As the foundation of quantum secure communication, the quantum key distribution (QKD) network is impossible to construct by using the operation mechanism of traditional networks. In the meantime, most of the existing QKD network routing schemes do not fit some specific quantum key practicality scenarios. Aiming at the special scenario of high concurrency and large differences in application requirements, we propose a new quantum key distribution network routing scheme based on application priority ranking (APR-QKDN). Firstly, the proposed APR-QKDN scheme comprehensively uses the application’s priority, the total amount of key requirements, and the key update rate for prioritizing a large number of concurrent requests. The resource utilization and service efficiency of the network are improved by adjusting the processing order of requests. Secondly, the queuing strategy of the request comprehensively considers the current network resource situation. This means the same key request may adopt different evaluation strategies based on different network resource environments. Finally, the performance of the APR-QKDN routing scheme is compared with the existing schemes through simulation experiments. The results show that the success rate of application key requests of the APR-QKDN routing scheme is improved by at least 5% in the scenario of high concurrency. Quantum key distribution (QKD) technology , which e(1)Data processing: During packet forwarding, classical network routing only needs to read the address field of the packet and the locally stored routing table, whereas trusted relay-based QKD network routing also needs to encrypt and decrypt the information carried in the packets, which has a much higher data processing overhead.(2)Forwarding capacity: The forwarding capacity of classical network routing depends mainly on the network bandwidth and is relatively fixed, while the forwarding capacity of trusted relay-based QKD network routing is also affected by the number of quantum keys stored inside the node and the associated links, so it is dynamically changing.(3)Success rate: The success rate of classical network routing is mainly influenced by network congestion, while the success rate of trusted relay-based QKD network routing is influenced by both the classical channel bandwidth and the quantum channel bandwidth.QKD networks can be separated into three major categories: optical node based QKD networks, quantum relay-based QKD networks, and trusted relay-based QKD network ,10,11. AIn addition, the link resources used by QKD networks in carrying user keys are considerably different from the bandwidth resources of classical networks. By combining the above descriptions, it can be concluded that the classical network routing scheme ,15,16,17There have been some studies on the routing problem in QKD networks, but most of them have not explored the relevant real-world scenarios, which are necessary for any application of quantum keys. Currently, most of the research results on the routing problem of trusted relay-based QKD networks have improved according to the characteristics of QKD networks by drawing on the proven classical network routing scheme. Yu et al.  transforSome routing schemes account for the changing demands of requests for secret keys and are better appropriate for practical applications. Cao et al.  first prThe above studies lack consideration for highly concurrent scenarios. In this respect, some routing schemes use dynamic information and are more adapted to rapidly changing QKD network environments, thereby complementing this deficiency. Yang et al.  propose In summary, the following problems remain in the present study on QKD network routing algorithms. Firstly, the secret key requests of all applications are treated equally, without considering the difference in the importance of different requests throughout the actual application. This can lead to a backlog of delayed requests with high-security requirements in high concurrency circumstances, thus causing a worsening of the overall network quality of service. Secondly, when a large number of secret key requests occur in a short period, requests with extremely high secret key demand may be prioritized based on the traditional first-come first-serve (FCFS) algorithm request processing strategy. This would result in a deviation in network resource allocation and a reduction in the request success rate of the overall network. Lastly, when the network resources are insufficient, the existing schemes adopt two strategies: direct rejection or never rejection. Direct rejection may lead to frequent repeated secret key requests. Never rejection may cause the application to block due to waiting for the secret key response, and the progress of secret key request processing will not be known for a long time.Considering the different importance and communication urgency of various applications in the actual use, we set a priority judgment criterion for application key requests in the scheme to quantify the priority of requests being processed.For specific scenarios of high concurrency, a fixed amount of secret key requests of applications reached in a short period are ranked by priority rather than merely relying on the order in which the key requests are reached. The requests will be processed later according to the ranking result.Depending on the permissible delay range for applications, secret key requests of applications with route failure are given a delayed retry instead of simply outright rejection and excessive infinite waiting.To tackle the aforesaid problems, this paper proposes a QKD network routing scheme based on the application priority ranking (APR-QKDN). Contributions are summarized as follows:Through simulation, it is proven that our scheme can acquire a greater request success rate compared with other schemes, better utilize the limited resources in the QKD network, and improve the actual quality of service of the quantum network. Accordingly, our work serves to advance the practical implementation of QKD technology.The remainder of this paper is organized as follows: In The trusted relay-based QKD network system used in this paper contains three layers: the application layer, the controller layer, and the QKD layer. The system model is illustrated in Application layer: The application layer consists of various application entities involved in data transfer. This layer is the service body of the QKD network as well as the bridge between the QKD network and real users. The application in this layer will initiate a secret key request at any moment. This secret key request is submitted to the controller, and when the controller gets the secret key request, the application enters the blocking and waiting phase. Only when the application receives the request response information back from the controller can it begin the subsequent key generation phase.Control layer: The control layer contains five modules: request management module, priority ranking module, delay retry module, network topology module, and route calculation module. The request management module is used to receive the secret key request of applications and send the details contained in requests to the priority ranking module. Subsequently, the priority ranking module will rank the huge number of requests arriving within a short period by the present resource status of the QKD network and the priority judging criterion defined in advance. After finishing the ranking, the priority ranking module delivers the request priority queue to the route calculation module for path selection, which is carried out under a specified strategy. When a request meets with route failure owing to insufficient resources, the request is delivered by the route calculation module to the delay retry module where the application will determine whether to rejoin the request queue according to its acceptable delay range. If the current delay is not exceeded, the request can be handled again by the priority module until the route is successful or timeout. The routing calculation module interfaces with the network topology module to gather the network resource status and give a full network topology for path selection.QKD layer: The quantum nodes in the QKD layer report the resource storage and operation status to the controller’s network topology module in real-time. When the controller’s route calculation module completes the path selection, it distributes the corresponding routing decision to each quantum node in the QKD layer quantum node. Quantum nodes would update the local routing table entries. When the application receives the key request response, it transmits the session key relay to the target application through each quantum node in the QKD layer. Finally, the two communicating parties would have the same session key.The APR-QKDN routing scheme serves specific scenarios with considerable variances in application secret key demands and establishes the priority judgment criterion for different requests. Therefore, the QKD network model used in this paper defines the application priority metrics. Meanwhile, the total key demand and the key update rate of the QKD network are evaluated. The nomenclature included in the model is defined as described below.Quantum link: a virtual link between neighboring quantum nodes abstracted for QKD, the underlying physical form is the combination of quantum channel and measurement-based channel, the process of QKD contains the information transmission of these two channels.Link time slice: each quantum link has a particular key generation rate that can be divided, meaning that the key generation cycle can be divided evenly to produce a time slice. Later, for different secret key requests of applications, the time slice is allocated on demand, i.e., occupying a period in the key generation cycle for updating its session key.Link key: the secret key generated by each quantum link is called the link key. It is generated by the negotiation of neighboring quantum nodes.Link key pool: the link key pool is a virtual concept that manifests as a pair of local key pools in neighboring quantum nodes. The quantum keys generated by the idle link time slice are stored in the link key pool, which can directly provide key services for applications and support the case of insufficient link time slice.Application priority: depending on the degree of importance, each application will have a distinct priority. The same application may have different levels of data transmission urgency at different times, thereby the application priority might change dynamically. A higher priority secret key request of an application signifies that the application has a higher security requirement or the application urgently needs to obtain a key for confidential communication.Total key request: the meaning of total key request is the total amount of key requests of an application. It