The loops of modulus defect and internal friction for ''loading-unloading'' in the saturation fatigue stage of iron and steels

Stiffness and ultrasonic attenuation measurements are frequently used for determining the fatigue damage of materials. This work is concerned with pure iron and Fe-C alloys cycled in push-pull mode at room temperature with a plastic strain amplitude between 10-7 and 8.10-4. A new method of analysis of dislocation structure transformation was suggested for fatigue tests, based on the study of ''loading-unloading'' loops of defect modulus (DM) and internal friction (IF). The main relationship of changes in the shape and size of loops in dependence on number of cycles was found. The results are discussed in the light of ''fresh'' dislocations appearing during strain cycling and their transformation to the dislocation network and pile-ups. (orig.)