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See detailValue Stream Mapping (VSM) to Evaluate and Visualize Interrelated Process-Chains Regarding Circular Economy
Mangers, Jeff UL; Minoufekr, Meysam UL; Plapper, Peter UL

in Advances in Production Management Systems. Artificial Intelligence for Sustainable and Resilient Production Systems (2021, August 31), 633

The concept of circular economy (CE) aims to close and slow resource loops without neglecting the goals of sustainable development. Recently, the concept received encouraging attention among researchers ... [more ▼]

The concept of circular economy (CE) aims to close and slow resource loops without neglecting the goals of sustainable development. Recently, the concept received encouraging attention among researchers and business experts to be a convenient solution to move away from the finite linear economy concept to a more sustainable solution. However, this change of paradigm is only possible if we consider systems in a holistic manner and can localize the preventing hurdles. Value stream mapping (VSM) is a commonly known lean method, used to develop current state visualization of product and information flows within organization, helping to seek weaknesses and improve process flows. The motivation of this paper is a new C-VSM model, which enables its users to evaluate and visualize connected process-chains regarding CE on different levels in a holistic manner. For this purpose, the traditional VSM model was adapted towards the needs and requirements of CE through the application of a new representation method, additional indicators, and an appropriated evaluation system. C-VSM is in line with the current political and industrial objectives to apply CE principles by enabling a holistic reflection and consideration of supply-chains (SCs) on different levels. The model itself is validated through an extensive cross-company case study. [less ▲]

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See detailDetection of fluid level in bores for batch size one assembly automation using convolutional neural network
Simeth, Alexej UL; Plaßmann, Jessica; Plapper, Peter UL

in Advances in Production Management Systems. Artificial Intelligence for Sustainable and Resilient Production Systems. (2021, August 31), 632

Increased customization and shortening product life cycles pose a challenge for automation, especially in assembly. In combination with the nature of assembly tasks, which may require high level of ... [more ▼]

Increased customization and shortening product life cycles pose a challenge for automation, especially in assembly. In combination with the nature of assembly tasks, which may require high level of perception, skill, and logical thinking, these tasks are often conducted manually, especially in certain industries (e.g. furniture, power tools) or small and medium-sized enterprises. One of such tasks is the liquid level monitoring in gluing processes. Existing non-manual solutions are based on conventional and less flexible algorithms to detect the current liquid level. In production environments with highly individualized products, a need for more performant models arises. With artificial intelligence (AI) it is possible to deduct decisions from unknown multidimensional correlations in sensor data, which is a key enabler for assembly automation for products with high degree of customization. In this paper, an AI-based model is proposed to automate a gluing process in a final assembly. Images of a gluing process are taken with a camera and a convolutional neural network is used to extract images features. The features are applied to train a support vector machine classifier to identify the liquid level. The developed model is tested and validated with a Monte-Carlo-simulation and used on a demonstrator to automate a gluing process. The developed model classifies images of liquid levels with over 98% accuracy. Similar results are achieved on the demonstrator. [less ▲]

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See detailObservability based data-fusion cascading filtering for urban network flow estimation
Rinaldi, Marco; Viti, Francesco UL

in IEEE Xplore (2021, June)

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See detailArtificial neural network to predict the weld status in laser welding of copper to aluminum
Mathivanan, Karthik UL; Plapper, Peter UL

in Procedia CIRP (2021)

Laser welding of copper to aluminum is challenging due to the formation of complex intermetallic phases. More Al (~18.5 at. %) can be dissolved in Cu, in contrast to Cu (~2.5 at. %) in Al. Therefore ... [more ▼]

Laser welding of copper to aluminum is challenging due to the formation of complex intermetallic phases. More Al (~18.5 at. %) can be dissolved in Cu, in contrast to Cu (~2.5 at. %) in Al. Therefore, welding from copper side, large melting of Al can be achieved. However optimum Cu and Al must be melted for a strong joint. Finding the right amount is difficult and time consuming by tradition analysis technique like inspection by weld cross-sections. Considering the speed of the welding process and complexity of analysis involving with metallography cross-sections, alternative rapid method to qualify the welds are necessary. The acoustic emission during laser welding can give proportional information of the Al, Cu melted. With such an approach the weld status can be obtained in real time. In this paper the acoustic welding signal using an airborne sensor in the audible range of 20 Hz to 20 kHz, is correlated to the weld strength and material mixing (Al, Cu melt). Finally, the weld status is predicted by an artificial neural network based on the acquired signal. [less ▲]

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See detailA cascading Kalman filtering framework for real-time urban network flow estimation
Rinaldi, Marco UL; Viti, Francesco UL

in 2020 IEEE 23rd International Conference on Intelligent Transportation Systems (ITSC) (2020, December)

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See detailSensitivity Analysis on Regularity Based Driver Advisory Systems
Laskaris, Georgios; Seredynski, Marcin; Viti, Francesco UL

in 2020 IEEE 23rd International Conference on Intelligent Transportation Systems (ITSC) (2020, December)

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See detailThe Framework of Security-Enhancing Friction: How UX Can Help Users Behave More Securely
Distler, Verena UL; Lenzini, Gabriele UL; Lallemand, Carine UL et al

in New Security Paradigms Workshop (2020, October 26)

A growing body of research in the usable privacy and security community addresses the question of how to best influence user behavior to reduce risk-taking.We propose to address this challenge by ... [more ▼]

A growing body of research in the usable privacy and security community addresses the question of how to best influence user behavior to reduce risk-taking.We propose to address this challenge by integrating the concept of user experience (UX) into empirical usable privacy and security studies that attempt to change risktaking behavior. UX enables us to study the complex interplay between user-related, system-related and contextual factors and provides insights into the experiential aspects underlying behavior change, including negative experiences. We first compare and contrast existing security-enhancing interventions (e.g., nudges, warnings, fear appeals) through the lens of friction. We then build on these insights to argue that it can be desirable to design for moments of negative UX in security-critical situations. For this purpose, we introduce the novel concept of security-enhancing friction, friction that effectively reduces the occurrence of risk-taking behavior and ensures that the overall UX (after use) is not compromised. We illustrate how security-enhancing friction provides an actionable way to systematically integrate the concept of UX into empirical usable privacy and security studies for meeting both the objectives of secure behavior and of overall acceptable experience. [less ▲]

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See detailRIVER MORPHOLOGY MONITORING OF A SMALL-SCALE ALPINE RIVERBED USING DRONE PHOTOGRAMMETRY AND LIDAR
Backes, Dietmar UL; Smigaj, Magdalena; Schimka, Marian et al

in The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences (2020, August 12), XLIII-B2-2020 Article Metrics Related articles(1017), 1024

An efficient alternative to labour-intensive terrestrial and costly airborne surveys is the use of small, inexpensive Unmanned Aerial Vehicles (UAVs) or Remotely Piloted Aerial Systems (RPAS). These low ... [more ▼]