plays a significant part in determining the request priority. A large total key request of an application suggests that there would be more difficulties to allocate resources for the current application and comprehensive consideration of the existing network resource state is needed.Key update rate: it is similar to the total key request but influences more the consumption of time slices of the quantum link rather than the consumption of the number of keys remaining in each quantum node. The higher key update rate that an application demand indicates, the more time slice resources that the application needs to occupy. Nevertheless, the key generation rate of the link is limited. Once the time slice resources are occupied by an application, they are difficult to be released in a short time.To address the problems of existing QKD network routing schemes, we propose a QKD network routing scheme based on application priority ranking (APR-QKDN) in this paper. Considering the situation that a large number of requests arrive in a short period, APR-QKDN sets a reasonable priority judgment criterion of requests and adjusts the processing order of requests so that the routing scheme can satisfy as many application requests as possible with the limited resources of the QKD network. Moreover, APR-QKDN grants a delayed retry chance to the request with route failure instead of rejecting it straightaway, intending to improve the overall service quality of the routing scheme. Since we sort requests based on priority, our solution is more suitable for scenarios with high concurrency and large gaps in application requirements than previous solutions. Since the range of sorting can be adjusted and the priority criteria can be customized and adjusted according to the usage requirements, our solution has high flexibility and low overhead, such as the ability to fall back to FCFS for non-high concurrency scenarios.In this section, the priority judgment criterion of requests and routing scheme will be explained in detail. In practical usage circumstances, both the importance of secret key requests and the specific secret key demands are diverse in different applications; performing the FCFS strategy cannot deliver adequate service quality and high network resource utilization. Under the foregoing scenario, APR-QKDN sets a reasonable priority judgment criterion based on the application priority and secret key demands, then decides the order of subsequent key requests according to this criterion.The priority judgment criterion considers three factors: the application priority, the total key request of applications, and the key update rate of applications. How to weigh the importance of these factors is a crucial issue. The specific evaluation formula is as follows:α, β, γ signify the weights of the three factors, respectively, and the larger the weight, the higher the impact on the priority of the request. The configuration of the weights needs to be altered dynamically according to the demands of applications, and the following are specific application cases:γ should be set to 0, and the priority judgment criterion can be expressed as follows:The total key request is less than the minimum number of remaining link keys in the link set: the request can be satisfied by the remaining link key solely and does not have to be allocated link time slices. Therefore, the priority judgment criterion of the request does not need to consider the effect of the key update rate. In this case, β should be set to 0, and the priority judgment criterion can be expressed as follows:The total key request is higher than the maximum number of remaining link keys in the link set: the request can only obtain a key by being allocated a link time slice. Therefore, the priority judgment criterion of the request does not need to consider the effect of the total key request. In this case, β = γ, and the priority judgment criterion can be expressed as Equation (1). The total key request is between the minimum and the maximum number of remaining link keys in the link set: the specific approach to satisfy the application request cannot be determined directly. It is necessary to consider the total key request and the key update rate. In this case, In Equation (1), In summary, the priority judgment criterion is influenced by three factors, and the weights of the contributing factors fluctuate as the application’s features and requirements change.APR-QKDN decides the routing order of requests according to the priority judgment criterion. Since there is a time interval between requests, it is necessary to set a suitable number of requests processed at one time  can fulfill the application demands (i.e., Step 8 Evaluate whether a request with route failure is eligible for a delayed retry. If the number of request retries is within 5 and the current delay of request does not exceed the maximum acceptable delay for requests (i.e., According to our experiment, we set the number of request retries at 5 in Step 8. It can be modified to satisfy the actual situation. To evaluate the performance of the APR-QKDN routing scheme proposed in this paper, we complete the performance comparison between different schemes through simulation experiments. The network topology used for the experiments is displayed in The comparison schemes employed in the simulation experiments are the KoD , the RWTOur APR-QKDN routing scheme targets the specific scenario with high concurrency and substantial differences in secret key demand between applications, hence it is necessary for this particular scene to be reproduced when comparing the performance of each scheme. In each simulation, the secret key requests of applications vary considerably. Consequently, there is no means to use the total key request and key update rate, which reflect the demand of applications, as fixed experimental parameters. Only application-independent environmental parameters can be changed dynamically. The experimental parameters considered for this simulation are the key pool expansion multiplier, interval of requests, and link key generation rate. The success rate of requests, which represents the request processing capability of the scheme, is the most essential service quality evaluation metric. The key efficiency is the ratio of the total key demand of a request to the total number of keys consumed to complete that request. A larger key efficiency rate indicates that the request itself has a larger share of the key demand and a lower level of quantum key waste. In a single experiment, one of the experimental parameters will be altered dynamically, and the success rate of requests under that particular parameter will be utilized as the performance assessment index of the four schemes. In the following different circumstances, the three parameters are used as a variable in turn. α, β and γ that influence total key request are all set to 0.33, the application duration satisfies a uniform distribution in the range of  s, the initial capacity of the key pool is 12,800 bit.In this circumstance, the key pool expansion multiplier varies in the range of , the key update rate varies in the range of  bps, the Poisson distribution parameter of the interval of requests is 100 ms, the number of requests processed at one time (As can be seen in As shown in α, β and γ that influence total key request are all 0.33, the application duration satisfies a uniform distribution in the range of  s, the initial capacity of the key pool is 12,800 bit.In this circumstance, the interval of requests varies in the range of  ms, the key update rate varies in the range of  bps, the key pool expansion multiplier of KoD is , the number of requests processed at one time (As shown in As shown in α, β and γ that influence total key request are all 0.33, the application duration satisfies a uniform distribution in the range of  s, the initial capacity of the key pool is 12,800 bit.In this circumstance, the link key generation rate varies in the range of  kbps, the key update rate varies in the range of  bps, the Poisson distribution parameter of the interval of requests is 100 ms, the key pool expansion multiplier of KoD is , the number of requests processed at one time (As shown in As shown in In general, the APR-QKDN routing scheme maintains good performance during the dynamic changes of the three experimental parameters with some performance improvement compared to the other three schemes and is more suitable for specific scenarios with high concurrency and large inter-application demand differences. From the above experimental results, we find that the request success rate of APR-QKDN has more opportunities to improve when the link key generation rate keeps increasing. The link key generation rate has a good prospect, so it is feasible to improve the request success rate of this scheme by increasing the link key generation rate.In addition, since different requests have different application priorities, how to integrate the importance of requests to evaluate the service quality of the scheme comprehensively and design new additional evaluation metrics is a question to be considered in our future work.In this paper, we focus on the research of QKD network routing schemes in specific scenarios with considerable disparities in demand between applications and high concurrency, and discuss the system model and application characteristics of the proposed APR-QKDN scheme. The priority judgment criteria and the delayed retry mechanism adopted by the APR-QKDN scheme are explained in detail. On the one hand, the application priority is adaptively and dynamically updated according to the characteristics of applications and the current network environment, enabling the system to adjust a suitable processing order for an immense number of requests arriving in a short period. On the other hand, considering that the application itself has a certain delay tolerance and there are unreasonable request rejection strategies in existing routing schemes, our scheme adds a delayed retry mechanism, which not only enables the application to follow up on the request processing progress in time but also avoids frequent and repeated network data transmission. Finally, the performance of the APR-QKDN scheme is experimentally compared with KoD, RWTA-FSL, and ADA-QKDN schemes through simulation. The experimental results demonstrate that the APR-QKDN scheme has a certain performance improvement compared to the other three schemes. The success rate performance improvement is the most noticeable compared to KoD, reaching up to 60%; compared to RWTA-FSL, it can enhance around 15%, and the performance gap between APR-QKDN and ADA-QKDN is about 5%. It is shown that the proposed APR-QKDN scheme has good performance in specific scenarios with intensive demands and big variances between requests, which is important for increasing the service quality of QKD networks.


