Fibre fracture in composites

Important results from the project

Polymer composites are in general brittle material and stresses around e.g. stress risers are relaxed by progressive damage instead of plasticity in a fracture process. Purely elastic analyses with material properties measured in coupon tests therefore underestimate the strength and instead it has to be measured on the laminate level. These strength values vary with material lay-up, thickness and so on and are therefore very expensive to determine. In order to drastically reduce the amount of testing, focus has therefore more and more been put on developing models and methods to simulate progressive damage growth. These models are also finding their way into the commercial Finite Element (FE) codes. These models require input in form of fracture stresses and fracture toughness on a lamina level, for the latter no accepted test method exists. Especially for a compressive failure mode this is a problem since it can be expected that the damaged zone always will be able to carry some load. Thus, one would like to, directly in an experiment, measure the stress-strain relation within the damage zone. The aim of this project is to apply the methods developed at University of Skövde to directly measure the softening behavior in the composite material at fiber fracture (cohesive law) in both compression and tension.

Expected long term effects

The project FiKom will deliver a verified test method for the determination of the energy dissipation at fiber fracture and material data from these tests for the VAC materials. These properties are necessary building blocks in the development of methods for progressive damage analyses in composites. In the end of the project, an analysis and description of what requirements such material data puts on the formulation of damage models will be performed together with an evaluation how well the commercially implemented models fulfill these requirements. For the University of Skövde, the project gives an opportunity to strengthen the research in the area of composites by using its position within cohesive modeling and especially experimental methods for measuring the cohesive laws. A successful project will give University of Skövde an international role within the area.

Approach and implementation

In order to reach the aim of the project, the work has been divided into three stages: evaluation of existing test coupon geometries and data reduction methods, the development of a test method and experimental work and conclusions on the testing in relation to simulation activities.