Cracks in Composite Materials

Researchers scrutinise cracks

in composite

materials

Windmills, tennis rackets, aircraft and windows. These are just some of the examples of where using composites is an advantage. The increasing use of composite products challenges our understanding of classical fracture mechanics. For example, how does carbon fibre react when it is subjected to stress? Researchers are working to develop a theoretical model that will describe the characteristics of materials with a much higher degree of detail than is possible today.

Today’s computer models can provide detailed information about the strength of constructions under different forms of load. This information is useful input for the design process. But these programmes are often inadequate to deal with a composite material that behaves much differently to a homogeneous material when subjected to stress.

The study of cracks and their significance for the durability of a construction has been going on for decades as classical fracture mechanics. But what actually happens when a modern composite breaks?

In the majority of cases, the problem is found in the joints but we do not have a detailed picture of its causes and effects. For example, how does a crack occur when a single glass fibre separates from plastic? How does it occur when two glass fibres simultaneously separate from the plastic? And what if a collection of fibres breaks? What significance will it have for the construction’s formation of cracks under various forms of stress and for its durability?

New theoretical model for the formation of cracks
Researchers from Aarhus University have entered virgin territory. They have already developed an existing theoretical model that can describe a random composite’s behaviour during the formation of cracks in two dimensions. Now they are busy expanding the model to three dimensions.

During the next three years, they have to continue feeding data into an existing FEM programme on the fracture mechanics of different composites in different directions during both monotonic and cyclic stress.

If the researchers succeed in developing a preliminary theoretical model, it will be possible for them to describe fatigue in structures far more precisely on the basis of both the formation of cracks and their expansion in the composites. In the future this research has a range of interesting perspectives. It can both give better perspectives for more precise design processes while at the same time providing basic knowledge about how composites behave.