Polymer-based three-dimensional (3D) printing—such as the UV-assisted layer-by-layer polymerization technique—enables fabrication of deformable microstructured materials with pre-designed properties. However, the properties of such materials require careful characterization. Thus, for example, in the polymerization process, a new interphase zone is formed at the boundary between two constituents. This article presents a study of the interphasial transition zone effect on the elastic instability phenomenon in hyperelastic layered composites. In this study, three different types of the shear modulus distribution through the thickness of the interphasial layer were considered. Numerical Bloch-Floquet analysis was employed, superimposed on finite deformations to detect the onset of instabilities and the associated critical wavelength. Significant changes in the buckling behavior of the composites were observed because of the existence of the interphasial inhomogeneous layers. Interphase properties influence the onset of instabilities and the buckling patterns. Numerical simulations showed that interlayer inhomogeneity may result in higher stability of composites with respect to classical layup constructions of identical shear stiffness. Moreover, we found that the critical wavelength of the buckling mode can be regulated by the inhomogeneous interphase properties. Finally, a qualitative illustration of the effect is presented for 3D-printed deformable composites with varying thickness of the stiff phase. Composite materials are deeply integrated in the modern life, due to their excellent mechanical and functional properties, which until recently were unachievable. Composites containing two or more constituents can be tailored to meet specific requirements by design of their geometry and smart selection of constituent materials. One of the most challenging problems in composite science is associated with the prediction of their failure. While delamination was historically considered to be the main failure mode, the phenomenon of local buckling or loss of stability attracted significant attention recently. In contrast with delamination, when composites lose the integrity catastrophically, the loss of stability can be considered as reversible microstructure transformation mode; once the external loads are removed, the initial undeformed state is restored thanks to the stored elastic energy. This phenomenon of instability-induced microstructure transformation has been employed to design materials with the switchable properties and functions ,5,6,7,8.The pioneering works of Rosen  and HillAt the same time, development of new composite manufacturing methods, especially appearance of multimaterial three-dimensional printing, opens the possibility to validate theoretical and numerical findings ,25,26. FTo analyze the mechanics of the layered composite with interphasial layers bonding two purely homogenous hyperelastic constituent materials, we utilized the finite element method with the help of COMSOL Multiphysics . Representative volume element (RVE) with dimensions Since the interphasial layer is inhomogeneous, we need to define the variation of the local elastic modulus through the thickness of the interface. In this study, we considered three different types of shear moduli distributions through thickness, denoted as A, B and C on on x see  is defin one see , μh=c for distribution A was lower than the one of the ideal composites; the total effective volume fraction for the combined stiffer zone was increased. However, the smooth change in the local shear modulus—in contrast to the sharp jump in the ideal composite—created a competing stabilizing effect. The contribution of these effects is reflected in the non-monotonic dependence of the normalized wave number for composites with distribution A. For the non-ideal composites with distribution A with higher values of relative thickness of interphasial layer λmacrFrom The numerical study presented in this paper showed that the existence of inhomogeneous interphases between two main constituents in hyperelastic layered composites significantly changed their buckling behavior, affecting the onsets of the instabilities as well as the developing buckling patterns. In particular, we found that for non-dilute as well as dilute cases, the “non-ideal” composites—which contain the mixing zone between two materials—were usually more stable in comparison with their “ideal” counterparts with the same effective shear modulus. However, it appears that the buckling characteristics depended not only on the thickness and effective shear modulus of the interphasial layer, but also on the distribution of the shear modulus through its thickness. Buckling responses predicted by our analysis were based on certain scenarios for distributions of the properties in the interphase material created due to mixing of the two phases during layer-by-layer curing. A careful experimental characterization is needed to provide the information on these actual distributions, and their dependence on various material fabrication and curing conditions. In the future, it is planned to perform a systematic experimental study of the formed wavy patterns in 3D-printed laminates with controllable interphase mixing zones. In


Advances of information and communication technologies in medical areas have led to the emergence of wireless body area network (WBAN). The high accessibility of media in WBAN can easily lead to the malicious tapping or tampering attacks, which may steal privacy data or inject wrong data. However, existing privacy protection mechanisms in WBAN depend on the third-party key management system and have a complex key exchange process. To enhance user privacy at a low cost and with high flexibility, a channel characteristic aware privacy protection mechanism is proposed for WBAN. In the proposed mechanism, the similarity of RSS is measured to authenticate nodes. The key extraction technique can reduce the cost of the key distribution process. Due to the half duplex communication mode of sensors, the biased random sequences are extracted from the RSS of sensor nodes and coordinator. To reduce the inconsistency, we propose the n-dimension quantification and fuzzy extraction, which can quickly encrypt the transmission information and effectively identify malicious nodes. Simulation results show that the proposed mechanism can effectively protect user privacy against tampering and eavesdropping attacks. Wireless Body Area Network (WBAN) employing wireless media to provide data transmission services is the core component of many telemedicine applications, such as personalized medicine and home-based mobile health . It replWBAN usually involves the star network topology like the centralized topology of peer to peer network . All senAt present, the solutions to the privacy leaks in WBAN mainly include the node authentication and information encryption. WBAN is usually composed of the coordinator and sensor nodes, where the coordinator is the so-called sink which serves as the gateway to another WBAN, a trust center or an access coordinator . The nodThe traditional node authentication exploits key sharing strategies to authenticate the node identity. However, malicious attackers can steal the key through wiretapping key exchange. Information encryption is another technology exploiting mathematical or physical means to avoid privacy leaks in the transmission process. The basic requirement of information encryption is the key generation and the most common key generation method is the asymmetric key. In this way, a third-party key management system is authorized to distribute asymmetric keys to nodes through the secure channel, by which the privacy data can be protected ,8,9. SenIn this paper, the independent key establishment scheme without third-party key system is proposed to prevent privacy leaks. The main contributions of this research work are as follows.the channel characteristic similarity estimation mechanism is proposed to authenticate the legitimate sensors, therefore malicious attackers cannot pass the authentication due to the channel characteristic difference between them and normal sensors.by utilizing the RSS between sensors and coordinator, extraction key is adopted to realize the node authentication and information encryption. Because of the instantaneous change of RSS in the wireless channel, the extraction key is dynamic and the information varies over time so that the privacy data can be protected.the inconsistency removal multidimensional quantification and key unification mechanisms are adopted to ensure the low bit inconsistency rate and high key generation rate.The main contributions of the paper are summarized as follows:The remainder of this paper is organized as follows. In In this section, we review existing node authentication and information encryption schemes.The work in  proposedIn this paper, to effectively guarantee the applicability and reliability of node authentication and information encryption in WBAN, and to meet the bit inconsistency and key generation rate requirements, a channel characteristic aware privacy protection mechanism is proposed. Simulation results show that the proposed privacy protection mechanism only relies on the physical layer characteristics, which can not only secure the secure communication, but also achieve low bit inconsistency rate and high key generation rate.In WBAN, each sensor node collects and sends physiological data to a coordinator through wireless channel. In particular, the computing power, memory space and energy supply of sensors are limited. The coordinator gathering the uploaded data can be a smartphone or a tablet computer, then it forwards data to the trusted local or remote data processing system, like an emergency service center. The coordinator close to the physical locations of all sensors is placed near the body, and the distances between the coordinator and sensors are mostly less than one meter.However, in actual application scenarios, malicious attackers randomly