Characterization and Modeling of Sheared Edge Failure in Advanced High Strength Steel

Edge failure is one of the major problems associated with forming of advanced high strength steels (AHSS) such as dual-phase (DP) steels. The development of ferritic-bainitic steels such as complex-phase (CP) steels have improved the performance of AHSS in industrial forming operations and is gaining attention in academia as well as industry. As a result, there is an interest in developing numerical techniques to predict sheared edge failure in forming simulations and optimize forming operations in the automotive industry for vehicle lightweighing. The primary objective of this thesis is to examine the influence of shearing on edge stretchability and damage evolution in two different grades of AHSS: CP and DP steels and develop damage-based models to predict sheared edge failure. The stretch-flangeability of DP and CP steels were evaluated using a hole expansion test for different edge conditions to isolate to the influence of a range of factors thought to influence edge formability. The results demonstrate that work hardening and void damage at the sheared edge govern formability while the sheared surface quality plays a minor or secondary role. A comparison of the edge stretching limits of DP and CP steels demonstrates the advantages of a ferritic-bainitic microstructure for forming operations with severe local deformation as in a stretch-flanging operation. The failure mechanisms in the CP and DP steels were systematically characterized by interrupting hole tension tests at different strain levels. Scanning electron microscope (SEM) analysis conducted on interrupted hole tension specimens revealed the ductile failure mechanism as being operative in the CP and DP steels for the different edge conditions and microstructures. Damage histories were developed from the interrupted samples using optical microscopy and quantitative stereology measurement of void nucleation, growth and coalescence, paired with in situ digital image correlation (DIC) strain measurements during the mechanical testing. The trend of damage evolution differs for the sheared edge in contrast with the reamed edge because the shearing process alters the microstructure in the shear affected zone (SAZ) by introducing work-hardening and damage behind the sheared edge. Two independent experimental techniques were applied to characterize the residual strain distribution within the shear-affected zone for CP800 and DP780 steels based on (a) the tendency of grains to orient in the direction of shearing and (b) work-hardening introduced within the deformed shear zone. The first technique was developed by applying finite strain theory to calculate the equivalent strain from microstructural measurements of grain rotation. The second strain measurement technique also involved using the same interrupted shear tests and DIC strains followed by microhardness measurements to develop a correlation between the equivalent strain and hardness. These techniques were applied to estimate the strain-distribution behind the sheared edge generated during the shearing process. The influence of stress-state on micro-void nucleation was evaluated experimentally for the CP and DP steels and a stress-state dependent nucleation model was developed. Stress state was varied by considering four specimen geometries: the equi-biaxial Nakazima test, a plane strain v-bend test, a central hole tension test for uniaxial loading and a simple shear test. 3D micro-tomography and quantitative stereology measurement of void nucleation paired with in situ digital image correlation (DIC) strain measurement was conducted on the interrupted samples to quantify damage as a function of equivalent strain. The influence of stress-state on damage evolution was observed for both materials with very little void nucleation under shear deformation but extensive void damage under biaxial tension. Of particular interest, Lode parameter-dependency of void nucleation was identified and a stress-state dependent nucleation model is proposed by introducing a nucleation strain surface as a function of stress-triaxiality and Lode parameter using a modified form of Chu and Needleman nucleation criterion. The critical damage parameters controlling the ductile failure process were identified from the void histories determined using 3D tomography to develop a micromechanics-based fracture model. An uncoupled anisotropic damage-based fracture model was formulated within an LS-DYNA user-defined material subroutine. The pre-strain and damage introduced during the shearing process were mapped onto finite element models of edge formability. The proposed model was validated for the hole tension experiments and found to predict failure efficiently and accurately for the CP800 and DP780 alloys with a reamed or sheared edge conditions

