Composites studied by moiré methods and high speed photography

Sammanfattning: Fibre composites are today used as construction material in a growing number of applications. Carbon fibres/epoxy matrix composites are employed in high performance applications as aircrafts where a high stiffness/weight ratio is important. The structures are made of laminates built by stacking thin sheets of the composite with unidirectional ordered fibres. Such laminates are vulnerable to impact damages caused by for example a dropped tool. It is desired to develop computational methods to model such impacts and predict the resulting reduction in the performance. Then accurate models of the mechanical behaviour of the material are needed, models based on measurements not only at quasi-static loading but also at high strain rates. In this thesis two versions of the optical moiré technique are applied to investigate the mechanical behaviour of carbon fibre/epoxy matrix composite laminates. The work is made in co-operation with the Aeronautical Research Institute of Sweden (FFA). With moiré methods the in-plane displacement field on a specimen surface can be measured with high accuracy and with high spatial resolution. Moiré photography is combined with high speed photography to study specimens loaded at high strain rates. Dynamically loaded delamination cracks are studied and numerical finite element models show good agreement to the measured displacement fields. Measurements of the mechanical properties transverse to the fibres show that the composite has a weak dependence of strain rate. Apparent crack speeds of more than 2000 m/s are observed. The dynamic loading is made with a tensile split Hopkinson bar apparatus. Moiré interferometry is used to measure the displacement fields at high magnification. Here mode I loaded delamination cracks and strain concentrations at fibre optics sensors embedded in composites are studied, in both cases at static load. Finite element models have acceptable agreement with the measured results.