for the inversion of the property parameters of FBG sensors based on the T-matrix method . PropertAs mentioned before, the wavelength shift extracted from the measured reflection intensity spectra is employed as a damage sensitive index. The baseline center wavelength is measured when no loading is applied, hence in a loading-free state. The numerical correlation model between the wavelength shift and crack size of Equation (21) is used to predict the crack size based on the measured FBG data. The baseline model obtained using the XFEM simulation deviates from the experiments, and therefore, a model updating is necessary. In this section, the Bayesian updating is used for this purpose with added information provided by the FBG1 sensor measurements. The calibrated and updated model is then applied for crack prediction with data obtained by the FBG2 sensor to investigate the influence of sensor location.In the aforementioned damage detection problem, the newly-observed information can be incorporated to obtain a better estimation of the system, including its parameter distribution, model accuracy and future performance. Bayes’ theorem has been extensively used in various structural health monitoring applications ,56,57,58For the crack detection problem in this paper, the sources of uncertainties usually include, but are not limited to, model parameters, the measurement data and the damage characterization. Accounting for these uncertainties is essential for detection accuracy and the reliability of the target system. The FE result is more sensitive to structural damage by eliminating the interference of other factors. With the computational efficiency and accuracy of the XFEM in crack propagation modeling, the simulation result is easily obtained. In our approach, the baseline detection model describes the strain profile change due to crack propagation with the corresponding specific physical meanings; therefore, the baseline model can be adapted to realistic engineering applications. However, the uncertainties in measurement data and crack orientation propagate to the detection results. The baseline detection model is not sufficient to directly detect and quantify crack damage for a realistic structure. Hence, the detection model in Equation (20) can be rewritten as:n groups of measurements, the likelihood function Assume that the error term variable , expressSubstituting Equation (25) into Equation (22), the posterior distribution of The posterior distribution of each parameter is estimated by the samples generated using the Markov chain Monte Carlo (MCMC) method. The posterior belief on crack size is then estimated from the posterior distribution of the updated parameters. A detailed discussion on MCMC is well documented in the open literature ,62 and iAs described in The prior distribution of ribution . The meau of these five parameters is obtained and given in Equation (28).The updating procedure using measurements is performed incrementally. The measurement here refers to the wavelength shift extracted from the FBG1 reflection intensity spectrum in the fatigue testing and the corresponding crack size. The updated crack size quantification model is given as:x-axis is the measured crack size by optical microscope, and the y-axis is the predicted crack size. It can be seen that the median prediction of the model generally characterizes the crack size, and all of the data points are within the 95% prediction bounds.To further validate the prediction and updating performance, the median and 95% bound predictions are shown in The reflection intensity spectrum is affected not only by the size and orientation of the crack defect itself, but also by its distance to the deployed FBG sensors. The FBG2 sensor are placed 15 mm from the hole-edge and 8 mm from the FBG1 sensor, respectively. The crack size versus the wavelength shift of FBG2 is shown in This paper presents a general procedure for a probabilistic updating of a crack size quantification method using fiber Bragg Grating sensor and extended finite element method (XFEM). A high-order XFEM method is employed to effectively improve the accuracy of the simulated strain field at the crack region. Based on the simulated strain field, the T-matrix method is used to construct the reflection intensity spectra of FBG sensors, for various crack sizes. The wavelength shift calculated from the reflection intensity spectra is identified as a damage sensitivity index. A baseline model is proposed to describe the correlation between the crack size and the wavelength shift. The Bayesian theory is employed to quantify the uncertainties in realistic applications and to update the detection model. The experimental measurement data from the fatigue testing of an aluminum alloy component is used as the application of the detection strategy proposed. The influence of measurement location is also included. Several conclusions are drawn based on the current study.The high-order XFEM shows great potential in structural health monitoring for modeling crack propagation with high accuracy and low computational cost. Combined with the T-matrix method, the reflection intensity spectrum interacting with different crack sizes can be constructed efficiently.The damage wavelength shift index is found to be effective for hole-edge crack size quantification. The detection model can yield satisfactory prediction results for FBG sensors with different locations.The proposed methodology uses simulation results as a baseline model and incorporates field measurement data to update the model for real structures. The example presented demonstrates the successful applicability of the approach in crack detection.The procedure described in this study is not limited to a particular model, and other data-driven models and with more complex physics can also be used. It should be noted that all of the FBG sensors are placed on the same side of the pre-crack. Therefore, the updated detection model can be used for FBG sensors with different locations. More complex configurations of FBG sensors are currently in ongoing research. Future studies will also be focused on more robust quantification methods for different crack propagation paths and more complex geometries. The crack generated from both sides of the hole commonly occurs for realistic engineering cases. The prediction model established from FBG1 should be applicable for sensor measurements from the same configuration of crack generated opposite the pre-crack. Uncertainties related with different sensor manufacturing, crack characterization, etc., are reduced through Bayesian updating by incorporating the wavelength shift and relative crack length of sensor FBG1 in fatigue testing. More thorough studies for multiple cracks and connection cracks between two holes are planned as future work.The FBG sensor senses the strain gradient along the grating due to the stress concentration at the crack tip. The closer the distance to the crack tip, the greater the impact on the reflection intensity spectrum. Therefore, ideally, the sensor should be placed as closer to the hot-spots as possible. However, the crack orientation is usually uncertain for realistic applications. In some extreme cases, the crack will break the FBG sensor. The reliability and durability of sensors is one of the dilemmas for the proposed method and also for the sensor-based structural health monitoring systems. In the current study, sensor location has a rare influence on the performance of the proposed method. Therefore, additional sensors installation can be an option for this problem. The applicability of the proposed method to other structural and material systems, such as composites, also needs further investigations.