appear around the patient or key management center, and try to disguise themselves as legitimate sensors to attack or wiretap the key distribution process. As shown in t, the RSS value of channel  between the coordinator C and sensor S is defined as:t, and The work in  affirmedTherefore, a sensor can obtain the observed value of RSS. The privacy protection mechanism proposed in this paper can use observed RSS values to carry out the node authentication and key generation for WBAN. First, the legitimate sensors have almost the same channel quality and RSS, which realize the fast sensor authentication. Second, the key pairs are randomly extracted from the RSS between nodes, which can prevent attackers from tampering and eavesdropping. The whole process does not involve the third-party key management system, complex settings, additional hardware or other changes, which is practical for WBAN.Node authentication and information encryption are designed in this section. The node authentication algorithm exploits the characteristics of wireless channel to authenticate legitimate nodes based on the correlation coefficient. The information encryption algorithm should meet three requirements: (1) The key of two node has the same sequence; (2) The key of two node has the appropriate length (128 bit to 512 bit); (3) The key has statistical randomness to encrypt the sending information. The proposed mechanism can prevent malicious attackers from wiretapping or impersonating legitimate nodes to forge data, and employs the observed RSS values to extract keys to avoid privacy leaks caused by key sharing.When sensors communicate with the coordinator, they obtain the observed RSS values by sending the detection packet. The RSS value can reflect the channel quality, which varies with the change of environment and body movements. The channel environment around the human body is complex and the RSS is influenced by many factors such as multipath and occlusion. Besides, the RSS may also be affected by body parts and their positions. However, the positions of attackers are much different from those of legitimate nodes and the attackers may only be influenced by environment factors such as multipath. Therefore, the RSS of a malicious attacker is not related to that of a legitimate node, and the attacker also cannot predict the observed RSS value of a legitimate node.d and reference distance f is the system loss factor, X is a normal random variable and The RSS can reflect the approximate degree between environments of two nodes . Let Y1dBm−The notations used in the system are summarized in n nodes are exploited to calculate the similarity between them, and the maximum mean square error Therefore, the off-line RSS samples After the authentication between the coordinator and legitimate sensors, the system sends the communication information. A new key establishment algorithm based on channel quality is proposed to symmetrically encrypt and transmit the information. In this algorithm, every sensor extracts effective key independently without key distribution process, which makes it impossible for a malicious attacker to acquire keys by eavesdropping.In order to extract the effective key, the coordinator and the sensor node, as a pair of transceiver nodes, obtain the random sequences separately by quantifying the RSS values. However, due to the half duplex communication mode of sensors, RSS can only be measured unidirectionally at the same time. Therefore the random sequences extracted by the sensor and coordinator are biased. Once the pair of transceiver nodes extract the random sequences from the measured values, the inconsistent random sequences have to be corrected to obtain the secret key between the coordinator and sensor.Therefore, an efficient key extraction method based on multidimensional quantification is proposed to correct the inconsistent bits in the two random sequences of the sensor and coordinator. Then increasing the generation length of secret keys can reduce the bit inconsistency rate and rise the key generation rate. First, we quantify the RSS with the inconsistency removal method and increase the sequence length with the n-dimensional quantifier. Finally, the random sequence obtained by the n-dimension quantifier is transformed into the same secret key between the coordinator and sensor through the fuzzy key unification method.v is the average RSS value and d is the standard deviation of the RSS value. The removal function Due to the half duplex communication mode, the traditional quantification of RSS values generates the inconsistent sequences. Specifically, traditional quantification schemes are bounded by the average value to quantify RSS, which is 1 (or 0) higher than the average value, and 0 (or 1) lower than the average value. However, the RSS values near the mean value measured by sensor node and coordinator easily shift. And the results of traditional quantification methods are inconsistent. To reduce the inconsistency, a quantification method is proposed in this paper with upper bound As shown in b. After the segmentation and quantification, the generated sequences are spliced to the final sequence. The sequence is used as input to the n-dimensional quantification process.Because there are too many RSS values to be collected, the average values of RSS in different time regions are diverse. To accurately quantify RSS, their values are divided into blocks and the block length is n-dimensional quantification method is proposed to improve the key generation rate without increasing the bit inconsistency.Because the inconsistency of RSS values is removed, the sequence generated is smaller. An  efficient n-dimensional vector is established to sample the sequences. The input values of the n-dimensional quantification scheme are d is the length of sequence Y, and then the n-dimensional vector is established as follows:j and Y, and then we rearrange them to generate the new random sequence. After multidimension sampling, the output value can be expressed as follows:The n-dimensional quantification with the random sequence as the input and the random sequence results as the output. The quantification method takes n-th value, and then it rearranges the values in the sequence to generate the new random sequence. The quantification outputs a new random sequence whose length is n and the bit generation rate of the n-dimensional quantification is 1:n. The pseudo code of the algorithm is shown in Algorithm 1. A two-dimensional quantification is taken as an example to illustrate the n-dimensional quantification process. The quantifier searches the second component The whole process can be regarded as an Algorithm 1n-dimensional quantification algorithm1:2:forddo3: 4: 5:fordo 6:  7:  8:end for 9:end for10:11:ENDn-dimensional quantification methods. To generate effective keys from the sequences of sensor node and coordinator for information encryption, the fuzzy key unification method is introduced, which corrects errors according to confusion and diffusion properties and selects the appropriate hamming distance to map to the same random sequence. It not only allows two similar random sequences to map to the same sequence, but also converts a low entropy sequence to a random sequence. The fuzzy extractor is shown in The sequences generated by the sensor and coordinator are processed by the inconsistency removal and M,l,t) contains the generation and regeneration process, namely Gen process and Rep process in this method, where M is a string of input sequences, l is the length of key R, and t indicates that the Hamming distance of two sequences can generate the same random sequence without exceeding t.The fuzzy extractor  and random key R with strong randomness is generated with the SHA-1 function. Then the coordinator sends public information P to the sensor, and the sensor restores its sequence Y according to public information P. And then the sensor generates the same strong random secret key R with the SHA-1 function. In the simulation, the length of the original BCH  code is 12 bits, the length of error correcting code P is 11 bits, and the minimum code distance is 7 bits which can correct 3 error bits. Therefore, the key generated by this method has very low inconsistent key rate.When the sensor and coordinator quantify the channel characteristics and RSS by using the inconsistency removal method separately, parameter n-dimensional quantization is used to increase the key generation rate without increasing bit inconsistency rate, where n is the quantification degree. The larger n signifies the higher bit generation rate and affects the randomness of the generated keys. In order to determine the impact on the key, the key generated by different dimensions is evaluated with the 9 test methods of NIST. For each test, if the value is greater than 0.01, the test is pass. The assessment results are shown in In this paper, the n-dimensional quantification. When the parameter is fixed to n-dimensional quantification method. It can be seen that the sequence already have relatively low bit quantification rate before the fuzzy extraction. Thus it provides a better input sequence for the fuzzy extractor. Because the bit inconsistency rate before the fuzzy extraction is very low, the fuzzy extractor can correctly output the same secret key.The average time of executing node authentication, key extraction and information encryption is calculated to reflect the complexity. The execution time that BANA and MASK-BAN consume are 74.5 × 10In this paper, a channel characteristic aware privacy protection mechanism is proposed for WBAN, which is lightweight and only relies on channel characteristics to achieve autonomous node authentication and extracts real-time keys to encrypt the information. The analysis and experimental results demonstrate the advantages of the proposed mechanism. At the same time, this mechanism is proved to be more suitable for resource constrained WBANs. In the further work, we plan to optimize the fuzzy extraction method to further reduce the computation cost.


