Volker Ulbricht

Simulating the Drape Behavior of Fabrics1

This paper is the result of an interdisciplinary research project at the Institute of Textile and Clothing Technology and the Institute of Solid Mechanics. The aim of this project is to describe and simulate the deformation behavior of flexural fabrics ( espe cially woven fabrics). For the simulation model, the shell theory is taken as a basis. Simulating drape behavior presents a geometrically nonlinear field problem with con siderable displacements. The deformation theory and its application for fabrics is used as a practicable approach for simulating drape. Investigations of the description of the materials behavior and its properties are a further necessary focus.

Simulation of drape behaviour of fabrics

In this paper a model is presented for the calculation by approximation of a drape test standardized in the textile industry. As woven fabric is of low thickness compared with the other dimensions, the fabric can be considered to be a two‐dimensional continuum. For the simulation model, the shell theory is taken as a basis. Simulating the drape behaviour presents a geometrically non‐linear field problem with considerable displacements.

Multi-Scale Modelling and Simulation of Textile Reinforced Materials

Novel textile reinforced composites provide an extremely high adaptability and allow for the development of materials whose features can be adjusted precisely to certain applications. A successful structural and material design process requires an integrated simulation of the material behaviour, the estimation of the effective properties which need to be assigned to the macroscopic model and the resulting features of the component.

Experimental Characterization of the Viscoplastic Material Behaviour of Thermosets and Thermoplastics

In this contribution the mechanical behaviour of polymeric matrix materials is analysed for both thermoplastics (Polypropylene) and thermosets (RTM6, RIM935). The results obtained from tensile tests carried out at different velocities indicate a nonlinear, inelastic material behaviour with strainrate dependence. For the clear identification and quantification of the nonlinearities, the experimental procedure has been extended to relaxation experiments and deformation controlled loading-unloadingprocesses with intermediate relaxations. Based on the experimental observations a small-strain viscoplastic material model is derived and material parameters are identified. The stress-strain-curves computed for different load histories are compared to the experimental results.

Computation of Effective Stiffness Properties for Textile-Reinforced Composites Using X-FEM

The macroscopic material behaviour of novel textile-reinforced composites is defined by its constituents (micro-level) and the design of the textile reinforcement (meso-level). Consequently, a multi-scale approach to the prediction of the material behaviour is necessary because only in this vein the adaptability of the textile reinforcement can be used to develop materials whose features can be adjusted precisely to certain applications.

Prevention of cyclic instability at the modeling of elasto-plastic deformation at large strains under proportional and non-proportional loading

The cyclic instability phenomenon is investigated at the modeling of large elasto-plastic strains. The instability is observed at large strains for some elasto-plastic material models with a kinematic hardening in contrast to the small strain case. The cyclic instability manifests itself as a changing of shape, sizes or location of the hysteresis loop during cycling. In partial case the cyclic instability leads to the stress ratcheting. Responses of 50 various models of elasto-plastic material have been considered under proportional loading (cyclic simple shear) and non-proportional loading (combined cyclic simple shear and tension-compression). Causes of the cyclic instability are analyzed and conditions ensuring the cyclic stability of elasto-plastic models are proposed.

Draw Bending of Load-Adapted Sheet Metal Profiles

The draw bending process represents an alternative for the flexible and inexpensive production of open sheet metal profiles. This paper introduces the draw bending process with its functional principle and specifies the most important characteristics. It includes the key results of several research projects dealing with draw bending to prove the applicability of this technique to produce customized profiles.

Flexible Draw Bending of Profiles

The technology of draw bending represents an alternative to produce flexible load-adapted sheet metal profiles. In addition to the adaptability of profile configuration low machine costs and an uncomplicated system control make this a very flexible production method. Hence it is particularly suitable for the production of profiles in small batch series or the fabrication of prototypes.

Material Characterization for Constituents of Fiber Reinforced Polymers by Experiments and Prediction of Effective Composite Properties

The present contribution aims to investigate the ability of drawing predictive conclusions from homogenization in case of damage. Therefor, two topics will be addressed. On the one hand, material properties for the constituents on the microscale have to be derived, to render a predictive homogenization possible. The investigation at hand is concerned with glass fiber reinforced epoxy resin. In this example the properties of the fiber and the matrix have to be studied individually by experiments. Furthermore, the interface between both materials needs to be examined. To this end experiments on several models of single fiber composites have been developed in the literature. For the present material combination single fiber fragmentation tests and pullout tests have been conducted and evaluated. On the other hand, boundary conditions are necessary, that allow for the strain localization in a volume element without leading to spurious localization zones.

Phase-Field Modelling of Damage and Fracture—Convergence and Local Mesh Refinement

In this contribution, we outline the combination of a phase-field model of brittle fracture with adaptive spline-based approximations. The phase-field method provides a convenient way to model crack propagation without topological updates of the used discretisation as the crack is represented implicitly in terms of an order parameter field that can be interpreted as damage variable. For the accurate approximation of the order parameter field that may exhibit steep gradients, we utilise locally refined hierarchical B-splines in conjunction with Bezier extraction. The latter allows for the implementation of the approach in any standard finite element code. Moreover, standard procedures of adaptive finite element analysis for error estimation and marking of elements are directly applicable due to the strict use of an element viewpoint. Two different demonstration problems are considered. At first we examine the convergence properties of the phase-field approach and explain the influence of the domain size and the discretisation for the one-dimensional problem of a bar. Afterwards, results of the adaptive local refinement are compared with uniformly refined Lagrangian and spline-based discretisations. In the second example, the developed algorithms are applied to simulate crack propagation in a two-dimensional single-edge notched, shear loaded plate.