Various lines of evidence have shown that nature exposure is beneficial for humans. Despite several empirical findings pointing out to cognitive and emotional positive effects, most of the evidence of these effects are correlational, and it has been challenging to identify a cause-effect relationship between nature exposure and cognitive and emotional benefits. Only few of the published studies use psychophysiological methods to assess the biological correlates of these positive effects. Establishing a connection between human physiology and contact with natural settings is important for identifying cause-effect relationships between exposure to natural environments and the positive effects commonly reported in connection to nature exposure. In the present study, we recorded physiological indexes of brain activity  and sympathetic nervous system , while the participants were presented with a series of videos displaying natural, urban, or neutral  scenes. Participants rated the scenes for their perceived relaxing value, and after each experimental condition, they performed a cognitive task (digit span backward). Participants rated natural videos as the most relaxing. Spectral analyses of EEG showed that natural scenes promoted alpha waves, especially over the central brain. The results suggest that experiencing natural environments virtually produces measurable and reliable brain activity markers which are known to be related to restorative processes. The literature on environmental psychology has defined a restorative environment as one that can help to restore drained emotional and/or cognitive resources. Outdoor environments that feature natural elements have been shown to effectively promote restorative effects . Human eA long line of empirical evidence has shown that natural settings can promote stress reduction and ease physiological arousal . PlacingCurrent scientific literature has extensively investigated the ability of natural settings to restore attentional and cognitive abilities. Current theories of the cognitive effects of exposure to natural stimuli see  propose The mechanisms underpinning the restorative effect of nature for humans have been extensively studied . A widelThe empirical findings of the benefits associated to the exposure to natural environments are often theoretically explained with two prominent theories . The firA considerable number of studies have examined human health and cognitive functions associated with viewing images, videos, and other simulations of nature , includiSeveral studies have used electroencephalography (EEG) to examine brain functional dynamics related to the experience of natural settings in different frequency bands. The frequency bands are delta (0.5–4 Hz), theta (4–8 Hz), alpha (8–13 Hz), beta (13–30 Hz), and gamma (>30 Hz) bands. The alpha activity has received most attention because it is related to relaxed state of mind. The alpha waves are understood to be produced from postsynaptic potentials in a neural network involving the dorsal anterior cingulate cortex, the anterior insula, the anterior prefrontal cortex, and the thalamus that is associated with alertness and attention . It has The physiological basis of the beta rhythms has been long being disputed. It has been proposed that beta waves are generated in basal ganglia and thalamic structures . Later sWhereas delta and gamma bands have not received practically any attention in studies on the psychological effects of nature, there is some evidence that theta band activity may be modulated during viewing natural stimuli. Theta activity has been associated with cognitive control and error processing, and an origin of increase theta power has been found in the anterior cingulate cortex . Sahni aOther studies have employed EEG as method to estimate the positive emotions related to the experience of natural settings , and theNon-brain physiological indexes such as heart rate HR; see , heart-rHeart rate variability (HRV) is defined as the variation in the intervals between heartbeats. Such physiological index is often used as an indicator of the activity of the autonomic nervous system (ANS). Higher HRV has been linked with an increased adaptability of the ANS and has been proposed to be associated with positive health factors . SeveralElectrodermal activity is controlled by the sympathetic nervous system, and it is modulated by the activity of the skin sweat glands. EDA is often used to measure psychological or physiological arousal . PreviouA small number of studies have considered the use of more than one physiological measure to cross-validate their results across different methods. The recently published study of One limitation that is shared by many of these studies is the lack of a control or neutral condition when comparing the influence of natural and urban environments. The studies that focus on comparing urban and natural environment  and non-brain (EDA) indexes  to understand physiological dynamics of the interaction with natural settings. This was done in a laboratory setting, where videos of nature (without audio) were showed to the participants while biological signals were recorded. Furthermore, we attempted to understand whether urban environments or natural environments were responsible for an eventual modulation of physiological brain and non-brain activity by adding control condition to the experiment. Our hypotheses were that the vision of nature video would modulate brain early alpha waves recorded over the central brain areas as shown in previous lab-controlled studies employing static images . We did Twenty-four neurologically healthy students with normal or corrected-to-normal vision participated in the study . They were selected from the pool of participants from the introductory psychology courses in University of Turku. The students received course credits as reward for their participation. The experiment was conducted in accordance with the Declaration of Helsinki and with the understanding and written consent of each participant and was approved by the Ethics Committee for Human Sciences at the University of Turku. Before the experimental session, the participants were asked to read an information sheet on the experiment, to read and sign the informed consent, and to self-assess their handedness. Only students reporting to be right-handed were tested. The sample size was determined based on the observed effect and n from previously published EEG studies on brain activity in response to environmental exposure . The numThe stimuli consisted of videos depicting natural, urban, or neutral settings . A total of nine videos per condition were selected by the study authors. Every participant was exposed to four consecutive videos of the same environment type  before passing to four consecutive videos of another environment, and four consecutive videos of the last environment. The four videos for each environment type were selected randomly among the nine available videos. The order of the experimental conditions  was randomized across participants. Every video lasted for 3 min, for a total of 12 min of exposure per each experimental condition. Natural settings videos depicted a variety of natural environments, among those, mountain lakes, beaches, forest, and fields. Urban environment videos showed city-like landscapes, and depicted for example traffic, buildings, and other types of common environments displaying urban or industrial buildings. Urban videos contained eventually some natural elements . Natural videos were selected appositely to avoid any type of non-natural element. The videos were selected from material retrieved from the internet. The authors SG and GS selected independently a list of suitable videos, and together reviewed and chose those that were judged as the most appropriate for the present study. Criteria that were used to selecting the videos were: high enough quality of details, absence of numbers/words in the environment, absence of close-ups of people/faces. Furthermore, markedly unpleasant urban environments were avoided, trying not to suggest a strong judgment bias to the participants. For the neutral setting condition, lines depicting slowly changing colorful shapes or patch of colors were selected, alike those often used in computer screensavers. All videos were displayed with a resolution of 1920 × 1080 pixels, and a frame rate of 24 frames/second, and were viewed at around 2 m of distance on a 17-inch LCD screen. After each video, the word “relaxing” was presented on screen and the participants rated how relaxing  they found the video. The “relax” score for each condition was then averaged across videos of the same type for each participant. The experiment was run using E-prime 2. After each condition , the participants were asked to perform a digitalized visual version of the Digit Span Backward Task (DSB). After the participants had been instructed to the task, they were asked to perform the DSB before the beginning of the experiment. This was done to allow the participants to familiarize with the task. In the DSB, a series of digits were displayed on the screen, following one another. The digit displayed were randomly chosen from 1 to 9. Participants were instructed to recall the digits backward . In the case the task was successful for two times in a row, participants were then asked to perform the task again with a longer list of digits. The task started with only 2 digits, and the participants could go to indefinitely long list of digits according with their performance. The digit list was made longer until the point when participant failed to inversely recall