An accurate initial alignment must be required for inertial navigation system (INS). The performance of initial alignment directly affects the following navigation accuracy. However, the rapid convergence of meridians and the small horizontal component of rotation of Earth make the traditional alignment methods ineffective in polar regions. In this paper, from the perspective of global inertial navigation, a novel alignment algorithm based on pseudo-Earth frame and backward process is proposed to implement the initial alignment in polar regions. Considering that an accurate coarse alignment of azimuth is difficult to obtain in polar regions, the dynamic error modeling with large azimuth misalignment angle is designed. At the end of alignment phase, the strapdown attitude matrix relative to local geographic frame is obtained without influence of position errors and cumbersome computation. As a result, it would be more convenient to access the following polar navigation system. Then, it is also expected to unify the polar alignment algorithm as much as possible, thereby further unifying the form of external reference information. Finally, semi-physical static simulation and in-motion tests with large azimuth misalignment angle assisted by unscented Kalman filter (UKF) validate the effectiveness of the proposed method. With the development of worldwide transportation, the importance of polar navigation is increasing. The polar navigation technology can ensure the safety and reliability of vehicles when they sail in polar regions. However, due to the special geographical and electromagnetic characteristics , common However, in polar regions, the rapid convergence of meridians makes the traditional north-oriented mechanization ineffective because the characteristic that the y-axis always points toward the north would result in both mechanical and mathematical difficulties in a platform instrument or strapdown attitude matrix ,7. As a Since strapdown INS (SINS) is a dead-reckoning navigation system, an accurate initial alignment is required before navigation . The perFor initial alignment operated in the polar regions, two major problems need to be addressed. Firstly, the navigation errors with the traditional north-oriented frame are enlarged sharply . In tradFor selection of attitude reference frame in polar regions, the wander frame, the grid frame, and the transverse frame have been proposed for polar navigation. However, the wander attitude reference frame just can be applied in most of polar regions because the definition of wander angle is invalid at 90-degree latitude . In the On the other hand, for polar alignment, the external observation information of filter also needs to be selected validly. Due to the small horizontal component of rotation of Earth, some observation information of filter applied usually in mid-low latitude would be invalid in polar regions, especially in and near pole. For initial alignment, the external position or velocity information is usually selected as the observation information of filter ,24. Howelignment ,21. ThisAccording to above analysis and considering the features of global navigation of SINS, a polar alignment algorithm, which can directly obtain the attitude matrix relative to local geographic frame when the initial alignment is finished, is expected to be proposed. The transformation of attitude matrix is not influenced by position errors. Moreover, a unified polar alignment algorithm is also expected to be explored as much as possible. Then the form of external observation information of filter can also be unified for polar alignment in future. Furthermore, it would also be advantage to implement the acquirement of the observation information in a form from the external navigation device. On the other hand, since the definition of coordinate system just like that in the traditional Earth frame, the transverse frame is more immediate and convenient . TherefoThe purpose of this research is expected to provide a new idea to solve the problem of polar alignment and explores a unified polar alignment algorithm as much as possible. The rest of this paper is organized as follows: the proposed polar alignment scheme based on pseudo-Earth frame and backward process is presented in The pseudo-Earth frame C˙Since the pseudo-Earth frame is a generalized transverse Earth frame and the pseudo latitude at the initial position  and (2) into Equation (6), the initial transformation matrix From Equation (7), we can obtain that the Remark. If the initial latitude of vehicle relative to-frame is 90°, the initial longitude of vehicle can choose the arbitrary value with. Then, the pseudo-Earth frame is built based on the selected longitude. The initial azimuth angle can still be obtained by the initial pseudo azimuth angle from Equation (7). Nevertheless, we should notice that the azimuth angle relative to-frame in pole is the angle between the vehicle direction and the selected longitude.With electronic technology development, the capabilities of the navigation computer become large. The backward process of data has been proposed for initial alignment ,28,29. BFrom Equations (3)–(5), the discrete form of forward pseudo navigation equations can be given as follows:Then, through further transposition, the backward pseudo navigation equations can also be obtained:Moreover, assuming the discrete time index for backward computation is Defining According to the analysis above, the backward strapdown attitude matrix should equal to the forward one at the same time. As a result, the corresponding terms of Equations (11) and (14) should also be equal. Comparing Equation (11) and Equation (14), we can obtain:Considering the corresponding relations of time index from Obviously, On the other hand, it is also expected that the backward pseudo position could be identical to the forward ones at the same time. Thus, comparing the corresponding terms of Equations (13) and (16), we can similarly obtain:Then, we can further have:From Equation (23), we can conclude that the sign of the pseudo velocity in backward process should be opposite to the corresponded one in forward process. Therefore, after taking opposite, Equation (12) should equal to Equation (15), correspondingly. Then we can get:From Equation (24), we have:From Equations (25) and (26), the signs of the outputs of accelerometer and the gravity in backward process should be the same as the forward ones. Moreover, substituting Equation (23) into Equation (27), we can obtain:From Equation (28), we can conclude that the sign of the angular rate of rotation of Earth in backward process needs to be taken negative. In addition, combining with Equations (21), (28), and (29), the outputs of gyro in backward process can be obtained by:From Equation (30), the sign of outputs of gyro should be opposite to the forward one in the backward process.According to above derivation, the expectation that the pseudo starpdown attitude matrix and the pseudo position in backward process should be the same as the forward ones at the same time can be realized. While the pseudo velocity of backward process is opposite to the forward one at the same time. Moreover, the signs of the outputs of gyro and the angular rate of rotation of Earth in backward process should be opposite to the corresponded ones with the forward process. The others are all the same as the forward ones. As a result, the expectation that the pseudo attitude matrix of vehicle at initial position is obtained at the end of alignment phase