Infrared Thermographic Assessment of Fatigue Behavior in Bolted Composite Joints

Abstract

This study investigates the temperature evolution of bolted thick glass fiber reinforced polymer composites during fatigue testing. Three different adherend thicknesses (8, 10, and 12 mm) and three different bolt tightening torques (30, 40, and 50 N $·$m were considered to evaluate the effects of adherend thickness and bolt tightening torque on fatigue life. Fatigue tests were conducted at stress levels corresponding to 80%, 70%, 60%, and 40% of the ultimate tensile strength of the samples. The temperature evolution during fatigue testing was recorded using an infrared thermal camera. The results indicate that increasing the bolt tightening torque enhances both the ultimate tensile strength and fatigue life of all samples across all load levels. A dramatic increase in the measured maximum temperature was observed in the final phase of the tests, accompanied by a significant reduction in sample stiffness. The maximum temperature measured in the specimens was attributed not only to friction between the joined materials but also to the propagation of matrix microcracks, with elevated temperature levels predominantly observed around the bolt hole regions. Overall, the results demonstrate that the fatigue lives of composite joints can be predicted using thermal imaging techniques.