Regularities of plastic deformation development in the crack tip vicinity

Abstract

When determining the service life of cyclically loaded metal structures at the stage of fatigue crack development, an important role is played by the accuracy of assessing the plastic deformation zone size at the crack tip formed by the tensile overload. Within this zone, after reducing the load to the initial level, the fatigue crack development slows down. Despite the large number of studies in this area, none of the proposed dependencies is currently fully confirmed by experimental results in determining the plasticity zone size. The aim of the work is to assess the size of the plastic deformation zone formed by a single tensile overload. In the work of the finite element method for central-notch specimens and compact tension specimens, an analysis of the stress-strain state in the crack top vicinity is performed. As a calculation, a flat and three-dimensional model was used. Calculations are performed in the Ansys 14.0. Data on the mechanical properties of steels were used: St20, VSt3sp, 09G2S, 15G2SF. With increasing load, the plastic zone size in front of the crack tip was estimated, in the process of reduction, the distribution and length of the fields of residual compressive stresses and the size of the cyclic plastic zone were recorded, the length of which was also determined experimentally using strain gauges with a base of 0.5 mm. To assess the plastic zone size formed by a single tensile overload, a dependence is proposed that takes into account the patterns of plastic deformation development in the vicinity of the fatigue crack tip, and the results of the calculation agree well with the experimental data given in the literature. The obtained expression makes it possible to predict with greater reliability the development of fatigue cracks at variable load amplitudes.