Multiscale simulation methodology for the forming behavior of biaxial weft-knitted fabrics

Trotz der guten Drapierbarkeit ist das Formen von flachen Mehrlagen-Gestricken (MLG) zu 3D-Preforms für schalenartige Faser-Kunststoff-Verbund (FKV) Bauteile immer noch eine Herausforderung, da einige Defekte wie Falten, Gassenbildung oder Faserschäden nicht vollständig vermieden werden können. Daher ist vor der Massenproduktion eine Optimierung erforderlich. Die virtuelle Optimierung des Umformprozesses mit Hilfe von Finite-Element-Methode (FEM) Modellen ist ein attraktiver Ansatz, da die Rechenkosten immer geringer werden. Dazu wurde ein auf Kontinuumsmechanik basierendes Makromodell erfolgreich für MLG implementiert. Der makroskalige Modellierungsansatz bietet angemessene Rechenkosten und kann gängige Defekte wie Faltenbildung vorhersagen. Weitere Defekte wie Faserversatz, ondulierte Fasern, Knicken von Fasern, Faserschädigung und Gassenbildung können jedoch mit dem Makromodell nicht vorhergesagt werden. Da die Komplexität von Bauteilen aus FKV und die Qualitätsanforderungen an die 3D-Preforms zunehmen, sind FEM-Modelle mit höherem Darstellungsgrad erforderlich. Im am weitesten entwickelten mesoskaligen FEM-Modell für MLG verhindert die zu starke Vereinfachung des Strickfadensystems mit Federelementen jedoch die Fähigkeit dieses FEM-Modells, Faserverschiebungen und Gassenbildung bei großer Verformung zu beschreiben, wobei das Gleiten zwischen den Fäden berücksichtigt werden muss. Ziel ist daher die Entwicklung, Validierung und Anwendung eines mesoskaligen FEM-Modells für MLG, um die derzeitigen Einschränkungen zu überwinden. Es werden neue Modellierungsstrategien für biaxiale MLG auf der Mesoskala entwickelt. Die mechanischen Eigenschaften von MLG werden durch eine Reihe von textilphysikalischen Prüfungen charakterisiert und analysiert, die alle notwendigen Daten für den Aufbau sowie die Validierung der FEM-Modelle liefern. Es sollen zwei Ansätze zur Modellierung des Verstärkungsgarns implementiert und verglichen werden: durch Balken- und durch Schalenelemente. Die validierten Modelle können für die Umformsimulation verwendet werden. Es folgt eine Benchmark-Studie über die Kapazität und Zuverlässigkeit der verfügbaren Makromodelle und der entwickelten Mesomodelle durch Umformsimulation. Als Grundlage für die Benchmark-Studie werden Umformversuche durchgeführt. Das zweite Ziel der Arbeit ist die Modellierung von FKV auf verschiedenen Skalen. Die Modellierung von FKV auf der Makroebene wird mit den Daten der Faserorientierung durchgeführt, die aus der Umformsimulation gewonnen werden. Eine Mapping-Methode hilft dabei, die vorhergesagte Faserorientierung aus der Umformsimulation von dem MLG Mesomodell auf das FKV-Makromodell zu übertragen. Um den FKV zu charakterisieren und die Parameter für das FKV Modell vorzubereiten, werden Versuche mit FKV durchgeführt und ausgewertet. Basierend auf dem Mesomodell des MLG wird eine weiteres FKV-Modell vorgeschlagen, wobei Garn und Matrix getrennt modelliert werden. Dieses mesoskalige FKV-Modell enthält auch eine Kontaktformulierung, mit der die Delamination im FKV-Bauteil vorhergesagt werden kann. Prüfungen von Schale-Rippen Strukturen dienen als Grundlage für die Modellvalidierung. Das validierte Modell wird erfolgreich zur Vorhersage des mechanischen Verhaltens weiterer Schale-Rippen Strukturen mit unterschiedlicher Höhe und Anordnung der Rippen verwendet.:Kapitel 1 stellt die Einleitung und Problemstellung von dem Thema FKV vor. Kapitel 2 gibt eine Übersicht über Stand-der-Technik von den Hochleistungsfasern, Herstellung von textilen Verstärkungen und Halbzeugen, Fertigung von FKV sowie von Prüftechnik für Textilien und FKV. Zunächst wurden in Kapitel 3 eine Einführung in die Modellierung mit FEM allgemein und Stand-der-Technik der Modellierung von technische Textilien gegeben. In Kapitel 4 wurden die Zielsetzung und das Forschungsprogramm festgelegt. Die experimentellen Arbeiten werden in Kapitel 5 vorgestellt. Der erste Schritt ist die Auswahl des Materials und der Konfiguration für die MLG. Sowohl das Ausgangsmaterial als auch die produzierten MLG sollten systematisch getestet werden. Als Referenz wird auch ein Leinwandgewebe in die Prüfprogramme aufgenommen. Neben der Charakterisierung von textilen Flächengebilden sollen auch deren gleichwertige FKV geprüft werden. Das erste Ziel des Forschungsprogramms wird in Kapitel 6 erreicht, wobei verschiedene Ansätze zur Modellierung von MLG vorgestellt und validiert werden. Die entwickelten und validierten FEM-Modelle werden für die Benchmark-Studie der Umformsimulation in Kapitel 7 verwendet. Kapitel 8 befasst sich mit der Modellierung von FKV in verschiedenen Skalen. Zunächst wird das Mapping-Verfahren vorgestellt. Es wird ein Mapping für ein schalenförmiges T-Napf-Bauteil durchgeführt. Die trukturanalyse für das T-Napf-Bauteil erfolgt für übliche Lastfälle. Zweitens wird ein mesoskaliges FEM Modell für MLG-verstärkte FKV vorgeschlagen. Dieses Modell wird auf der Grundlage der Prüfdaten aus Kapitel 5 validiert. Das validierte Modell wird dann zur Vorhersage des mechanischen Verhaltens eines Schale-Rippen-FKV-Bauteils unter Biegebelastung verwendet. Kapitel 9 gibt eine Zusammenfassung von den Forschungsergebnissen und Vorschlägen für mögliche weitere Forschungen rund um dem Thema MLG als Verstärkung für FKV. Die Kombination von vorhandenen Makro-und Mesomodellen in einer einzigen Simulation kann die Berechnungskosten senken, ohne die Vorhersagenfähigkeiten des Modelles kompromittiert zu werden

