Acoustic emission (AE) technology is widely applied for monitoring the damage of materials and structures. In this work, tensile tests were carried out to investigate the shear properties of adhesively bonded single-lap composite joints with adhesive defects. AE technology was used to evaluate the damage progress in real time. Besides, linear AE source location was also performed to study the damage initiation and propagation. Results show that the shear strength of adhesively bonded single-lap composite joints reduces as a result of the defects in the adhesive layers. Adhesion failure is found to be the dominant failure mode and most of the damage initiates and propagates at the edge of the adhesive layers and aggregates around the defects in the adhesive layer. Furthermore, the AE relative energy, amplitude distribution, cumulative hits and duration time are related to the damage process. The linear AE source location results are in consistent with the initiation and propagation of shear damage. For the adhesive joints with defects, AE signals with the amplitude from 60 to 80 dB increase at the evolution stage. It is suggested that the dynamic characteristics of AE signals can be effectively used to describe the damage mechanisms of adhesive bonded structure for composites.

Accurate characterization of delamination damage and evolution plays a significant role in studying the failure behaviors of composite laminates. In the present research, both acoustic emission and digital image correlation technology are used to simultaneously monitor the buckling process of multi-delaminated composites under compression. Three kinds of composite specimens are carried out to investigate the influence of delamination lengths and positions on the compressive behaviors of the composites. Meanwhile, the buckling load, micro-displacement fields of interfacial zone and acoustic emission response characterizations are also obtained. The results indicate that acoustic emission parameters such as hits, amplitude, duration and relative energy are correlated with the damage process of composite specimens, while the critical damage deformation of delamination regions is clearly exhibited from digital image correlation results. Furthermore, the buckling behaviors are significantly influenced by the thickness of sub-layer and the lengths and positions of delaminations. The complementary nondestructive testing technologies combining acoustic emission with digital image correlation are beneficial for in situ damage monitoring of the composites.

Realizing the accurate characterization for the dynamic damage process is a great challenge. Here we carry out testing simultaneously for dynamic monitoring and acoustic emission (AE) statistical analysis towards fiber composites under mode-II delamination damage. The load curve, AE relative energy, amplitude distribution, and amplitude spectrum are obtained and the delamination damage mechanism of the composites is investigated by the microscopic observation of a fractured specimen. The results show that the micro-damage accumulation around the crack tip region has a great effect on the evolutionary process of delamination. AE characteristics and amplitude spectrum represent the damage and the physical mechanism originating from the hierarchical microstructure. Our finding provides a novel and feasible strategy to simultaneously evaluate the dynamic response and micro-damage mechanism for fiber composites.

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