Nondestructive Evaluation for Damage Tolerance Life Management of Composite Structures

Polymer matrix composites (PMCs) are experiencing a growth in their use for civilian and military aircraft. However, the certification process for PMCs leads to new requirements for nondestructive evaluation/inspection (NDE/I). For example, in metallic structures current practice as defined in MIL STD 1530Dc1 uses slow crack growth analysis requiring the NDE/I technique to have a defined probability of detection (POD) curve to enable risk calculations. However, certification processes used to date for PMCs are closer to safe-life methods. However, there is a desire to alter the approach for managing PMCs structures in the US Air Force (USAF) to follow slow damage evolution criteria as is done for metallic structures today. To realize this desired capability, predictive modeling is being developed for slow damage evolution in PMCs. As a key input to these models, metrics of damage are required from NDE/I-based methods that characterize the geometry of the defects. Explored technical approaches include conventional pulse-echo methods, including resolution of tip diffraction from delaminations at the individual ply level, but amplitudes of these responses are quite small. An alternative approach uses localized single-sided pitch-catch methods to evaluate ultrasound propagating through and around a damaged region, such as typical damage from impacts. It uses the signal transmitted through damaged regions to extract features indicating matrix cracking or internal geometric attributes of delaminations that supplement the conventional one-sided pulse echo measurement. The methods includes using model-based methods and feature extraction approaches. Initial results show significant promise and damage state verification is obtained from destructive methods based on serial sectioning. This provides ground truth for measurements and is a reference for metrics of performance. With this capability, the Building blocks are in place to realize slow damage growth damage tolerance for PMC structures.