In most industrial applications, adhesive joints experience impact fatigue loads in service. The energy of each impact is too low to cause joint failure. Although the repetitive impacts usually do not apparently affect the joints, they can significantly reduce the strength of adhesively bonded structures. Accordingly, understanding the effect of impact fatigue on the residual strength of the bonded components is crucial in real applications. This study deals with this issue where the effect of impact fatigue on the residual static strength of single lap joints is analyzed. To achieve this, the manufactured single lap joints were categorized into four different groups. Group 1 joints were tested under static loading conditions. Joints in group 2 were tested under impact to obtain the impact strength of the single lap joints. To analyze the impact fatigue life of the joints, the single lap joints in group 3 were tested under cyclic impact at different energy levels until failure. To investigate the effect of impact fatigue on the residual static strength of the joints, single lap joints in group 4 were tested under a specific number of impact cycles followed by a static tensile test. Using microscopic analysis, the fracture surfaces of the tested specimens were analyzed. The results showed that cracks initiate from the middle of the bonded area as a result of cyclic impact stress waves. Then, by increasing the number of impacts, a large number of cracks nucleate from the edges of the joints and grow along the width to the middle of the overlap. A 3D finite element method was employed to analyze the stress distribution along the bondline under impact loads.

One of the most common loading conditions that bonded joints experience in service is repeated impact. Despite the destructive effects of impact fatigue, the behavior of metal-composite bonded joints subjected to repeated impact loads has rarely been studied in the literature. Therefore, it is of utmost importance to pay attention to this phenomenon on the one hand and to find solutions to improve the impact fatigue life of bonded composite metal components on the other hand. Accordingly, in this study, the use of the bi-adhesive technique is proposed to improve the durability of composite-metal single-lap joints (SLJs) under impact fatigue loading conditions. J-N (energy-life) method is also used to analyze the experimental data obtained. Accordingly, in the present study, the impact fatigue behavior of single adhesive metal to composite joints was analyzed experimentally based on the J-N method and also numerically using the finite element method (FEM). By using two adhesives along a single overlap, the impact fatigue life of joints between dissimilar composite and metal joints was also analyzed experimentally. The results show that the double adhesives technique can significantly improve the impact fatigue life of the tested joints. It was also found that the optimum length ratio of the adhesives (the length covered by the ductile adhesive relative to the total overlap size) is a function of the stiffness of the joint and is more pronounced for less stiff bonded joints. A linear elastic numerical analysis was also conducted to evaluate the stress state along the bloodline of the bonded joints. Results show that the compressive peel stress made at the boundary of the two adhesives can be a possible reason behind the different results observed.

One of the most efficient methods of improving the strength of adhesive bonds is the bi-adhesive technique. The main aim of the current study is to investigate the fatigue life of bi-adhesive joints under cyclic impact loads based on the J-N (impact energy vs. fatigue life) methodology. To achieve this, single lap joints were subjected to cyclic low energy impacts. The J-N curves were constructed using the experimental data considering Basquin's equation.According to the results, the durability of the joints against impact fatigue loads can be improved significantly (up to 17 times higher for the tested joints) using the bi-adhesive technique. The results also showed that the impact fatigue life is very sensitive to the length ratio of the ductile to the brittle adhesives in bi-adhesive joints. It was found that the total absorbed energy of the joints increases significantly by the reduction of the impact energy.

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