Microscopic investigation of stress triaxility of interface fracture

An experimental study on the delamination of bilayers that is coupled to simulations with digital image correlation (DIC). A comparison is done on the performance of cohesive zones and cohesive bands.

Master Thesis Candidate: S. Krekel
Supervisors: dr.ir. J.P.M. Hoefnagels, dr.ir. J.J.C. Remmers
Project Period: 3/2015 - 12/2015

One of the mayor mechanical engineering issues is to predict the moment of failure in structures. Fracture mechanics has been a successful theory to describe brittle fracture. However, quasi-brittle and ductile materials have a fracture process zone that is too large to be described with linear elastic fracture mechanics. One of the most powerful methods to solve that problem is a cohesive zone model. When looking at a crack, a cohesive zone projects damage mechanisms in and around the crack tip on the interface, see figure 2. This model was introduced by Barenblatt (1959) and Dugdale (1960), since then there have been mayor developments in this field. 

For instance, Remmers et al. (2013) introduced a new concept: a cohesive band. In principle the method adds an extra dimension to the cohesive zone, namely a thickness, so the cohesive zone evolves into a cohesive band. The advantage of this method is that it is depended on the hydrostatic stress. This means the method can predict behaviour for problems in which stress triaxilities are important, such as crack branching and crazing in polymers.

The mechanical performance of the cohesive band model will be compared to that of the original cohesive zone model. The performance will be tested by predicting the behaviour of a bilayer during a delamination experiment. The experiments will be done on the experimental setup of Kolluri et al. (2011) (see figure 1). A detailed comparison is done between the experiments and the simulations done in Dawn with the aid of digital image correlation (DIC).