Ankylosing spondylitis (AS) and the related spondyloarthritis (SpA) have become an extremely productive field in immunogenetic research over the last several years, justifying an update. This timely special issue will thoroughly cover the strong heritability of SpA by tackling the explosion of new genetic discoveries in SpA, paving the way for functional studies and also interrogating the striking but still mysterious role of HLA-B27 in disease susceptibility .Costantino et al. the role of genetic factors in SpA pathogenesis is discussed. The association of SpA with HLA-B27 and other MHC/non-MHC genes is reviewed in detail. This is followed by a section dealing with the functional studies exploring the pathogenic role of these novel genetic discoveries. Finally, in the section on the clinical relevance, the potential diagnostic, and prognostic utility of these genetic markers is outlined.In the exhaustive review by Vecellio et al. start their review by discussing the role of HLA-B27, aminopeptidases, and IL-23R in influencing AS susceptibility. A particularly interesting section is dedicated to the analysis of the functional consequences of the genetic association of AS with TBX21 and RUNX3, two transcription factors relevant for tuning the functions of multiple immune cell types.More than 100 genes have been identified to contribute to AS susceptibility even though the mechanisms underlying most of the associations are unknown. López de Castro is focused on the contribution given by the immunopeptidomics to understand how the Endoplasmic Reticulum Aminopeptidases ERAP1 and 2, major risk factors for AS and other immune-mediated disorders, shape the peptidomes of the related disease-associated HLA-I molecules. Despite the advances in the structural, biochemical, and functional knowledge of ERAP1 and 2, there is an emerging need to define the influence of ERAP1 polymorphisms and ERAP2 expression in shaping the constitutive HLA-I-bound peptidomes expressed in live cells and eventually involved in disease pathogenesis.The article by Paladini et al. stems from the observation that the ancestral HLA-B*2705 allele and the derived B27 subtypes show an opposite gradient in the worldwide distribution and includes into the scheme the ERAP variants known to be associated with AS. Of note, those variants lowering ERAP2 gene expression, have a distribution similar to that of the alternative HLA-B27 subtypes. Hence, focusing on Sardinia island, the authors made the hypothesis that these genes, both involved in AS pathogenesis, co-evolved under the environmental pressure by pathogens such as Plasmodium falciparum, and that the disease results as a trade-off of these interconnections.The perspective by Watad et al. conceptualized the initial stages of AS which link the skeletal mechanical stress and the tissue microdamage to an abnormal immunological response and exuberant repair process. These are indicated as major events which cause enthesitis and osteitis in the sacroiliac joints and are followed by post-inflammatory chronic new bone formation in genetically predisposed subjects. Particular emphasis is given to the gut inflammation and to the role of altered microbiota which are linked to joint manifestations through a still undefined immunological axis. The role of entheseal resident cells and pivotal cytokines for disease initiation is also discussed.A very important matter for any chronic disease concerns the understanding of early events predating full disease establishment, since this may open the door for preventive care. Accordingly, Maksymowych comprehensively covers the field by a systematic overview of the recent studies focused on the findings of new candidate biomarkers . It emerges that CRP remains the only reliable predictive biomarker of disease progression while fecal calprotectin is a consistent indicator of gut inflammation in Axial SpA.A crucial topic that emerges from this article collection is the promise of novel predictive biomarkers, particularly but not exclusively blood-derived, since the gut might also provide an interesting contribution. This will be of interest not only to help diagnosis but also to predict prognosis and therapeutic success, notably with the widely used anti-TNF therapies as well as to personalize treatment options. The review by Liu et al., in their exhaustive review summarize the recent evidence on the role of anti-CD74 autoantibodies in AS, also presenting their data which discard these autoantibodies as good biomarker for SpA diagnosis in Asian population.Historically, AS has been considered the prototype of seronegative arthritis. Recent evidence indicating the presence of anti-CD74 autoantibodies in patients with AS, suggests that the concept of seronegativity in AS should be reviewed. CD74 is the receptor of the macrophage migration inhibitory factor (MIF) which is involved in the pathogenesis of AS and mediates its proinflammatory function. Menegatti et al. explore the important area of personalized medicine discussing different treatment modalities in spondyloarthritis and drawing parallels from treatment of Rheumatoid Arthritis patients. Starting with the history of the usage of TNF inhibitors in rheumatology, they extend the review to their role in spondyloarthritis. The current literature on predicting which patients are likely to respond to TNF inhibitors is discussed. The review concludes with a description of the more recent drugs that block type 3 immunity.Many unanswered questions dictate the priorities that need to be addressed. Diligent integration of susceptibility gene polymorphisms with cell/tissue expression data is needed to understand their functional consequence. Epigenetics is an emerging approach in the field of SpA that will require more investment. Well-characterized cohorts of patients and pre-clinical models, as well as up-to-date techniques are required to improve the discovery of efficient biomarkers. From these novel and emerging approaches suitable targets for curative or, even better, preventive interventions are likely to emerge. This is what we need at the end.All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


Global data sets on the geographic distribution of livestock are essential for diverse applications in agricultural socio-economics, food security, environmental impact assessment and epidemiology. We present a new version of the Gridded Livestock of the World (GLW 3) database, reflecting the most recently compiled and harmonized subnational livestock distribution data for 2010. GLW 3 provides global population densities of cattle, buffaloes, horses, sheep, goats, pigs, chickens and ducks in each land pixel at a spatial resolution of 0.083333 decimal degrees (approximately 10 km at the equator). They are accompanied by detailed metadata on the year, spatial resolution and source of the input census data. Two versions of each species distribution are produced. In the first version, livestock numbers are disaggregated within census polygons according to weights established by statistical models using high resolution spatial covariates (dasymetric weighting). In the second version, animal numbers are distributed homogeneously with equal densities within their census polygons  to provide spatial data layers free of any assumptions linking them to other spatial variables. Livestock farming has a major impact on the environment, through greenhouse gas (GHG) emissions from enteric fermentation and manure, disruption of nitrogen and phosphorous cycles and indirect impacts on biodiversity and other ecosystem services through overgrazing and land-use change2. Livestock farming also bears public health implications through its role in food-borne disease transmission, the emergence and spread of infectious zoonotic diseases3 such as avian influenza4, Q-fever and MERS and its contribution to the global burden of antimicrobial resistance, linked to the routine abuse of those drugs in livestock production.6 Detailed, contemporary data sets on the global distribution of the most important species of farmed animals have a wide range of applications in understanding the social, economic, environmental, epidemiological and public health impacts of the livestock sector.Livestock play a key role in global food systems as the main source of animal protein , contribute to crop productivity through the provision of draught power and manure, and to the livelihoods and nutrition of poor households in low- and middle-income countries7: i) to collect, harmonize and disseminate subnational global livestock data, ii) to predict livestock numbers in areas with missing census counts (gap-filling), and iii) to provide a statistically-informed estimate of how livestock may be distributed within census units . GLW 1 was produced at a spatial resolution of 0.0416666 decimal degrees (approximately 5 km at the equator). In 2014, an updated GLW 2 was published, benefiting from the availability of finer-scale and more contemporary input census data, from the improvement of the processing and from higher spatial resolution predictor variables that were used for downscaling8.The gridded livestock of the world database (GLW 1) produced in 2007 had three objectives9. In addition to that important change in methodology, GLW 3 differs from the previous ones in three ways.In this paper, we describe a new global, subnational livestock dataset (GLW 3) generated using Random Forests (RF), a machine-learning technique recently shown to provided more accurate gap-filling and disaggregation of livestock data than did the previously-used multivariate regression methodsFor each species, we now provide a detailed report that includes comprehensive metadata on the input census data for each country  and goodness-of-fit metrics of the models by continent and by the size of the administrative unit from which the census data came. This enables users to assess the quality of the estimates for each combination of species, country and size of census unit.2 of suitable land in the census unit. The AW models were introduced because the spatial predictor variables used in the downscaling algorithms  may introduce uncontrolled confounding effects or circularity for users wishing to study livestock distribution numbers independently of any other spatial variables. The AW models are free of the influence of other spatial predictor variables, at the cost of displaying cruder distribution patterns, especially in large census areas containing a wide range of different environmental, land-use and farming conditions. In polygons where input census data were missing, the AW model simply includes the aggregated predictions of the DA models, and a separate layer is provided for the user that distinguishes between predictions and census observations.All species distributions are now available in two representations, termed dasymetric (DA) and areal-weighted (AW). The DA models correspond to previous GLW versions, whereby different animal densities are assigned to different pixels within a given census polygon according to the RF models. In contrast, the AW models simply spread individuals of a census polygon evenly, and the density of animals in each pixel corresponds to the average number of animals per kmGLW 3 provides global data  at a spatial resolution of 0.083333 decimal degrees (approximately 10 km at the equator), as the higher spatial resolution of previous GLW versions could be