can be realized by backward process with pseudo navigation mechanization. Further, the initial transformation matrix From the analysis above, the alignment algorithm based on pseudo-Earth frame and backward process can be applied to solve the problem of polar alignment. The attitude matrix relative to local geographic frame is obtained when the initial alignment is completed. Moreover, it is also easy to implement without the influence of position errors and the cumbersome computation since the initial transformation matrix is a fixed known constant matrix and never influenced by initial position of vehicle from Equation (7). Supposing the IMU data are saved in first forward fine alignment and the sampling index of saved data sequence is 1 to m. Moreover, the computed pseudo attitude matrix In addition, the implementation of the proposed alignment algorithm is also illustrated in Considering that an accurate coarse alignment of azimuth is difficult to obtain in polar regions, the dynamic error modeling with large azimuth misalignment angle is designed in this section. Defining the pseudo misalignment angle between computed navigational frame (itten as :(31)Cnn′Defining the angular velocity of given by :(32)ωnn′From Equation (32), we have:Only considering that the pseudo-azimuth angle is a large misalignment angle here, Equations (31) and (34) can be rewritten by:Due to the existence of misalignment angles and gyro drifts, the differential equation of actual computed pseudo strapdown attitude matrix can be written as:SinceSubstituting Equations (3), (37), and Multiplying the left by Then, Equation (42) can be rewritten by:Substituting Equation (38) and From Equation (33), the pseudo attitude error equation with large azimuth misalignment angle can be obtained by:Since the measurement errors and calculation errors exist, the differential equation of actual computed pseudo velocity can be written as:Then the pseudo velocity error equation can be obtained as:Ignoring Defining Aiming at the nonlinear error models derived above, a suitable nonlinear filtering should be selected to replace the standard Kalman filter (KF). Because the extended KF (EKF) has to bear a number of drawbacks such as cumbersome derivation, evaluation of Jacobian matrices, and larger linearized errors and so on ,32, as aAssuming the measurement errors of accelerometer and gyro are mainly composed of the constant error and zero mean Gaussian white noise. Then we have According to the nonlinear pseudo error equations in Further, the general nonlinear system dynamic model can be written by:In addition, the differences of the pseudo velocity projected in On the other hand, the block diagram of polar alignment assisted by UKF is also shown in In order to confirm the performance of the proposed polar alignment scheme, both semi-physical static simulation and in-motion tests, which are assisted by UKF under the large azimuth misalignment angle, are conducted in this section.Due to the restriction of geography, the semi-physical static simulation is conducted. As attributed to the rotation of Earth, the real static angular velocity measured by gyro at different geographical positions can be exactly known. The angular velocity measured by gyro can be expressed as:Therefore, the static outputs of gyro in polar regions can be simulated by the measured angular velocity in non-polar regions from Equation (57). The static experimental data are collected with fiber optical gyroscope (FOG) SINS, which fixed in a tri-axis high-precision turntable. The attitude reference of IMU is provided by turntable. The gyro drift and the accelerometer bias of test IMU are less 0.01°/h, 100 μg respectively. The local longitude and latitude is 126.6778° and 45.7778°. The pitch, roll and azimuth of IMU relative to the Then, the static alignment with the pseudo-Earth frame is carried out, which is assisted by UKF under the condition of large azimuth misalignment angle. In this simulation, the initial pseudo misalignment angles As shown in Due to the restriction of geography, the actual test data of in-motion alignment are difficult to obtain. Consequently, only a vehicle test of low latitude, which is assisted by UKF under the condition of large azimuth misalignment angle, is firstly conducted to verify the feasibility of the proposed alignment scheme. Then, the polar alignment tests with the simulation data are carried out to validate the superior performance of the proposed method. In addition, in the process of polar alignment, the external velocity observation information projected in the navigation frame  SINSs as shown in found in ,28,29. HAs shown in Next, the superior performance of the proposed method for polar alignment is verified by simulation data due to restriction of geography. In polar in-motion alignment simulations, the outputs of gyro and accelerometer are generated by a strapdown INS simulator. The constant and random drifts of gyro are 0.01 °/h and 0.001 °/h, respectively, and the constant and random biases of the accelerometer are 100 μg and 10 μg, respectively. The initial values of attitude relative to As shown in On the other hand, in Furthermore, comparing In this paper, from the perspective of global inertial navigation, a generalized transverse Earth frame built based on the initial position of vehicle  is proposed as the attitude reference frame for polar alignment. Then, a polar alignment algorithm based on the pseudo-Earth frame and backward process is proposed to implement polar alignment. Through the proposed algorithm, the attitude matrix relative to local geographic frame is obtained without influence of position errors when the initial alignment is completed, and there is less cumbersome derivation and low computational complexity. As a result, it is very convenient to access any one of the following polar navigation systems without any additional workloads and complexities. Moreover, it would also be advantageous to unify the forms of polar alignment and the external observation information of filter, thereby further unifying the implemented form of the reference information acquired from the external navigation device. In addition, the backward process can still accelerate the filtering estimation and decrease the alignment time. Finally, the semi-physical static simulation and in-motion tests with large azimuth misalignment angle assisted by UKF are carried out to verify the proposed polar alignment algorithm, and the results exhibit the superior performance of the proposed alignment algorithm in polar regions.