An efficient alternative to labour-intensive terrestrial and costly airborne surveys is the use of small, inexpensive Unmanned Aerial Vehicles (UAVs) or Remotely Piloted Aerial Systems (RPAS). These low-altitude remote sensing platforms, commonly known as drones, can carry lightweight optical and LiDAR sensors. Even though UAV systems still have limited endurance, they can provide a flexible and relatively inexpensive monitoring solution for a limited area of interest. This study investigated the applicability of monitoring the morphology of a frequently changing glacial stream using high-resolution topographic surface models derived from low-altitude UAV-based photogrammetry and LiDAR. An understanding of river-channel morphology and its response to anthropogenic and natural disturbances is imperative for effective watershed management and conservation. We focus on the data acquisition, processing workflow and highlight identified challenges and shortcomings. Additionally, we demonstrate how LiDAR data acquisition simulations can help decide which laser scanning approach to use and help optimise data collection to ensure full coverage with desired level of detail. Lastly, we showcase a case study of 3D surface change analysis in an alpine stream environment with UAV-based photogrammetry. The datasets used in this study were collected as part of the ISPRS Summer School of Alpine Research, which will continue to add new data layers on a biyearly basis. This growing data repository is freely available for research. [less ▲]

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See detailInferring Urban Mobility and Habits from User Location History
Cantelmo, Guido; Vitello, Piergiorgio UL; Toader, Bogdan et al

in Transportation Research Procedia (2020, January), 47

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See detailInferring Urban Mobility and Habits from User Location History
Cantelmo, Guido; Vitello, Piergiorgio UL; Toader, Bogdan et al

in Transportation Research Procedia (2020, January), 47

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See detailInferring Urban Mobility and Habits from User Location History
Cantelmo, Guido; Vitello, Piergiorgio UL; Toader, Bogdan et al

in Transportation Research Procedia (2020, January), 47

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See detailInferring Urban Mobility and Habits from User Location History
Cantelmo, Guido; Vitello, Piergiorgio UL; Toader, Bogdan et al

in Transportation Research Procedia (2020, January), 47

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See detailModel and Solution Methods for the Mixed-Fleet Multi-Terminal Bus Scheduling Problem
Picarelli, Erika; Rinaldi, Marco UL; D'Ariano, Andrea et al

in Transportation Research Procedia (2020, January), 47

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See detailIvy: Reading a critical design for sedentary behavior in the office context
Damen, Ida; Menheere, Daphne; Lallemand, Carine UL et al

in DIS 2020 Companion - Companion Publication of the 2020 ACM Designing Interactive Systems Conference (2020)

In this paper, we present and discuss Ivy, a critical artifact offering a novel design perspective on interventions that aim to reduce sedentary behavior in office workers. Ivy is an interactive office ... [more ▼]

In this paper, we present and discuss Ivy, a critical artifact offering a novel design perspective on interventions that aim to reduce sedentary behavior in office workers. Ivy is an interactive office chair that represents the amount of sitting time through growing ivy strands. Using the matrix of common argument types by Bardzell et al., we propose a structured "reading" of Ivy, as an example supporting reasoned and accessible conversations about criticality in design. Our reading of Ivy emphasized that its criticality emerges mainly from data physicalization as a new form of interactivity intended to trigger reflectiveness. The insights of this design study contribute towards a critical perspective on designing interventions to reduce sedentary time and spark discussion amongst designers and researchers in the field of Human-Computer Interaction. © 2020 Owner/Author. [less ▲]

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See detailIvy: A qualitative interface to reduce sedentary behavior in the office context
Menheere, Daphne; Damen, Ida; Lallemand, Carine UL et al

in DIS 2020 Companion - Companion Publication of the 2020 ACM Designing Interactive Systems Conference (2020)

This paper describes Ivy, an office chair that represents sitting time of an office worker through growing ivy strands. The longer one sits, the more strands will grow onto the chair. By means of a ... [more ▼]

This paper describes Ivy, an office chair that represents sitting time of an office worker through growing ivy strands. The longer one sits, the more strands will grow onto the chair. By means of a qualitative interface called Ivy, we illustrate a design approach that is currently underrepresented in sedentary behavior interventions. With this approach, we counter the current trend of digitalization and quantification of health interventions. Instead of graphs and numbers, Ivy uses data physicalization as a qualitative interface that represents sitting. We describe the design, the process, and future research steps of Ivy as a critical perspective on sedentary behavior interventions. We aim to spark discussion amongst designers and researchers in the field of Human-Computer Interaction to use qualitative interfaces as a promising approach to deepen the user's relationship with the targeted behavior and enrich the ability to construct meaning from the feedback. © 2020 Owner/Author. [less ▲]

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See detailThe hub: Facilitating walking meetings through a network of interactive devices
Damen, Ida; Kok, Anika; Vink, Bas et al

in DIS 2020 Companion - Companion Publication of the 2020 ACM Designing Interactive Systems Conference (2020)

Walking meetings are a promising means to improve healthy behavior at work. By providing a physically active way of working, walking meetings can reduce our sitting time. Several obstacles that limit the ... [more ▼]

Walking meetings are a promising means to improve healthy behavior at work. By providing a physically active way of working, walking meetings can reduce our sitting time. Several obstacles that limit the social acceptance and wider adoption of walking meeting practice have been highlighted in previous research. Amongst these, the difficulty to take notes or present files is a recurring concern for office workers. To address these barriers, we designed the Hub, a network of stand-up meeting stations that accommodate different work-related tasks during walking meetings. We report on two pilot user tests investigating users' experiences and ideas for improvement, and present future research steps. We discuss the usefulness and relevance of the Hub concept to overcome the obstacles associated with walking meetings. © 2020 Owner/Author. [less ▲]

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See detailTowards a high-resolution drone-based 3D mapping dataset to optimise flood hazard modelling
Backes, Dietmar UL; Schumann, Guy; Teferle, Felix Norman UL et al

in International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences (2019, June), XLII-2/W13

The occurrence of urban flooding following strong rainfall events may increase as a result of climate change. Urban expansion, ageing infrastructure and an increasing number of impervious surfaces are ... [more ▼]

The occurrence of urban flooding following strong rainfall events may increase as a result of climate change. Urban expansion, ageing infrastructure and an increasing number of impervious surfaces are further exacerbating flooding. To increase resilience and support flood mitigation, bespoke accurate flood modelling and reliable prediction is required. However, flooding in urban areas is most challenging. State-of-the-art flood inundation modelling is still often based on relatively low-resolution 2.5 D bare earth models with 2-5m GSD. Current systems suffer from a lack of precise input data and numerical instabilities and lack of other important data, such as drainage networks. Especially, the quality and resolution of the topographic input data represents a major source of uncertainty in urban flood modelling. A benchmark study is needed that defines the accuracy requirements for highly detailed urban flood modelling and to improve our understanding of important threshold processes and limitations of current methods and 3D mapping data alike. This paper presents the first steps in establishing a new, innovative multiscale data set suitable to benchmark urban flood modelling. The final data set will consist of high-resolution 3D mapping data acquired from different airborne platforms, focusing on the use of drones (optical and LiDAR). The case study includes residential as well as rural areas in Dudelange/Luxembourg, which have been prone to localized flash flooding following strong rainfall events in recent years. The project also represents a cross-disciplinary collaboration between the geospatial and flood modelling community. In this paper, we introduce the first steps to build up a new benchmark data set together with some initial flood modelling results. More detailed investigations will follow in the next phases of this project. [less ▲]

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See detailMaximum flow of complex manufacturing networks
Omar, Yamila UL; Plapper, Peter UL

in Procedia CIRP (2019)

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See detailScalable Gaussian Process Inference with Finite-data Mean and Variance Guarantees
Huggins, Jonathan; Campbell, Trevor; Kasprzak, Mikolaj UL et al

in Proceedings of the 22nd International Conference on Artificial Intelligence and Statistics (AISTATS) 2019 (2019)