the list for two succeeding trials. The score for each participant was calculated on the length of the longest list the participant successfully performed. The DSB task aimed to detect cognitive restorative effect from the exposure to the videos (working memory). Several studies have implemented this type of cognitive task to assess the restorative effect arising from the exposure to natural settings .Electroencephalographic brain activity was assessed at 64 sensors placed according to the international 10–10 electrode system. The electrodes were placed on the head using a sintered Ag/AgCl cap featuring active electrodes . NeurOne 1.3.1.26 software and Tesla #MRI 2013011 and #MRI 2013012 amplifiers  were used for data collection. Signal was referenced online to Cz, and the ground electrode was placed on position AFz. The recording sampling rate was set to 500 Hz.Electroencephalography data was processed offline using MATLAB  and with the EEGLAB toolbox version v2021.0 . IntervaThe EEG data pre-processing pipeline was created adapting the suggestions reported in Electrodermal activity was measured using an electrode couples connected  to a wireless handcuff placed on the left, non-dominant hand. The recording baseline was individually adjusted using the Biopac default function for each participant prior to the experiment. The equipment used was the Biopac MP150 recording system  and the amplifier BN-PPGED-R with 16-bit resolution and a sampling rate of 2 kHz. Participants were asked to avoid movements and to keep the left hand as still and relaxed as possible. The EDA data was operationalized computing two different variables exploring different aspects of the activity of sympathetic nervous system, the skin conductance level (SCL), and the number of skin conductance responses (SCRs). The SCL shows tonic and long-lasting activity of the sympathetic nervous system, while SCRs indicate phasic activity reflecting the number of stimulated nerve impulses regulating the secretion of sweat . SCRs wep-values were obtained with parametric ANOVAs, while multiple comparisons were corrected with spatial clustering, using the traditional threshold of significance level of p < 0.05  as provided in the Matlab toolbox Fieldtrip (Fieldtrip Lite v20210330). Repeated-measures ANOVA was computed using experimental conditions (3) as factor, and post hoc t-tests were computed to study differences of activity in low alpha between the experimental conditions.Electroencephalography results were analyzed using the data analysis package integrated in the EEGLAB study function. The difference between the three experimental conditions were first analyzed for the early alpha waves (8–11 Hz), as brain waves at these frequencies were found to be the most affected by the presentation of natural vs. non-natural sceneries in static photos . The difpost hoc t-tests.To control for the specificity of the effect for early-alpha waves  = 43.05, p < 0.001, e2 = 0.652. Post hoc t-tests revealed that natural environment videos were rated as the most relaxing and differed statistically both from the neutral (p < 0.001) and urban (p < 0.001) ones. Ratings did not differ between neutral and urban environments (p = 0.184).The relaxation ratings given after each of the four videos/condition were averaged to obtain a single score for each of the three experimental conditions. The scores ranged from a minimum of 1 to a maximum of 9. p = 0.189). Descriptive data are reported in ANOVA analysis showed that there were not statistically significant differences between the DSB task scores between the experimental conditions (Post hoc t-tests (Oscillatory activity during the display of the videos (3) were compared for every recorded electrode (64), and for the early-alpha frequency band (from 8 to 11 Hz). Based on the differences revealed by the ANOVA in the previous analyses, two electrode cluster were individuated  and the signal was analyzed in the spectral domain from 2 to 30 Hz. This analysis aimed to study whether the effects of viewing nature videos on spectral power was restricted specifically to the lower alpha band or whether they could be observed also in other frequencies. For the central cluster , the ANOFor the occipital cluster , ANOVA sp = 0.068  = 3.45, p = 0.041. Post hoc t-tests, revealed that the lowest tonic activity was recorded in response to urban videos, and it statistically differed from the tonic activity recorded during the vision of the neutral videos (p = 0.025) but not from tonic activity related to the vision of nature videos (p = 0.256). Nature videos produced the second lowest level of EDA-SCL, but it did not differ from the one associated with neutral videos (p = 0.146). Data averages and distribution is shown in The number of SCRs recorded for each of the experimental conditions were averaged and a singular numerical score was obtained for each condition. EDA – SCR scores were of 198.33 (SD = 32.57) for nature videos, 200.65 (SD = 33.53) nSCR for neutral videos, and 201.61 (SD = 33.86) for urban videos. One-way ANOVA showed that phasic activity SCRs did not differ between the three types of videos, Even though it is widely acknowledged in the scientific literature that natural environments provide restoration and promote positive feelings , it is sThe recently published studies of The present study aimed to confirm previous findings using more immersive scenes compared to some of the previous studies . FurtherIn the present experiment we recorded EEG and electrodermal activity while the participants viewed passively a series of scenes from natural, urban, or “control” environments. The control environment was obtained using computerized non-realistic scenes. After viewing each video, the participants were asked to self-report how relaxing they found the video. In line with several previous findings, participants rated as more relaxing the presentation of natural environments compared to those of urban environments. Ratings of perceived relaxation did not differ between neutral and urban environments video.Our results are in line with our prediction concerning the modulation of early-alpha waves during the experience of videos of natural environments. The distribution of the early-alpha waves modulation showed to be mainly over central brain areas, in line with the findings of Visual examination of spectral power density patterns in the scalp topographic activity map showed that the difference in early-alphas was not limited to the central brain areas but extended also for the activity recorded over the occipital-central brain area. We performed exploratory analysis for this brain area, and we found similar patterns than those for the centralized alpha activity. However, in this case the difference between the conditions extended more toward the beta powers, and brain activity in response to nature and urban videos were very similar over all the frequency spectra, while different from the activity recorded for the control condition. A speculative explanation may be related to the very different visual content (fast lines and colors) of the neutral condition, which may be able to modulate in a very different manner the visual cortex compared to the other two more slowly changing and realistic experimental conditions.Further visual inspection of the brainwaves pattern in graphical presentation of the power density × frequency analyses for the central cluster revealed that, alike early-alpha, a difference between the condition was also shown in late-theta spectral power density. This difference may be because of the higher perceived relaxation in response to the nature video condition. Several investigations have found that both alpha and theta power are associated to the activities of mindfulness and meditation , and theA recent study  recordedThe increase in alpha waves observed in our experiment suggests that the experience of nature video may be related to an increase activity of the inhibitory mechanism related to the activity of the thalamic-cortical circuit . In inhit-tests showed that the difference was statistically significant only in the comparison against the neutral condition. These findings are difficult to explain and are against the expected results  systematically affecting one of the other experimental conditions. Furthermore, the “neutral” condition may arguably not be neutral but depicting man-made computerized objects which are processed similarly to urban ones .The results presented in our study, together with the ones presented in previous investigations , showed The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.The studies involving human participants were reviewed and approved by the Ethics Committee for Human Sciences at the University of Turku. The patients/participants provided their written informed consent to participate in this study.SG developed the study idea, conceptualized the study design, responsible for the choice of the study methodology, developed the experimental paradigm, trained and supervised the laboratory assistants, responsible for the data analysis, data pre-processing scripts, and data curation, and had the main role in writing the manuscript draft. GS assisted the study planning and selection of experimental stimuli and responsible for the laboratory tests. MK assisted the development of the study, supervised the research project, and assisted the writing of the final version of the manuscript and the revisions of it. All authors contributed to the article and approved the submitted version.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.All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.