Latest Hydroforming Technology of Metallic Tubes and Sheets

This Special Issue and Book, ‘Latest Hydroforming Technology of Metallic Tubes and Sheets’, includes 16 papers, which cover the state of the art of forming technologies in the relevant topics in the field. The technologies and methodologies presented in these papers will be very helpful for scientists, engineers, and technicians in product development or forming technology innovation related to tube hydroforming processes

3D printing assisted development of bioinspired structure and device for advanced engineering

Smart materials with bio-inspired structure and stimuli responsive features can sense the external and internal condition changes, such as temperature, light intensity, pH or ion concentration. Those unique functions have been widely utilized in cutting edge engineering applications, such as flexible sensors, soft robotics and tissue engineering. Meanwhile, conventional manufacturing methods such as moulding, and lithography-based microfabrication still represent the mainstream force in scale up manufacturing. Considerable limitations for these technologies, such as on demand rapid prototyping, the high cost and low-volume production, remain to be overcome. In this PhD project, I explored the advanced manufacturing in facilitating the complex structure, with higher controllability, lower prototyping cost and extended applications (flexible sensors, soft robots, biomedical devices, etc.). The key practice is to utilize the high resolution 3D printing technology to create dedicated bio inspired structures based on functional materials. Combined with advanced micro/nano engineering, we have achieved a variety of techniques/prototypes for future applications, such as optical control, micro-fluidic and bio-medical systems, etc

The prediction of wrinkle formation in non-crimp fabrics during double diaphragm forming

