Fatigue – Institute of Continuum Mechanics – Leibniz University Hannover

The demand for application of complex materials such as high strength concrete or fiber-reinforced composites (FRPs) is growing continuously for instance in advanced structures, high rise structures, aerospace and energy industries. Especially for those complex materials the question arises how long their service life will be, or how to predict their failure?

These questions are impossible to answer by relying merely on experimental tests due to different aspects, such as high costs of manufacturing parts and machines, extremely time consuming procedures, and even incapability of the test in capturing the events of interest for real structures, in some cases.

In the group of “Life time prediction and failure of modern materials” at the IKM, the focus of the projects is on developing, adapting and testing numerical tools to simulate damage, degradation, and failure behavior of materials and structures, with an eye on building an extensive, efficient, and accurate integrated tool for analyzing and predicting the life time of structures. In collaboration with other groups, the projects attempt to cover as many significant aspects as possible that need to be considered in failure of a structure, such as:

Loading conditions (static, dynamic, fatigue, etc.)
Environmental effects (water, temperature, etc.)
Size effects (multiscale simulations)
Time scaling (multi time stepping techniques
Presence and evolution of defects, cracks, material degradation and instabilities

On the way towards design and optimization of complex materials and structures, utilizing such numerical tools makes the process much cheaper and faster, and permits more accurate and reliable predictions.

Life time prediction and failure of modern complex materials and structures

Multiscale Modeling of Buckling of Fiber-Reinforced Polymers

This project is about multiscale modeling of fiber kinking in unidirectional fiber reinforced composites.

Led by: S. Löhnert, P. Wriggers

Team: S. Hosseini

Year: 2018

Funding: DFG (Graduiertenkolleg 1627)
Fatigue lifetime prediction using Wavelet transformation induced multi-time scaling (WATMUS)

A fast and accurate numerical method for fatigue lifetime prediction using eXtended Finite Element Method(XFEM) and WATMUS

Led by: S. Löhnert, P. Wriggers

Team: Tengfei Lyu

Year: 2017
Modelling the temperature development and crack propagation during sheet-bulk metal forming

n metal forming processes a large amount of mechanical work is dissipated due to large plastic deformations. The accompanying temperature rise leads to thermal strains and a change in the material behaviour which can influence the mechanical behaviour during the forming process and the final shape of the part. For this reason it is important to consider temperature effects and heat conduction in the material modelling of the polycrystalline microstructure. The resulting thermomechanical problem exhibits a strong coupling since on the one hand through mechanical deformation heat sources are introduced and on the other hand material parameters may depend on the temperature and also large thermal strains can emerge. Experimentally the temperature influence can be analysed by performing experiments at IW, IFUM, LFT or IUL with material specimens at different temperatures. The results can be used to develop a thermomechanical material model for the microstructure. By using homogenization techniques the macroscopic effective material model developed in period 1 of the SFB/TR73 will be extended by temperature effects. Another critical effect occurring during the forming process is the initiation and propagation of microcracks. This effect will lead to a stiffness reduction or even to failure of the entire structure. Therefore it is essential to study the degradation mechanisms of the crystallographic microstructure. A nonlocal damage model will be used to induce microcracks. For propagating the crack existing models have to be extended to nonlinear anisotropic and inelastic materials. Especially a criterion has to be found when cracks collide with grain boundaries. For the case of stable crack growth with a statistical simulation series a representative volume element can be found. This is used to produce a micromechanically motivated stress strain relationship by a homogenization procedure. With this material response the effective material model of period 1 of the SFB/TR73 will be extended. Here it is important to capture the softening effects with a nonlocal damage model which is for example used and developed at the IUL. In a last step the two approaches will be combined for the construction of a material model capturing thermomechanical effects and cracks on the microstructural level.

Led by: S. Löhnert, P. Wriggers

Team: S. Beese, S. Zeller

Year: 2013