Improved Transverse Crack Detection in Composites

A modified ultrasonic C-scan technique was implemented for improving the detection of a certain type of damage in composite specimens. The type of damage being studied is transverse (through the thickness) cracking of unidirectional off-axis graphite-epoxy specimens. These cracks are difficult to detect using standard through-transmission C-scan techniques. The modification is based on mode conversion to produce transmitted shear waves from incident longitudinal waves. While mode conversion is used extensively with isotropic materials, its use with composites is more limited. This is largely because the computation of wave propagation parameters is significantly more complicated with highly anisotropic materials than with isotropic materials. The appropriate incident angles to produce the desired mode conversion were computed based on the mechanical properties of the composite. Once the angles were computed the technique was simple to implement and resulted in marked improvement in detection of the transverse cracks being studied.