Development of a photomicroscope method for in situ damage monitoring under ultrasonic fatigue test

Purpose Mechanical issues related to the information and growth of small cracks are considered to play a major role in very high cycle fatigue (VHCF) for metallic materials. Further efforts on better understanding in early stage of a crack are beneficial to estimating and preventing catastrophic damage for a long period service. Design/methodology/approach Dependent on the ultrasonic loading system, a novel method of in situ photomicroscope is established to study the crack behaviors in VHCF regime. Findings This in situ photomicroscope method provides advantages in combination with fatigue damage monitoring at high magnification, a large number of cycles, and efficiency. Visional investigation with attached image proceeding code proves that the method has high resolution on both size and time, which permits reliable accuracy on small crack growth rate. It is observed that the crack propagation trends slower in the overall small crack stage down to the level of 10-11 m/cycle. Strain analysis relays on a real-time recording which is applied by using digital image correlation. Infrared camera recording indicates the method is also suitable for thermodynamic study while growth of damage. Originality/value Benefiting from this method, it is more convenient and efficient to study the short crack propagation in VHCF regime.