Xenopus laevis, depends on circumblastoporal convergence forces generated by the marginal zone (MZ), but direct evidence is lacking. We show that explanted MZs generate tensile convergence forces up to 1.5 μN during gastrulation and over 4 μN thereafter. These forces are generated by convergent thickening (CT) until the midgastrula and increasingly by convergent extension (CE) thereafter. Explants from ventralized embryos, which lack tissues expressing CE but close their blastopores, produce up to 2 μN of tensile force, showing that CT alone generates forces sufficient to close the blastopore. Uniaxial tensile stress relaxation assays show stiffening of mesodermal and ectodermal tissues around the onset of neurulation, potentially enhancing long-range transmission of convergence forces. These results illuminate the mechanobiology of early vertebrate morphogenic mechanisms, aid interpretation of phenotypes, and give insight into the evolution of blastopore closure mechanisms.Indirect evidence suggests that blastopore closure during gastrulation of anamniotes, including amphibians such as  Major morphogenic (shape-generating) movements in the development of multicellular organisms occur by integration of local, force-generating activities and force-transmitting properties of individual cells into ‘morphogenic machines’ that act across the tissue-level length scale. Understanding the physical aspects of tissue movements is essential for understanding how cells and gene products function in morphogenesis , and thuXenopus laevis, in many species of invertebrates and anamniotes, as well as in a few amniotes , and it was thought to occur only ventrally  initiated in the form of the VAZ  and neural (NIMZ) tissues in unencumbered (un-restrained) giant sandwich explants e.g.  undergo The generation of force by IMZ tissues lying around the blastopore  was assayed by cutting these tissues from the embryo and making stable constructs (explants) with plastic attachment strips. As this tissue converged mediolaterally, the tension generated was measured by observing the deflection of a probe pulled by a cleat on one of the attachment strips , Video 4Explants for Giant sandwiches were made by cutting the embryo ventrally and trimming off most of the vegetal endoderm and some of the animal cap region . Two FibAssays of standard giant sandwich explants , beginniImmunohistochemical staining showed normal differentiation of somitic and notochordal tissues undergoing CE in mechanically loaded (encumbered) sandwiches  extension through G+2hr (stage 10.5) . To measLate giant explants, made as giant sandwich explants  but remaUnencumbered giants converge (=negative strain) uniformly along their mediolateral axis  and by the yield strength of the tissues involved. We use ‘stall’ here in the sense that cellular convergence can no longer proceed, e.g. because the tension across individual cells engaged in MIB is high enough that they can no longer pull themselves between each other; it is less clear what factor limits convergence by CT. Once the yield stress (force/area) for a tissue is reached, forces generated by CT and CE result in plastic deformation of that tissue, and convergence in one region is balanced by strain in another. Understanding the behavior of the explant requires an understanding of the biomechanically complex structure of the explant, and its dynamic changes over time. Also, it may be assumed that yield strength and stall force as they apply to the explant are not ‘all or nothing’ effects; there are likely to be multiple structures with different rates of viscous flow leading to different rates of plastic deformation across a range of tensions. And because each cell is a motor, and is differently arranged within the tissue, they will reach their stall force at different over-all tensions across the explant.We propose that initially (first 20–30 min) convergence proceeds rapidly until the stall force of the machine(s) is reached. At this point, force increase slows, advancing only as additional cells are recruited, either by the progression of MIB into more posterior tissues, or as shear allows convergence to proceed such that more MIB expressing cells are acting in parallel see , increasOur model above, that encumbered explants are increasing tensile force as they recruit more cells into MIB, predicts that applying additional exogenous tension to an explant should prevent further force generation by the explant until shear has allowed enough convergence such that the instantaneous stall force rises above the current level of tension. To test this, we applied additional tension to explants at various stages through the end of the plateau, by increasing the strain on them . ExplantIf the force developed by the explant at a given time does represent an instantaneous stall force, we expect that decreasing tension by decreasing strain should allow more rapid convergence until the system maximum is reached again. Reducing strain by moving the anchor toward the probe  enough tWe tested the idea that less force was being generated during the plateau phase, based on the premise that reduced force generation results in lower stored elastic energy, which is presumably continuously dissipated by long term viscous tissue flow, loss to heat, etc. While not quantitative, the immediate, rapid elastic recoil  strains when a known amount of tensile stress is applied to it. By contrast, structural stiffness of a composite structure depends on the mechanical properties and arrangement of the individual elements that make up the structure, such as a bridge made of steel beams, or in our case, of the individual tissues and cell types and ECM that make up the explant. Elastic modulus of the individual elements may be inferred from a measurement of structural stiffness if the elements and architecture of the structure can be accurately modeled and the strains in each element measured, an approach that has been applied to embryonic tissues  when a known amount of tensile stress (force per unit area) is applied to it. Structural stiffness and the elastic modulus are different specific mechanical properties that describe a material’s stiffness and that can be measured experimentally with similar methods. The ues e.g. , but is We could not measure a true elastic modulus of embryonic tissues because the tests are not isometric nor are the explants uniform or homogenous (see Appendix 5). Instead we estimated structural stiffness along the mediolateral axis of the explants with a uniaxial, tensile stress relaxation test , and estFrom the time-dependent stress decay , parametD180° and V180° sandwich explants also showed a trend of increasing stiffness from gastrula to neurula stages  with botAlthough a bulk measure for the explant as a whole, our measures of structural stiffness provide baseline estimates for mechanical behaviors of embryonic tissues. Somewhat surprisingly, all explants types measured, although comprised of different sets of tissues, exhibit similar structural stiffness, and their stiffness increases by about the same amount from gastrulation to neurulation.To understand how differential strain of tissues, as found above in our investigation of the plateau, relates to blastopore closure, we measured spring stiffness for each explant type over time. Tissue spring stiffness (force/strain) reflects the deformation or strain that a tissue of a specific cross sectional area will undergo in response to a force, and so can be used to predict the strain in different tissues across the explant when subjected to the same tension. Standard giants, D180°, V180° and AC explants all showed a trend of increasing spring stiffness between gastrula and neurula stages . StandarGiant sandwich explants (IMZ and NIMZ) generate and maintain a consistent pattern of convergence force throughout gastrulation, neurulation, and into the tailbud stage. Correction for drift of the two probes of different stiffness’s yielded similar force profiles, suggesting that our results are robust to different sources of error. Wound healing at the edges of the explants  or a resXenopus gastrula .Previously, post-involution CE was thought to generate the convergence force driving blastopore closure, as well as the post-involution anterior-posterior extension that elongates the body axis . CT was gastrula  but its gastrula , before gastrula . Second,gastrula , which igastrula . Thus CTgastrula . These f embryos  and normiobambae  close thiobambae  and symmAnalysis of the force profiles of the various explants, in the context of the degree to which they express CT and CE, illuminates much about the mechanics of gastrulation and blastopore closure. We represent the