Finite element study on the static and fatigue behaviour of wide single lap bonded joints with semi-circular defect

The use of adhesively bonded joints is increasing in aeronautical industrial applications due to its high strength-to-weight ratio and excellent stress transfer. Failure of composite bonded joints under fatigue loading occurs when a crack in the joint grows to a length that significantly reduces joint strength. However, it is difficult to accurately predict the crack growth rate in adhesive joints when subjected to cyclic loading due to the presence of defects in the adhesive. Several types of defect could occur in adhesive bonds, which could be generated by inadequate joint preparation during the fabrication procedure, accidental impact in service, etc., all of which reduce the joint quality and influence the joint strength. It is therefore important to understand the joint behaviour under cyclic loads when these defects are present in a structural joint.
Published research has conducted analytical and experimental works on evaluating the influence of defects on the joint strength. However, few researchers have studied joints with initial strip defects under fatigue cyclic loads. It was observed that fabrication defects have a significant effect on the fatigue life uncertainty and average fatigue life while having a relatively small effect on the static strength. Besides strip defects, void-like defects also commonly arise from accidental impact or lack of adhesive. The lack of appropriate methods for analysing defects with complex geometry under fatigue cyclic loads has motivated this study.
This paper presents an experimental and numerical study of static and fatigue behaviour of a wide single lap joint (SLJ) configuration (e.g. 50 mm wide) with a pre-designed semi-circular defect in the adhesive layer, centred along the width direction. Static tests were performed with laser extensometer to measure extensions and the joint strength was reduced approximately 20% by comparing to an equivalent specimen without any pre-existing defect. Fatigue tests were also conducted on the joint specimens and crack lengths were observed from the four runout corners. Non-destructive inspection techniques, e.g. ultrasonic C-scan and X-ray, were used to examine the defect geometry and position inside the material during the initial fatigue cycles. A novel in-situ crack growth measurement technique has been developed using red ink injection through a needle nozzle through the interface over the lap in order to identify the crack propagation profile during tensile-tensile fatigue loading. The experimental observations showed that an initial semi-circular defect in a wide joint will initially propagate towards the two free edges to develop a through-width defect. The crack growth was modelled under quasi-static conditions in Virtual Crack Closure Technique (VCCT) in finite element analysis. The combination of mode I and mode II was taken into account using two mixed-mode failure criteria, the maximum and equivalent strain energy release rate (SERR). The crack growth law was determined experimentally using double cantilever beam (DCB), end notch flexure (ENF) and mixed mode bending (MMB) specimens bonded with the same adhesive material as the wide SLJs. The numerical models based on the crack propagation in the adherend-adhesive interface and crack growth data from mixed mode tests, allowed the fatigue crack growth rate (FCGR) of the wide SLJ, (as observed during fatigue tests) to be predicted with good accuracy.
The main innovations of the proposed work over the previously mentioned studies are related to the novel initial defect configuration design and the monitored ingredient damage growth pattern, which will enable a better understanding and assessment of fatigue life prediction inside single lap joints with defects.