Estimation of impact-induced damage in CFRR laminates through ultrasonic imaging

Ultrasonic C-scans are used frequently to detect defects in composite components caused during fabrication and damage resulting from service conditions. Evaluation and characterization of defects and damage require experience and good understanding of the composites. Under low-velocity impact loading delamination is observed to be a major failure mode. In this paper, the results of extensive experimental work on damage of carbon fiber reinforced plastic (CFRP) laminates due to low-velocity impact are presented. The study was carried out on 10 different layups over four different thickness. Both drop-weight and projectile impacts simulating practical conditions like tool-drop and runway debris, respectively, were carried out at energy levels varying from 3 to 30 J. The ensuing delamination damage was determined by ultrasonic: C-scans using the pulse-echo immersion method for both projected and layer-wise distribution. While projected delamination was obtained by placing a gate over the backwall echo, layerwise distribution was obtained by successive time delay from the frontwall to the backwall echo covering each interface. Delamination areas were quantified accurately by processing the raw image data using a digital image processing technique. Based on the data obtained an empirical relation was established between the delamination area and the impact energy.