capacity of CT and CE to generate and transmit tensile force in the explants at different stages of expression of CT and CE (A-D) as a linear array of motors and springs  as they involute, beginning in the presumptive anterior somitic mesoderm with formation of the Vegetal Alignment Zone (VAZ), and progressing medially and posteriorly  Figure . At thisIn the embryo, however, the stronger MIB/CE region never acts in direct series with an intervening, weaker CT region; instead MIB originates at the lateral boundary of the somitic mesoderm at its junction with the vegetal endoderm, and is anchored there  is under-estimated by our measurements, because CE results in extension of many of the cells expressing MIB away from the zone directly between the sleds, and these cells therefore pull on the sled/anchor system at an increasing angle . HoweverThe convergence of the somitic mesoderm during late neurulation involves MIB but also columnarization (thickening) see , which fFinally, when we explant tissue it expends its stored elastic energy as it converges rapidly, and consequently its subsequent convergence is slower than intact embryos and the additional force is measured. Observed force is further reduced by friction, perhaps by 0.2 μN, despite the slick agarose pad beneath the explant and by the glass beads beneath the sled. Therefore force measured here should moderately underestimate that generated at the instantaneous stall force or yield strength of the embryo, for a given extent of morphogenic progress.Xenopus embryonic tissues stiffen about two fold along their mediolateral axis around the end of gastrulation, regardless of the tissue type, suggesting a systemic mechanism. Such increases may have many causes, such as increased ECM deposition  of the deep mesoderm, the cell population we expect is effective in bulk force production during the tractor pull . We estiXenopus . In ongoing work, we are testing the idea that the deployment of CT and CE varies with the egg size and the amount and distribution of yolk and the mechanical challenges changes in these parameters present to the morphogenic machines of gastrulation.The temporal, spatial parameters of the CT to CE transition, and its biomechanical implications, represent a major morphogenic and regulatory transition. The force traces show that CT-generated forces occur early and throughout the IMZ prior to its involution, whereas CE, and its underlying cell behavior, MIB, are expressed after involution and progressively, with increasing numbers of cells acting in parallel with time . This prXenopus  and 6 exla stage . At the ternally . Others,Our findings illustrate that CT is one of the morphogenic machines that contribute to blastopore closure, along with CE and Vegetal Rotation  and thatX. laevis embryos were obtained and cultured by standard methods X. laevis methods . Explant methods . Animal De-jellied embryos were placed in dishes made of 15 mm transverse sections of 60 mm diameter PVC pipe with Saran wrap stretched across the bottom, irradiated 6 or 7 min from below at about 35 min post fertilization on a UV trans-illuminator  and left undisturbed for at least an hour to avoid accidentally rotating and thus dorsalizing them . Embryoshttp://rsb.info.nih.gov/nih-image/; RRID:SCR_003073), Object Image  or Image J .Explant morphometrics (see Appendix 3) were done with NIH Explants were attached to two polyester shim stock strips , both 25 μm thick x 0.8 to 1.5 mm wide, and one, the ‘anchor’, 4 to 8 mm long, and the other, the ‘sled’, 3 to 5 mm long . A cleatMediolateral tensile force was measured as the explants pulled the cleat of the sled against the probe. In most cases, the probe was placed adjacent to the cleat such that both probe and explant were unloaded at the start of the experiment. In others, the explant was ‘pre-strained’ about 25%, by moving the cleat against the probe, and then away from it, or the explant was left under a pre-tension. Measurements were made to tailbud stages (~20 hr) when the probe deflection generally ceased to increase significantly. The cleat was then backed off from the probe to determine the resting position of the probe. Tension could be adjusted during a force measurement by moving the anchor away from or toward the sled, to decrease or increase strain as in .P is the spring constant of the probe in μN/μm. Drift was determined from the unstressed position of the probe before and after the assay, the difference interpolated linearly over the duration of the test and subtracted from the probe movement. Means of force traces were plotted, with the mean of hourly intervals and the standard error of the hourly mean shown as error bars.Probe position was recorded every six minutes, and probe displacement was translated into force by the following:For estimates of stiffness , anchor-inf or stiffness at infinite time , Ssp or instantaneous stiffness, and τ, the relaxation time constant, representing the half life of stress-relaxation. η, the coefficient of viscosity, is related to τ by:We modeled the time (t) dependent structural stiffness (SS(t), Pascals) using the following viscoelastic spring and dashpot model . Alternatively, an explant ‘spring stiffness’ constant (KE) was also calculated. See Appendix 4 for further details.The model assumes that instantaneous stiffness is reduced by viscous flow or remodeling of intra- or inter-cellular structures, until a residual stiffness, representing stable elastic elements of intra- or inter-cellular structure, is reached. SS(t) was calculated using the cross-sectional area of the tissue, force measurement, and observed strain over time, and two analytical techniques were used to generate the parameters S In the interests of transparency, eLife includes the editorial decision letter and accompanying author responses. A lightly edited version of the letter sent to the authors after peer review is shown, indicating the most substantive concerns; minor comments are not usually included.Xenopus blastopore closure and body axis elongation" for consideration by eLife. Your article has been reviewed by three peer reviewers, and the evaluation has been overseen by a Reviewing Editor and Didier Stainier as the Senior Editor. The reviewers have opted to remain anonymous.Thank you for submitting your article "Large, long range tensile forces drive convergence during The reviewers have discussed the reviews with one another and the Reviewing Editor has drafted this decision to help you prepare a revised submission.Summary:Xenopus embryo morphogenesis during gastrulation. The results of the work address important, long-standing questions in the field regarding tissue mechanics of vertebrate development and the studies appear to be rigorously performed and analyzed. Overall the work is of high quality and should represent an important contribution to the literature provided that most of the suggested revisions by the reviewers can be addressed.In this manuscript the authors clarify the specific biomechanical role of convergent thickening and convergent extension that impacts Essential revisions:Much of the writing and presentation of the results is hard for non-specialized researchers to easily understand, thus revising the text throughout, and particularly the Introduction and Discussion, is needed to make the fundamental concepts more readily accessible and comprehensible. Consultation with a non-expert in the details of this work could help to make suggestions to the figures and text, or at least specifically point out elements that could benefit from some such clarification.Validation and/or application of the proposed model