On the applicability of infrared thermography for investigating the fatigue behaviour under variable amplitude loading on the example of HFMI-treated longitudinal stiffeners

Abstract Imaging techniques that capture the surface of a specimen are well suited for investigating localised phenomena, such as the initiation of fatigue damage, as well as monitoring the progress of fatigue damage. One approach is the use of an infrared camera, which allows a full-field investigation of the temperature response of a specimen surface. Due to the thermoelastic effect, the temperature response of a material subjected to cyclic loading is synchronous and proportional to the applied loading and represents a linear relationship between loading and temperature signal. Structural nonlinearities arising from fatigue damage processes, such as plasticisation of the crack tips as well as crack opening and closure, cause nonlinearities in the temperature response. Thus, those nonlinear effects have proven to be a suitable indicator for fatigue-induced damage processes. A graphical representation of the thermoelastic temperature response and the signal component resulting from dissipative effects can be used to visualise the fatigue behaviour, particularly crack initiation and crack growth. Existing approaches enable the evaluation of the fatigue behaviour of a specimen under constant amplitude loading using thermography. An application of infrared thermography (IRT) for fatigue tests respecting variable amplitude loading is still pending. Therefore, this study focuses on the adaption and extension of existing evaluation methods to variable amplitude loading scenarios, followed by a graphical evaluation of the fatigue behaviour under variable amplitude loading. The proposed evaluation methodology enables the visualisation of the fatigue behaviour under variable amplitude loading scenarios. Through subsequent graphical evaluation, both crack initiation and crack propagation can be illustrated and further analysed under variable amplitude loading conditions. The development and application of the adapted methodology were investigated using one-sided longitudinal stiffeners made of S355 with the post-weld treatment high-frequency mechanical impact treatment (HFMI).