Liquid composite moulding (LCM) processes are an economical alternative to autoclave-cured prepreg, as intermediate material forms are less expensive, capital equipment costs are lower and cycle times are shorter. LCM is suitable for producing aerospace quality components, but the process chain is dependent on a separate preforming process to convert 2D fibre formats into complex 3D shapes prior to moulding. Preforming is difficult to automate to ensure defect-free architectures, as multiple plies are often formed at the same time according to predefined forming loads and constraints. Corrections to the draping direction, force and sequence can be easily refined during manual hand layup, as the laminator works on one ply at a time. Corrections to the forming sequence cannot be easily made during automated forming, therefore numerical models are required during the design phase to refine process parameters to ensure that defects do not evolve. This thesis seeks to develop a robust simulation methodology for modelling the forming behaviour of biaxial fabrics, in order to enable effective identification of forming related defects and assist in process design for improving the quality of preforms. Research has been conducted in three main areas: (I) Material characterisation and model development. A macroscale constitutive model has been developed to simulate the forming behaviour of biaxial fabrics, based on an explicit finite element scheme, incorporating the effects of bending stiffness to predict wrinkling. Cantilever tests were employed to characterise the bending behaviours of a twill-weave woven fabric and a non-crimp fabric (NCF) with pillar stitches, providing linear (a constant rigidity) and nonlinear bending stiffness models to represent the fabric materials. Experimental and numerical studies have shown that the bending behaviour of the fabrics is nonlinear, which is dependent on the fibre curvature along the bending direction. Forming simulations using a constant bending stiffness from the standard cantilever test (BS EN ISO 9073-7; 1998) produced unrealistic predictions for fabric bending and wrinkling behaviours, while the nonlinear model produced more accurate forming induced wrinkle patterns compared to the experimental data. The nodal distance between the deformed fabric mesh and the tool surface was identified to be a suitable method to locate areas containing out-of-plane defects, using the principal curvature to further isolate wrinkles from areas of fabric bridging (poor conformity). (II) Multi-resolution modelling approach for defect identification. A multi-resolution modelling strategy was developed for determining forming induced defects in large-scale DDF components, using a macroscopic global-to-local sub-modelling technique. The fabric constitutive model developed in Stage (I) above was used for predicting macro-scale defects (i.e. wrinkling and bridging defects at the ply level) during double diaphragm forming (DDF) of a generic geometry comprising local changes in cross-sectional shape. Comparisons between simulations and experimental results confirmed the accuracy of the forming model, but the runtime of the full-scale shell-element model was found to be impractical. Therefore, a multi-resolution modelling strategy was developed to improve the overall computational efficiency. Areas containing potential defects were initially determined by a full-scale global simulation using a coarse membrane-element mesh (element edge length of ~5mm). Results with higher resolution (wrinkle amplitude of ~1mm) and more realistic shapes were subsequently obtained by local sub modelling, using higher order shell-element meshes, based on boundary conditions derived from the global simulation. The applied methodology enabled an 87% saving in the runtime compared to the high fidelity full-scale shell-element model for the same geometry, with the length of wrinkles and area of fabric bridging predicted within 10% compared to experimental data. These reductions will become more significant as the overall length scale is increased for components produced by DDF, as defects will tend to be within more localised regions. (III) Defect formation and mitigation during multi-ply NCF forming. Forming experiments and simulations were performed to investigate the mechanism of wrinkling in multi-layered NCF plies during DDF. Simulation results indicate that in-plane fibre compression, caused by dissimilar shear deformation between adjacent plies, can lead to out-of-plane wrinkles, where the wrinkle length is a function of the relative fibre angle at the ply-ply contact interface. The most severe wrinkles occurred when the inter-ply angle was 45° for a multi-ply biaxial NCF preform. Numerical and experimental studies have shown that out-of-plane wrinkles are sensitive to the friction resistance between NCF plies and therefore lubricating the fibres can minimise wrinkling defects caused by dissimilar inter-ply deformation. In summary, results from the first part of the thesis (Chapter 3 and Chapter 4) demonstrate the importance of incorporating a curvature-dependent bending behaviour into fabric constitutive modelling for correctly predicting forming behaviour of bi-axial fabrics. The multi-resolution forming simulation strategy (Chapter 5) extends the capability of the proposed fabric model to predict macroscale defects in large structures more efficiently. The study on multi-ply fabric forming (Chapter 6) provides further understanding about the effect of inter-ply sliding on ply wrinkling, enabling a feasible solution for wrinkle mitigation. The results from this work can be directly extended for industrial application to improve the performance of composite structures made via fabric preforms

The prediction of wrinkle formation in non-crimp fabrics during double diaphragm forming