should be performed to demonstrate its relevance to tissue morphogenesis.The rationale for, and implications of, the stiffness calculations for CT/CE-driven morphogenesis should be further clarified.Reviewer #1:Xenopus embryos. They focus on two different morphogenetic machines believed to drive this convergence in the involuting marginal zone (IMZ) tissue of the gastrula: convergent thickening (CT) and convergent extension (CE).In this manuscript, the authors define the biomechanical basis of the convergence (tissue shortening) forces that drive blastopore closure and extension of the body axis in Using a custom-built mechanical measuring device  to directly quantify the forces generated by IMZ explants, the authors show that, during early gastrulation, CT generates circumblastoporal tension throughout the (pre-involution) IMZ. They then identify a transition point at mid-gastrulation, after which CE contributes progressively more force .The manuscript also describes stress-relaxation tests to calculate the tensile stiffness of IMZ tissues . From these endeavors, the authors conclude that all tissues of the embryo gradually increase in stiffness by the end of gastrulation.Xenopus gastrulation.Previously, the relative contribution of CT vs. CE in convergence was not known, and CT was thought to be limited to only the ventral sector of the gastrula. The data presented in this manuscript clarifies the independent spatiotemporal influences of CT and CE on convergence, and quantifies the force dynamics involved at different stages of development. The authors conclude with an elegant model of the function of CT and CE as integrated spring (CE) and motor (CT) pulling mechanisms, offering a deeper, biomechanical understanding of the convergence forces likely acting in vivo during 1) It is not entirely clear (to me) how the tensile stiffness calculations are related to the force measurements, nor how stiffness was used to inform the model in Figure 6. The rationale for, and implications of, the stiffness calculations for CT/CE-driven morphogenesis should be further clarified.2) The conclusions regarding the timing and independence of CT vs. CE are interesting and could provide a reinterpretation and refinement of our understanding of the biomechanical mechanisms driving gastrulation. However, it is difficult to predict the impact and significance of this work without some sort of validation or application of the proposed model. For example, the tractor pull apparatus could be used to biomechanically tease apart a "non-specific" blastopore closure/gastrulation phenotype , thus testing the validity of the model and demonstrating its relevance for understanding morphogenesis.Reviewer #2:Xenopus gastrulation literature on three levels. First, it provides new data on the patterning of forces during gastrulation in one of the key model system for understanding this important developmental process. This is important, as the key role of mechanics is now well appreciated, but we still lack solid data on many aspects of gastrulation in vertebrates.This paper is a welcome addition to the canon of Second, it brings to the fore the previously under-appreciated process of convergent thickening, which relies on radial intercalation rather than mediolateral cell intercalation. Indeed, we are seeing in the last year something of a renaissance of interest in radial intercalation after over a decade of obsession specifically with the mediolateral variety. So this aspect sis quite timely.Xenopus embryos close their blastopores and how do other frogs – that express convergent extension only very late – close their blastopores.Third, through the first two, the paper provides answers to several longstanding puzzles in the field, including but not limited to: why do ventralized Thus, the paper has impact on developmental and evolutionary issues, as well tissue mechanics.The data appear solid throughout (I say "appear" only because I am not an expert in mechanics). And the while the writing is dense, so too are the data, so it seems reasonable here.Reviewer #3:Xenopus blastopore closure and body axis elongation" by Shook et al., provides direct evidence that two morphogenetic movements, convergent thickening (CT) and convergent extension (CE) of the marginal zone (MZ), generate tensile convergence forces to close the blastopore and elongate the body axis in Xenopus. The authors, measured these forces in several MZ explants  that undergo CT and/or CE using a customized "Tractor Pull" device. The authors show that the tension developed by giant explants is increasing in two distinct phases separated by a plateau phase. Interestingly, the first phase of force increase is conserved in explants that do not undergo CE , suggesting that during this period (onset of gastrulation) CT is the main source of the circumblastoporal convergence tension, providing thus new insights into the yet non-well defined role of CT in Xenopus development. The second phase of force increase is at late neurulation which mainly coincides with CE behavior and stiffening of these tissues. Overall, this work confirms the long-standing notion that convergence tensile forces close the blastopore in amphibians. The finding that CT plays a major role in this process is interesting especially to the researchers of Xenopus developmental biology, since it is a morphogenetic movement that was not well-described before and it could provide a new avenue of interpreting phenotypes related to blastopore closure. The experiments are technically well performed and describe nicely the mechanical input of the two morphogenetic events (CT and CE) in Xenopus morphogenesis. However, the following major concerns need to be taken into consideration before publishing this study in eLife.The manuscript "Large, long range tensile forces drive convergence during Xenopus developmental biology, it would be valuable to characterize at the cell and molecular level the differences between CT and CE (i.e. which cell behaviors and which molecules can polarize the cells to first converge and thick the tissue and then switch to elongation). Such mechanistic insight will be extremely interesting for the field of mechanobiology, since it will explain how different cell/molecular behaviors lead to distinct force generating processes that shape differently the tissue (please see below).A) Although the authors mention that CT and CE are evolutionary conserved, in order for this study to be highly appreciated by a wider audience beyond the field of Xenopus gastrulation (for specific comments please see below).B) Although the authors provide several diagrams to explain to the reader the complexity of blastopore closure, the manuscript and figures are still very hard to be understood by researchers non-specialized in Major points:1) It was previously shown that inhibition of the PCP pathway in the dorsal MZ leads to inhibition of CE and defects in blastopore closure. In contrast, inhibition of the PCP pathway in the ventral MZ only leads to a slight delay of blastopore closure until mid-gastrula stages (until which CT is mainly occurring), indicating that CT might not rely on PCP signaling . In order for the authors to provide cellular and molecular mechanistic insights, it would be worth to express Xdsh either dorsally or ventrally and examine if and how PCP signaling affects the convergence force generated by CT in all four types of explants, and how this can be correlated with defects in morphogenesis of the sibling embryos. Such experiments would not only support the impact of this study for the interpretation of the phenotypes associated with