Damage Evolution in Complex-Phase and Dual-Phase Steels during Edge Stretching

The role of microstructural damage in controlling the edge stretchability of Complex-Phase (CP) and Dual-Phase (DP) steels was evaluated using hole tension experiments. The experiments considered a tensile specimen with a hole at the center of specimen that is either sheared (sheared edge condition) or drilled and then reamed (reamed edge condition). The damage mechanism and accumulation in the CP and DP steels were systematically characterized by interrupting the hole tension tests at different strain levels using scanning electron microscope (SEM) analysis and optical microscopy. Martensite cracking and decohesion of ferrite-martensite interfaces are the dominant nucleation mechanisms in the DP780. The primary source of void nucleation in the CP800 is nucleation at TiN particles, with secondary void formation at martensite/bainite interfaces near the failure strain. The rate of damage evolution is considerably higher for the sheared edge in contrast with the reamed edge since the shearing process alters the microstructure in the shear affected zone (SAZ) by introducing work-hardening and initial damage behind the sheared edge. The CP microstructures were shown to be less prone to shear-induced damage than the DP materials resulting in much higher sheared edge formability. Microstructural damage in the CP and DP steels was characterized to understand the interaction between microstructure, damage evolution and edge formability during edge stretching. An analytical model for void evolution and coalescence was developed and applied to predict the damage rate in these rather diverse microstructures.Natural Sciences and Engineering Research Council of Canada (NSERC) AUTO21 Network of Centres of Excellence Canada Research Chairs Secretaria

Experimental Techniques for Finite Shear Strain Measurement within Two Advanced High Strength Steels

This Preprint article has been submitted for consideration.There is a growing need to experimentally characterize local shear deformation in advanced high strength steels (AHSS) for the calibration of stress-state dependent fracture criteria and to better understand sheared edge cracking during secondary forming operations. Planar shear test specimens with digital image correlation (DIC) strain measurement are now commonly performed tests but may not be able to resolve the local strains during the final stage of fracture when the macroscopic shear band collapses to a micro-shear band with intense local strains. Studies of sheared edge stretching of AHSS have shown that the microstructure at the sheared edge experiences extreme local shear deformation with the shear-affected zone (SAZ) that can be much larger than the macroscopic strains reported using DIC on planar shear tests. In this work, two independent experimental techniques are proposed to characterize the residual strain distribution within the shear-affected zone for two AHSS grades with a similar strength level: a complex-phase (CP) steel, CP800, and a dual-phase (DP) steel, DP780. The first method uses finite strain theory to calculate the work-conjugate equivalent strain from grain rotations within the shear bands of interrupted in-plane shear tests. A comparison between the local DIC strain measurements and the grain rotation measurements were found to be in excellent agreement until just prior to failure. The second technique used micro-hardness measurements taken from the interrupted shear tests to develop correlations with the measured equivalent strain from the DIC system. The hardness and grain rotation techniques were then used to characterize the local strain distribution within the SAZ of hole expansion test specimens for punch clearances of 12% and 28%. Both methods provided similar strain distributions with the grain rotation method having the highest strain resolution. The residual strain field within the SAZ of both AHSS was found to be strongly dependent upon the punch clearance. Finally, a homogenization procedure was applied to the SAZ strain distributions to facilitate the length scale transition from the grain-level to length scales appropriate for finite-element modelling of sheet metal forming operations with sheared edges.Natural Sciences and Engineering Research Council of Canada (NSERC) AUTO21 Network of Centres of Excellence Canada Research Chairs Secretaria

Failure parameter identification and validation for a dual-phase 780 steel sheet

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.ijsolstr.2017.06.018 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/A hybrid experimental-numerical procedure was implemented to determine the failure surface of a dual-phase 780 steel sheet as a function of the effective plastic strain, triaxiality, and Lode parameter using butterfly specimens with in situ digital image correlation strain measurement and supporting finite element calculations. A butterfly-type test specimen was employed to experimentally obtain stress states ranging from simple shear to plane strain tension including mixed tensile and shear loading. The numerically-derived failure surface was implemented into the phenomenological GISSMO damage model in the commercial finite element code LS-DYNA and the accuracy of the failure surface was determined using finite element predictions of the characterization experiments. A series of independent validation experiments related to sheet metal forming were performed including a hole tension test, a conical and flat punch hole expansion test, and a hemispherical punch test. The finite element models utilizing the damage model were able to accurately reproduce the load–displacement and surface strains of the sheet material for both the characterization and validation experiments. Prediction of the failure orientation and location compared favorably to each of the validation tests

