Relationship between characteristics of local deformed state and damage parameters of structural steels 20 and 45 under static loading

Tensile tests were performed on the samples with stress concentrators made of structural steels of 20 and 45 grades, with an analysis of the local deformed state using the digital image correlation method and damage assessment by optical microscopy. Strain dependences were plotted for the areas of the plastic deformation zone S Y , the values of the principal strain ε 1 , and the damage parameter S * , defined as the relative area of microcracks in the image. The authors analyzed the stage-wise fracture of the studied specimens and proposed a metho­dology for calculating the total damage S tot in the plastic zone, based on the experimentally established relationship between damage and value of the principal strain. It was established that under loading, microcrack initiation in steel 45 occurs at an earlier fracture stage compared to steel 20. At the same time, at the final loading stage, the growth rate of the total microcrack area S tot in steel 20 specimen is higher than in steel 45 sample, which is reflected in the exponent of the strain dependence of S tot , which is almost twice as high as the corresponding parameter for steel 45 ( m = 5.5 and 2.97, respectively). The observed differences in damage kinetics are associated with the strain-hardening coefficient and structure of the studied steels. The approach applied in this study can be used for structural integrity diagnostics, as it allows evaluating of material damage over a larger surface area of the structural element compared to optical microscopy, which has limitations on the size of the investigation area and imposes high requirements on quality of the studied surface preparation.