misleading in areas where the census data were of poor quality.Future versions of GLW will differentiate stocks according to production systems for ruminant (meat vs. dairy) and monogastric species . Higher resolution models for individual countries where the census data can support such predictions will also be produced.et al.8, is that in GLW 3 RF models have replaced stratified linear multiple regressions for predictions9.The only change to the overall workflow, which was fully detailed for GLW 2 in Robinson 2), and countries such as Russia or South Africa with very coarse subnational census data (ASR > 250 km and mean area > 62 500 km2). There are also important differences in the census years. The oldest subnational census data are for the Democratic Republic of the Congo (1994), whilst some countries, for example Turkey, have very recent data (2014). Both the ASR and year of the census data depend on the species in question, as sometimes data can be available for one species and not for others. The two types of information are essential indicators of data quality and are therefore provided in the metadata. Summarized distributions of ASR and census years of GLW 3 are shown for all species in Detailed livestock census statistics are mined from agricultural yearbooks or through direct contacts with ministries or statistical bureaux. The census statistics are usually found in the form of numbers per administrative unit, in which case they need to be linked to corresponding geographic information system (GIS) boundaries. Data are increasingly found though as pre-prepared GIS files that are then integrated in a centralized database. These individual country data are combined into a global database, which often implies resolving typology issues; miss-matched, split or merged polygons, for example. In compiling GIS data from subnational census counts priority is given to censuses that most closely match the reference year (2010 for GLW 3) and those with the highest level of spatial detail. This results in a global mosaic of data from different spatial resolutions and different years. For example, -2 as defined by the human population data layer , corrected by a mask excluding unsuitable areas. The suitability mask is very conservative and only excludes permanent water  a third of the variables are used to build each tree with a minimum of 5 variables; ii) the number of trees is set to 1/20th of the number of sampling points, with a minimum of 100 trees; and iii) the node size is set to 1/1,000th of the number of sampling points, with a minimum node size of 5. The 5 bootstrapped RF models are then applied to the raster predictor variables to estimate a density value in each pixel, and the 5 predicted values are used to estimate the prediction mean and standard deviation in each pixel.The sample points are divided between training and validation sets according to the subnational census polygons, using sample points from 70 percent of the polygons for training the models and from 30 percent of the polygons for assessing the model accuracy. This operation is repeated 5 times, each time selecting a different set of polygons to train the RF models and goodness-of-fit (GOF) measurements. The parameters of the RF models were investigated in Nicolas The 30 percent of polygon data that were held back for each bootstrap are used to estimate GOF metrics. For each of these polygons and each bootstrap, the predicted values of the pixels falling into the polygon are summed to calculate a predicted total per polygon. The predicted and observed animal numbers of these 30% validation polygons are then used to estimate the Root Mean Squared Error (RMSE) and correlation coefficient between the observed and predicted totals, as measures, respectively, of accuracy and precision .The entire analysis is stratified by developing 5 bootstrapped models for each continent: North America, South America, Europe, Africa, Asia and Oceania. In total 30 RF models are produced and applied to six continents. The GOF metrics are produced separately for each continent.2 at the equator). In these cases, the sum of the animals from those small census polygons falling within a pixel is estimated and assigned to that pixel; replacing the RF model prediction. This can result in two situations. When pixels are smaller than their corresponding polygon, the sum of the pixel values matches the observed animal total of the polygon. Conversely, when pixels are larger than their corresponding polygons, the pixel value matches the sum of the intersecting smaller polygons. In both situations, issues of polygon boundaries going across pixel boundaries are resolved according to the proportion of the intersecting surface area.For the dasymetric product, the average values predicted by the RF models are used as weights to distribute the animals within each subnational census unit at the pixel level. For each polygon, the pixel weights are multiplied by the ratio of the total number of animals per census unit to the sum of pixel weights in the polygon . In polygons with missing census values the RF mean predicted density is used (i.e. the factor applied to the weights=1). In some countries, the spatial census units have smaller areas than the area of a single pixel . The data records are grouped by species  and through the FAO livestock systems World Web site  gives the most accurate estimate of the prediction accuracy of the pixel-level predictions. These ranged between 0.60 and 0.78, meaning that a significant part of the variability is not captured by the model and that pixel-level estimates cannot be assumed to fully represent what is on the ground. This could be linked to important predictor variables that are absent from the model, or to the stochastic nature of the spatial allocation of farms. As spatial units become larger, the variability gets filtered out and the observed values become easier to predict, with correlation coefficients close to 0.90 for the largest units. So, the gap-filling capacity of the models can be assumed to be good, which benefits both the DA and AW products. The GOF metrics by continent and species sometime reflect very different model qualities depending on the continent. Future studies should evaluate whether alternative stratifications could better harmonize the quality of the models across geographic regions. For example, groupings based on the economic status of countries may prove more appropriate than those based on continents. Livestock farming is constrained by production factors such as land, capital and manpower, and the last two are strongly associated with countries’ economic status, which influences how farms can be distributed across the landscape.The GOF was moderate to high, depending on species and size of census administrative unit . Since oThe GOF metrics need to be interpreted with care because they result from internal cross-validation. They do not measure the correspondence between the predicted densities of animals and what is actually on the ground. If the census itself is of poor quality, there could be discrepancies between the recorded numbers and what is actually on the ground, let alone between the census and the predicted values. Furthermore, census data are mostly based on where animals are registered to their owner, not necessarily where they are raised or spend most of their lifetime. For ruminants raised in pastoral systems, or ducks raised in free-grazing systems, for example, there could be significant seasonal changes in the spatial distributions of animals that would not be captured in the models.The outputs also assume no livestock to occur in IUCN protected areas, and imposing a density of zero in such areas. It is, however, known that livestock encroach on protected areas and the validity of these assumptions depends on how effectively these restrictions are enforced. This varies greatly from country to country and even within countries.18.One possible way to validate the models would be to use household demographic and health surveys (DHS) or living standards measurement studies (LSMS) data on livestock ownership. These data are typically geo-referenced at the cluster level, follow a completely different sampling approach and would therefore provide a different base for the development of livestock models. Cross-checking the results of models derived from large-scale census and from point-based surveys would help to identify areas of convergence where predictions would be consolidated, and areas of divergence where there would be higher uncertainties associated with the predictions. Field observations and aerial surveys provide efficient means of collecting high-quality data that could be used to validate the models, however, they are costly, especially when carried out over large areas. The cost-effectiveness of remote sensing for counting large animals was recently reviewed, and appears currently to be of limited utility1 and regional19 ruminant production systems data are available and global monogastric systems are available for pigs and chickens20.This version of GLW is suited to applications in the domains of socio-economics, environment and health. The data are most appropriate for applications at global and continental scales. Decisions regarding the use of this version of GLW over smaller spatial extents should be taken in relation to the ASR of the underlying census data. For example, analyses of GLW data in Brazil, Spain or Thailand would be appropriate because their respective ASRs are small relative to the size of the country. In contrast, we would discourage the country-level use of GLW data in countries such as Russia or Mali, because the ASR values are particularly high (>250 km). It is also important that users of these data are mindful of the fact that the type of production system is not accounted for in these livestock distribution data. The diverse contexts in which livestock are raised have major bearings on their primary uses, their productivity, the benefits they confer, the constraints to production and the impacts they have. The distribution data should therefore be used in conjunction with information on production systems. Currently, globalThe DA version is recommended for applications where spatial detail matters more than concerns about circularity in the analytical workflow in relation to spatial predictor covariates. However, we warn potential users against over-interpretation of spatial accuracy of the DA product. As indicated by the GOF metrics, much variability was not captured by the models and the downscaled densities of the DA version only imperfectly represent what may actually be on the ground. When circularity concerns are more important than detailed spatial resolution, it is recommended that the AW versions be used.This is the third version of GLW, and the previous version had reference years of 2002 and 2006, respectively. However, since the three version of GLW differ in the type of input data, the predictor covariates and modelling methods, we would discourage their use for time-series analysis. Future studies will develop appropriate models to map how livestock distributions have changed over time.2) by dividing each pixel value by the pixel area in km2. For convenience, a global Geotiff file of pixel areas, expressed in km2/pixel, is provided along with each species data file .Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.


