Acoustic Emission from Microcrack Initiation in Polymer Matrix Composites in Short Beam Shear Test

Acoustic emission (AE) was applied for detection of microcrack initiation in carbon fiber reinforced polymer composites subjected to shear stresses. Experimental materials were prepared from polyester bonded unidirectional (1D) non-crimp fabric and 2D plain-weave carbon fiber fabrics, using the resin transfer moulding technology. Control of epoxy resin/carbon textile proportions enabled variation of fiber volume content from small (34/35% for 2D/1D), through medium (51%) to high (68%). Rectangular samples (45 x 4 x 2 mm) were cut from 1D plates along [0] and across [90] fibers. Similar size samples from 2D plates were cut along warp/weft axes as well as in two orthogonal bias directions. Selected side surfaces were polished for microscopic (SEM) observations. Short-beam-strength tests were performed in 3-point bending (l/h = 4), with two AE sensors attached for damage monitoring, which allowed to interrupt loading sequence before final failure. The acoustic emission historic index was the most effective AE parameter in damage initiation control. Microcracks developing on polished composite side-surfaces were observed under the SEM and direct microscopic evidence confirmed fiber debonding to be the principal mechanism of crack initiation in these materials and testing conditions before any further damage.