Fatigue life estimation of double lap simple bolted and hybrid (bolted/bonded) joints using several multiaxial fatigue criteria

In this investigation, the effects of torque tightening on the fatigue life of double lap simple bolted and hybrid (bolted/bonded) joints have been studied via experimental and multi-axial fatigue analysis. To do so, two kinds of joints, i.e. simple double lap and hybrid joints were studied. For each kind of the joints, three sets of specimens were prepared and subjected to tightening torques of 1, 2.5 and 5 Nm and then fatigue tests were carried out at different cyclic longitudinal load levels. A nonlinear finite element ANSYS code was used to obtain stress and strain distribution in the joint plates due to torque tightening of the bolt and longitudinal applied loads. Fatigue lives of the specimens were predicted with six different multi-axial fatigue criteria by means of local stress and strain distribution obtained from the finite element analysis. Experimental tests revealed that the hybrid joints have higher fatigue life in comparison with the simple bolted joints. In addition, multi-axial fatigue analysis and experimental results revealed that the fatigue life of specimens was improved by increasing the clamping force resulting from the compressive stresses created around the hole. Furthermore, the investigation revealed the positive role of clamping force resulting from torque tightening on the fatigue life of both simple and hybrid joints.