A coalition may be bony, fibrous, cartilaginous, or a mixture of these tissues and may be partial or complete, i.e. affect a complete joint (facet) surface area or a fraction of it.The term Segmentation refers to a process in the unborn when the future bones’ centers start to separate out of a common cartilaginous plate or volume in the foot. This process is thought to occur by constrictions progressing from the periphery towards the center of the common bone plate until a new bone center is separated out of the common plate with the trace of the constrictions delineating the future joint space. It is hypothesized that the time and extent that this process is disturbed or interrupted determines the type of coalition: early disturbance will result in a partial or complete bony coalition; disturbance later in the process of segmentation will result in a fibrous, fibrocartilaginous, or cartilaginous partial or complete coalition.While originally the incidence of tarsal coalitions was thought to be around 1%, more recently, it is estimated to be up to 13%. Coalitions may occur bilaterally in up to 60–70% and two coalitions in the same foot are described in 10–15%. This is clinically relevant if a surgical resection/correction of a coalition is considered. Inheritance is considered unifactorial, autosomal dominant with high penetrance. The most common forms of coalitions are the talocalcaneal – especially of the middle subtalar joint facet – and calcaneonavicular forms. Talonavicular and calcaneocuboidal or tarsometatarsal coalitions are less common.Coalitions in one group of patients tend to become symptomatic, i.e. painful, with progressive ossification of the normal bones as well as of potential coalitions, with less general intrinsic flexibility (ossifying skeleton) as well as locally restricted movement  in the foot. Hence the age of presentation mostly is in adolescence. Following the age pattern of foot ossification, calcaneonavicular coalitions tend more to become symptomatic in the earlier teen years; talocalcaneal ones become symptomatic more towards the later teen years . Yet, anAlthough the amount of foot deformity varies considerably within and between any type of tarsal coalitions, one frequent and central pathomechanism is (direct or indirect) limitation of subtalar joint movement, a condition that considerably increases stress on the adjacent joints and that may ultimately result in a flatfoot configuration . ExcessiTherapeutic strategies include restricting any physical activities that seem to provoke the symptoms; also orthoses, temporary cast immobilization, especially in children, may significantly improve the condition. NSAID can contribute to relief secondary inflammatory changes and pain. Talocalcaneal coalitions appear to respond better to conservative treatment options than to other forms. Surgical treatment after unsuccessful conservative therapy can be excision of the coalition  – in the absence of secondary degenerative change – and/or arthrodesis of the involved joints can be performed, sometimes guided by diagnostic injections of local anesthetics to determine the (most) painful joints.Imaging should be based on radiographs followed by MRI and – if relevant for therapy – by CT Figure . The rolTalar beaking is a bony prominence or spur of variable extent at the distal dorsal aspect of the talus attributed to increased traction on the dorsal talonavicular ligament and its periosteal insertion following restricted motion in the subtalar joints. This finding must not be confounded with an osteophyte in the talonavicular joint, which would be located at the joint surface/margin and be an argument against a potential surgical procedure.anteater sign, an extension of the prominent anterior process of the calcaneus towards the lateral aspect of the navicular, indicative of a calcaneonavicular coalition is well seen in the oblique view. Similarly, a bony prominence on the lateral aspect of the navicular towards the anterior process of the calcaneus may be present, the so-called reverse anteater sign. In the case of a complete bony calcaneonavicular coalition, that bony bridge may present as the bony bar sign.The absent middle facet sign relates to an ill-defined or completely obscured middle subtalar joint facet in well-obtained lateral radiographs. The C-sign is a C-shaped projected contour in complete or incomplete continuity from the sustentaculum tali to the posterior projection of the talus in the lateral view. Finally, the dysmorphic sustentaculum describes a roundish and broad projection of the sustentaculum tali as opposed to its regular flat rectangular aspect in the lateral view resulting from the dysmorphic and abnormally tilted middle subtalar joint. The three latter findings may indicate a talocalcaneal coalition.The bone marrow edema pattern around a non-osseous coalition or around adjacent joints; this can be used for the detection of coalitions and illustrates some of the mechanical impairment and stress brought upon the foot by a coalition. And of course, MRI may demonstrate associated soft-tissue changes such as tendon pathologies in foot deformity or sinus tarsi syndrome. In terms of MRI analysis, it is helpful to systematically look for the presence or absence/disturbance of the typical sequence of linear patterns reflecting a joint: trabecular bone, subchondral bone plate, articular cartilage, joint space, articular cartilage, subchondral bone plate, trabecular bone. This approach may help to avoid overlooking subtle coalitions.The technical evolution of MRI enables this modality to depict and accurately describe any coalition and its morphologic features, reducing the need for CT in the work-up of coalitions. Both CT and MRI have greatly facilitated the diagnosis of tarsal coalitions compared to radiography; this is also reflected by the higher incidence of coalitions in cross-sectional imaging studies. MRI shows In specific situations, such as pre-operatively, CT is useful in deciding whether a resection or arthrodesis is feasible, specifically planning a procedure, assessing the fine bony details of a coalition, or to assessing secondary subtle degenerative change . It may


Hydrogels based on reversible covalent bonds represent an attractive topic for research at both academic and industrial level. While the concept of reversible covalent bonds dates back a few decades, novel developments continue to appear in the general research area of gels and especially hydrogels. The reversible character of the bonds, when translated at the general level of the polymeric network, allows reversible interaction with substrates as well as responsiveness to variety of external stimuli . These represent crucial characteristics in applications such as drug delivery and, more generally, in the biomedical world. Furthermore, the several possible choices that can be made in terms of reversible interactions generate an almost endless number of possibilities in terms of final product structure and properties. In the present work, we aim at reviewing the latest developments in this field  by focusing on the chemistry of the systems at hand. As such, this should allow molecular designers to develop a toolbox for the synthesis of new systems with tailored properties for a given application. In the last few years, a renewed interest in hydrogels has arisen due to an extended range of applications . This stParticularly important is the adhesion at the interface between the polymeric chains and several different substrates to be then released (where and when needed) upon action of an external stimulus. On a molecular level, this translates in the presence of reversible bonds between the polymeric network and such “loading.” For example, the catechol chemistry  can actuIn this particular case, the crosslinking via metal free click chemistry is practically quantitative and does not result in the formation of any by-product, and this rendering it ideal for biomedical applications ,17,18. The catechol chemistry is not the only example of a reversible yet covalent bond that can be formed and broken on command upon an external stimulus. Indeed, along the same strategy, the imine formation chemistry can be used to tune the adhesion .The molecular reversibility can be actually achieved in two different ways: either by making use of equilibrium reactions  or through dynamic exchange reactions . Both approaches have been widely used for tuning the adhesion of different substrates on different polymeric networks and for the network formations (and disruption) itself. These are discussed in the following paragraph.The same concept of covalent reversible bonds can be used for the hydrogel formation and disruption. Examples of interactions used in this case are  electrosThe general idea is that the use of dynamic covalent bonds allows the polymeric network to adjust itself as a result of an external stimulus. This can be achieved in principle through other weaker interactions, e.g., hydrogen bonding. In particular, a clear trend is detected in the last year, according to which self-assembly driven processes ,37 can bThe general concept behind the use of reversible interactions for the hydrogel polymeric chains is the (reversible) network disruption with immediate release of any loading .Reversible bonds can be incorporated along the backbone (red circles) or at the crosslinking point (green triangles). The network, when subjected to an appropriate external stimulus, can then break at the crosslinking point (route A) or along the backbone (route B). This generates network fragments that can be quite different in terms of chemical structure even if in both cases the loading (blue circles) will be released. As a result of the network disruption, the load is released as the polymeric chains become soluble and not able anymore to entrap the load. In the specific case of drug delivery, this mechanism entails a controlled release rate depending on the kinetics of the network disruption, which in turn can be linked, at the molecular level, to the kinetics of the reversible bonds. 