Experimental Stress State-Dependent Void Nucleation Behaviour for Two 800 MPa Advanced High Strength Steels

The influence of microstructure and stress state, as defined by the stress triaxiality and Lode parameter, on micro-void nucleation was evaluated experimentally for two 800 MPa Advanced High Strength Steels (AHSS), one a Complex-Phase CP800 alloy, with a ferritic-bainitic microstructure, and the other a Dual-Phase DP780 ferritic-martensitic steel. Four plane stress specimen geometries (simple shear, hole tension, v-bend and biaxial Nakazima) were adopted, providing stress triaxiality and Lode parameter values ranging from in-plane shear to biaxial tension under approximately constant stress states until failure. This approach facilitated determination of the relationship between void nucleation and macroscopic stress state. Damage histories were developed from interrupted samples using 3D micro-tomography and quantitative stereology measurement of void nucleation paired with in situ digital image correlation (DIC) strain measurements during the mechanical testing. The trends in damage evolution are strongly linked to the stress state, with very little void nucleation under shear deformation but extensive void damage under biaxial tension for both materials. A dependency of the nucleation rate on Lode parameter was also demonstrated. A higher rate of damage accumulation was observed for the DP780 steel compared to damage in the CP800 steel for all loading conditions highlighting the strong influence of initial microstructure. An analytical framework is proposed to obtain the local stress-state and equivalent plastic strain history from direction integration of the measured DIC strain histories, using a measured hardening law and assumed anisotropic yield function (Yld91) to develop the link between nucleation and the macroscopic stress state. A stress-state dependent nucleation model is proposed by introducing a nucleation strain surface as a function of stress-triaxiality and Lode parameter using a modified form of the strain-based Chu and Needleman nucleation criterion.Natural Sciences and Engineering Research Council of Canada (NSERC) AUTO21 Network of Centers of Excellence Ontario Research Fund Canada Research Chairs Secretaria

Micromechanical Modelling of Edge Failure in 800 MPa Advanced High Strength Steels

Sheared edge failure is one of the major problems associated with the forming of advanced high strength steels (AHSS) such as dual-phase (DP) steels. To improve the performance of AHSS in industrial forming operations, ferritic-bainitic complex-phase (CP) steels have been developed and are gaining attention in academia as well as industry. The present work aims to investigate the influence of microstructure on micro-void (damage) evolution during the edge stretching of CP800 and DP780 steels and develop a micromechanics-based fracture model to predict edge failure for both reamed and sheared holes. Three-dimensional damage histories were obtained using x-ray microtomography on a series of hole tension specimens interrupted at different strain levels. These experiments considered a tensile specimen with a sheared hole at the center of specimen (sheared edge condition) or reamed (ideal edge condition). Void damage measurements, such as void area fraction, number of voids, void diameter, and void aspect ratio, were conducted and the results are compared to the reamed specimens to isolate the sheared edge effect on damage. The void measurements were used to implement a stress-state dependent model for nucleation, and coupled with the Ragab (2004) model for void growth and the Benzerga and Leblond (2014) model for void coalescence to develop a damage-based material model. Higher damage accumulation was observed behind the sheared edge compared to the reamed edge at a given strain for both materials considered. An uncoupled anisotropic damage-based fracture model was formulated in a LS-DYNA user-defined material subroutine. As an alternative to computationally expensive multi-stage sheared edge stretching simulations, the measured strain-distribution of the sheared edge was mapped into a finite-element model to predict sheared edge failure. The proposed model was validated for the hole tension testing simulations and found to predict failure accurately for both the CP800 and DP780 for both the edge conditions.Natural Sciences and Engineering Research Council of Canada (NSERC) AUTO21 Network of Centers of Excellence Ontario Research Fund Canada Research Chairs Secretaria