Acoustic emission method to study fracture (Mode-I, II) and residual strength characteristics in composite-to-metal and metal-to-metal adhesively bonded joints

Abstract

Failure behaviour of two types of adhesively bonded joints (composite-to-metal, metal-to-metal) has been studied under failure modes (Mode I: double cantilever beam (DCB) and Mode II: three-point end notch flexures (3-ENF)) using acoustic emission (AE) technique. The bonded specimens were prepared using two types of adhesive bond materials with three variations of adhesive bond quality. The effect of the presence of interfacial defects along the interface on the residual strength of the joint has also been studied. It was possible using the maximum AE amplitude method to select the AE events of mechanical significance. However, it proved difficult to propose a definitive AE trait for the mechanical phenomena occurring within specific AE event signals, for all adhesive types, bond qualities, and substrate configurations; therefore, all specimen combinations. There was a notable shift in spectral energy proportion as the AE source of mechanical significance varied along the specimen length for specimen combinations. However, it was difficult to confirm this distinctive trait for all specimen combinations due to difficulty in confirming the location and exact mechanical source. The proposed measurement technique can be useful to assess the overall structural health of a bonded system and may allow identification of defects.

Analysis of Acoustic Emission Propagation in Metal-to-Metal Adhesively Bonded Joints

“…The use of a second sensor also allowed for source location of each hit for further validation of the failure mechanisms. While this method appears successful in the small specimens tested (25.4 mm square bond area), where the source-sensor distance experiences minimal variation, the results presented by Droubi et al [6] indicate that changes in source-sensor separation may lead to changes in spectral content, and thus in the failure-mechanism recognised. For application of this method to larger bond areas the effects of propagation distance may need to be accounted for to ensure reliability of results.…”

“…Therefore, a commercial mechanical pencil with an in-house machined guide-ring was used to generate simulated AE sources by breaking a 0.5 mm diameter and 2-3 mm length 2H pencil lead, as recommended by ASTM standards (E976-99) [17]. A Hsu-Nielsen source provides a signal with a broad frequency spectrum, with significant content in the bandwidths previously demonstrated to be associated with adhesive failure [3,6,11]. This, combined with its good repeatability, makes it an appropriate choice of source for these experiments.…”

“…Droubi et al [6,7]; Senthil et al [10]) have recognised the correspondence between the initiation of AE activity and changes in the load-displacement curves resulting from the onset of adhesive failure, during mechanical testing of various bonded specimens. Droubi et al [6] investigated the mode I and II failure of bonded metalto-metal and metal-to-composite specimens through use of AE instrumented double cantilever beam and three-point end-notch-flexure tests. Both a ductile and a brittle adhesive were investigated with varying levels of bond quality, introduced by use of Polytetrafluoroethylene (PTFE) spray to reduce the effective bond area.…”

“…It is capable of detecting multiple damage mechanisms (e.g. adhesive failure/interfacial debonding [3,6,9], cohesive failure/adhesive cracking [9,10], adherend matrix cracking [11], fibre breakage [3,11], sandwich core failure [11]) and is not limited by a minimum detectable defect size.…”

“…One technique which avoids this issue is acoustic emission. Acoustic emission (AE) has proven promising for condition monitoring of large structures and has been shown to be effective in detecting failure of adhesive joints [3][4][5][6][7][8][9]. AE is a passive technique based on the detection of transient elastic waves released by the sudden redistribution of stress resulting from processes such as crack initiation and growth.…”