Interfaces in Thermoplastic Composites Probed by Laser-Induced Acoustic Emission

The mechanical properties and durability of carbon Þber composites depend not only on the properties of the constituent Þber and matrix but also on the quality of the interfacial bond. Many of the techniques for evaluating this bonding rely on model (e. g. single Þlament) composites (1-3). Besides being difÞcult to prepare and test, these specimens are subject to the criticism that they may not reßect actual behavior in a full-scale composite with a reasonable volume fraction of Þbers. This is an important consideration especially in thermoplastics, whose morphology may be sensitive to processing details. A few interface measurement techniques use actual composites, but are destructive (4-6). The thermoacoustic technique of Wu, on the other hand, is applied to actual laminates and probes a very small area (7). Thus, although it is not entirely nondestructive, it could be used for quality control of manufactured parts, for example. This letter reports on application of Wu's technique to some well-characterized amorphous thermoplastic composites. Unidirectional composite panels were fabricated by molding thoroughly dried solution-impregnated, drum-wound prepreg in matched metal molds. Composite Þber volume fractions were calculated from prepreg Þber areal weights. The materials studied are listed in Table I.* The two Þbers chosen have similar nominal tensile properties, but embedded single-Þlament tests (8) and fracture toughness results (9,10) indicate that they differ in their afÞnities for thermoplastic resins. An unfocused 6 watt Argon Ion laser beam was trained on a specimen for 10 seconds. Acoustic emission (AE) was detected by a 150 kHz resonant AE sensor (Physical Acoustics Corporation model R15). The signals were ampliÞed 40 dB by a preampliÞer (Physical Acoustics Corporation model 1220A) which had a bandpass Þlter of 100-300 kHz. The distance from the center of the laser spot to the center of the