Liquid composite moulding (LCM) processes are an economical alternative to autoclave-cured prepreg, as intermediate material forms are less expensive, capital equipment costs are lower and cycle times are shorter. LCM is suitable for producing aerospace quality components, but the process chain is dependent on a separate preforming process to convert 2D fibre formats into complex 3D shapes prior to moulding. Preforming is difficult to automate to ensure defect-free architectures, as multiple plies are often formed at the same time according to predefined forming loads and constraints. Corrections to the draping direction, force and sequence can be easily refined during manual hand layup, as the laminator works on one ply at a time. Corrections to the forming sequence cannot be easily made during automated forming, therefore numerical models are required during the design phase to refine process parameters to ensure that defects do not evolve. This thesis seeks to develop a robust simulation methodology for modelling the forming behaviour of biaxial fabrics, in order to enable effective identification of forming related defects and assist in process design for improving the quality of preforms. Research has been conducted in three main areas: (I) Material characterisation and model development. A macroscale constitutive model has been developed to simulate the forming behaviour of biaxial fabrics, based on an explicit finite element scheme, incorporating the effects of bending stiffness to predict wrinkling. Cantilever tests were employed to characterise the bending behaviours of a twill-weave woven fabric and a non-crimp fabric (NCF) with pillar stitches, providing linear (a constant rigidity) and nonlinear bending stiffness models to represent the fabric materials. Experimental and numerical studies have shown that the bending behaviour of the fabrics is nonlinear, which is dependent on the fibre curvature along the bending direction. Forming simulations using a constant bending stiffness from the standard cantilever test (BS EN ISO 9073-7; 1998) produced unrealistic predictions for fabric bending and wrinkling behaviours, while the nonlinear model produced more accurate forming induced wrinkle patterns compared to the experimental data. The nodal distance between the deformed fabric mesh and the tool surface was identified to be a suitable method to locate areas containing out-of-plane defects, using the principal curvature to further isolate wrinkles from areas of fabric bridging (poor conformity). (II) Multi-resolution modelling approach for defect identification. A multi-resolution modelling strategy was developed for determining forming induced defects in large-scale DDF components, using a macroscopic global-to-local sub-modelling technique. The fabric constitutive model developed in Stage (I) above was used for predicting macro-scale defects (i.e. wrinkling and bridging defects at the ply level) during double diaphragm forming (DDF) of a generic geometry comprising local changes in cross-sectional shape. Comparisons between simulations and experimental results confirmed the accuracy of the forming model, but the runtime of the full-scale shell-element model was found to be impractical. Therefore, a multi-resolution modelling strategy was developed to improve the overall computational efficiency. Areas containing potential defects were initially determined by a full-scale global simulation using a coarse membrane-element mesh (element edge length of ~5mm). Results with higher resolution (wrinkle amplitude of ~1mm) and more realistic shapes were subsequently obtained by local sub modelling, using higher order shell-element meshes, based on boundary conditions derived from the global simulation. The applied methodology enabled an 87% saving in the runtime compared to the high fidelity full-scale shell-element model for the same geometry, with the length of wrinkles and area of fabric bridging predicted within 10% compared to experimental data. These reductions will become more significant as the overall length scale is increased for components produced by DDF, as defects will tend to be within more localised regions. (III) Defect formation and mitigation during multi-ply NCF forming. Forming experiments and simulations were performed to investigate the mechanism of wrinkling in multi-layered NCF plies during DDF. Simulation results indicate that in-plane fibre compression, caused by dissimilar shear deformation between adjacent plies, can lead to out-of-plane wrinkles, where the wrinkle length is a function of the relative fibre angle at the ply-ply contact interface. The most severe wrinkles occurred when the inter-ply angle was 45° for a multi-ply biaxial NCF preform. Numerical and experimental studies have shown that out-of-plane wrinkles are sensitive to the friction resistance between NCF plies and therefore lubricating the fibres can minimise wrinkling defects caused by dissimilar inter-ply deformation. In summary, results from the first part of the thesis (Chapter 3 and Chapter 4) demonstrate the importance of incorporating a curvature-dependent bending behaviour into fabric constitutive modelling for correctly predicting forming behaviour of bi-axial fabrics. The multi-resolution forming simulation strategy (Chapter 5) extends the capability of the proposed fabric model to predict macroscale defects in large structures more efficiently. The study on multi-ply fabric forming (Chapter 6) provides further understanding about the effect of inter-ply sliding on ply wrinkling, enabling a feasible solution for wrinkle mitigation. The results from this work can be directly extended for industrial application to improve the performance of composite structures made via fabric preforms