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See detailA17044 Community health workers for non-communicable disease interventions in the digital age
Mishra, Shiva Raj; Lygidakis, Charilaos UL; Neupane, Dinesh et al

in Journal of Hypertension (2018, October)

Objectives: In this study, we review the evidence and discuss how the digitalization affects the CHWs programs for tackling non-communicable diseases (NCDs) in low-and-middle income countries (LMICs ... [more ▼]

Objectives: In this study, we review the evidence and discuss how the digitalization affects the CHWs programs for tackling non-communicable diseases (NCDs) in low-and-middle income countries (LMICs). Methods: We conducted a review of literature covering two databases: PubMED and Embase. A total of 97 articles were abstracted for full text review of which 21 are included in the analysis. Existing theories were used to construct a conceptual framework for understanding how digitalization affects the prospects of CHW programs for NCDs. Results: We identified three benefits and three challenges of digitalization. Firstly, it will help improve the access and quality of services, notwithstanding its higher establishment and maintenance costs. Secondly, it will add efficiency in training and personnel management. Thirdly, it will leverage the use of data generated across grass-roots platforms to further research and evaluation. The challenges posed are related to funding, health literacy of CHWs, and systemic challenges related to motivating CHWs. More than 60 digital platforms were identified, including mobile based networking devices (used for behavioral change communication), Web-applications (used for contact tracking, reminder system, adherence tracing, data collection, and decision support), videoconference (used for decision support) and mobile applications (used for reminder system, supervision, patients’ management, hearing screening, and tele-consultation). Conclusion: The digitalization efforts of CHW programs are afflicted by many challenges, yet the rapid technological penetration and acceptability coupled with the gradual fall in costs constitute encouraging signals for the LMICs. Both CHWs interventions and digital technologies are not inexpensive, but they may provide better value for the money. [less ▲]

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See detailA Contribution to Cyber-Physical Systems Security: an Event-based Attack-tolerant Control Approach
Bezzaoucha, Souad UL; Voos, Holger UL

in Proceedings of 10th IFAC Symposium on Fault Detection, Supervision and Safety for Technical Processes, SAFEPROCESS 2018 (2018, August 29)

In the present paper, a model-based fault/attack tolerant scheme is proposed to deal with cyber-threats on Cyber Physicals Systems. A common scheme based on observers is designed and a state feedback ... [more ▼]

In the present paper, a model-based fault/attack tolerant scheme is proposed to deal with cyber-threats on Cyber Physicals Systems. A common scheme based on observers is designed and a state feedback control based on an event-triggered framework is given with control synthesis and condition on the switching time. An event-based implementation is proposed in order to achieve novel security strategy. Observer and controller gains are deduced by solving su cient Bilinear Matrix Inequality (BMI) condition. Simulation results on a real-time laboratory three tank system are given to show the attack-tolerant control ability despite data deception attacks on both actuators and sensors. [less ▲]

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See detailAssessing the performance of coordinated predictive control strategies on urban-motorway networks
Rinaldi, Marco UL; Viti, Francesco UL

in IFAC-PapersOnLine (2018, July), 51(9), 285-290

Coordination and integration of different traffic control policies have been of considerable interest in research in the last decades and, recently, have been object of large scale implementation trials ... [more ▼]

Coordination and integration of different traffic control policies have been of considerable interest in research in the last decades and, recently, have been object of large scale implementation trials. In the setting of peri-urban motorway systems, coordination of various kinds of controllers must however be accompanied by accurate prediction of both propagation of flows and queues in the network, as well as the users’ response in terms of route choice. In this paper, we showcase through a real-life case study how coordination and prediction are both essential when performing hybrid urban-motorway control. Simulation results of a Model Predictive Control application are compared to simpler local control approaches, and the impact of coordinated intersection control and, additionally, Ramp Metering is evaluated. [less ▲]

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See detailCyber Security and Vulnerability Analysis of Networked Control System subject to False-Data injection
Gerard, Benjamin; Bezzaoucha, Souad UL; Voos, Holger UL et al

in Proceedings of the 2018 American Control Conference (2018, June 27)

In the present paper, the problem of networked control system (NCS) cyber security is considered. The geometric approach is used to evaluate the security and vulnerability level of the controlled system ... [more ▼]

In the present paper, the problem of networked control system (NCS) cyber security is considered. The geometric approach is used to evaluate the security and vulnerability level of the controlled system. The proposed results are about the so-called false data injection attacks and show how imperfectly known disturbances can be used to perform undetectable, or at least stealthy, attacks that can make the NCS vulnerable to attacks from malicious outsiders. A numerical example is given to illustrate the approach. [less ▲]

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See detailA Decoupling Approach to Design Observers for Polytopic Takagi-Sugeno Models Subject to Unknown Inputs
Bezzaoucha, Souad UL; Voos, Holger UL; Davila, Jorge et al

in Proceedings of the 2018 American Control Conference (2018, June 27)

A decoupling approach for state estimation of nonlinear systems represented in the polytopic Takagi-Sugeno with unmeasurable premise variables subject to unknown inputs is proposed in this paper. The idea ... [more ▼]

A decoupling approach for state estimation of nonlinear systems represented in the polytopic Takagi-Sugeno with unmeasurable premise variables subject to unknown inputs is proposed in this paper. The idea consists in defining a state and unknown input transformations in order to divide the state vector into two parts, a measurable part and an observable one (decoupled from the unknown input). A classical Luenberger observer to estimate the unmeasurable part is then designed and given in terms of Linear Matrix Inequalities (LMIs) conditions. A numerical example is also presented in order to illustrate the proposed approach. [less ▲]

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See detailA UX Approach to Privacy and Security: the Impact of User, Contextual and System-Related Factors
Distler, Verena UL; Lallemand, Carine UL; Koenig, Vincent UL

in CHI Workshop Exploring Individual Diffferences in Privacy (2018, April)

This position paper lays out current and future studies which we conduct on the UX aspects of security and privacy, our goal being to understand which factors influence privacy-related decision-making. We ... [more ▼]

This position paper lays out current and future studies which we conduct on the UX aspects of security and privacy, our goal being to understand which factors influence privacy-related decision-making. We advocate using UX design methods in order to study interindividual differences, system-related and contextual factors involved in privacy and security attitudes and behaviors. These results will contribute to user-tailored and personalized privacy initiatives and guide the design of future technologies. [less ▲]

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See detailA UX Approach to Privacy and Security: the Impact of User, Contextual and System-Related Factors
Distler, Verena UL; Lallemand, Carine UL; Koenig, Vincent UL

in CHI Workshop Exploring Individual Diffferences in Privacy (2018, April)

This position paper lays out current and future studies which we conduct on the UX aspects of security and privacy, our goal being to understand which factors influence privacy-related decision-making. We ... [more ▼]

This position paper lays out current and future studies which we conduct on the UX aspects of security and privacy, our goal being to understand which factors influence privacy-related decision-making. We advocate using UX design methods in order to study interindividual differences, system-related and contextual factors involved in privacy and security attitudes and behaviors. These results will contribute to user-tailored and personalized privacy initiatives and guide the design of future technologies. [less ▲]

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See detailAcceptability and Acceptance of Autonomous Mobility on Demand: The Impact of an Immersive Experience
Distler, Verena UL; Lallemand, Carine UL; Thierry, Bellet

in Proceedings of the 2018 CHI Conference on Human Factors in Computing Systems (2018, April)

Autonomous vehicles have the potential to fundamentally change existing transportation systems. Beyond legal concerns, these societal evolutions will critically depend on user acceptance. As an emerging ... [more ▼]

