Numerical modelling of additive manufacturing process for stainless steel tension testing samples

Nowadays additive manufacturing (AM) technologies including 3D printing grow rapidly and they are expected to replace conventional subtractive manufacturing technologies to some extents. During a selective laser melting (SLM) process as one of popular AM technologies for metals, large amount of heats is required to melt metal powders, and this leads to distortions and/or shrinkages of additively manufactured parts. It is useful to predict the 3D printed parts to control unwanted distortions and shrinkages before their 3D printing. This study develops a two-phase numerical modelling and simulation process of AM process for 17-4PH stainless steel and it considers the importance of post-processing and the need for calibration to achieve a high-quality printing at the end. By using this proposed AM modelling and simulation process, optimal process parameters, material properties, and topology can be obtained to ensure a part 3D printed successfully

Intuitive, iterative and assisted virtual guides programming for human-robot comanipulation

Pendant très longtemps, l'automatisation a été assujettie à l'usage de robots industriels traditionnels placés dans des cages et programmés pour répéter des tâches plus ou moins complexes au maximum de leur vitesse et de leur précision. Cette automatisation, dite rigide, possède deux inconvénients majeurs : elle est chronophage dû aux contraintes contextuelles applicatives et proscrit la présence humaine. Il existe désormais une nouvelle génération de robots avec des systèmes moins encombrants, peu coûteux et plus flexibles. De par leur structure et leurs modes de fonctionnement ils sont intrinsèquement sûrs ce qui leurs permettent de travailler main dans la main avec les humains. Dans ces nouveaux espaces de travail collaboratifs, l'homme peut être inclus dans la boucle comme un agent décisionnel actif. En tant qu'instructeur ou collaborateur il peut influencer le processus décisionnel du robot : on parle de robots collaboratifs (ou cobots). Dans ce nouveau contexte, nous faisons usage de guides virtuels. Ils permettent aux cobots de soulager les efforts physiques et la charge cognitive des opérateurs. Cependant, la définition d'un guide virtuel nécessite souvent une expertise et une modélisation précise de la tâche. Cela restreint leur utilité aux scénarios à contraintes fixes. Pour palier ce problème et améliorer la flexibilité de la programmation du guide virtuel, cette thèse présente une nouvelle approche par démonstration : nous faisons usage de l'apprentissage kinesthésique de façon itérative et construisons le guide virtuel avec une spline 6D. Grâce à cette approche, l'opérateur peut modifier itérativement les guides tout en gardant leur assistance. Cela permet de rendre le processus plus intuitif et naturel ainsi que de réduire la pénibilité. La modification locale d'un guide virtuel en trajectoire est possible par interaction physique avec le robot. L'utilisateur peut déplacer un point clé cartésien ou modifier une portion entière du guide avec une nouvelle démonstration partielle. Nous avons également étendu notre approche aux guides virtuels 6D, où les splines en déplacement sont définies via une interpolation Akima (pour la translation) et une 'interpolation quadratique des quaternions (pour l'orientation). L'opérateur peut initialement définir un guide virtuel en trajectoire, puis utiliser l'assistance en translation pour ne se concentrer que sur la démonstration de l'orientation. Nous avons appliqué notre approche dans deux scénarios industriels utilisant un cobot. Nous avons ainsi démontré l'intérêt de notre méthode qui améliore le confort de l'opérateur lors de la comanipulation.For a very long time, automation was driven by the use of traditional industrial robots placed in cages, programmed to repeat more or less complex tasks at their highest speed and with maximum accuracy. This robot-oriented solution is heavily dependent on hard automation which requires pre-specified fixtures and time consuming programming, hindering robots from becoming flexible and versatile tools. These robots have evolved towards a new generation of small, inexpensive, inherently safe and flexible systems that work hand in hand with humans. In these new collaborative workspaces the human can be included in the loop as an active agent. As a teacher and as a co-worker he can influence the decision-making process of the robot. In this context, virtual guides are an important tool used to assist the human worker by reducing physical effort and cognitive overload during tasks accomplishment. However, the construction of virtual guides often requires expert knowledge and modeling of the task. These limitations restrict the usefulness of virtual guides to scenarios with unchanging constraints. To overcome these challenges and enhance the flexibility of virtual guides programming, this thesis presents a novel approach that allows the worker to create virtual guides by demonstration through an iterative method based on kinesthetic teaching and displacement splines. Thanks to this approach, the worker is able to iteratively modify the guides while being assisted by them, making the process more intuitive and natural while reducing its painfulness. Our approach allows local refinement of virtual guiding trajectories through physical interaction with the robots. We can modify a specific cartesian keypoint of the guide or re- demonstrate a portion. We also extended our approach to 6D virtual guides, where displacement splines are defined via Akima interpolation (for translation) and quadratic interpolation of quaternions (for orientation). The worker can initially define a virtual guiding trajectory and then use the assistance in translation to only concentrate on defining the orientation along the path. We demonstrated that these innovations provide a novel and intuitive solution to increase the human's comfort during human-robot comanipulation in two industrial scenarios with a collaborative robot (cobot)

