Acoustic Emission Testing of Polymer Composites

Acoustic Emission (AE) enables real-time monitoring of defects in polymer composites. The inherent anisotropy of composites makes signal interpretation complex. The case study reveals that failure modes of polymer composite laminates correlate with ply structure and multiple failure modes can coexist. This case study of a carbon fibre reinforced polymer composite laminate illustrates the relationship between failure mechanisms and the fabrication method of the laminate.

Microstructural changes in any material, due to mechanical stresses or conditions such as corrosion or phase-change will release elastic stress or pressure waves (acoustic waves). Acoustic Emission (AE) monitoring and testing utilizes highly sensitive transducers that convert these acoustic waves to corresponding electrical signals. The frequencies of interest are typically in the ultrasound range between 20 KHz and 1 MHz. Embedded AE sensors enable micro-level, real-time monitoring of initiation and progress of damage in materials. A typical AE monitoring or test system would comprise signal detection, data acquisition, processing, and analysis units. While piezoelectric sensors are commonly used, other types of sensors include accelerometers, magnetostrictive type transducers and fibre-optic sensors.

AE testing is performed by applying a loading stimulus and monitoring progressive damage by identifying the locations, time, intensity and rate of change of AE emission sources in the sample. When AE testing is performed on anisotropic materials such as polymer composites, correlating signals to sources is challenging, since the acoustic signals do not propagate uniformly in all directions.

Polymer composites are growing in importance in infrastructure and transportation applications. AE monitoring and testing is invaluable in these cases, for real-time monitoring of mechanical integrity and quality control. Failure mechanisms in polymer composites that can be detected by AE techniques include matrix cracking, fibre break, fibre/matrix interface debonding and delamination. Each failure mode has a characteristic AE frequency range, based on which the failure mode can be analysed.

This case study of a carbon fibre reinforced polymer composite laminate illustrates the relationship between failure mechanisms and the fabrication method of the laminate. It also demonstrates the interaction between different modes of failure. The study was conducted on carbon/epoxy resin laminated composites using four different fibre/matrix layups and manufactured using compression moulding and curing. Two types of specified defects, namely a crack and a hole, were introduced into test samples. Twenty specimens of 250mm × 36mm × 4 mm size, were subjected to tensile and three-point bending tests and the spectrogram and dispersion curves from AE signals were analysed. Figure 1 shows the set-up and testing equipment. The results indicate that carbon fibre parallel to loading direction improves tensile strength, while ±45° ply is vulnerable to delamination. Further, the failure modes correlate with ply structure and multiple failure modes can co-exist.

Figure 1. (a) Sketch of tensile test set-up done on an Instron 1342 test bed and Micro-SHM AE sensors. (b) Photo of tensile test equipment shows damping rubber to reduce background signals from structure. (c) Sketch of three-point bending test set-up. (d) Photo of bending test equipment.

References:

Xinye Liu, Xinyue Yao, Jinhui Cai, Jiusun Zeng, and Wingkong Chiu, Failure Mode Analysis of Carbon Fiber Composite Laminates by Acoustic Emission Signals, Advances in Materials Science and Engineering Volume 2021, Article ID 6611868