Mixed-mode fracture characteristics of metal-to-metal adhesively bonded joints: experimental and simulation methods

Fracture behavior of adhesively bonded joints subjected to mixed-mode (i.e. mode I+II) loading conditions is of importance in many industrial applications. This research therefore aims to characterise the failure behaviour of metal-to-metal (i.e. both aluminium adherends) adhesive joints using the mixed mode bending test (MMB), adapted from ASTM D6671/D6671M standard, along with instrumentation using acoustic emission (AE) sensor. Twenty-four adhesively bonded specimens were prepared using two types of adhesive bond materials (acrylic, cyanoacrylate) with two different bonded area 65% and 100%. To understand the effect of mixed-mode loading conditions on the failure behavior, two different mixity ratios were achieved through the design of the MMB test fixture and tested for each bonded joint. The AE results during mechanical testing shows that the time domain signals were spread over the loading phase with distinct features for different mixity ratios. They successfully identified the moment of adhesive fracture during every test. Also, the fracture behavior of the bonded joints was simulated using virtual crack closure technique (VCCT) method using finite element method to understand the loading dynamics in specimen when considering a combination of various design parameters. In addition, an analytical method (e.g. corrected beam theory or CBT) was used to determine strain energy release rates of each specimen. The results show that both the brittle and ductile specimens exhibited higher energy release rates when mode II proportion of loading was increased during the crack initiation phase. The proposed measurement can be useful to assess the overall structural health of bonded systems.