3+ ions  on the pH values. Such outstanding mechanical behavior  can also be attributed to the presence of the micelles, which actually act as physical crosslinking points for the all system.This approach, i.e., the use of two different reversible covalent bonding, represents a very popular choice in the last five years ,41. ThisBorax acts in this case as catalyst for the thiol-ene reaction, thus factually helping in building the polymeric backbone, but also as crosslinking agent via the reversible borax-diol chemistry. The reversibility of the crosslinking was demonstrated by self-healing experiments and confirmed by rheology measurements. As typical for these systems, i.e., covalently and reversibly crosslinked gels, a cross over point between the elastic (G′) and loss (G″) modulus is observed for relatively moderate frequency values (1 < ω < 100 rad/s). This is in fact a very general characterization technique and observation  that canVery recently host-guest chemistry (molecular recognition) has been also reported for the synthesis of hydrogels ,43. In oThe versatility of this method and its compatibility with several different monomeric precursors and polymeric end-products render it particularly attractive.An interesting consequence of the dynamic character of these bonds is the fact that this significantly favors the adhesion between hydrogels and organo-gels (when in contact with each other), provided that such reaction can take place at the interface between these two gels .A shown above, the choice of covalent reversible bonds for the network formation endows the final product with a multifaceted portfolio of properties . This can be achieved on the basis of several different substrates. Indeed, synthetic polymers are still widely used for the preparation of hydrogels, with polyacrylamide representing the most popular choice . On the The choice of PEG as the main constituent of the polymeric backbone is an obvious one ,50 when The presence of an amino group along the chitosan chain is exploited here in order to achieve high reactivity (even at room temperature) with modified cellulose through enamine formation, in this case or via imine formation in other reported examples ,52. A coAs clearly seen form the example discussed above, the use of reversible covalent bonds in hydrogels endows the final product with very peculiar chemical structures, which in turns translate into a kaleidoscopic ensemble of possibilities in terms final properties. As also observed in other chemistry-related research fields, the study of synergistic phenomena seems to represent a very popular trend. In the specific case of hydrogels, this translates into the combination of multiple reversible interactions  in the same final product. This allows control of the network structure in processes such as self-healing in response to several different external stimuli. This is particularly attractive for biomedical applications where physiological parameters  might change in a simultaneous manner. On a molecular level, this generates the need for synthetic strategies allowing the incorporation of such reversible covalent bonds on the polymeric network. The above review of the most recent trends clearly show on one side the high sophistication level of the synthetic strategies employed and the multifaceted properties toolbox achievable in this way.It must be stressed here that such research trends will probably continue in the coming years as the number of possibilities in terms of chemical bonds is certainly not yet exhausted. On the other hand, it is also conceivable that, as the topic will reach more scientific maturity, more attention will be paid to industrially feasible preparation routes. In this context, the choice of suitable substrates  as well as synthetic strategies will probably constitute a focal point of future research projects.


Laser-generated ultrasound is a modern non-destructive testing technique. It has been investigated over recent years as an alternative to classical ultrasonic methods, mainly in industrial maintenance and quality control procedures. In this study, the detection and reconstruction of internal defects in a metallic sample is performed by means of a time-frequency analysis of ultrasonic waves generated by a laser-induced thermal mechanism. In the proposed methodology, we used wavelet transform due to its multi-resolution time frequency characteristics. In order to isolate and estimate the corresponding time of flight of eventual ultrasonic echoes related to internal defects, a density-based spatial clustering was applied to the resulting time frequency maps. Using the laser scan beam’s position, the ultrasonic transducer’s location and the echoes’ arrival times were determined, the estimation of the defect’s position was carried out afterwards. Finally, clustering algorithms were applied to the resulting geometric solutions from the set of the laser scan points which was proposed to obtain a two-dimensional projection of the defect outline over the scan plane. The study demonstrates that the proposed method of wavelet transform ultrasonic imaging can be effectively applied to detect and size internal defects without any reference information, which represents a valuable outcome for various applications in the industry. Embedded structural damage is a typical defect in metallic parts and structures exposed to a defective manufacturing procedure or subjected to a complex and cyclic loading during their service. Among various non-destructive evaluation (NDE) and structural health monitoring (SHM) techniques, ultrasound-based approaches have been widely applied in the last decade for metallic materials inspection ,2. In faThe emitter-receiver ultrasonic inspection scheme, based on ultrasonic transducers, has been classically adopted for damage detection and failure localization as a cost-effective NDE strategy ,6. HowevIn recent years, as an alternative to exclusive ultrasonic approaches, photonic strategies, based on laser-induced ultrasound, have been revealed as a powerful remote NDE technique. These approaches offer the possibility of inspection at higher resolution ratios, even when the power density of the pulsed laser is lower than the ablation threshold of the material under test, i.e. working on the thermo-elastic regime . Hence, Indeed, this approach allows ultrasonic wave generation at a far distance from the object, enabling remote testing without the need for a direct contact with the excitation source. The resulting broad excitation frequency bandwidth covers the majority of the ultrasonic bandwidths of interest for available applications involving material characterization. Moreover, the ultrasound transducers for reception, already integrated in the structure following common SHM approaches or strategically positioned in the component under inspection, may cover the measuring of resulting responses . ResearcAlthough these procedures show good qualitative results over rotating surfaces, the damage quantization is limited. The detection and quantization of embedded damages in metallic components is still a challenging field of research in terms of applying suitable signal processing techniques and defect reconstruction methodologies to extract characteristic ultrasonic wave contents and generate reliable defect estimations. In this regard, time-domain analysis represents a set of powerful tools which are applied often for damage detection purposes . If the In this paper, a defective metallic component for damage detection and visualization through a laser-ultrasonic approach is studied. For this purpose, the wavelet transform is proposed as a time-frequency processing tool. The characteristic damage frequency patterns are studied, identified, and tracked back to the boundaries of the source, i.e., the damage spatial location in the component under inspection. Then, a damage reconstruction procedure is proposed based on the estimated time of flights of the ultrasonic damage propagation patterns. The contribution of this study lies in providing a new approach of damage detection, localization, and shape reconstruction based on detection and processing of the damage ultrasonic echo characteristics. An adaptive resolution method using wavelet transform was chosen in order to check the proposed methodology under a laser-ultrasonic scheme.Novelties of this work include a new damage pattern tracking method over the WT-based time-frequency maps, and a time of flight-based damage localization and visualization procedure; both are supported by the application of a density-based spatial clustering algorithm in order to identify the highest probability regions of damage location. It must be noticed that for the first time, to the authors’ knowledge, this processing scheme and this damage visualization procedure have been used in the laser-ultrasonic inspection of metallic specimens. According to the obtained results, the proposed methodology is reliable and feasible for defect detection and quantification of metallic damages in industrial applications since the proposed method does not require any reference information.This paper is organized as follows: the theoretical basis and its suitability for ultrasound processing of the wavelet transform are presented in Spectral analysis of A-scan signals is very important to get information about both time and frequency responses of the signal in the same diagram. Many time-frequency transforms are available for this purpose. One of the most famous related