Response of sub-surface fatigue damage under sonic load – a computational study

Graphite/epoxy composite samples with a center hole were subjected to fatigue loading, and were tested for damage using the sonic infrared (sonic IR) imaging technique. The technique revealed the presence of semi-elliptical delaminations near the circular notch. In the sonic IR technique, a short pulse of sound infuses into the material and results in frictional heating at the delamination surfaces. The heating at the damage locations reaches the surface of the sample by conduction, and is captured by an IR camera. In the present analysis, the finite element technique was employed to model a notched sample with two semi-elliptical delaminations. The delaminations were modeled using a surface-to-surface contact algorithm, and the frictional energy developed during the infusion of sound pulse was graphed. A separate finite element model was developed for transient heat transfer analysis by utilizing the calculated frictional energy as a heat source. Variation in surface temperature within the delamination regions was plotted with time, and was compared with the temperature–time plot obtained from the sonic IR technique.