Autonomous vehicles have the potential to fundamentally change existing transportation systems. Beyond legal concerns, these societal evolutions will critically depend on user acceptance. As an emerging mode of public transportation [7], Autonomous mobility on demand (AMoD) is of particular interest in this context. The aim of the present study is to identify the main components of acceptability (before first use) and acceptance (after first use) of AMoD, following a user experience (UX) framework. To address this goal, we conducted three workshops (N=14) involving open discussions and a ride in an experimental autonomous shuttle. Using a mixed-methods approach, we measured pre-immersion acceptability before immersing the participants in an on-demand transport scenario, and eventually measured post-immersion acceptance of AMoD. Results show that participants were reassured about safety concerns, however they perceived the AMoD experience as ineffective. Our findings highlight key factors to be taken into account when designing AMoD experiences. [less ▲]

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See detailDirectionality of Attacks in Natural Language Argumentation
Cramer, Marcos UL; Guillaume, Mathieu UL

in CEUR Workshop Proceedings (2018)

In formal (abstract and structured) argumentation theory, a central notion is that of an attack between a counterargument and the argument that it is challenging. Unlike the notion of an inconsistency ... [more ▼]

In formal (abstract and structured) argumentation theory, a central notion is that of an attack between a counterargument and the argument that it is challenging. Unlike the notion of an inconsistency between two statements in classical logic, this notion of an attack between arguments can be asymmetric, i.e. an argument A can attack an argument B without B attacking A. While this property of the formal systems studied by argumentation theorist has been motivated by considerations about the human practice of argumentation in natural language, there have not been any systematic studies on the connection between the directionality of attacks in argumentation-theoretic formalisms and the way humans actually interpret conflicts between arguments in a non-symmetric way. In this paper, we report on the result of two empirical cognitive studies that aim at filling this gap, one study with ordinary adults (undergraduate students) and one study with adult experts in formal argumentation theory. We interpret the results in light of the notions and distinctions defined in the ASPIC+ framework for structured argumentation, and discuss the relevance of our findings to past and future empirical studies about the link between human argumentation and formal argumentation theory. [less ▲]

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See detailA Parallel Multiscale DEM-VOF Method For Large-Scale Simulations Of Three-Phase Flows
Pozzetti, Gabriele UL; Besseron, Xavier UL; Rousset, Alban UL et al

in Proceedings of ECCM-ECFD 2018 (2018)

A parallel dual-grid multiscale DEM-VOF coupling is here investigated. Dual- grid multiscale couplings have been recently used to address different engineering problems involving the interaction between ... [more ▼]

A parallel dual-grid multiscale DEM-VOF coupling is here investigated. Dual- grid multiscale couplings have been recently used to address different engineering problems involving the interaction between granular phases and complex fluid flows. Nevertheless, previous studies did not focus on the parallel performance of such a coupling and were, therefore, limited to relatively small applications. In this contribution, we propose an insight into the performance of the dual-grid multiscale DEM-VOF method for three- phase flows when operated in parallel. In particular,we focus on a famous benchmark case for three-phase flows and assess the influence of the partitioning algorithm on the scalability of the dual-grid algorithm. [less ▲]

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See detailComparison of Several RANS Modelling for the Pavia TRIGA Mark II Research Reactor
Introini, Carolina; Cammi, Antonio; Lorenzi, Stefano et al

in Journal of Nuclear Engineering and Radiation Science (2018)

Aim of this work is the comparison of different turbulent models based on the Reynolds Averaged Navier-Stokes (RANS) equations in order to find out which model is the most suitable for the study of the ... [more ▼]

Aim of this work is the comparison of different turbulent models based on the Reynolds Averaged Navier-Stokes (RANS) equations in order to find out which model is the most suitable for the study of the channel thermal-hydraulics of the TRIGA Mark II reactor. Only the steady state behaviour (i.e. the full power stationary operational conditions) of the reactor has been considered. To this end, the RAS (Reynolds-Averaged Simulation) models available in the open source CFD software OpenFOAM have been applied to the most internal channel of the TRIGA and assessed against a Large Eddy Simulation (LES) model. The results of the latter approach, expressed in terms of axial velocity, turbulent viscosity, turbulent kinetic energy, and temperature have been compared with the results obtained by the RAS models available in OpenFOAM (k − ε, k − ω and Reynolds Stress Transport). Heat transfer is taken into account as well by means of the turbulent energy diffusivity parameter. The simulation results demonstrate how, amongst the RAS models, the k − ω SST is the one whose results are closer to the LES simulation. This model seems to be the best one for the treatment of turbulent flow within the TRIGA subchannel, offering a good compromise between accuracy and computational requirements. Since it is much less expensive than an LES model, it can be applied even to full core calculation, in order to obtain accurate results with less computational effort. [less ▲]

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See detailUnderstanding human need fulfilment to support the design of secure experiences
Distler, Verena UL

in Proceedings of NordiCHI' 18 Doctoral Consortium (2018)

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See detailEmpirical Cognitive Study on Abstract Argumentation Semantics
Cramer, Marcos UL; Guillaume, Mathieu UL

in Frontiers in Artificial Intelligence and Applications (2018)

In abstract argumentation theory, multiple argumentation semantics have been proposed that allow to select sets of jointly acceptable arguments from a given set of arguments based on the attack relation ... [more ▼]

In abstract argumentation theory, multiple argumentation semantics have been proposed that allow to select sets of jointly acceptable arguments from a given set of arguments based on the attack relation between arguments. The existence of multiple argumentation semantics raises the question which of these semantics predicts best how humans evaluate arguments, possibly depending on the thematic con- text of the arguments. In this study we report on an empirical cognitive study in which we tested how humans evaluate sets of arguments de- pending on the abstract structure of the attack relation between them. Two pilot studies were performed to validate the intended link between argumentation frameworks and sets of natural language arguments. The main experiment involved a group deliberation phase and made use of three different thematic contexts of the argument sets involved. The data strongly suggest that independently of the thematic contexts that we have considered, strong acceptance and strong rejection according to the CF2 and preferred semantics are a better predictor for human argument acceptance than the grounded semantics (which is identical to strong acceptance/rejection with respect to complete semantics). Furthermore, the data suggest that CF2 semantics predicts human argument acceptance better than preferred semantics, but the data for this comparison is limited to a single thematic context. [less ▲]

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See detailDiscretisation and Model Selection for Interface Problems in Mechanics
Bordas, Stéphane UL

in International Journal of Computational Methods (2017, August 04)

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See detailEffectiveness of the Two-Step Dynamic Demand Estimation model on large networks
Cantelmo, Guido UL; Viti, Francesco UL; Derrmann, Thierry UL

in Proceedings of 2017 5th IEEE International Conference on Models and Technologies for Intelligent Transportation Systems (MT-ITS) (2017, June 28)

In this paper, the authors present a Two-Step approach that sequentially adjusts generation and distribution values of the (dynamic) OD matrix. While the proposed methodology already provided excellent ... [more ▼]

In this paper, the authors present a Two-Step approach that sequentially adjusts generation and distribution values of the (dynamic) OD matrix. While the proposed methodology already provided excellent results for updating demand flows on a motorway, the aim of this paper is to validate this conclusion on a real network: Luxembourg City. This network represents the typical middle-sized European city in terms of network dimension. Moreover, Luxembourg City has the typical structure of a metropolitan area, composed of a city centre, ring, and suburb areas. An innovative element of this paper is to use mobile network data to create a time-dependent profile of the generated demand inside and outside the ring. To support the claim that the model is ready for practical implementation, it is interfaced with PTV Visum, one of the most widely adopted software tools for traffic analysis. Results of these experiments provide a solid empirical ground in order to further develop this model and to understand if its assumptions hold for urban scenarios. [less ▲]

