Acoustic characterization of damage modes in CFRP laminates with mesh interlayers

Inserting polyamide (PA) meshes between plies of CFRP laminates to introduce mesh interlayers is an effective method for improving their interlaminar fracture toughness. To understand acoustic emission (AE) frequency characteristics of each damage mode generated in CFRP laminates with mesh interlayers, tensile tests, double cantilever beam (DCB) tests, and four-point end-notched flexure (4-ENF) tests were conducted for carbon/epoxy laminates with mesh interlayers while performing AE monitoring. In addition, the damage modes generated in each test were observed by optical microscopy and scanning electron microscopy (SEM). The peak frequency band of each damage mode related to carbon/epoxy plies or mesh interlayers with PA meshes was classified from multiple combinations of stress-strain curves or load-time diagrams, peak frequency band obtained by fast Fourier transform (FFT) of AE signals and damage modes observed in each test. The frequency band of AE signals of PA/epoxy interfacial debonding generated in the mesh interlayers was successfully classified as well as the well-known damage modes found in CFRP laminates such as CF/epoxy interfacial debonding, tensile and shear fracture of epoxy, fiber breakage. AE signals by PA/epoxy interfacial debonding in tension and shear were shown to correspond to 750-950 kHz and 500 kHz, respectively, though they overlapped in frequency with AE signals by damage originating in CFRP plies.