Effect of preload force on heat generation of fatigue crack in ultrasonic infrared thermography

The heat generation behaviors of fatigue crack are deeply investigated under different preload forces combing numerical simulation and experiment. Firstly, a multi-contact simulation model is applied to stimulate the crack surfaces contact and the horn-sample contact under ultrasonic excitation for calculating the temperature fields. Then, the ultrasonic infrared thermography testing and the microscope testing are carried out for the heat generation and the plastic deformation behaviors of crack region under different preload forces. On this basis, an indirect observation method based on dots distribution is proposed to estimate the plastic deformation on crack contact surfaces. The obtained results show that the temperature rise of crack region increases with the increase of preload force when the preload force is less than 250 N, while the temperature rise rapidly declines due to the plastic deformation on crack contact surfaces and the inhibition effect when the preload force is 280 N. Moreover, the plastic deformation does not lead to the crack propagation, but reduces the detection repeatability of fatigue crack. This work provides an effective method for optimizing testing conditions in practical testing processes, which will be helpful to the establishment of testing standards for batches of test objects in ultrasonic infrared thermography testing.