Metal Coercivity as a Measure of Microdamages for Evaluation of Fatigue and Recovery of Mechanical Properties

Introduction. The problem of determining the current state of metal parts mechanical properties is always relevant. For its solution a simple, cheap and sufficiently informative NDT method is needed. Today the method of magnetic coercive force seems to be the most suitable. The purpose of this work was to determine patterns of change in a coercive force after cyclic loading of steel samples in the initial state and after surface plastic deformation (work hardening). Method. Two batches of 40X steel cylindrical samples in the delivery condition were put to cyclic tests. The both batches were tested in the same range of tension amplitudes but for the second batch the tests were carried out in two steps with intermediate surface plastic deformation. The change in the level of accumulated fatigue damage was determined by the coercive force method. Results. The most significant coercive force increase (up to 1.5%) was observed in the initial stages of the specimens’ elongation. For samples subjected to preliminary cyclic loading, surface plastic deformation leads to lower coercive force values (by 9%–17%) and to 2.8–3.4 times longer operating life. This effect can be explained by “healing” the accumulated defects by the surface treatment. Discussion. The results indicate high sensitivity of the coercive force to changes in structural state of the metal as a result of both fatigue damage accumulation and partial recovery of the metal structure due to defect “healing”.