In this study, an elastic solution of an axisymmetrically surface-loaded thin layer resting on a rigid substrate is established by taking the surface stress and material microstructural effects into account. Derived solutions provide not only a means to investigate the size effects on the mechanical response but also a set of fundamental solutions essential for tackling contact problems in a micro/nano scale. In the formulation, the couple stress and surface elasticity theories are adopted to simulate the microstructured bulk layer and the surface material, respectively. A general solution of an elastic field within the bulk layer is obtained first by Hankel transform method and subsequently used together with the surface equations and boundary conditions to form a set of conditions essential for determining all unknown constants. After being fully tested with available benchmark solutions, results are used to study the role of surface and couple stresses on the load transferring mechanism to the substrate and its size-dependent characteristic for a wide range of external length scales relative to the internal length scales. In recent years, applications of nanotechnology to enhance performance of surface coatings have grown remarkably. Such continuous developments and uses of nanoscale coatings result directly from the increasing availability of nanoscale/nanostructured materials and advances in the coating processes. For instance, silver nanoparticles embedded in textiles can kill odor-causing bacteria; nanofiber coatings on textiles can stop liquid penetration; novel nanomaterials on fabrics can also absorb perspiration and wick it away; and titanium nanoparticles embedded in textiles can inhibit UV rays from penetrating through the fabric, etc3, plates5, surface coating8 and indentations10. Most of the existing studies can be divided into three main groups based on the underlying methodology and procedure employed: one associated with experimental investigations13 and the other two concerning discrete-based18 and continuum-based mathematical modelings. In the past decades, simulations based on continuum-based mathematical models have been progressively offered as viable alternatives. Various size-dependent elasticity theories, such as the couple stress theory23, the strain-gradient-based elasticity theory25, the surface stress elasticity theory28, and the nonlocal elasticity theory31, have been proposed to account for the influence of material small-scale structures in a continuum manner. Although the results and findings from mathematical models are considered only as of the first/rough response estimation, these predicted trends can be used to provide preliminary data for more accurate experiments.Many researches have been extensively conducted to understand the fundamental behavior of micro- and nano-structures such as micro-/nano-scale beams19, Toupin21, Mindlin22, and Koiter23 and has been received attention from researchers due to its capability to tackle problems at micro-scale. Muki and Sternberg6 first applied the theory to investigate the role of couple stresses on the response of an elastic half plane under surface loads and simple contacts. Since then, studies have been significantly expanded to handle more complex scenarios including indentation problems37 and layered media42. The nontrivial extension to three-dimensional cases has also been documented46. Nevertheless, the number of studies is still relatively few in comparison with that of two-dimensional problems.Fundamental problems in solid mechanics at micro/nano scales are extensively studied, especially those involving surface loads and contacts. Several groups of researchers have studied the size-dependent effects using various theories. Couple-stress-based theories, in which an additional deformation measure termed the curvature is introduced along with its conjugate pair known as the couple stresses, are commonly used in the literature to simulate the influence of material microstructures of small-scale objects. The original (indeterminate) couple stress theory was proposed by Mindlin and Tiersten28 by following the fundamental idea of Gibbs47 and its modeling capability in comparison with atomistic and molecular static simulations has been confirmed by several studies50. Within the context of surface mechanics, applications of such theory to study the near-surface responses have also been well recognized; for instance, problems related to half plane, half space, and layered media under surface loads (e.g.54) and surface contacts62. Results from existing studies have confirmed the significant role of both residual surface tension and surface elasticity on the predicted responses and the size dependent characteristics as the relevant external length scales become comparable to the intrinsic length scale of the material surface. In such tiny scales, the need to replace the conventional, size-independent mechanics theory with models capable of accounting for size effects is apparent.The surface/interface elasticity theory is one among available frameworks widely adopted to simulate the mechanical response of tiny-scale objects in which the surface free energy is observed to be significant. The solid mathematical foundation of such theory was laid down by Gurtin and his co-workers63 and Le et al.64 applied both couple stress and surface elasticity theories to investigate the size dependent response of a homogeneous half plane excited by surface loads and tilted flat indenters, respectively. The extension to treat a surface loaded homogeneous half space accounting for both couple and surface stresses was achieved by Lawongkerd et al.65. In above mentioned studies, it was clearly demonstrated that both effects are significant when the internal length scales of the bulk and surface materials are comparable. The simultaneous effects must be taken into account in the modeling when the relevant external length scales fall within the range of the two material length scales. While the role of both surface and couple stresses were extensively explored in above investigations, the medium was modeled either by a homogeneous half-plane or half-space, and such simplified settings clearly pose a key limitation on their practical applications. For instance, key response and characteristics of coated objects with a very thin coating layer under surface excitations  is not possible from such limited settings. On the basis of an extensive literature survey, the authours are unaware of any further development from the studies mentioned.While both microstructure of bulk materials and surface free energy have been addressed to be responsible for the size-dependent characteristics of the response of micro/nano-scale homogeneous and layered media, work toward the integration of both effects in the simulations, within the continuum-based framework, has been still relatively few. Recently, Le et al.In the present study, a size-dependent elastic response of a surface loaded material layer resting on a substrate is investigated. Both surface free energy and bulk material microstructures are taken into account in the modeling as those responsible for size effects. The treatment of a medium as a finite-thickness layer clearly broadens its practical applications from available half-plane and half-space cases, especially for studying surface coating problems. Besides their direct contribution to gain an in-depth understanding of the mechanical response of a very thin layer medium, established results form an essential and sufficient basis for the development of a solution scheme to tackle surface contact problems.h (representing a thin coating layer) resting on a rigid substrate (representing a coated substrate) as shown schematically in Fig. O located at the center of the loading region, the r-axis directing along the infinite direction of the layer, and the z-axis directing downward is employed.Consider a three-dimensional, elastic layer of finite thickness 19 and Koiter23 is adopted. Basic equations  governing the elastic field under the axisymmetric deformation and zero body force and couple are given by67To simulate the elastic response of the bulk material with microstructures, a fundamental couple stress theory proposed by Mindlin and Tiersten26, Gurtin and Murdoch27, and Gurtin et al.28. For an axisymmetric case, the non-zero surface displacements A material surface adhered to the top of the bulk is modeled by the surface elasticity theory proposed by Gurtin and MurdochSince the surface part is perfectly bonded to the bulk layer, the surface displacements By applying the continuity conditions Eqs.  and 10)10) togetSince the surface part is considered infinitesimally thin and has no bending resistance, the applied couple traction The boundary conditions at the bottom of the bulk layer can be readily expressed asEquations –16) for for16) f67To obtain the closed-form solution of an elastic field within the bulk layer, a method of Hankel transform together with the representation of the displacement field is adopted. In particular, the vertical and radial displacements of the bulk layer undergoing the axisymmetric deformation admit the following representations68 and the final results are given bym; The closed-form general solution of Eq.  can be r of Eqs. , 18),  tog tog16) t54 and Lawongkerd et al.65 is adopted.The solution of the system Eq.  for eachary Eqs. –A12). T. T\documComputed results for certain cases are first compared with existing benchmark solutions to verify both the formulation and solution procedure. The influence of surface and couple stresses on the elastic field within a thin material layer under various surface loads is subsequently investigated. To clearly demonstrate the individual and simultaneous effects on the size-dependent characteristics, results for four different models , the normalized vertical stress attains the maximum magnitude at the center of the loading region and monotonically decays to zero as C and load Case D, the magnitude of the normalized shear stress A and load Case B can be drawn. In particular, as the size of