A study was undertaken to determine if oblique incidence ultrasonic C-scans could predict changes in the tensile properties of carbon-epoxy laminates that had been subjected to low level impacts. Angle-ply laminates were manufactured to provide roughly a 3:1 ratio of longitudinal-to-transverse extensional stiffness. The specimens were divided into two groups for impacting, a delamination group and a rigid group. After impacting, each specimen underwent a normal incidence C-scan and an oblique incidence C-scan using a focusing transducer to reduce beam spread. The normal incidence scan was used to map the damage area and the oblique incidence scan was used to quantify the changes in extensional stiffness. Tensile tests were performed on each specimen and strength, extensional stiffness, and Poisson's ratios were determined. A comparison of the ultrasonic data with the tensile test data reveals that the variations in mechanical properties were too small and unmeasurable for the damage levels and types used in this study.
Composite materials, especially laminated composites, have generated much interest in recent years and for good reason. These composites have high strength-to-weight and high stiffness-to-weight ratios, thus lending themselves to applications where weight is a premium, such as automobiles, mass transit, and aerospace. In addition laminated composites can be specifically designed beforehand for the intended loads and/or deflections that will be seen in service. This last characteristic provides the engineer the rare opportunity of optimizing the design provided he or she knows what the design constraints are. One aspect of composite use that has always initiated debate is with damage assessment and evaluation. The indentation could be from a tool drop, or a hail strike, or from flying debris, but the impact was not sufficient to visually break up the surface.
