4D X-ray CT for Evaluation of Progressive Damage Growth of Composites under Fatigue Loading

Abstract

Due to the heterogeneous microstructures, fatigue damage growth mechanics of composites are complex and highly dependent on the type of loading mode and load sequencing. Unlike for metals where striation marks can be used for understanding the damage growth rates and mechanics, the final destructive failure mode of composites tends to overshadow damage initiation sites and propagation details. Damage initiation and propagation in composites typically occur from an internal defect/feature or from an area where there is a stress concentration (i.e., fastener hole or impact damage) and are not visible with standard nondestructive inspection (NDI) techniques until they are within the inspection threshold of the equipment or the inspection method, which is also a function of materials, geometry, and location of the defect/feature. Although standard NDI techniques such as ultrasonic testing (UT) and pulse thermography are adequate for detecting such defects, in-depth information related to manufacturing defects (including certain manufacturing features that could potentially be failure initiation or nucleation sites) is required to fully understand the structural capability (i.e., strength and durability) of composites. In addition to high-fidelity post-manufacturing inspections, detailed inspections are required to detect damage initiation and monitor damage propagation through the heterogeneous microstructures of composites. Therefore, a high-fidelity 3D inspection technique such as X-ray computed tomography is required to investigate the damage growth mechanics and interaction of multiple failure modes. Such detailed damage characteristics can then be used for development and validation of high-fidelity progressive damage growth analysis (PDA) techniques and life prediction tools. More importantly, the damage accumulation and growth in composites need to be fully characterized to aid in certification as well as establishing damage tolerance testing and analysis protocols.