Transformation and interaction of microcracks in metal under high-energy pulsed electromagnetic field

Simulation of processes occurring in metals when they are treated with short pulses of high density electric current is of interest primarily due to studying the phenomenon of electroplasticity; the physical mechanism of which is still unknown to researchers. The effect of healing micro-defects in metals is one of existing explanations for this phenomenon. The present paper considers the processes of transformation and interaction related to flat microcracks with linear sizes of about 10 microns when processing metal samples with short pulses of high-density currents. The investigation is carried out numerically on basis of coupled quasi-stationary model of impact using high-energy electromagnetic field on the pre-damaged thermal elastoplastic material with defects. The model accounts for melting and evaporation of the metal and the dependence of its physical and mechanical properties on the temperature [1]. The system of equations is solved numerically by finite elements method with adaptive mesh using alternative Euler-Lagrange’s method. The calculations show that the crack welding and micro-defects healing occur under the short pulse of current. The healing occurs due to a simultaneous reduction in length, the ejection of the molten metal into the cracks and closing of micro-crack shores which leads to the fact that the shores of the crack come into contact with the jet stream; and in the end of these processes the jet’s material is completely jammed by the cracks shores [1]. This paper studies the influence of distance between the cracks and their relative position with respect to each other on deformation and healing of micro-defects; also the choice of the integration regions and conditions at its boundaries. Numerical modeling shows that it is enough to study microcracks healing by considering one representative element (or one-quarter of the axisymmetric representative element) as a region of integration by setting the electrical potential which is certain for the element without defects (when it is "unperturbed" by the presence of microcracks) on its borders that are the axes of symmetry. When the distances between the cracks exceed their lengths by 5-6 times, the healing processes will occur in the same manner regardless of the fact that we model them in the region of integration consisting of one or several representative elements. When the distance between the cracks increases, the influence of mutual arrangement of micro-cracks on the healing process is decreased. Thus, if the distances between the microcracks exceed their lengths by six times, in fact, the healing of microcracks is the same for any position of cracks compared to each other. Interaction between microcracks begins to significantly affect their healing processes when the distance between them is reduced to 5-6 lengths of microcracks. If the distance between the cracks exceeds their six lengths, the processes of microcracks healing become practically independent of the distance between the defects or the position of defects with regard to each other. Decreasing the distance between the cracks up to 1-2 of their linear sizes (taking into account their relative position changes) does not qualitatively change the described healing process of microcracks, however it results in a considerable slowing down: the ejection of a molten material in a crack is retained, but the crack reduction is significantly reduced especially in the transverse direction.