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See detailA contribution to Cyber-Security of Networked Control Systems: an Event-based Control Approach
Bezzaoucha, Souad UL; Voos, Holger UL; Darouach, Mohamed

in Proceedings of 2017 3rd International Conference on Event-Based Control, Communication and Signal Processing (EBCCSP 2017) (2017, May)

In the present paper, a networked control system under both cyber and physical attacks is considered. An adapted formulation of the problem under physical attacks, data deception and false data injection ... [more ▼]

In the present paper, a networked control system under both cyber and physical attacks is considered. An adapted formulation of the problem under physical attacks, data deception and false data injection attacks, is used for controller synthesis. Based on the classical fault tolerant detection (FTD) tools, a residual generator for attack/fault detection based on observers is proposed. An event-triggered and Bilinear Matrix Inequality (BMI) implementation is proposed in order to achieve novel and better security strategy. The purpose in using this implementation would be to reduce (limit) the total number of transmissions to only instances when the networked control system (NCS) needs attention. It is important to note that the main contribution of this paper is to establish the adequate event-triggered and BMI-based methodology so that the particular structure of the mixed attacked/faulty structure can be re-formulated within the classical FTD paradigm. Experimental results are given to illustrate the developed approach efficiency on a pilot three-tank system. The plant model is presented and the proposed control design is applied to the system. [less ▲]

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See detailCoping with turn-taking:Investigating breakdowns in human-robot interaction from a Conversation Analysis (CA) perspective
Arend, Béatrice UL; Sunnen, Patrick UL

in Proceedings of the 8th International Conference on Society and Information Technologies (2017, March)

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See detailOn characterizing the relationship between route choice behavior and optimal traffic control solution space
Rinaldi, Marco UL; Tampère, C. M. J.; Viti, Francesco UL

in Transportation Research Procedia (2017), 23

Explicitly including the dynamics of users' route choice behaviour in optimal traffic control applications has been of interest for researchers in the last five decades. This has been recognized as a very ... [more ▼]

Explicitly including the dynamics of users' route choice behaviour in optimal traffic control applications has been of interest for researchers in the last five decades. This has been recognized as a very challenging problem, due to the added layer of complexity and the considerable non-convexity of the resulting problem, even when dealing with simple static assignment and analytical link cost functions. In this work we establish a direct behavioural connection between the different shapes and structures emerging in the solution space of such problems and the underlying route choice behaviour. We specifically investigate how changes in the active equilibrium route set exert direct influence on the solution space's structure and behaviour. Based on this result, we then formulate and validate a constrained version of the original problem, yielding desirable properties in terms of solution space regularity. © 2017 The Authors. [less ▲]

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See detailNovel C-ITS support for Electric Buses with Opportunity Charging
Seredynski, Marcin; Viti, Francesco UL

in Proceedings of IEEE-ITS Conference (2017)

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See detailBoard Game Prototyping to Co-Design a Better Location-Based Digital Game
Jones, Catherine UL; Liapis; Lykourentzou, Ioanna et al

in Abstract book of the 2017 CHI Conference Extended Abstracts on Human Factors in Computing Systems - CHI EA '17 (2017)

In this case study we describe the iterative process of paper prototyping, using a board game, to co-design a location-based mobile application. The end goal of the application is to motivate reflection ... [more ▼]

In this case study we describe the iterative process of paper prototyping, using a board game, to co-design a location-based mobile application. The end goal of the application is to motivate reflection on historical topics about migration. The board game serves to capture the core concerns of this application by simulating movement through the city. Three play tests highlighted the users' interest and issues with the historical content, the way this content is represented, and the players' responses to the interactions and motivating mechanisms of the application. Results show that the board game helped capture important design preferences and problems, ensuring the improvement of our scenario. This feedback can help reduce development effort and implement a future technology prototype closer to the needs of our end users. [less ▲]

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See detailOptimising time-series experimental design for modelling of circadian rhythms: the value of transient data
Mombaerts, Laurent UL; Mauroy, Alexandre UL; Goncalves, Jorge UL

in IFAC-PapersOnLine (2016, October)

Circadian clocks consist of complex networks that coordinate the daily cycle of most organisms. In light/dark cycles, the clock is synchronized (or entrained) by the environment, which corresponds to a ... [more ▼]

Circadian clocks consist of complex networks that coordinate the daily cycle of most organisms. In light/dark cycles, the clock is synchronized (or entrained) by the environment, which corresponds to a constant rephasing of the oscillations and leads to a steady state regime. Some circadian clocks are endogenous oscillators with rhythms of about 24-hours that persist in constant light or constant darkness. This operating mechanism with and without entrainment provides flexibility and robustness to the clock against perturbations. Most of the clock-oriented experiments are performed under constant photoperiodic regime, overlooking the transitory regime that takes place between light/dark cycles and constant light or darkness. This paper provides a comparative analysis of the informative potential of the transient time-series data with the other regimes for clock modelling. Realistic data were simulated from 2 experimentally validated plant circadian clock models and sliced into several time windows. These windows represent the different regimes that take place before, meanwhile and after the switch to constant light. Then, a network inference tool was used over each window and its capability of retrieving the ground-truth of the network was compared for each window. The results suggest that including the transient data to the network inference technique significally improves its performance. [less ▲]

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See detailTextual Inference with Deep Learning Technique
Adebayo, Kolawole; Di Caro, Luigi; Robaldo, Livio UL et al

in Proceedings of the 28th Annual Benelux Conference on Artificial Intelligence (BNAIC2016). (2016)

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See detailSystematic assessment of local & global signal control policies: A methodological perspective
Cantelmo, Guido UL; Viti, Francesco UL; Rinaldi, Marco UL et al

in Proceedings of 2015 International Conference on Models and Technologies for Intelligent Transportation Systems, MT-ITS 2015 (2015)

Traffic control performance on networks depends on the flow response to the policy adopted, which in turn contributes to determine the optimal signal settings. This paper focuses on the relationship ... [more ▼]

Traffic control performance on networks depends on the flow response to the policy adopted, which in turn contributes to determine the optimal signal settings. This paper focuses on the relationship between local and network wide traffic control policies within the combined traffic control and assignment problem. Through a full exploration of the solution space, an in depth cross comparison is performed between the well-known local policies P0 and Equisaturation, versus the global policies Maximum Throughput and Minimum Delay, to verify how the two local policies approximate the optimal settings for signalized intersections. Realistic traffic dynamics, such as congestion, multiple controllers and spillback are considered, to empirically determine the conditions under which the local policies are able to approximate global performances. After presenting the different local and global control policies, experiments are performed on simple toy networks. The complexity of the underlying network and, therefore, of the problems' boundary conditions is then increased, allowing us to showcase how the different metrics perform in different situations. Finally, conclusions on the results are drawn. © 2015 BME. [less ▲]

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See detailAnticipation of brain shift in Deep Brain Stimulation automatic planning
Bilger, Alexandre UL; Cotin, Stephane; Duriez, Christian et al

in IEEE Engineering in Medicine and Biology Society (2015)

