Advanced composite structures

Deformation of the corner of a helicopter structure, an example of a stress concentration. The red color in the right-hand image indicates an area of higher strain.

Composites consisting of slender fibrous reinforcements in a polymer matrix have become increasingly important because they enable stiff, strong, lightweight, corrosion-resistant structures with reduced life-cycle costs in applications ranging from aerospace and marine to infrastructure construction, from defence to sporting equipment.  But because composites combine multiple ingredients their behaviour is significantly more complex than that of more conventional structural materials such as steel and concrete.

Our research into composite structures aims at developing means of predicting the mechanical response, and particularly the failure modes of these fibre-reinforced polymer composites, in order to facilitate the design of applications by New Zealand industry.

As part of the Composite Cluster of New Zealand, we are collaborating with the University of Auckland's Centre for Advanced Composite Materials, industry, and the New Zealand Defence Forces' Defence Technology Agency.

The problem

Joints, openings, corners and load introduction points (such as fittings bolted to the structure) all cause stress concentrations that often are the ‘weak link’ in composite structures and therefore critical to the performance of the structure as a whole. The failure processes in these regions are complex and difficult to predict, which necessitates an empirically based design process whose results are costly, time consuming, and significantly affect structural performance leading to a loss of the composite advantage.

Our research

The primary goal of our current research is to improve analytical and empirical modelling methods for stress concentrations in composite sandwich structures, to understand the effect of the variability of materials and geometry on the local stress distribution and failure processes, and to incorporate variability or uncertainty into design calculations.

Local structure around a loading or attachment point in a composite sandwich panel. Typically there will be in-plane and out-of-plane forces applied to the insert. Note that the insert can be either blind (shown above) or through thickness.

In the finite element analysis of failure, an alternative to fracture and damage mechanics models is the cohesive zone approach. In this approach, cohesive elements are placed in regions where failure is expected; usually at the interface of finite elements. These cohesive elements are characterised by traction-separation rules that simulate crack growth as shown above.

A two-parameter response surface showing the results of an uncertainty analysis produced using a direct Monte Carlo simulation with 20,000 samples. In this case, the uncertain parameters are the Young’s modulus of the adhesive and the magnitude of the applied pullout force. The colors represent different ranges of maximum strain. Here, yellow (lower right) is a low maximum strain, and black (upper left) is a high maximum strain.