<p>The problem of sheared edge fracture in forming operations of advanced high strength steels (AHSS) is critical due to damage accumulated in the shear affected zone (SAZ) during cutting. The sheared edges of the blank are subjected to various operations such as in-plane or axisymmetric stretching, in-plane bending, and hole extrusion. Differences in imposed strain gradients across these deformation modes and their interaction with the SAZ can lead to non-unique fracture limits for a fixed punching condition. It also remains an open question whether the boundary condition effects only reflect true material behavior or arise from experimental measurement techniques and lengthscales. This study presents an experimental and numerical methodology to quantify the influence of deformation mode and lengthscale on edge fracture limits with application to a 1.4 mm thick 3rd generation 980GEN3 steel. In-plane bending, conical and flat punch hole expansion, hole and edge fracture tension tests were performed after punching with 12% clearance. Fracture limits for the in-plane tests converged when measured using digital image correlation (DIC) with the same virtual strain gauge length (VSG) while differences with respect to the out-of-plane conical hole expansion test persisted. Measures of global edge stretch were developed to map the DIC lengthscale to geometric deformation for implementation into finite-element models of the tests using shell elements. The uniaxial tensile stretching mode produced the lowest thinning limit of approximately&#xa0;7% in the simulations with a boundary condition effect emerging across tests despite experimental convergence. These findings highlight the strong dependence of edge fracture limits on both the deformation mode and measurement lengthscales in experiments and simulations and provide guidance for selecting appropriate VSGs and shell mesh sizes for virtual stamping process design.</p>

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On the influence of boundary condition on sheared edge fracture limits: an experimental and numerical study with application to 980GEN3 steel

  • Advaith Narayanan,
  • Rhys Northcote,
  • Patrick Cleary,
  • Cliff Butcher

摘要

The problem of sheared edge fracture in forming operations of advanced high strength steels (AHSS) is critical due to damage accumulated in the shear affected zone (SAZ) during cutting. The sheared edges of the blank are subjected to various operations such as in-plane or axisymmetric stretching, in-plane bending, and hole extrusion. Differences in imposed strain gradients across these deformation modes and their interaction with the SAZ can lead to non-unique fracture limits for a fixed punching condition. It also remains an open question whether the boundary condition effects only reflect true material behavior or arise from experimental measurement techniques and lengthscales. This study presents an experimental and numerical methodology to quantify the influence of deformation mode and lengthscale on edge fracture limits with application to a 1.4 mm thick 3rd generation 980GEN3 steel. In-plane bending, conical and flat punch hole expansion, hole and edge fracture tension tests were performed after punching with 12% clearance. Fracture limits for the in-plane tests converged when measured using digital image correlation (DIC) with the same virtual strain gauge length (VSG) while differences with respect to the out-of-plane conical hole expansion test persisted. Measures of global edge stretch were developed to map the DIC lengthscale to geometric deformation for implementation into finite-element models of the tests using shell elements. The uniaxial tensile stretching mode produced the lowest thinning limit of approximately 7% in the simulations with a boundary condition effect emerging across tests despite experimental convergence. These findings highlight the strong dependence of edge fracture limits on both the deformation mode and measurement lengthscales in experiments and simulations and provide guidance for selecting appropriate VSGs and shell mesh sizes for virtual stamping process design.