algorithms is the Short-Time Fourier Transform (STFT). It simply applies the Fourier Transform approach to the measured signal multiplied by a non-zero function for a short-time window.By shifting these time windows across the whole temporal axis of the measured signal, we obtain the spectrogram of the signal with spectral data at each time. One major disadvantage of this technique is the fact that the time window is fixed during the whole analysis. This fixed window results in a high resolution only in either the frequency or the temporal spectrum, i.e., a wider window results in a better frequency resolution with a poor time resolution and vice versa for a shorter window. Other techniques that use non-parametric and bilinear transforms are also good examples of these spectral analysis topologies that overcome the disadvantages of STFT. They can be expressed generally as Cohen’s class distribution functions that generate a function representing intensity per time and per frequency . The outThere are other irrelative and undesirable components defined as cross-terms that are oscillating components found between the auto-terms. Cross-terms hide some information of the main signal and reduce the resolution, making it difficult to interpret the resulting distribution ,33. An ax(t) is the target time series, ωx is the resulting wavelet analysis, ψ* is the complex conjugation of the mother wavelet that has to be a continuous function in both time and frequency domains, a is a scale factor that either stretches (large a) or compresses  the wavelet and, b is the signal’s shifting parameter in time. By choosing a different mother wavelet, different characteristics of the time signal can be emphasized as an output. The flexibility of choosing the optimal mother wavelet is one of the advantages of using WT, for the choice of the mother wavelet for a particular problem improves the signal processing capability of the technique. If the shape of the signal to be detected is known a priori, a replica of the set can be utilized as the mother wavelet function, or the mother wavelet can be chosen from a set of theoretical signals. In this regard, the Morlet wavelet is extensively used and ultrasonic signals are investigated due to their behavior which is close to the detected signals. It has been proven to be efficient in improving the signal strength and reducing the noise, making the WT-based technique extremely useful for damage detection [The main purpose of the initially selected mother wavelet is to provide a source function to generate the daughter wavelets which are simply the translated and scaled versions of the mother wavelet . The WT etection .The proposed laser ultrasound imaging methodology is shown in The first stage is the acquisition of ultrasonic signals resulting from the thermal mechanism induced by each considered laser scan point over the specimen scan plane. In this stage, the acquired signals are digitally pre-processed by means of a bandpass filtering in order to increase the resulting signal to noise ratio in the frequency band of interest. Second, the wavelet transform is computed for each time-based ultrasonic signal. As aforementioned, a Morlet wavelet is proposed as a good trade-off between performance and simplicity . Thus, iIn the third stage of the proposed methodology, it is expected to automatically identify such ultrasonic echoes corresponding to the presence of defect in the considered area of inspection. In this stage, a high-performing clustering algorithm, commonly used in image processing applications, is proposed in order to identify the time instant of eventual ultrasonic echoes over the resulting wavelet-based time-frequency maps. ε corresponding to the maximum radius of the neighborhood to be considered during the analysis of a point p, and the minimum number of points to consider a dense region, minp. The density-based spatial clustering of applications with noise (DBSCAN) ,38 does As a result of this stage, the presence of characteristic damage echoes is detected through the clustering algorithm and its corresponding time is obtained from all scan points. The identified echoes, pertinent to the interaction between the ultrasonic wave front and the defect, correspond to the sum of the TOF between the laser scan point to the defect and back to the ultrasonic transducer. N ellipsoids generated by N different measurements resulting from N different scan points. A total number of intersections, I, considering any pair of ellipsoid intersection will be obtained, following:In the fourth stage, we automatically identify the corresponding TOFs of the second detected group of echoes by detecting the time of the second cluster. For each resultant TOF, which corresponds to the sum of the individual TOFs from the laser to the defect and the individual TOFs from the defect point to the sensor, an ellipsoid is estimated, as a locus of defect location, which represents a set of possible geometrical solutions matching the computed TOF. An ellipsoid, by definition, is the locus of a point whose sum of displacement from the two well-determined points, the foci of the ellipsoid, is constant. Actually, the proposed method is based on the consideration of discretized three-dimensional curves resulting from the intersections of each pair of ellipsoids. From a geometrical point of view, the embedded defect generating the corresponding time of flight is estimated to be at any point in such a curve. So, every point of the resulting curve is a potential locus of the defect. This continuous three-dimensional intersection curve, when discretized, is converted to points that become the locus of intersection. Thus, considering Finally, the fifth stage proposes the identification of coherent intersections among the ellipsoids resulting from all laser scan point computations through a density analysis. In this stage, it must be noted that a previous filtering is proposed to reject the ellipsoids resulting from TOFs larger than the dimensions of the considered scan area. Then, the re-iterative intersections in the considered scan volume are projected in the scan plane x-y, where the DBscan technique is proposed to extract maximum density areas corresponding to a higher probability of defect location. The analysis of the resulting set of ellipsoid intersections is proposed to be faced with the application of the DBSCAN-based clustering algorithm once more in our methodology. Then, the spatial localization of the defect is obtained by the geometrical center calculation among the resulting clustered intersection points, and the defect reconstruction for visualization is carried out by means of the outline of the clustered intersection points.The schematic diagram of the entire laser-ultrasonic inspection system configuration, considered for method validation, is shown in As shown in An example of the ultrasonic waves resulting from the thermal mechanism induced during the pulsed laser excitation scan is shown in The time of the initial ultrasonic wavefront arrival to the ultrasonic sensor in In this regard, the corresponding wavelet transform is carried out for each acquired ultrasonic wave during the laser scan process using Morlet mother wavelet. Some resulting time-frequency maps corresponding to ultrasonic waves induced in the aluminum specimen at different laser scan points are shown in As an example, Considering all 10 × 10 ellipsoid estimations the intersections among them correspond to potential locations where the interaction between the laser-generated ultrasonic wave and the defect took place during the inspection. In this regard, x = 62 mm and y = 69.5 mm, while the actual defect location center is at x = 61.5 mm and y = 65 mm, thus, revealing a positioning error of 4.53 mm. Considering the size of the defect, the proposed methodology results in an estimation of 578 mm2, while the actual dimension of the defect’s cross section area is 520 mm2, thus, resulting in an error of 11.1%.In This paper presents a novel methodology to detect, quantify, and reconstruct internal defects on metallic samples based on laser-generated ultrasound. There are three important aspects of this new method. The first one is the ultrasonic wave signal analysis through wavelet processing. The multi-resolution capabilities of the wavelet analysis allow the identification of the main ultrasonic modes during the front wave propagation from the excitation point to the defect, and finally to the sensor; thus, enhancing characteristic echo detection for each point considered in the scan. The second is the extraction of characteristics of damage echoes and the methodological estimation of ellipses through the resulting time of flight for each laser excitation point. The third is the application of a clustering strategy as a processing stage in order to enhance the significance of the intersection analysis among ellipses. The application of the proposed clustering method over the resulting points of intersection among ellipses allows the identification of the most coherent region and elimination of the less significant intersections. An internal defect at 95 mm from the surface of inspection is considered, which represents a challenging condition. Under such a scenario, the proposed methodology shows reliable fault diagnosis results, with a location and sizing error of 4.53 mm and 11.1%, respectively. The results obtained in this work suggest that this methodology may be also useful for further 3D imaging reconstruction of defects. However, the three-dimensional reconstruction will require a further analysis of multiple echo intersections in order to estimate the depth of the defect. The future work will focus on the analysis of the proposed laser ultrasound imaging method considering boundary interaction within the scan area.