defective blastopore closure but will also provide evidence for differential molecular control of polarity during CT and CE (point A).2) The giant explants exhibit two force increase phases separated by a plateau phase. Although the authors suggest that this could be associated with the progression of MIB, it would be relevant to show with high resolution time lapse imaging how cell behavior is changed in the different phases  (point A).3) As mentioned in the text, CT was previously described to take place at the ventral MZ and later in development , but here the authors show that it takes place in the whole MZ earlier in gastrulation. It would be useful to describe this finding further by showing images or time lapses of how CT progresses in both embryos and explants. Sagittal sections of ventral vs. dorsal explants will also be useful to address any differences in the timing and extent of CT in the two regions.Xenopus blastopore closure and body axis elongation" for further consideration at eLife. Your revised article has been favorably evaluated by Didier Stainier (Senior Editor), a Reviewing Editor, and three reviewers.Thank you for resubmitting your work entitled "Large, long range tensile forces drive convergence during The manuscript has been improved and it can be accepted pending the deposition of the other referenced manuscript  to bioRxiv, as previously discussed. Please work on this and resubmit when you're ready, citing the preprint and including a formal reference in the References list.The full reviews are appended below for your consideration.Reviewer #1:This revised manuscript by Shook et al. investigates the biomechanics of convergent thickening (CT), a force-generating process previously implicated in blastopore closure during amphibian gastrulation. In this paper, the authors illuminate the nature of this process with quantitative biomechanical analyses of explanted amphibian embryonic tissues.The reviews of the original manuscript asked authors to 1) revise the text throughout to provide clarification for non-specialists in the field, 2) include an experimental application/validation of the model, and 3) explain the rationale for, and implications of, the stiffness calculations. Below I address the perceived effectiveness of the revised manuscript in addressing each of these points.1) The Introduction and Discussion of the revised manuscript are not any more streamlined than the original; however, the flow of information is more accessible to the non-specialist.My main concern is that the new version of the Introduction now refers to unpublished data/conclusions about CT in an as-yet-unfinished additional manuscript .For example:- "…our understanding of CT is that the underlying surface tension based mechanism does not require a polarized cell behavior, only motility to maximize high-affinity cell-cell contacts ." - "These events are based on…characterization of CT ." - "IMZ explants from ventralized embryos show a rapid, near uniform CT throughout the IMZ…."eLife manuscript  is somewhat misleading as, technically, it is not peer reviewed, published knowledge. In general, it is difficult to discern what is known (published), unknown (unpublished), and perhaps known only by the authors (but still unpublished) about CT as a morphogenetic machine.Thus, some of the background information about CT now included as the foundation of the present 2) Rather than include application/validation studies in the present manuscript, the authors chose to refer reviewers to independent data contained in the as-yet-unpublished, second manuscript .eLife manuscript, when published, would not have access to these data. It does not seem prudent to allow the validity of the CT/CE model in the current eLife article to rely on findings that (while promising) have not yet been formally peer-reviewed/published.Although the existence of this manuscript in preparation provides confidence that relevant experiments are ongoing and feasible, readers of the current Unfortunately, the current organization of all the relevant data regarding CT into two highly interrelated and interdependent studies, one of which is still unfinished, is simply less than ideal. 3) The rationale for performing the stiffness calculations, and the implications of stiffness for CT/CE-driven morphogenesis, are much clearer.Reviewer #2:This revision addressed my previous concerns, all of which were minor. I strongly support publication.Reviewer #3:The manuscript has been revised along the lines suggested by the different referees. The revisions have substantially improved the manuscript – it is now much easier to read and the link to the other not yet published manuscript also helps in understanding the molecular and cellular basis of the described CT and CE mechanisms. That said, I now fully support publication of this manuscript as is. Essential revisions:Much of the writing and presentation of the results is hard for non-specialized researchers to easily understand, thus revising the text throughout, and particularly the Introduction and Discussion, is needed to make the fundamental concepts more readily accessible and comprehensible. Consultation with a non-expert in the details of this work could help to make suggestions to the figures and text, or at least specifically point out elements that could benefit from some such clarification.Based on consultation with non-experts, Figure 1 has been revised to more clearly illustrate the morphogenic mechanisms involved, as has the figure legend and text describing it in the Introduction. The relationship between CT and RI has also been clarified in the Introduction section; to be clear, they are to some extent opposite processes, driven by very different mechanisms, not two facets of the same process. The biomechanical measurements of stiffness have been better explained for understanding by non-specialists, as have their rationale and implications, in the Results and Discussion sections. The explanation of our model for the differential use of CT and CE  in the Discussion section has also been revised to clarify the model and its implications.We welcome further suggestions from the reviewers for points that could be better clarified.Validation and/or application of the proposed model should be performed to demonstrate its relevance to tissue morphogenesis.As noted above, we have performed the experiments suggested by reviewers and include them in the CT paper, along with other that serve to validate our model and demonstrate its relevance to morphogenesis.The rationale for, and implications of, the stiffness calculations for CT/CE-driven morphogenesis should be further clarified.As noted above, we have included explanatory text for non-specialists, both more clearly explaining what we are measuring, why we are measuring it, and how we measure it, particularly in the portion of the Results section describing these experiments, and in the Discussion with our interpretation of these results.Reviewer #1:[…] 1) It is not entirely clear (to me) how the tensile stiffness calculations are related to the force measurements, nor how stiffness was used to inform the model in Figure 6. The rationale for, and implications of, the stiffness calculations for CT/CE-driven morphogenesis should be further clarified.An overview of force and stiffness measurements in “lay-biologists” terms are now given at the beginning of the relevant sections within the Results, before going into more specific detail. The reason for making stiffness measurements, and their implications are now spelled out at the beginning and ends of the relevant sections.Our model in