Use of Finite Element Analysis to Create Robust Composite Designs – Going Beyond First Ply Failure

The history of fibre reinforced composite product development has shown an evolution from the classical laminate analysis approach through to modern techniques. This paper shows how finite elements can be used to create composite structures which perform better and provide enhanced safety in critical applications. The paper shows how a structure can be made more robust by the analysis of failure through to ultimate load. Understanding how this failure occurs allows us to make a better assessment of the structures performance in extreme load conditions. Failure prediction methods such as progressive failure analysis (PFA), virtual crack closure (VCCT) and cohesive zone modelling (CZM) have been available for years. This paper looks at the practical use of these methods and how material behaviour influences the results. It also looks at what material testing can be needed to make these new techniques more accurate. These methods are illustrated with examples such as a plate with a hole. In this example we can change the behaviour of progressive failure by modifying FE solver settings for the way that the material degrades. Understanding this is important to ensure that we get better analysis results. Practical comparison of analysis with theory and test of some typical L-sections from an aircraft structure is given. Of particular interest is the ability of structures to survive beyond initial failure. This can be due to loads being re-distributed due to the failure and also due to material non-linearity in certain failure modes. Using this practical example we can see how composite failure can involve a combination of different failures including resin cracking, delamination, buckling and fibre failure. A complete finite element solution needs to be capable of including all of these effects. The conclusion suggests some questions which the analyst can ask of a design to ensure that it is safe, robust and damage tolerant. All analyses described in this paper use MSC Software solutions. CLASSICAL ANALYSIS METHODS Initial composite analysis approaches were based upon classical laminate theory. This is broadly based upon assumptions similar to those for engineers bending theory. It is well documented (ref 1) and is currently accepted practice in most composite applications. Failure analysis is classically performed using a “first ply failure” criteria. This assumes that as soon as there is some degradation to the composite then it is failed and no longer acceptable. This approach is suitable for normal operating loads but may be too conservative if you are looking at single high load events. In these scenarios it would be good to predict the capability of the structure to go beyond this initial failure. The failure theories developed for composites are often empirical derivations based upon observed tested behaviour. For example the Tsai-Wu failure theory creates a rotated ellipsoidal failure envelope for the material. This rotation is based upon an interaction term which is nominally derived from test. However most people don’t do this testing and assume a value of