the loading region reduces to be comparable to (or much less than) the two length scales To demonstrate the intensity of load transferring to the coated substrate, the vertical stress A and load Case B and the shear stress C and load Case D are also reported in Fig. A and load Case B) or approximately attains (for the load Case C and load Case D) its maximum. It is evident from Fig. C and load Case D (see Fig. Through-the-thickness profiles of the vertical stress A and load Case C are chosen as representative load cases for applied normal and radial shear tractions, respectively. For the load Case A, the transferring vertical stress C, it is chosen, for convenience, to report the transferring radial shear stress To further illustrate the influence of the coating-layer thickness on the reduction of the transferring stress on the substrate when both surface and couple stress effects are present, the transferring vertical and shear stresses for the case of applied normal and radial shear stresses, obtained from the Model-1, are reported as a function of the normalized thickness see Fig. a. For thA and the transferring shear stress at C are reported as a function of A drops quite rapidly and monotonically from the value predicted by the classical model. The different behavior is observed for the load Case C. The variation of the transferring shear stress over a wide range of the ratio Finally, the size dependent characteristics of the predicted transferring stresses to the substrate are also investigated. To clearly illustrate such behavior, the maximum transferring vertical stress for the load Case The analytical solution of an elastic field of a thin material layer coating a rigid substrate and excited by axisymmetrically distributed surface loads have been derived. Such established results are considered novel in that both surface free energy and material microstructures, which are recognized to be responsible for the size effects in small-scale objects, are taken into account simultaneously to model a medium of finite thickness. This allows the direct application to simulate the mechanical response of components coated by a very thin material layer. A continuum-based model integrating the couple stress elasticity theory for tackling the inherent microstructural effect and Gurtin–Murdoch surface elasticity theory for capturing the surface effect has been formulated and solved by an analytical scheme based on Hankel transform and the displacement representation. Obtained results are explicit in an integral form, highly accurate as useful benchmark solutions, and an essential basis for the development of solution schemes to tackle surface contact problems.Results from an extensive numerical study have revealed that the surface and couple stresses significantly affect both the characteristics and maximum value of load transferring to the coated substrate in comparison with the classical case when the size of the loading region is comparable to or much smaller than the material length scales. For a coated system under normal tractions, the presence of surface and couple stresses can significantly boost the reduction of the transferring vertical stress to the substrate especially when the size of the loading region is much less than the length scale of the bulk and surface materials. A different trend has been observed in the case of applied shear loads. The transferring shear stress to the substrate predicted by the model integrating both surface and couple stress effects can be either lower or higher than the classical solution depending on the ratio between the size of the loading region and the material length scales. This results directly from that the surface effect lowers the transferring shear stress but the couple stress effect causes the reverse trend. In addition, as the size of the loading region become much smaller than the two material length scales, the surface effect is much more pronounced than the couple stress effect.Supplementary Information.


Hand gestures are a common means of communication in daily life, and many attempts have been made to recognize them automatically. Developing systems and algorithms to recognize hand gestures is expected to enhance the experience of human–computer interfaces, especially when there are difficulties in communicating vocally. A popular system for recognizing hand gestures is the air-writing method, where people write letters in the air by hand. The arm movements are tracked with a smartwatch/band with embedded acceleration and gyro sensors; a computer system then recognizes the written letters. One of the greatest difficulties in developing algorithms for air writing is the diversity of human hand/arm movements, which makes it difficult to build signal templates for air-written characters or network models. This paper proposes a method for recognizing air-written characters using an artificial neural network. We utilized uni-stroke-designed characters and presented a network model with inception modules and an ensemble structure. The proposed method was successfully evaluated using the data of air-written characters  from 18 people with 91.06% accuracy, which reduced the error rate of recent studies by approximately half. Hand gestures along with speech are important tools in human communication. Notably, hand gestures have allowed people to express and describe their thoughts more accurately. In addition, gestures are more compelling in articulating one’s opinions and beliefs, especially when there are difficulties in voice communication, the environment is noisy, or the communication occurs in an environment where silence is required.In recent decades, studies have used gestures as communication tools between humans and computers. Air writing, where researchers write in the air with hand gestures, is recorded, analyzed, and recognized automatically using computer algorithms. The most popular method for recording hand gestures is the camera-based approach using a pen and an optical camera. Detecting a pen-tip or fingertip is one of the most important tasks in such computer vision-based approaches, as the shape of the written characters is very similar to conventional handwriting on paper. Handwriting recognition on paper is a traditional research topic that has been studied in the last decades. Various methods, such as linear discriminant analysis, multi-layer perceptron, tangent distance, and support vector machine ,2, have In , the penThe detection of the hand or fingertips can be enhanced by substituting the optical camera with a depth sensor. For example,  used theA drawback of the camera- or the depth sensor-based method is that they mostly require users to stand or sit in front of a camera or a sensor. Though head-mounted cameras, proposed in , mitigatAnother approach to recording hand gestures is using accelerometer/gyro sensors, which provide users freedom of movement. Because accelerometer/gyro sensors are embedded in most present-day smartwatches/bands, users can easily utilize them as input devices. This approach is also stable irrespective of the light conditions.xyz coordinates. Because the sensors move or rotate while obtaining the signals, the accelerometer values follow the local coordinates of the sensors. Therefore, [One of the biggest issues with the accelerometer/gyro sensor-based approach is tracing the position of the sensors in the global erefore,  proposedTherefore, the accelerometer/gyro sensor-based methods have lower accuracy than camera-based methods. For example, the maximum accuracies of camera-based approaches are 99.571% for Arabic numbers and 16 directional symbols . By contAnother issue with the sensor-based approach is the diversity of the signals obtained among individuals. Because the speed and rotating angles of a hand differ for each person, it is difficult to recognize the signal of an unknown person. For example, recognition accuracy dropped significantly from 89.2% to 83.2% in  and fromThis study focused on increasing the accuracy of the air-writing method for this writer-independent case. We proposed a method to process the obtained signal by designing a novel network structure, training method, and boosting algorithm for the trained network model.The remainder of this paper is organized as follows: A Huawei Watch 2 and an Android tablet (Nexus 7) were used to collect the data. The watch has a built-in 3-axis accelerometer and gyro sensors and can transfer the data into the Android tablet using Bluetooth see .The data were recorded on the tablet as there was insufficient space to record data in the watch. A user wears the watch on a wrist and virtually draws characters with hand movements see .Twenty people aged from 19 to 24 participated in the air-writing activity. Each person virtually wrote the characters freely in the air five times each, after having the shape of each character explained to them. There was no guidance on arm posture or speed, and the participants were allowed to rest freely during the experiments. Data from the smartwatch were transferred to the tablet and recorded. Data from two participants were excluded from further evaluation because of recording errors found after the experiments. All participants were right-handed.The gestures of some characters are the same or slightly different see . The gesThe data were transferred and recorded at 40 Hz from the watch to the tablet. The data include accelerometer and gyro values in three axes and the gravity-removed accelerometer values provided by the watch using a linear filter.An example of the collected data in a tabular form is shown in The collected data were smoothed using a Savitzky–Golay filter ; this way axis) such that the value at the beginning is zero.Examples of normal and corrupted signals of the accelerometer are shown in (1)Calculate detail coefficients from single-level wavelet transform for each channel of signals;(2)Interpolate and smooth with a Savitzky–Golay filter  as the l(3)Calculate the standard deviation of the signal for each channel, and set the minimum value among the channels as the threshold for the data;(4)Signal