Bolted spherical joints are widely used to form space steel structures. The stiffness and load capacity of the structures are affected by the looseness of bolted spherical joint connections in the structures. The looseness of the connections, which can be caused by fabrication error, low modeling accuracy, and “false twist” in the installation process, may negatively impact the load capacity of the structure and even lead to severe accidents. Furthermore, it is difficult to detect bolted spherical joint connection looseness from the outside since the bolts connect spheres with rods together from the inside. Active sensing methods are proposed in this paper to monitor the tightness status of the bolted spherical connection using piezoceramic transducers. A triangle-on-triangle offset grid composed of bolted spherical joints and steel tube bars was fabricated as the specimen and was used to validate the active sensing methods. Lead Zirconate Titanate (PZT) patches were used as sensors and actuators to monitor the bolted spherical joint tightness status. One PZT patch mounted on the central bolted sphere at the upper chord was used as an actuator to generate a stress wave. Another PZT patch mounted on the bar was used as a sensor to detect the propagated waves through the bolted spherical connection. The looseness of the connection can impact the energy of the stress wave propagated through the connection. The wavelet packet analysis and time reversal (TR) method were used to quantify the energy of the transmitted signal between the PZT patches by which the tightness status of the connection can be detected. In order to verify the effectiveness, repeatability, and consistency of the proposed methods, the experiments were repeated six times in different bolted spherical connection positions. The experimental results showed that the wavelet packet analysis and TR method are effective in detecting the tightness status of the connections. The proposed active monitoring method using PZT transducers can monitor the tightness levels of bolted spherical joint connections efficiently and shows its potential to guarantee the safety of space steel structures in construction and service. Space steel structures are widely used in stadiums, theaters, exhibition centers, airport terminals, and many other large-scale structures. Any one of such structures can simultaneously house a large number of occupants. It is very important to guarantee the integrity of these structures. Space steel structures are composed of joints and bars. Bolted spherical joints have been widely used to form space steel structures due to such advantages as good mechanical behavior, flexible connection, and high installation speed. However, there are a variety of issues that arise during the design and installation process of the bolted spherical joints, including low modeling accuracy, joint manufacturing error, and the “false tightening” phenomenon. The issues are difficult to avoid, and may lead to connection looseness and structural failure if not resolved in time . FurtherCurrently, most research on joint connection monitoring is focused on individual bolt connection ,4,5 or fStructure Health Monitoring (SHM) is the process of identifying the damage and evaluating the health status of the structure in real time ,16. The In the active sensing method, wavelet packet analysis ,54 and tIn this paper, a novel application of health monitoring method based on active sensing technique is developed to monitor the tightness status of the bolted spherical connection using piezoceramic transducers. Experiments on a triangle-on-triangle offset grid composed of bolted spherical joints and steel tube bars were conducted to verify the efficiency of the proposed method. The rest of the paper is organized as follows. The detection principle for bolt-sphere joint tightness is presented in A space steel structure often consists of hundreds of bolted spherical joints. As shown in The cross-sectional view of the bolted spherical joint connection is shown in PZT1 is surface-bonded on the central bolted sphere at the upper chord and acts as an actuator to generate stress waves. PZT2 is surface-bonded on the end of the bar, which is near the bolt sphere, and acts as a sensor for receiving stress waves. The stress wave propagation is as follows: At first, upon receiving an input signal, PZT1 generates a stress wave. Then, the stress wave propagates through the contact interface between the bolted sphere and the sleeve. After that, the stress wave continues to propagate through the contact interface between the sleeve and the bar. Finally, the stress wave is detected by PZT2. In this process, the tightness of the joint connection will directly impact the stress wave signal intensity received by PZT patches.Stress wave propagation is essentially the process of energy transmission, just as shown in Wavelet packet analysis is an extension of wavelet analysis. It can further decompose the high-frequency components unprocessed by the wavelet transform into independent frequency bands. This decomposition has the advantages of no omission, no residue, and orthogonality. Wavelet packet analysis is a more precise method for signal processing.n—layer wavelet packet into 2n frequency bands, If a signal The connection is divided into The looseness index As the bolted spherical joint is tightened to reach the required preload level , the corresponding energy vector can be expressed as:Based on the Formulas (2) and (3), when the connection is at the The looseness index The time reversal is a reverse operation on the received signal in the time domain. It is a process of reversing acoustic or electromagnetic wave signals from the object and then sending it back. The reversal signals have the characteristics of spatial and temporal focusing properties. These properties can solve the multiscattering problem of acoustic detection in complex environments.In this paper, the time reversal method has three steps.In the first step, a pulse input is applied to PZT1. Then PZT1 (acting as an actuator) generates a stress wave. The generated stress wave propagates through the bolted spherical joint connection. At last, the stress wave is captured by PZT2 (acting as a sensor). The process can be expressed as follows:In the second step, the signal received is reversed in time domain, which is the time reversal process and this process can be expressed as follows:In the third step, the reversed signal According to the principle of convolution and correlation, For the reason that the input signal is supposed as a pulse signal, it can be written as To simulate the bolted spherical joint connection, a triangle-on-triangle offset grid model was designed and constructed. It was 0.2 m in height and composed of three orthogonal triangular pyramid units, as shown in Experimental setup mainly included a data acquisition system (NI USB-6363) manufactured by National Instruments Company, a power amplifier (Model 2100HF PIEZO DRIVER) made by TREK Company, a torque wrench made by Craftsman Company, and a laptop, as shown in In this experiment, the process of tightening the bolted spherical connection was divided into two steps. In the first step, the sleeve was screwed by hand, which is called the initial tightening. In the second step, the sleeve was gradually screwed by a torque wrench, which is called the final tightening. In this research, the experiments were all started after the initial tightening. Tightening status in the process of final tightening was divided into 8 tightening degrees according to the torque value applied on the sleeve, as shown in A repeated swept sine wave was used as an excitation signal for the experiment and parameters of the wave are listed in It can be seen from A Gaussian pulse was used as an excitation signal for the experiment, as shown in It is clear that the focused peak value increased with the increase in torque level from As shown in The experimental results show that the proposed method based on PZT and TR is effective and stable for monitoring the tightness degree of bolted spherical joint connections. The TR method has excellent repeatability, consistency, and anti-noise ability . For civIn this paper, the authors proposed a method based on active sensing technique using piezoceramic transducers to monitor the tightness degree of bolted spherical joint connections. Wavelet packet analysis and time reversal technique were used to process the stress wave signals. Taking a triangle-on-triangle offset grid with bolt-sphere joints as an example, six repeated experiments were conducted on six bars in different positions. The results are consistent. The looseness index calculated by wavelet packet analysis decreases and the focused peak value based on the TR method increases as the tightness level increases. It is clear that the monitoring method of bolted spherical joints based on the piezoceramic sensing technique is feasible and repeatable. The proposed method has the potential to be implemented in the health monitoring of space steel structures connected with bolted spherical joints.