Influence of Boundary Conditions on the Sheared Edge Fracture Limits of a 3rd Generation Advanced High Strength Steel.

Abstract

A fundamental trade-off between strength and ductility exists in advanced high strength steels (AHSS), particularly for sheared edge splitting in automotive forming operations. The widely used ISO16630 conical hole expansion test for edge stretchability is known to be a poor representation of the in-plane deformation modes that are the primary source of edge splitting in stamping, leading to an overestimation of formability in virtual tryouts. Additionally, virtual experiments rely upon the input of a single fracture strain value to predict edge cracking in stamped parts, disregarding the effects of deformation mode and element size. An efficient and reliable modeling approach for edge failure is required without having to simulate the shear cutting process. The present work addresses some of these challenges through four interrelated tasks aimed at developing guidelines to efficiently characterize the anisotropic plasticity behavior and edge fracture limits, to support reliable experimental assessment and finite-element modelling of sheared edge fracture in practical forming applications.

There is a need to develop efficient strategies for anisotropic plasticity characterization of sheet materials to be able to accurately simulate the various tensile edge stretching modes ranging from splitting without necking to potential localization before fracture. To this end, the baseline plasticity characterization of four approximately pressure-independent aluminum alloys (AA) and steels with varying ductilities and anisotropy levels: AA5182-O, AA7075-T6, DC04, and 980GEN3 steels were performed using uniaxial tensile tests in multiple orientations. Using digital image correlation (DIC), the area strain at the neck center was monitored to measure the flow stress response to strain levels more than twice the uniform elongation, with the added advantage of probing anisotropic hardening effects. A hybrid inverse analysis procedure was further developed and applied to notch tensile tests to obtain the major stress under plane strain tension while constraining the minor-to-major principal stress ratio to remain near 1:2. Anisotropic yield functions were subsequently calibrated using data from a range of stress states with emphasis on plane strain tension. The calibrated yield functions and hardening responses were shown to accurately reproduce both the local and global behavior in flat punch hole expansion tests, which activate a wide range of tensile-dominated stress states. Flat punch hole expansion simulations using yield functions calibrated without plane strain data consistently deviated from the DIC in-plane strain magnitudes with absolute differences of up to 15% for DC04 steel. The proposed methods provide general guidelines for efficient calibration of anisotropic constitutive models for approximately pressure-independent materials that are accurate to large deformation levels.

Next, the mechanics of the conical hole expansion test were examined to assess the role of necking and anisotropy and to develop methodologies for fracture strain estimation. Finite-element (FE) models of the test were created in LS-DYNA software for two AHSS grades with differing plastic strain anisotropies using hexahedral solid elements. An analysis of through-thickness stress and strain gradients from the numerical models revealed that localization is suppressed until a hole expansion ratio of 200%, with the outer hole edge exhibiting a proportional uniaxial tensile stress state. Any non-uniformity in hole shape or thickness around the circumference of the extruded hole was found to be a manifestation of the tensile plastic strain anisotropy distribution and not necking. The hole expansion ratio was found to be suboptimal for quantifying edge stretchability since the inner hole edge undergoes a non-linear strain path transitioning from compression to uniaxial tension. Furthermore, when using the HER as a fracture metric, the local outer hole edge element strains from FE simulations were underpredicted with absolute differences of up to 10%. An analytical technique was proposed to obtain the local major fracture strain from conical hole expansion using the outer hole diameter measured at the crack location, with the equivalent failure strain then obtained using plastic work equivalence. The strains obtained using the proposed method were in excellent agreement with the elemental strains from numerical models with a maximum difference of 4% for the highly anisotropic CP800, confirming its suitability for fracture strain measurement from the test.

Subsequently, a novel four-point fixture and specimen geometry that promotes failure under the deformation mode of in-plane bending was developed to characterize the uniaxial fracture limits of moderate ductility materials. The in-plane bending mode is also representative of edge splitting at peripheral regions of stamped parts. Techniques to detect the onset of fracture and accurately measure the edge strains from the in-plane bend tests were proposed that is applicable to a wide range of material ductilities. The uniaxial fracture strain measured in the in-plane bend test conducted with a machined edge was found to agree closely with the conical hole expansion true fracture strain of 0.68 for a 3rd generation 980GEN3 advanced high strength steel. The in-plane bend test also showed promise for plastic strain anisotropy characterization under uniaxial tension and compression to strain levels much larger than the material uniform elongation. A gauge height-to-thickness ratio of 4.0 or lower is recommended as a specimen design guideline to mitigate buckling based on a comprehensive experimental study conducted on multiple materials and thicknesses.

Finally, the influence of loading conditions on the sheared edge fracture limits of 980GEN3 steel punched with a 5.0 mm hole and 12% clearance was investigated using five different test methods that imposed different stress and strain gradients in the vicinity of the sheared edge. A convergent fracture strain value of approximately 0.30 was observed across the in-plane edge fracture tests, with the conical hole expansion test exhibiting a higher strain of 0.45 due to out-of-plane deformation and fracture being defined at through-thickness cracking. Differences in fracture strains between the in-plane tests were also magnified by the choice of DIC lengthscale or virtual strain gauge length, reflecting each test’s varying sensitivity to DIC strain averaging. Global stretchability metrics were proposed for each deformation mode, enabling edge crack assessment in industrial applications without the need for DIC. The global edge stretch metrics were also found to inform the appropriate choice of DIC lengthscale for design and FE modelling. Finally, FE simulations of the edge fracture tests were conducted using multiple mesh sizes, revealing that a boundary condition dependence can also manifest in simulations with the added influence of lengthscale sensitivity. The predicted major strains at the experimental fracture instant varied with mesh size, suggesting that a single strain value may be insufficient to describe edge fracture. The elemental thinning strain showed reduced dependence on mesh size, making it a more reliable parameter for assessment of edge fracture in simulations. Importantly, the simulations indicated that the edge fracture strain cannot be represented by a unique value but is rather a function of the imposed loading condition. The in-plane stretching mode exhibited the lowest engineering thinning strain limit of 8.8%, making it the critical deformation mode for edge crack initiation in 980GEN3 steel.

A key outcome of this work is the quantitative understanding of the effect of boundary conditions and lengthscale on the edge fracture limits. Prediction of sheared edge fracture must account for both the imposed loading and numerical lengthscale, with thinning strain offering a more robust metric for use in simulations. The developed methodologies provide practical and efficient guidelines that can be implemented in industrial environments for edge crack assessment and prediction in stamping simulations.