3D tensile fracture analysis of tailored fibre placement composites using digital volume correlation

Tailored Fibre Placement (TFP) composites, manufactured using one of the latest dry-fibre technologies, feature unique 3D fibre bundle architectures and complex layup configurations involving stitching threads and base materials. Damage development under load is therefore challenging to observe and failure hard to predict. In this study, in-situ tensile testing of a TFP composite was conducted for the first time within an X-ray Computed Tomography microscope to capture the 3D damage evolution up to failure. Segmented image-based models were used to correlate crack locations with strain distributions measured using Digital Volume Correlation by post-processing X-ray images recorded at regular load intervals throughout the test. Results revealed that initial cracks appeared at applied stresses below 50% UTS, and propagated mainly within fibre bundles. The first initiation site matched the location of the maximum value (about 0.008) of the local strain component transverse to the fibre direction within the ply where the first crack appeared. Propagation followed bands of strain concentration along the fibre bundle direction, first in off-axis plies and then in longitudinal plies towards the end of the test, with the crack volume fraction increasing exponentially after initiation. Very limited delamination was observed after 90 % UTS, with stitching threads thought to be responsible for this retardation. Results therefore showed that fracture can be controlled through optimisation of the manufacturing process and laminate design. They also provide the foundation for the future development of physically-based multiscale strength prediction models needed for this optimal design of TFP composites for lightweight structural applications.