Detecting low-velocity impact damage in composite plates using infrared thermography

This research investigated low velocity impact damage in fiber-reinforced polymer (FRP) composites. Small-scale glass/epoxy laminates (approximately 210mm x 210mm x 2mm) were subjected to varying degrees of dynamic impact energies ranging from 5 to 20 J and infrared thermography inspections were performed on the damaged specimens. Three distinct damage modes were observed: penetration resulting in highly localized fiber rupture through the thickness of the composite; penetration/delamination in which localized fiber rupture was observed on the impacted surface and additional delamination occurred around the point of impact; and delamination/reverse side fiber rupture in which no visible damage occurred on the impacted surface but fiber rupture and delamination occurred beneath the surface. A modified lock-in thermography procedure was used in the nondestructive evaluation (NDE). Phase images were constructed by applying a least-squares sinusoidal curve fit to a series of thermal images collected over one cycle of sinusoidal heating. This method was shown to increase contrast for subsurface delaminations compared to raw thermal data. Finally, thermography results for FRP composite samples containing simulated damage (back-drilled holes) were compared with thermography results from impact-damaged samples.