Deep Brain Stimulation is a neurosurgery procedure consisting in implanting an electrode in a deep structure of the brain. This intervention requires a preoperative planning phase, with a millimetric ... [more ▼]

Deep Brain Stimulation is a neurosurgery procedure consisting in implanting an electrode in a deep structure of the brain. This intervention requires a preoperative planning phase, with a millimetric accuracy, in which surgeons decide the best placement of the electrode depending on a set of surgical rules. However, brain tissues may deform during the surgery because of the brain shift phenomenon, leading the electrode to mistake the target, or moreover to damage a vital anatomical structure. In this paper, we present a patient-specific automatic planning approach for DBS procedures which accounts for brain deformation. Our approach couples an optimization algorithm with FEM based brain shift simulation. The system was tested successfully on a patient-specific 3D model, and was compared to a planning without considering brain shift. The obtained results point out the importance of performing planning in dynamic conditions. [less ▲]

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See detailWorkplace Relocation and Mobility Changes in a Transnational Metropolitan Area: The Case of the University of Luxembourg
Sprumont, François UL; Viti, Francesco UL; Caruso, Geoffrey UL et al

in Transportation Research Procedia (2014, December), 4

The aim of this paper is to study the utility variation related to the commuting mobility of University staff members due to their future workplace relocation. During the year 2012, a travel survey was ... [more ▼]

The aim of this paper is to study the utility variation related to the commuting mobility of University staff members due to their future workplace relocation. During the year 2012, a travel survey was completed by a total of 397 staff members, representing 36.4% of the university employees, who filled in a questionnaire which revealed complex decision making patterns due to the special traveling scenario involving four countries at once. A Multinomial Logit model has been used to anticipate the impact of university relocation from the capital city to a developing area in the south of the country which will happen between 2015 and 2018 and that will affect most of the employees. The effects of several Travel Demand Management measures are discussed based on the analysis of alternative scenarios [less ▲]

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See detailSpatial play effects in a tangible game with an f-formation of multiple players
Jungmann, Manuela UL; Cox, Richard; Fitzpatrick, Geraldine

in Proceedings of the Fifteenth Australasian User Interface Conference (2014, January), 150

Drawing on Kendon's F-formation framework of social interaction, we analysed the game-space activity of collocated players engaged in a tangible multiplayer game. Game input from groups of 3 players ... [more ▼]

Drawing on Kendon's F-formation framework of social interaction, we analysed the game-space activity of collocated players engaged in a tangible multiplayer game. Game input from groups of 3 players interacting competitively in a natural spatial arrangement via balance-boards requiring whole-body movements was logged and analysed quantitatively. The spatial analysis of a range of players' activities in game-space revealed synergistic effects combining perceptual-motor factors with game-strategy behaviour which were reflected in preferred game-board playing regions. The findings illustrate the importance for HCI designers of considering interactions between human spatial behaviour, physical space and virtual game-space as games become increasingly embodied and social. [less ▲]

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See detailIsogeometric finite element analysis using polynomial splines over hierarchical T-meshes
Nguyen-Thanh, Nhon; Nguyen-Xuan, Hung; Bordas, Stéphane UL et al

in IOP Conference Series: Materials Science and Engineering (2014), 10(1),

Isogeometric finite element analysis has become a powerful alternative to standard finite elements due to their flexibility in handling complex geometries. One major drawback of NURBS based isogeometric ... [more ▼]

Isogeometric finite element analysis has become a powerful alternative to standard finite elements due to their flexibility in handling complex geometries. One major drawback of NURBS based isogeometric finite elements is their less effectiveness of local refinement. In this study, we present an alternative to NURBS based isogeometric finite elements that allow for local refinement. The idea is based on polynomial splines and exploits the flexibility of T-meshes for local refinement. The shape functions satisfy important properties such as non-negativity, local support and partition of unity. We will demonstrate the efficiency of the proposed method by two numerical examples. [less ▲]

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See detailPOD-based model order reduction for the simulation of strong nonlinear evolutions in structures: Application to damage propagation
Kerfriden, P.; Gosselet, P.; Adhikari, S. et al

in IOP Conference Series: Materials Science and Engineering (2014), 10(1),

In this paper, we develop a bridge between POD-based model order reduction techniques and the classical Newton-Krylov solvers to derive an efficient solution procedure for highly nonlinear problems ... [more ▼]

In this paper, we develop a bridge between POD-based model order reduction techniques and the classical Newton-Krylov solvers to derive an efficient solution procedure for highly nonlinear problems undergoing strong topological changes. [less ▲]

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See detailFinite-time road grade computation for a vehicle platoon
Yang, Tao; Yuan, Ye; Li, Kezhi et al

in IEEE (2014)

Given a platoon of vehicles traveling uphill, this paper considers the finite-time road grade computation problem. We propose a decentralized algorithm for an arbitrarily chosen vehicle to compute the ... [more ▼]

Given a platoon of vehicles traveling uphill, this paper considers the finite-time road grade computation problem. We propose a decentralized algorithm for an arbitrarily chosen vehicle to compute the road grade in a finite number of time-steps by using only its own successive velocity measurements. Simulations then illustrate the theoretical results. These new results can be applied to real-world vehicle platooning problems to reduce fuel consumption and carbon dioxide emissions. [less ▲]

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See detailEquilibrium in capacitated network models with queueing delays, queue-storage, blocking back and control
Smith, Mike J.; Huang, Wei; Viti, Francesco UL

in Procedia Social and Behavioral Sciences (2013), 80

This paper considers a steady-state, link-based, fixed (or inelastic) demand equilibrium model with explicit link-exit capacities, explicit bottleneck or queuing delays and explicit bounds on queue ... [more ▼]

This paper considers a steady-state, link-based, fixed (or inelastic) demand equilibrium model with explicit link-exit capacities, explicit bottleneck or queuing delays and explicit bounds on queue storage capacities. The (spatial queueing) model at the heart of this equilibrium model takes account of the space taken up by queues both when there is no blocking back and also when there is blocking back. The paper shows in theorem 1 that a feasible traffic assignment model has an equilibrium solution provided prices are used to impose capacity restrictions and utilises this result to show that there is an equilibrium with the spatial queueing model, provided queue-storage capacities are sufficiently large. Other results are obtained by changing the variables and sets in theorem 1 suitably. These results include: (1) existence of equilibrium results (in both a steady state and a dynamic context) which allow signal green-times to respond to prices and (2) an existence of equilibrium result which allow signal green-times to respond to spatial queues; provided this response follows the P0 control policy in Smith (1979, 1987). These results show that under certain conditions the P0 control policy maximises network capacity. The spatial queueing model is illustrated on a simple network. Finally the paper includes elastic demand; this is necessary for long-run evaluations. Each of the steady state models here may be thought of as a stationary solution to the dynamic assignment problem either with or without blocking back. [less ▲]

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See detailXFEM coupling of granular flows interacting with surrounding fluids
Pasenow, F.; Zilian, Andreas UL; Dinkler, D.

in ECCOMAS 2012 - European Congress on Computational Methods in Applied Sciences and Engineering, e-Book Full Papers (2012)

In this paper, ideas for the simulation of sliding dry granular materials interacting with surrounding fluids are presented and first results are presented. The compressible granular material is modeled ... [more ▼]

