Performance evaluation of thin acoustic emission sensors in a laminated composite for structural health monitoring

Acoustic emission (AE) is a viable technique for structural health monitoring. Structures can be monitored by placing the AE sensors on the surface. However, with polymer-reinforced composites, it is possible to embed the acoustic emission sensor within the structure. The embedding process is postulated to increase the sensor’s sensitivity and thus detect subtle structural changes. However, researchers have differed on whether the sensitivity increases or not. To investigate the sensitivity of the sensors, the acoustic emission source used can be either artificial or natural. Artificial acoustic emission sources such as the pencil lead breaking and transducer methods were investigated for their precision and accuracy. To increase the versatility of the transducer method, repositionable double-sided and thin double-sided tapes were assessed as removable and repositionable couplants. The PLB and transducer methods were found to have different frequency responses on both the metal and the composite. The results also indicate that the transducer must be coupled permanently or at least not repositioned for best performance. The artificial acoustic emission sources, PLB and transducer methods were used to investigate the sensitivity of the fully embedded AE sensor. A test was set up with pre-determined AE initiation locations (surface and sub-surface) and pre- determined receiving sensor’s location (surface and sub-surface) to ensure any sensitivity increase was evident. The results indicated that the sensitivity of the sensors was influenced by the frequency of the artificial acoustic emission source. The sensitivity was then analysed using natural acoustic emission sources by inducing failure using mechanical loading. Two mechanical testing methods are used: three-point bend and double cantilever beam (DCB) mode I. Three AE sensors were tested. The results suggested that the sensitivity is more related to the crack’s frequency than the crack initiation point.