Degradation models for the collapse analysis of composite aerospace structures

For the next generation of aircraft, the use of fibre-reinforced polymer composites and the design of "postbuckling" structures to withstand immense loads after buckling are key technologies for considerable weight and cost savings. However, the application of postbuckling composite structures has been limited, as today's analysis tools are not capable of accurately predicting the collapse of these structures under compression. The major objective of this PhD work was the development of an analysis methodology and complementary software package for composite postbuckling structures, which included the degrading effects of the critical damage mechanisms. From a comprehensive literature review and extensive benchmark study, a methodology was developed for analysing postbuckling composite structures that was capable of representing the critical damage mechanisms. One aspect of this was a degradation model to represent the growth of interlaminar damage such as delaminations and skin-stiffener debonds. In this degradation model, layers of shell elements were tied with user-defined multi-point constraints (MPCs), and fracture mechanics calculations using the Virtual Crack Closure Technique were applied to control the release of these MPCs to model crack growth. Another aspect of the methodology was an approach to predict the initiation of interlaminar damage in intact structures. This was implemented using strength-based criteria, and demonstrated on cross-section models of skin-stiffener interfaces, on which the input of deformations from a global postbuckling analysis of an entire panel was also shown. Separately, a degradation model was developed to capture pl y damage such as matrix cracking and fibre fracture, which was based on the Hashin criteria for damage prediction and the Chang-Chang approach for material softening. The analysis methodology was implemented into the finite element (FE) code MSC.Marc, using a combination of nine user subroutines and several external data files. The methodology was incorporated into a user-friendly software tool in MSC.Patran, which provided a suite of functions for including the damage representations into nonlinear FE analysis. Extensive validation was performed with experimental results from the European Union project COCOMAT, which included fracture mechanics characterisation coupons, single-stiffener flat panels, and large, multi-stiffener curved panels representative of composite fuselage designs. This demonstrated the applicability of the methodology for the design and analysis of both intact and pre-damaged postbuckling composite structures, and the capacity for accurate and detailed analysis of the critical damage mechanisms