Experimental study of strain fields during shearing of medium and high-strength steel sheet

Abstract

Background: There is a shortage of experimentally determined strains during sheet metal shearing. These kinds of data are a requisite to validate shearing models and to simulate the shearing process. Methods: In this work, strain fields were continuously measured during shearing of a medium and a high strength steel sheet, using digital image correlation. Preliminary studies based on finite element simulations, suggested that the effective surface strains are a good approximation of the bulk strains below the surface. The experiments were performed in a symmetric set-up with large stiffness and stable tool clearances, using various combinations of tool clearance and clamping configuration. Due to large deformations, strains were measured from images captured in a series of steps from shearing start to final fracture. Both the Cauchy and Hencky strain measures were considered, but the difference between these were found negligible with the number of increments used (about 20 to 50). Force-displacement curves were also determined for the various experimental conditions. Results: The measured strain fields displayed a thin band of large strain between the tool edges. Shearing with two clamps resulted in a symmetric strain band whereas there was an extended area with large strains around the tool at the unclamped side when shearing with one clamp. Furthermore, one or two cracks were visible on most of the samples close to the tool edges well before final fracture. Conclusions: The fracture strain was larger for the medium strength material compared with the high-strength material and increased with increasing clearance.

Strain and stress conditions at crack initiation during shearing of medium- and high-strength steel sheet

“…(2)). Furthermore, effective strain and tool forces versus tool displacement were presented for comparison with corresponding experimental data presented by Gustafsson et al (2016b). An agreement between these simulations and experiments (effective strain and force vs. displacement) should be seen as a validation of the FE model.…”

“…For this purpose, the xy-surface of the sheet samples was prepared with a random speckle pattern for subsequent digital image correlation. Further details on the experiments are described by Gustafsson et al (2016b).…”

“…where D p ij is the plastic component of the rate of deformation tensor, which can be compared with the effective strains measured by Gustafsson et al (2016b), since the elastic part of the latter is negligible.…”

“…The yield stress for the sheet materials was described with the exponential hardening law suggested by Hollomon (1945), σ Y = Kε n p , whereε p is the effective plastic strain and K and n are material specific parameters. These parameters were fitted to compression test data provided by Gustafsson et al (2016b) and are shown in Table 1 for the sheared material grades.…”

“…Strains were measured during planar blanking by Stegeman et al (1999) and in experiments with a symmetric shearing set-up by Gustafsson et al (2016b). The purpose of the present work was to extend these experiments with stress calculations using the method described by Marth et al (2016), to determine the strain and stress conditions at crack initiation.…”