Coercimetric Control in the Assessment of the Structure Metal Damage Under Mechanical Loading

Experimental results are presented for paramagnetic and ferromagnetic steels’ coercive force (Hc) response to static and cyclic loads. The physical nature of its response to mechanical loads differed essentially for both steels. The application of nondestructive coercimetric control to assess the damage of structures from paramagnetic and ferromagnetic steels under mechanical loading was experimentally substantiated. For paramagnetic steels, the damage accumulation stages were determined. An increase in the coercive force on loading is associated with the change in the ratio of ferromagnetic and paramagnetic phases of the metal, corresponding to elastic and elastoplastic deformation (crack initiation), and its decrease is related to the loss of metal uniformity induced by pores or cracks (crack extension). From the change in the coercive force kinetics on loading, the yield limit of the metal could be established, the endurance limit evaluation procedures were greatly simplified, and the irreversible damaging line was constructed (after French). The coercimetric method also effected with a single device determining the direction of principal stresses, detecting the surface and subsurface cracks, and estimating damage level by stages, regardless of origin. The change in coercive force under static and cyclic deformation stems from the metal domain structure ordering (from random to ordered) for ferromagnetic steels. The coercive force of ferromagnetic steels is growing on their loading, and with unloading after applying stresses exceeding the yield limit of the metal, a 90° rotation of the domains is taking place, and, accordingly, maximum Hc values. Under cyclic loading over yield limit stresses, the change in the coercive force direction and values is reversible. Under elastic loading with unloading, the direction of maximum coercive force values is not changed. Such behavior of the coercive force concerning the load direction is inherent in the domain orientation response of ferromagnetic steels to those loads. The damages accumulated on loading of ferromagnetic steels should be assessed not by the changes in absolute coercive force values, but by the change in kinetics of the structural anisotropy, defined as the ratio of coercive force values in two mutually perpendicular directions. The change in the kinetics of the anisotropy factor under elastic and elastoplastic deformation governs the safe range of mechanical loading due to reversible damage processes, as well as the ranges with risks of accumulation of irreversible high and low-cycle fatigue and quasistatic damages giving rise to corresponding fracture modes. The nondestructive coercimetric method permits setting the metal endurance, yield limit, and transition stress from low-cycle fatigue to low-cycle quasistatic fracture. In the coercimetric assessment of the serviceable condition of structures from ferromagnetic and paramagnetic steels, the effect of a sharp change in the coercive force kinetics with stresses exceeding the yield limit, eliminates the need to determine the mechanical characteristics of the metal since the relative load capacity is diagnosed concerning the yield limit.