regions where the detail coefficients of all the channels are less than or equal to the threshold are marked as non-movement regions;(5)Remove the signals in the non-movement regions.Because the obtained signals include the non-movement data, the following algorithm was developed to trim the signals using wavelet transforms:This algorithm is applied for each datum separately. Removing the non-movement region means that the signal changes in this region are ignored and the data points are removed. As the lengths of the processed signal varied, all signals were resampled to have the same number of data points. This process was necessary to input the data into a convolutional layer. The resampled signal length was 129, which is the maximum length of the hand movement regions for all the data.The data were normalized between 0 and 1, which is the recommended input for artificial neural networks. Normalization was conducted for each channel group of accelerometers, linear accelerometers (gravity-removed accelerometers), and gyro values. The minimum and maximum values were calculated for each group and then normalized to between 0 and 1. The minimum and maximum values were then calculated for each air-writing datum.The proposed network model with inception architecture , inspireFully connected layers were connected with the dropout layers to prevent overfitting, as the amount of data is limited in our experiment. Parameters such as the number of filters, filter sizes, and dropout ratios are shown in The accuracy of artificial neural networks may vary because of the randomness of the training algorithms, especially when the size of data is small. We employed an ensemble model to stabilize and increase the overall accuracy ,22. We tThe proposed method was validated independently for writers by training the network with the data of 17 participants and then testing using the data of the last participant. This process was repeated 18 times such that each participant’s data were used as test data at least once.The number of epochs and batch size were set to 400 and 16, respectively. The ADAM optimizer with a learning rate of 0.001 and the AMSGrad option  was usedThe proposed method was evaluated in a user-independent manner, following the leave-one-subject-out cross-validation (LOSOCV) approach . In thisThe experiment was conducted on a computer with Intel i9 and Nvidia RTX 3090ti. The GPU memory was 24GB and all the program codes were written in Python with the TensorFlow library.The proposed method was evaluated with and without the preprocessing trimming algorithm, as it is often reported that CNNs show improved performance without intensive preprocessing algorithms. The accuracy with and without the trimming procedure was 88.27% and 91.06%, respectively, indicating the ineffectiveness of the trimming algorithm in our dataset. This may have been caused by writing speed changes, as removing the non-movement regions was successful in our visual examinations. The convolutional operators of the networks could fail to detect signal changes when the writing speed changes when adjusting the length of the signals after trimming.The ensemble structure was effective and stable according to the experiment. The accuracy dropped to 89.51% when the ensemble structure was removed from the proposed method and did not improve even with repeated experiments. This is because we utilized the 18-fold validation policy where the accuracies were averaged from 18 different trained models.It is remarkable that the proposed method achieved accuracy higher than 90% with data from accelerometer and gyro sensors in a user-independent manner. Higher accuracies have been reported with depth sensors or optical cameras, but the best accuracies with accelerometer and gyro sensors have been limited to 83.2% when the systems were validated user-independently. The high accuracy was achieved by two factors: the dataset and network structure. We employed the uni-stroke characters designed for PDA, which are simpler than the original alphabets. Because the uni-stroke characters were designed for finger gestures, we expect that it was easier for the participants to be accustomed to air writing. The design of the proposed network structure also affected the accuracy. A simplified inception structure and the sizes and numbers of filters affect the accuracy of the neural networks.The evaluation was conducted in a user-independent manner to overcome the limitations of the small size of the dataset. The same accuracies are expected to be achieved when new users’ data are included for additional evaluations. Because of the posture variations during the air writing, the accuracies of the user-independent validations were lower than the user-dependent validations ,16.Ninety models were trained during the experiment as the five trained models were utilized for each fold. The five models were trained with the same network structure and data, but the predicted outputs were different because of the randomness of the weight initializations. Accuracy without the ensemble algorithm was slightly reduced to 89.51% on average.The differences in the accuracies were observed for each character. One of the reasons for this low accuracy can be found in the confusion matrix in Accuracies also varied according to the participants see . The accIt should be noted that the data of two participants were excluded from the experiments because of a recording failure, since similar situations could occur during real-time application in the future. Firstly, this recording issue could be recognized by checking the data communication status, as the data are corrupted by network failure. Secondly, recording failure can be recognized by checking the length of the obtained data. Because recording failure can occur for unknown reasons, we need to check if all the data were transferred by checking the length of the obtained signals.This paper proposed a method for recognizing the air-written characters of English alphabets and Arabic numbers from accelerometer and gyro sensor data. Accuracies of air writing with accelerometer and gyro sensor data have been lower than depth sensors or optical cameras because of data variations among users. This study utilized a uni-stroke design for the characters and a deep neural network structure to solve this problem. The network structure was newly designed by utilizing simplified inception modules. The method was evaluated using user-independent data from 18 people to validate whether the proposed method overcomes the variation problems among users. The proposed user-independent air-writing system achieved 91.06% accuracy, which is higher than what has been reported in previous studies using acetometer/gyro sensor data, reducing the error rates by approximately half.One of the limitations of the proposed system is the variation in accuracies according to the characters and the participants. Designing new shapes for English alphabets could be a solution because much confusion occurred between characters with similar shapes in the air writing. Combining boosting algorithms such as AdaBoost with the convolutional neural network  could be


Thermoplastic composites (TPCs) are predestined for use in lightweight structures, especially for high-volume applications. In many cases, joining is a key factor for the successful application of TPCs in multi-material systems. Many joining processes for this material group are based on warm forming the joining zone. This results in a change of the local material structure characterised by modified fibre paths, as well as varying fibre contents, which significantly influences the load-bearing behaviour. During the forming process, many different phenomena occur simultaneously at different scales. In this paper, the deformation modes and flow mechanisms of TPCs during forming described in the literature are first analysed. Based on this, three different joining processes are investigated: embedding of inserts, moulding of contour joints, and hotclinching. In order to identify the phenomena occurring in each process and to describe the characteristic resulting material structure in the joining zones, micrographs as well as computed tomography (CT) analyses are performed for both individual process stages and final joining zones. Continuous-fibre-reinforced plastics have a high potential for use in lightweight structures due to their good specific mechanical properties and the adjustable characteristics. In particular, thermoplastic composites (TPCs) enable both highly integrative part design and efficient large-scale production . MoreoveThe resultant material structure during the forming process depends on the material properties of the single constituents , the process parameters , and the design . Caused by the composition of two heterogeneous materials (fibre and matrix), a macroscopic point of view leads to a lack of information in terms of the occurring phenomena and material structure. Furthermore, the dimensions of the fibres ; see Depending on the deformations and flow phenomena at a single fibre level, three different deformation modes can be described . Fibre eOn the fibre bundle scale, interactions  affect the material behaviour due to the large number of fibres in a yarn or a fibre bundle. A distinction of the fibre and fibre bundle behaviour is made by . In formThe deformation and interactions of yarns in prepreg layers or dry fabrics at the mesoscopic scale are discussed in ,32. The All the phenomena described in the literature, which occur during the forming of TPCs, are summarized in For the embedding of inserts, the process temperature is above the melting temperature of the matrix (Temperature Section I) and an isothermal mould is used.Moulding of contour joints is also carried out above the melting temperature of the matrix (Temperature Section I), but in a variothermal mould.The hotclinching, on the other hand, is performed below the melting temperature of the matrix (Temperature Section II) using a variothermal mould.Therefore, joining technologies using thermally supported forming of TPCs are discussed