Real-time simulation and visualisation of cloth using edge-based adaptive meshes

Real-time rendering and the animation of realistic virtual environments and characters has progressed at a great pace, following advances in computer graphics hardware in the last decade. The role of cloth simulation is becoming ever more important in the quest to improve the realism of virtual environments. The real-time simulation of cloth and clothing is important for many applications such as virtual reality, crowd simulation, games and software for online clothes shopping. A large number of polygons are necessary to depict the highly exible nature of cloth with wrinkling and frequent changes in its curvature. In combination with the physical calculations which model the deformations, the effort required to simulate cloth in detail is very computationally expensive resulting in much diffculty for its realistic simulation at interactive frame rates. Real-time cloth simulations can lack quality and realism compared to their offline counterparts, since coarse meshes must often be employed for performance reasons. The focus of this thesis is to develop techniques to allow the real-time simulation of realistic cloth and clothing. Adaptive meshes have previously been developed to act as a bridge between low and high polygon meshes, aiming to adaptively exploit variations in the shape of the cloth. The mesh complexity is dynamically increased or refined to balance quality against computational cost during a simulation. A limitation of many approaches is they do not often consider the decimation or coarsening of previously refined areas, or otherwise are not fast enough for real-time applications. A novel edge-based adaptive mesh is developed for the fast incremental refinement and coarsening of a triangular mesh. A mass-spring network is integrated into the mesh permitting the real-time adaptive simulation of cloth, and techniques are developed for the simulation of clothing on an animated character

Textile materials

In this specialised publication, the reader will find research results and real engineering developments in the field of modern technical textiles. Modern technical textile materials, ranging from ordinary reinforcing fabrics in the construction and production of modern composite materials to specialised textile materials in the composition of electronics, sensors and other intelligent devices, play an important role in many areas of human technical activity. The use of specialized textiles, for example, in medicine makes it possible to achieve important results in diagnostics, prosthetics, surgical practice and the practice of using specialized fabrics at the health recovery stage. The use of reinforcing fabrics in construction can significantly improve the mechanical properties of concrete and various plaster mixtures, which increases the reliability and durability of various structures and buildings in general. In mechanical engineering, the use of composite materials reinforced with special textiles can simultaneously reduce weight and improve the mechanical properties of machine parts. Fabric- reinforced composites occupy a significant place in the automotive industry, aerospace engineering, and shipbuilding. Here, the mechanical reliability and thermal resistance of the body material of the product, along with its low weight, are very relevant. The presented edition will be useful and interesting for engineers and researchers whose activities are related to the design, production and application of various technical textile materials

Design and Development of a Soft Robotic Gripper for Fabric Material Handling

Fabric and textile materials are widely used in many industrial applications, especially in automotive, aviation and consumer goods. Currently, there is no semi-automatic or automatic solution for rapid, effective, and reconfigurable pick and place activities for limp, air permeable flexible components in industry. The production of these light-weight flexible textile or composite fiber products highly rely on manual operations, which lead to high production costs, workplace safety issues, and process bottlenecks. As a bio-inspired novel technology, soft robotic grippers provide new opportunities for the automation of fabric handling tasks. In this research, the characteristics of fabric pick and place tasks using the clamping grippers are quantitatively investigated. Experiments on a carbon fiber fabric are performed with a collaborative robot to explore the damage, slippage, draping, and wrinkling during basic pick and place operations. Based on the experimental results, multiple soft robotic gripper configurations are developed, including a compliant glove set that can improve the performance of traditional rigid grippers, an elastomer-based soft gripper, and a linkage-based underactuated gripper. The gripper designs are analyzed and refined based on finite element simulation. Prototypes of the grippers are fabricated using a rapid tooling solution for an overmolding strategy to verify their functionality. Through the research, it is proven feasible to reliably perform flexible fabric handling operations using soft grippers with appropriate toolpath planning. Finite element simulation and additive manufacturing have shown to be useful tools during the gripper design and development procedure, and the methodologies developed and applied in this work should be expanded for more flexible material handling challenges