Low velocity impact damage assessment in IM7/977-3 cross-ply composites using 3D computed tomography

As composite materials become more common in sporting goods, automotive applications, and aerospace structures, reliable methods of damage assessment will become increasingly important. Composite structures, unlike metals, tend to develop internal damage and fail in different ways. In the present work low-velocity impact damage, like that which could be produced from a dropped tool, was investigated for aerospace-grade carbon fiber reinforced polymer composite plates. Each plate was composed of 32 alternating-direction layers (plies) and was symmetric about the center plies. Each plate was impacted using a drop-weight test machine, and the impact force, deflection, and energy were measured. Two impact energies were investigated: One that produced slightly visible damage (in the form of cracks and back surface damage) and one that produced barely visible damage (in the form of a small front surface indent). To determine the extent of the internal damage the plates were scanned using an industrial CT imaging system, which produced a 3D x-ray image. As the scan of the original specimens (152.4 mm square plates) had too low of a resolution to uniquely identify all internal damage features, the plates were cut to a smaller size (45 mm square plates) and scanned again. The images produced from the smaller plates had a resolution approximately 2.5 times smaller than the thickness of the individual plies, which allowed for a more accurate internal damage assessment. Results showed that the amount of internal damage increased as the impact energy increased, but that the relationship was generally weak.