Composite Material Characterization using Acoustic Wave Speed Measurements

Composites are commonly found in a wide variety of applications such as construction materials, sporting equipment, and aerospace components. This class of material is multilayered and anisotropic both on the microscopic and macroscopic scale. Solid woven carbon fiber reinforced plastic (CFRP) composite consists of a polymeric binder and a carbon fiber weave. The carbon fiber weave acts as the reinforcing agent within the polymeric binder. This woven material is unique when compared to bi-directional composites due to its flexibility to wrap around small or large radii while maintaining a high strength to weight ratio. The thickness and strength of the CFRP are determined by the design parameters of the component and are varied by the number of woven carbon plys and the orientation of the plys within the solid laminate respectively. Pre-impregnated (“pre-preg”) woven carbon plys are becoming more common because they can be stacked in a predefined sequence before being placed in an autoclave with a specific pressure-temperature profile. Ultrasonic inspections are very important for finding defects within post-cured composite components. One parameter used in ultrasonic inspections is the velocity of sound which is unique to every material. The interaction of sound dispersion and absorption within the composite, due to the plys, affects its measured sound wave velocity. Ultrasonic inspections of composites are well established but, the reliability is hampered by the travel distance in the anisotropic, nonhomogeneous ply layers that lie between the interfaces. This paper will explore two influences of the solid woven CFRP composite’s velocity of sound: 1) the ultrasonic wave propagation through multiple specimens of varying thickness (ply number) with a single frequency and 2) the ultrasonic wave propagation using varying frequencies with a single thickness specimen.

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