in more detail in this work. The focus here is on processes where forming takes place above the Vicat softening temperature of the matrix. In order to investigate which phenomena occur during different processes, three technologies were selected, which differ in particular in terms of temperature management:Typical matrix systems for TPC applications with moderate thermal and mechanical requirements are polypropylene (PP) and polyamide 6 (PA6). Glass fibres (GFs) and carbon fibres (CFs) are usually used as reinforcement.The joints with the different materials considered in this paper are shown in The laminate layup for the embedding of inserts is made of unidirectional (UD) tapes, which are processed into TPC sheets in an autoclave process. The inserts are made of steel, and the height corresponds to the thickness of the laminate, so that they are flush with the laminate surface a. The laMetal inserts can be embedded into TPCs during the part manufacturing process using the principle of moulding holes . TherebyIn this study, a pilot rig at the laboratory scale consisting of an infrared heating device (TPC temperature 210 °C) and a tempered steel mould ( 40 °C) was applied. The pin tool and the counterpunch were pneumatically actuated. The feed rate of the pin tool was 230 In the integral-bladder-assisted moulding process, two semi-finished parts are intrinsically joined during the manufacture of the composite structure. A tape-braided preform is inserted into a multi-scale-structured metallic load introduction (LI) element and positioned inside a mould a. By pre (RGE30)  with a dthe tool c.The thermally supported clinching process (hotclinching) uses a tool concept consisting of a conventional punch, as well as a blankholder and a novel split die . The dieThe schematic illustration of the process is given in eeve top b. Causedeeve top c, where eeve top d. In totAn analysis of the joining zone with imaging techniques is essential to identify and evaluate the process phenomena and their impact on the material’s structure. Micrograph analyses of cross-sections are often used for quality assessment or measurement of characteristic geometrical parameters . In the Micrograph analyses have the disadvantage that the joining zone can only be evaluated in one section plane. Three-dimensional phenomena, such as fibre reorientations, can thus hardly be evaluated. For this reason, additional computed tomography (CT) analyses were carried out to investigate the joining zones. The specimens were investigated with a CT system V|TOME|X L450CT ( 300 kV micro-focus X-ray tube). The scanning parameters are summarized in To achieve a high image quality and to avoid artefacts, the joining zones with embedded inserts were sectioned. The insert was removed, and only the TPC of a quarter joining zone (22 mm × 22 mm × 4.3 mm) was scanned . To analIn The subsequent movement of the pin displaces the fibres and the matrix, causing friction and sliding between the pin tool and the TPC sheet (4). Thereby, the fibres are shifted (5), bent (6), and elongated, which results in a significant three-dimensional change of the fibre orientation b. WhereaAfterwards, the material displaced in the thickness direction is pressed back into the laminate plane by the counterpunch c. Due toFor a three-dimensional analysis of the resulting material structure with locally varying fibre orientations and the FVC, CT analyses were carried out on the final joining zone. It can be identified that the fibres are not only reoriented in the thickness direction, but also in the laminate plane a, as in As the micrograph analyses of the forming process showed, matrix-rich zones occur in the joining process. These areas were also detected in the CT analysis . FurtherAn examination of the specimen edge proves that inter-ply slip also occurs when the inserts are embedded. In A multi-scale form closure is targeted with the presented hybrid contour joints. Photomicrographs provide insights into the moulding phenomena after the bladder-assisted moulding process. In The main phenomena during the moulding of macroscopic structures is the forming of the TPC in the thickness direction, creating the desired form closure. By intra-ply shear deformation of the braided textile, expansion of the preform is enabled. During expansion in the bladder-assisted moulding process, fibre shifting on the fibre bundle scale occurs, as well as inter-ply slip and tool–ply slip on the multi-layer scale. These phenomena are necessary to mould the composite into the structures of the aluminium LI element.5] bandage is displayed. Bridging effects due to insufficient fibre bending occur in tight corners and grooves of the meso-structure pyramids. Due to matrix percolation, the cavities in the structured LI element are filled by the matrix material of the composite, which results in matrix-rich zones and a locally deviating FVC.Depending on the shape of the meso-structure, certain geometries such as sharp edges or corners are not fully mouldable by the fibres via bending. In In The initial material structure of the multi-layered GF-PA6 is characterised by unidirectional yarns with matrix-rich zones in between . The addThe hotclinching process operates in Temperature Section II below the melting temperature of the matrix. The temperature next to the Vicat softening temperature of the matrix just leads to a softening of the thermoplastic PA6 matrix and, therefore, to a more ductile material behaviour. This enables higher degrees of deformation without matrix failure and enables the joining by plastic deformations . For detDuring heating, the fibres and the ductile matrix are clamped between the blankholder and the heated rigid die in the outer areas (1). The joining process starts when the punch moves downwards and offsets the sheets in the out-of-plane direction a,b. Due With the further downward stroke of the punch, a neck area is generated in the upsetting phase c,d. The It can be stated that the fibre fracture in the bending areas also increases especially in the ring channel and the neck area (9 and 10), caused by the compression force and contour of the die and the anvil. The radii of joining tools are below the critical bending radius .In the flow pressing phase e,f, the The CT analysis of the finished joint is shown in All three joining processes considered result in a three-dimensional reorientation of the fibres in the joining zone across scales. The analyses of the joining zones were carried out on the fibre bundle and laminate scale; an evaluation on the single fibre level was not executed. However, it can be stated that a deformation at the fibre bundle scale also implies deformations of the single fibres.Fibre bundle elongations cannot be determined with the applied characterisation methods . HoweverIn all three processes, transverse compaction occurs due to the out-of-plane forces on the joining zone. Depending on the matrix conditions (molten or not molten), this is accompanied by different flow processes . Deformation of fibre bundles can be detected successfully using CT analysis. Here, for example, the local fibre orientation in the joining zone can be evaluated qualitatively and quantitatively. As has been shown, all three processes lead to shifting and bending of fibre bundles. Torsion of fibre bundles was detected by CT analysis for hotclinching. In future works, fibre bundle torsion may be identified by analysing the paths of individual fibres in a fibre bundle during an in situ CT analysis of the forming process. When the breaking elongation of the fibres is exceeded in the process, fibre failure occurs. This could only be observed in the case of hotclinching. Here, the flexibility of the fibres is restricted due to the low temperature of the matrix during forming, which induces fibre failure in the joining zone.In addition to the deformation of the fibre bundles, the flow processes of the matrix also occur during local forming if the matrix is flowable due to heating of the TPC . For hotDeconsolidation is observed only for embedding of inserts, because heating in an infrared heating device above the melting temperature of the matrix takes place without pressure on the TPC. Voids and detachments of laminate layers that occur during deconsolidation are closed in the pressure-impacted subsequent forming process, which was detected in the micrograph analyses of partially formed specimens . The heaThe deformation and interactions of fibres and fibre bundles, as well as the matrix flow mechanisms at the microscale and mesoscopic scale lead to multiple deformation modes at the macroscopic scale, as also described in the literature . In all The contributions of the individual phenomena during forming depend on the material properties, such as the textile architecture, the process parameters, such as the TPC temperature and tool velocity, and the design, such as the geometry of the tools or the structured surface.In this paper, the process phenomena at different scales occurring during the forming of TPCs as described in the literature were analysed. On this basis, three different joining processes were investigated regarding the deformation modes and flow mechanisms of the TPC during forming. Although the joining processes were very different, especially with respect to the temperature of the TPC (below and above the melting temperature), it was shown that almost all the forming phenomena described in the literature can occur also during joining. The superposition of the forming phenomena on the different scales results in locally complex material structures with varying properties such as the fibre orientation and FVC. This local material structure affects the load-bearing behaviour of the joint. The numerical description of those joining processes with fibre–matrix interaction is not possible yet. For this purpose, it is necessary to reduce the complexity by focusing on the individual phenomena first. Within this work, a major step is made by describing complex forming processes with a superposition of single phenomena taking into account the common deformation theories of forming.