Internal fatigue crack monitoring during ultrasonic fatigue test using temperature measurements and tomography

Very high cycle fatigue fracture is often associated with internal crack propagation and one major problem to study the initiation and the propagation of this internal crack is to detect its initiation and quantify its propagation rate. The objective of the present work is to develop an experimental methodology to follow the initiation and propagation of the internal crack. The experimental technique presented here is based on temperature field measurement on the specimen surface during an ultrasonic fatigue test using infrared thermography. Indeed, plasticity in the reverse cyclic plastic zone near the crack front generates heat sources due to the dissipation of plastic energy rate and thus an increase of the temperature which propagates through the specimen thanks to heat conduction. By considering the heat source located in the reverse cyclic plastic zone and the geometry of the crack obtained by tomographic observations, finite element simulation of the heat transfer problem enables us to establish a relationship between the internal crack growth and the temperature field evolution on the specimen surface. The results obtained for a cast aluminum alloy are presented and analyzed.