In this paper, ideas for the simulation of sliding dry granular materials interacting with surrounding fluids are presented and first results are presented. The compressible granular material is modeled as a medium which can show solid-like and fluid-like characteristics. Therefore a weighted decomposition of stress tensors of a solid-like and a fluid-like phase is applied. The surrounding incompressible fluids are described with a Newtonian constitutive model. Interface dynamics are handled with the level-set method. The model equations are discretized with the space-time finite element method. Discontinuous solution characteristics across interfaces are captured numerically by the extended finite element method (XFEM). For all discontinuities the space of ansatz functions is enriched with Heaviside functions. [less ▲]

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See detailA force-based large increment method for 2D continuum solids and the mesh convergence study
Long, D.; Guo, Z.; Liu, X. et al

in AIP Conference Proceedings (2012), 1504

In this paper, a triangular plane stress element is implemented based on the large increment method (LIM) to solve 2D continuum mechanics problems. In the LIM, after the governing equations are ... [more ▼]

In this paper, a triangular plane stress element is implemented based on the large increment method (LIM) to solve 2D continuum mechanics problems. In the LIM, after the governing equations are established using the generalized elemental force variables as primary unknowns, an iteration procedure is employed to obtain an optimised approximate solution of the problem. Two numerical examples are investigated to study the mesh convergence of the proposed triangular LIM element. Structured meshes as well as unstructured meshes with different element densities are generated to illustrate the convergence of the total strain energy in both examples. The numerical results obtained from the LIM (including the total strain energy, the displacement and the stress) are compared with the analytical solutions as well as the results from the commercial FEM software ABAQUS. All the results show that the performance of the LIM is as good as the FEM in linear elastic problems. A simple elastoplastic example suggests that the LIM may obtain better result than the FEM. © 2012 American Institute of Physics. [less ▲]

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See detailError estimation and error bounding in quantities of interest based on equilibrated recovered displacement fields
Nadal, E.; González-Estrada, O. A.; Ródenas, J. J. et al

in ECCOMAS 2012 - European Congress on Computational Methods in Applied Sciences and Engineering, e-Book Full Papers (2012)

Over the last ten years there has been an increase on the use of goal-oriented error estimates aimed to quantify the local error on a (non)linear quantity of interest (QoI) that might result relevant for ... [more ▼]

Over the last ten years there has been an increase on the use of goal-oriented error estimates aimed to quantify the local error on a (non)linear quantity of interest (QoI) that might result relevant for design purposes. Residual-based error estimators have been used recursively to obtain upper and lower bounds of the error in quantities of interest for finite element approximations. In this paper, we present a recovery technique for 2D linear elasticity problems, based on the superconvergent patch recovery (SPR), which provides recovered displacement and stress fields that are then utilised to evaluate practical upper and lower error bounds in QoI. [less ▲]

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See detailEquilibrated patch recovery for accurate evaluation of upper error bounds in quantities of interest
González-Estrada, O. A.; Ródenas, J. J.; Nadal, E. et al

in ECCOMAS Thematic Conference - ADMOS 2011: International Conference on Adaptive Modeling and Simulation, An IACM Special Interest Conference (2012)

There is an increasing interest on the use of goal-oriented error estimates which help to measure and control the local error on a linear or non-linear quantity of interest (QoI) that might result ... [more ▼]

There is an increasing interest on the use of goal-oriented error estimates which help to measure and control the local error on a linear or non-linear quantity of interest (QoI) that might result relevant for design purposes (e.g. the mean stress value in a particular area, displacements, the stress intensity factor for fracture problems,⋯). In general, residual-based error estimators have been used to obtain upper and lower bounds of the error in quantities of interest for finite element approximations. In this work, we propose a novel a posteriori recovery technique to obtain an upper error bound of the QoI. We use a recovery procedure based on the superconvergent patch recovery (SPR) technique to obtain nearly statically admissible recovered stress fields for the primal and dual problems. This recovery technique was previously used to obtain upper bounds of the error in energy norm and has been used in this paper to obtain a computable version of the upper bound for the quantity of interest. [less ▲]

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See detailError estimation and error bounding in energy norm based on a displacement recovery technique
Nadal, E.; González-Estrada, O. A.; Ródenas, J. J. et al

in ECCOMAS 2012 - European Congress on Computational Methods in Applied Sciences and Engineering, e-Book Full Papers (2012)

Traditionally, recovery based error estimators in linear elasticity have considered the evaluation of an enhanced stress field obtained from the raw Finite Element (FE) stress solution. Instead of that ... [more ▼]

Traditionally, recovery based error estimators in linear elasticity have considered the evaluation of an enhanced stress field obtained from the raw Finite Element (FE) stress solution. Instead of that, one can also obtain a recovered displacement field from the FE displacements. Herein, we describe a superconvergent patch recovery of the displacement field which considers the local fulfilment of boundary and internal equilibrium equations, Dirichlet constraints and, for singular problems, the splitting of the displacement and stress fields into singular and smooth parts. Numerical tests using problems with known analytical solution have been carried out to validate the proposed technique for error estimation and error bounding in energy norm and quantities of interest. [less ▲]

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See detailLifetime prediction for solder joints with the extended finite element method
Menk, Alexander; Pearce, Chris J.; Lanier, Olivier et al

in Proceedings of 12th Int. Conf. on Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2011 (2011)

Predicting the lifetime of solder joints undergoing thermal cycling is crucial for the electronics industry in order to guarantee a certain performance of their products in the field. Semi-empirical ... [more ▼]

Predicting the lifetime of solder joints undergoing thermal cycling is crucial for the electronics industry in order to guarantee a certain performance of their products in the field. Semi-empirical methods are often used to predict the average lifetime of the critical joints. However, to get a reliable failure probability the standard deviation must also be addressed. The deviation of the lifetime from the mean value is a consequence of the variation in microstructure found in actual joints. We therefore propose a new methodology that calculates crack growth based on microstructural features of the joint. A series of random microstructures is generated. Crack growth calculations are performed for each of these structures. The structural problem is solved numerically with the extended finite element method which allows a complete automation of the process. The mean crack length and standard deviation are calculated from the crack growth simulations and the result is compared to experimental data. [less ▲]

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See detailOn numerical integration of discontinuous approximations in partition of unity finite elements
Natarajan, S.; Bordas, Stéphane UL; Mahapatra, D. R.

in IUTAM Bookseries (2010), 19

This contribution presents two advances in the formulation of discontinuous approximations in finite elements. The first method relies on Schwarz-Christoffel mapping for integration on arbitrary polygonal ... [more ▼]

This contribution presents two advances in the formulation of discontinuous approximations in finite elements. The first method relies on Schwarz-Christoffel mapping for integration on arbitrary polygonal domains [1]. When an element is split into two subdomains by a piecewise continuous discontinuity, each of these polygonal domains is mapped onto a unit disk on which cubature rules are utilized. This suppresses the need for the usual two-level isoparametric mapping. The second method relies on strain smoothing applied to discontinuous finite element approximations. By writing the strain field as a non-local weighted average of the compatible strain field, integration on the surface of the finite elements is transformed into boundary integration, so that the usual subdivision into integration cells is not required, an isoparametric mapping is not needed and the derivatives of the shape (enrichment) functions do not need to be computed. Results in fracture mechanics and composite materials are presented and both methods are compared in terms of accuracy and simplicity. The interested reader is referred to [1,6,13] for more details and should contact the authors to receive a version of the MATLAB codes used to obtain the results herein. © 2010 Springer Science+Business Media B.V. [less ▲]

