Estimation of Impact Damage Threshold to Cause Internal Damage in CFRP Laminates by Means of Acoustic Emission

The impact damage threshold to cause internal damage in cross-ply CFRP laminates has been determined, by monitoring dynamic surface strain and Lamb-wave AE (Acoustic Emission) along with the impact force history in SACMA-type impact tests. Four types of cross-ply CFRP plates ([0o n /90o n]sym., n=4,6,8,10) are impacted with a spherical steel tup, 16 mm diameter, at 0.8 and 1 m/s. Surface strain of a test laminate is measured using a strain gauge and Lamb-wave AE signals are monitored by small AE sensors on both surfaces. The time history of surface strain matches with the impact force history when no damage occurs as predicted by theory of impact dynamics. Here, only impact-induced AE (or Impact AE) is obtained when the impact tup contacts the specimen. When internal fracture occurs, measured strain history deviated from that expected from the dynamic force and both Impact AE and fracture-induced AE (Fracture AE) are detected. Fracture AE is detected when the dynamic force and strain history indicates the initiation of internal CFRP damage. Impact damage threshold values for the four types of cross-ply CFRPs are measured by comparing the impact force, strain and Fracture AE. Initial AE signals monitored simultaneously on both sample surfaces are symmetric Lamb waves. Wave simulation analysis has revealed the nature of the onset of fracture to be the matrix cracking of the center plies. Complex AE signals following the initial fracture for a few ms are expected to be from delamination damage, which was confirmed with an ultrasonic C-scan study.