Study of Generation Mechanisms for Laser Ultrasonics

The aerospace industry is beginning to use advanced composite materials for primary load bearing structures and their failure mechanisms must be better understood to predict their behavior in service. The Combined Loads Tests (COLTS) facility is being constructed at the NASA Langley Research Center to characterize these failure mechanisms. Laser based ultrasonic NDE can monitor the samples during dynamic loading without interfering with the structural tests. However, the constraints of implementing laser ultrasound in a structures laboratory reduces the efficiency of the technique. The system has to be “eye-safe” because many people will be present during the structural tests. Consequently, laser light has to be delivered through fiber optics and a significant amount of light is lost. Also, the nature of the composite materials makes laser ultrasonic inspection difficult. The composites of interest are formed from woven layers that are stitched through the laminate thickness and bound in a resin matrix. These materials attenuate ultrasound strongly and exhibit a high degree of scattering. In this paper, we describe a field-deployable laser based ultrasonic NDE system that we developed to investigate structures during testing in the COLTS facility. We illustrate the design constraints that reduced efficiency and present some ultrasonic data from thick stitched composites