Advanced, Waveform Based Acoustic Emission Detection of Matrix Cracking in Composites

An advanced, waveform based acoustic emission (AE) system was used to study the initiation of transverse matrix cracking in cross-ply graphite/epoxy (gr/ep) composites. The AE signals were detected with broad band, high fidelity sensors, and digitized for analysis. Plate wave propagation analysis was used to discriminate noise signals from those generated by cracks. The noise signals were confirmed to have originated in the specimen grip region by a new, highly accurate form of location analysis which was independent of threshold setting. Six different specimen thicknesses ([0 n , 90 n , 0 n ], n = 1 to 6) were tested under stroke controlled, quasi-static tensile loading. The presence and location of the cracks were confirmed post test by microscopy. Back scatter ultrasonics, penetrant enhanced X-ray techniques, and in limited cases, destructive sectioning and microscopy were also used to determine the length of the cracks. For thicker specimens (n > 2), there was an exact, one to one correspondence between AE crack signals and observed cracks. The length of the cracks in these specimens extended the full specimen width. Precise linear location of the crack position was demonstrated. The average absolute value of the difference between the microscopy determined crack location and the AE crack location was 3.2 mm. for a nominal sensor gage length of 152 mm. A four sensor array was used which not only improved the linear location accuracy, but also provided the lateral position of the crack initiation site. This allowed determination of whether the cracks initiated in the interior bulk of the specimens or along the free edges. For all cracks, the location of the crack initiation site was at one of the edges of the specimen. The cracks were more difficult to detect with AE in the thin specimens (n ≤ 2). The cracks in these specimens also initiated at the specimen edge, but did not immediately propagate across the specimen width. They generated significantly smaller amplitude AE signals. These measurements demonstrated that the same source mechanism can generate a wide range of AE signal amplitudes.