A Computational Modeling Approach of Fracture-Induced Acoustic Emission

Acoustic Emission (AE) has become a prominent Nondestructive Testing (NDT) technique with capabilities to be used for Structural Health Monitoring (SHM) applications that entail in-service monitoring, detecting damage-prone areas, and establishing damage prognostics of structures. The next generation of acoustics-based techniques for SHM will rely upon the reliable and quantitative characterization of AE signals related to dominant damage mechanisms. In this context, the forward problem of simulating AE activity is addressed herein by proposing advanced finite element models for damage-induced stress wave generation and propagation. Acoustic emission for this purpose is viewed as part of the dynamic process of energy release caused by damage initiation. To form the computational approach, full field experimental information obtained from monitoring the damage initiation process using digital image correlation is used to construct constitutive laws, e.g. traction-separation law, and to define other damage related parameters. Subsequently, 3D FE simulations based on such experimental data are implemented using cohesive zone modeling and extended finite element method to create an initial failure. Numerically simulated AE signals from the dynamic response due to the onset of damage are evaluated in the context of the inverse problem of source identification and localization. The results successfully demonstrate material and geometry effects of the propagating source and describe completely the AE process from crack-induced isolated source to transient and steady-state dynamic response. Furthermore, the computational model is used to provide quantified measures of the energy release associated with crack. In addition, the effect of plasticity on simulated traveling waves ahead of the crack tip was investigated and revealed nonlinear interactions that had been postulated to exist. Ultimately, the forward AE methodology is applied to an aerospace structural component to recreate the debonding process and associated stress release propagation. All damage-induced wave propagation simulations presented in this dissertation create a pathway for the quantitative comparison between experimental and theoretical predictions of AE.Ph.D., Mechanical Engineering and Mechanics -- Drexel University, 201

Monitoring companion for industrial robotic processes

For system integrators, optimizing complex industrial robotic applications (e.g. robotised welding) is a difficult and time-consuming task. This procedure is rendered tedious and often very hard to achieve when the operator cannot access the robotic system once in operation, perhaps because the installation is far away or because of the operational environment. In these circumstances, as an alternative to physically visiting the installation site, the system integrator may rely on additional nearby sensors to remotely acquire the necessary process information. While it is hard to completely replace this trial and error approach, it is possible to provide a way to gather process information more effectively that can be used in several robotic installations.This thesis investigates the use of a "monitoring robot" in addition to the task robot(s) that belong to the industrial process to be optimized. The monitoring robot can be equipped with several different sensors and can be moved into close proximity of any installed task robot so that it can be used to collect information from that process during and/or after the operation without interfering. The thesis reviews related work in the industry and in the field of teleoperation to identify the most important challenges in remote monitoring and teleoperation. From the background investigation it is clear that two very important issues are: i) the nature of the teleoperator’s interface and; ii) the efficiency of the shared control between the human operator and the monitoring system. In order to investigate these two issues efficiently it was necessary to create experimental scenarios that operate independently from any application scenario, so an abstract problem domain is created. This way the monitoring system's control and interface can be evaluated in a context that presents challenges that are typical of a remote monitoring task but are not application domain specific. Therefore the validity of the proposed approach can be assessed from a generic and, therefore, more powerful and widely applicable perspective. The monitoring framework developed in this thesis is described, both in the shared control design choices based on virtual fixtures (VF) and the implementation in a 3D visualization environment. The monitoring system developed is evaluated with a usability study with user participants. The usability study aims at assessing the system's performance along with its acceptance and ease of use in a static monitoring task, accompanied by user\hyp{}filled TLX questionnaires. Since future work will apply this system in real robotic welding scenarios, this thesis finally reports some preliminary work in such an application

CLT-Steel Composite Floors for Sustainable Multi-Storey Construction

This thesis encompasses the investigation of a novel proposed construction system that pairs Cross-Laminated Timber (CLT) floor panels with steel-frame multi-storey construction. As a substitute for concrete floors, the promise of CLT lies in its reduced embodied carbon, and its lightness, which gives the promise of reduced material usage and a reduction in the environmental impact of a multi-storey building overall. In addition to clarifying the environmental benefit such a system provides, this thesis seeks to determine whether forming composite sections from the CLT panels and the steel beam, can lead to meaningful enhancements to the section stiffness in bending. Finite element analysis is applied for this task, necessitating first a methodology for modelling CLT to capture its relevant complexities as a material, and subsequently incorporation of the behaviour of the joints between structural elements in the proposed system, with the key characteristics of the materials and joint models validated against physical test data. Focusing predominantly on a slimfloor arrangement of panels and beams, the study provides the first evaluation of the effective width of CLT floor panels when acting compositely with steel beams, provides a prediction for composite action CLT panels and Asymmetric Steel Beams (ASBs) with a series of connectors, and identifies areas of development within the system that could bring about increased composite benefits. The outcomes of the study are that in a slimfloor arrangement with contemporary panels, beams and connectors, the composite enhancement is small but measurable, and that with changes and development to the system components, much larger composite enhancement effects could be generated

Understanding and modelling damage and fracture in nuclear grade graphite

This thesis studied the crack initiation and propagation characteristics of Nuclear Block Graphite 10 (NBG10) and Gilsocarbon (IM1-24), using the DoubleTorsion (DT) technique