Figure 6 is supported by our stiffness measurements in that the differential “spring stiffness” of dorsal vs. ventrolateral tissues is used to explain the plateau, and the increasing spring stiffness of standard giants and dorsal 180s, but not ventral 180s, between about 9 and 12 hours, is used to explain the end of the plateau, with the eventual transition from CT to MIB in the ventral tissues of a standard giant.The section in the Discussion section titled “The force profile reflects the transition from CT to CE and illuminates the mechanics of gastrulation”, as well as minor elements of Figure 6 have been changed to better explain the implications of our understanding of the forces produced by and the stiffness of embryonic tissues, both for the behavior of explants in the tractor pull and in the intact embryo.2) The conclusions regarding the timing and independence of CT vs. CE are interesting and could provide a reinterpretation and refinement of our understanding of the biomechanical mechanisms driving gastrulation. However, it is difficult to predict the impact and significance of this work without some sort of validation or application of the proposed model. For example, the tractor pull apparatus could be used to biomechanically tease apart a "non-specific" blastopore closure/gastrulation phenotype , thus testing the validity of the model and demonstrating its relevance for understanding morphogenesis.Experiments validating our model are now in the CT paper, as described above. MIB-driven CE clearly has a much greater dependence on PCP signaling than does CT. Further, CE is also more dependent on Myosin II B.Reviewer #2:[…] The data appear solid throughout (I say "appear" only because I am not an expert in mechanics). And the while the writing is dense, so too are the data, so it seems reasonable here.We thank the reviewer for these comments.Reviewer #3:[…] A) Although the authors mention that CT and CE are evolutionary conserved, in order for this study to be highly appreciated by a wider audience beyond the field of Xenopus developmental biology, it would be valuable to characterize at the cell and molecular level the differences between CT and CE (i.e. which cell behaviors and which molecules can polarize the cells to first converge and thick the tissue and then switch to elongation). Such mechanistic insight will be extremely interesting for the field of mechanobiology, since it will explain how different cell/molecular behaviors lead to distinct force generating processes that shape differently the tissue (please see below).Xenopus as well as other amphibian species, and ultimately in invertebrate species; Hemichordates appear to use some variation of CT to close their blastopore.We agree, and have begun this analysis in the CT paper. And as soon as Dr. Shook is able to obtain funding, this work will be carried forward, in B) Although the authors provide several diagrams to explain to the reader the complexity of blastopore closure, the manuscript and figures are still very hard to be understood by researchers non-specialized in Xenopus gastrulation (for specific comments please see below).We hope our new and improved Figure 1 and our explanation for it in the text addresses this problem. It is worth noting that amphibian gastrulation involves a complex set of morphogenic mechanisms, which one research paper cannot hope to thoroughly explain. We have however tried to distill the most important elements of these mechanism and explain them as clearly as possible, and to explain their relevance to blastopore closure.Major points:1) It was previously shown that inhibition of the PCP pathway in the dorsal MZ leads to inhibition of CE and defects in blastopore closure. In contrast, inhibition of the PCP pathway in the ventral MZ only leads to a slight delay of blastopore closure until mid-gastrula stages (until which CT is mainly occurring), indicating that CT might not rely on PCP signaling . In order for the authors to provide cellular and molecular mechanistic insights, it would be worth to express Xdsh either dorsally or ventrally and examine if and how PCP signaling affects the convergence force generated by CT in all four types of explants, and how this can be correlated with defects in morphogenesis of the sibling embryos. Such experiments would not only support the impact of this study for the interpretation of the phenotypes associated with defective blastopore closure but will also provide evidence for differential molecular control of polarity during CT and CE (point A).As noted above, this set of experiments has been carried out, and will be included in the CT paper. Comparing normal to ventralized embryos, Xdd1 overexpression strongly interferes with blastopore closure in normal embryos, which express both CE and CT, but not in ventralized embryos, which rely on only CT to close their blastopore. Similar results were found with force measurements when comparing the effects of Xdd1 on D180 vs. V180 sandwich explants; Xdd1 strongly inhibits force production in D180s, which rely on both CT and CE to generate force, but only weakly inhibits force production in V180s, which rely on only CT.This is a somewhat cleaner experiment than injecting Xdd1 ventrally in normal embryos, since this tissue will eventually express MIB-driven CE.2) The giant explants exhibit two force increase phases separated by a plateau phase. Although the authors suggest that this could be associated with the progression of MIB, it would be relevant to show with high resolution time lapse imaging how cell behavior is changed in the different phases  (point A).The normal progression of MIB in the IMZ has been extensively documented by the Keller lab, in references listed in the introduction at the end of the explanation for Figure 1, as well as the failure of MIB to advance as rapidly as normal when CE is mechanically retarded, as in the tractor pull (but using other mechanisms). While it would be nice to be able to directly correlate the cell behaviors that are happening in explants in the tractor pull with force generation, the experimental preparations that allow high resolution imaging of cell behaviors are incompatible with force measurements. Technological advances may some day allow this, but it is not currently possible.3) As mentioned in the text, CT was previously described to take place at the ventral MZ and later in development , but here the authors show that it takes place in the whole MZ earlier in gastrulation. It would be useful to describe this finding further by showing images or time lapses of how CT progresses in both embryos and explants. Sagittal sections of ventral vs. dorsal explants will also be useful to address any differences in the timing and extent of CT in the two regions.Video 2  shows CT in the whole IMZ during early gastrulation. This has now been explicitly pointed out in the text. Video 1 shows whole embryos, but this provides little clarity on its own. These issues are also extensively documented in the CT paper.The manuscript has been improved and it can be accepted pending the deposition of the other referenced manuscript  to bioRxiv, as previously discussed. Please work on this and resubmit when you're ready, citing the preprint and including a formal reference in the References list.https://doi.org/10.1101/270892) within the current manuscript (the Tractor Pull paper). We also added some acknowledgements and added DOIs to some of the references. Otherwise, the Tractor Pull paper is unchanged.We have submitted a more complete, polished version of the draft of our Convergent Thickening paper on BioRxiv and cited it as  (