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See detailOn the structure of a new superhard hexagonal carbon phase
Zhang, Bin; Liang, Yongcheng; Guo, Zaoyang et al

in AIP Conference Proceedings (2010), 1233(PART 1), 489-493

Molecular dynamics simulations show that graphite will transform into a superhard phase under cold compression. Recent experiments show that there is a sp 3-rich hexagonal carbon polymorph (a 0=2.496 Å, c ... [more ▼]

Molecular dynamics simulations show that graphite will transform into a superhard phase under cold compression. Recent experiments show that there is a sp 3-rich hexagonal carbon polymorph (a 0=2.496 Å, c 0=4.123Å) with a bulk modulus of 447 GPa and average density about 3.6g/cm 3, restricted to the space group of P-62c (No. 190), but the detailed atomic structure was not obtained [Wang et al., P. Natl. Acad. Sci. 101(38), 13699]. Here we set carbon atoms occupying P-62c 4f Wyckoff positions of P-62c, and calculate the total energy of the different structures changing the internal parameter z by first-principles calculations using geometry optimisation algorithm in CASTEP code, which shows that the stable structures in energy (at local minimum points) are hexagonal carbon (z=1/4) and hexagonal diamond (z=1/16). The calculated mechanical properties and lattice parameters of the structure P-62c 4f (z=1/4) are in good agreement with those of the new hexagonal carbon proposed by Wang et al., which indicates that the atomic structure is a possible candidate. © 2010 American Institute of Physics. [less ▲]

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See detailAn element nodal force-based large increment method for elastoplasticity
Long, Danbin; Guo, Zaoyang; Liu, Xila et al

in AIP Conference Proceedings (2010), 1233(PART 1), 1401-1405

This paper presents a new method for establishing the basic equations in the novel force-based large increment method (LIM) for continuum elastoplastic problems. In LIM, unlike traditional displacement ... [more ▼]

This paper presents a new method for establishing the basic equations in the novel force-based large increment method (LIM) for continuum elastoplastic problems. In LIM, unlike traditional displacement methods, the (generalised) elemental force variables are adopted as system unknowns. The equilibrium equations can then be obtained directly at every nodal degree of freedom without physical equations (i.e., constitutive equations) involved. The generalised inverse of the non-square system of equations is employed to obtain the set of solutions of the non-square matrix equations directly. A conjugate gradient procedure is then used to find the correct solution from this set of solutions by optimising the compatibility of the solution based on the fact that the correct solution should also satisfy the constitutive equations and the compatibility equations. In this paper, the generalised elemental force variables are defined based on the element nodal forces. The LIM framework is therefore successfully applied to elements based on this definition. The efficiency and accuracy of the LIM are illustrated with a few benchmark problems and the results are compared with the analytical solution and the conventional displacement-based finite element method. [less ▲]

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See detailComputational heuristics for simplifying a biological model
Petre, Ion; Mizera, Andrzej UL; Back, Ralph-Johan

in Lecture Notes in Computer Science (2009), 5635

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See detailAPPLICATION OF EXTENDED ELEMENT-FREE GALERKIN METHOD TO MULTIPLE FLAWS UNDER BRITTLE FRACTURE CONDITIONS
RABCZUK, T.; BEZENSEK, B.; Bordas, Stéphane UL

in PROCEEDINGS OF THE ASME PRESSURE VESSELS AND PIPING CONFERENCE - 2008, VOL 6, PT A AND B (2009)

The extended element-free Galerkin (XEFG) method incorporates cracks through partition of unity enrichment of the standard basis functions. Discontinuous functions are added to capture the jump through ... [more ▼]

The extended element-free Galerkin (XEFG) method incorporates cracks through partition of unity enrichment of the standard basis functions. Discontinuous functions are added to capture the jump through the crack faces and near-front enrichment is added to capture the asymptotic behaviour in the vicinity of the crack fronts. Depending on the material behaviour, these functions can be of various type. The method can treat initiation, growth and coalescence of cracks seamlessly in both linear elastic and non-linear settings. The method is a powerful tool for modelling and studying crack paths, which are a central feature in the assessment of multiple flaws.The method is applied to the problem of multiple non-aligned flaws in a ferritic plate under cleavage failure. Fracture paths from two nonaligned notches in a plate are modelled. Based on the observations of crack paths the critical flaw alignment distance is established for nonaligned through-wall flaws. [less ▲]

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See detailInfluence of the microstructure on the stress state of solder joints dusing thermal cycling
Menk, A.; Bordas, Stéphane UL

in Proceedings of 10th International Conference on Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2009 (2009)

The lifetime of a solder joint is mainly determined by its creep behaviour. Creep arises from the stresses inside the solder joints as a consequence of the thermomechanical mismatch of the board and the ... [more ▼]

The lifetime of a solder joint is mainly determined by its creep behaviour. Creep arises from the stresses inside the solder joints as a consequence of the thermomechanical mismatch of the board and the substrate. The stress state is heavily influenced by the anisotropy of tin. To understand the damage process in solder joints, the influence of the anisotropic microstructure must be understood. In this paper the influence of different grain sizes, shapes and orientations on the stress state is evaluated, based on numerical experiments. [less ▲]

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See detailThe smoothed extended finite element method
Natarajan, S.; Bordas, Stéphane UL; Minh, Q. D. et al

in Proceedings of the 6th International Conference on Engineering Computational Technology (2008)

This paper shows how the strain smoothing technique recently proposed by G.R.Liu [1] coined as smoothed finite element method (SFEM) can be coupled to partition of unity methods, namely extended finite ... [more ▼]

This paper shows how the strain smoothing technique recently proposed by G.R.Liu [1] coined as smoothed finite element method (SFEM) can be coupled to partition of unity methods, namely extended finite element method (XFEM) [2] to give birth to the smoothed extended finite element method (SmXFEM), which shares properties both with the SFEM and the XFEM. The proposed method suppresses the need to compute and integrate the derivatives of shape functions (which are singular at the tip in linear elastic fracture mechanics). Additionally, integration is performed along the boundary of the finite elements or smoothing cells and no isoparametric mapping is required, which allows elements of arbitrary shape. We present numerical results for cracks in linear elastic fracture mechanics problems. The method is verified on several examples and comparisons are made to the conventional XFEM. © 2008 Civil-Comp Press. [less ▲]

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See detailMechanical failure in microstructural heterogeneous materials
Bordas, Stéphane UL; Hoppe, R. H. W.; Petrova, S. I.

in Lecture Notes in Computer Science (2007), 4310 LNCS

Various heterogeneous materials with multiple scales and multiple phases in the microstructure have been produced in the recent years. We consider a mechanical failure due to the initiation and ... [more ▼]

Various heterogeneous materials with multiple scales and multiple phases in the microstructure have been produced in the recent years. We consider a mechanical failure due to the initiation and propagation of cracks in places of high pore density in the microstructures. A multi-scale method based on the asymptotic homogenization theory together with the mesh superposition method (s-version of FEM) is presented for modeling of cracks. The homogenization approach is used on the global domain excluding the vicinity of the crack where the periodicity of the microstructures is lost and this approach fails. The multiple scale method relies on efficient combination of both macroscopic and microscopic models. The mesh superposition method uses two independent (global and local) finite element meshes and the concept of superposing the local mesh onto the global continuous mesh in such a way that both meshes not necessarily coincide. The homogenized material model is considered on the global mesh while the crack is analyzed in the local domain (patch) which allows to have an arbitrary geometry with respect to the underlying global finite elements. Numerical experiments for biomorphic cellular ceramics with porous microstructures produced from natural wood are presented. [less ▲]

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