Clifford Butcher

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.

Evaluation of the VDA 238-100 Tight Radius Bending Test using Digital Image Correlation Strain Measurement

The VDA238-100 standard for tight radius bending (v-bending) of sheet materials has received widespread acceptance with automotive suppliers and material producers to characterize local formability. However, the test fixture and tooling in the v-bend test standard is not amenable to direct strain measurement and the operator cannot terminate the test at the onset of crack initiation as the outer bend surface is not visible. Consequently, fracture is identified using a load threshold and the bend angle estimated from an analytical formula based upon the punch displacement and tooling geometry. Bend angles are not directly transferable and must be interpreted relative to the sheet thickness and bend radius unlike a strain measurement. By obtaining an in-situ strain measurement on the surface using digital image correlation (DIC), the plane strain fracture limit can be accurately identified at the onset of cracking and remove ambiguity in translating the bend angles to practical forming operations and simulations. A novel inverted VDA test frame was developed to incorporate DIC strain measurement during the bend test and a variety of advanced high strength sheet materials were evaluated. It was observed that the VDA bend test creates a homogeneous strain state of plane strain across the width of the sample along with a proportional strain path to fracture without necking that is ideal for fracture characterization. A correlation is developed to relate the bend angle with the major strain for the materials considered and accounts for the sheet thickness and bend radius. A comparison of the bend angle obtained using the formula in the VDA standard based on the punch displacement was in very good agreement with manual measurements and an algorithm to measure the bend angle using DIC analysis was developed.

Assessment of the Critical Parameters Influencing the Edge Stretchability of Advanced High-Strength Steel Sheet

The edge formability of ferritic-martensitic DP (dual-phase) and ferritic-bainitic CP (complex-phase) steels was evaluated using a hole expansion test for different edge conditions. Hole expansion tests involving the standard conical punch as well as a custom flat punch were performed to investigate formability when the hole is expanded out-of-plane (conical punch) and in-plane using the flat punch. A range of edge conditions were considered, in order to isolate 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. A comparison of a traditional DP780 steel with a CP steel of similar strength showed that the edge stretching limit of the CP steel was three times larger than that of the DP780.

Prediction of edge failure of dual phase 780 steel subjected to hole expansion

The edge failure of a dual phase steel, DP780, under hole expansion (stretch flange) deformation conditions is examined. In order to assess the effect of damage processes without the presence of notches due to shearing, machined holes with a smooth edge condition were expanded with a conical punch. Images of the expansion as well as punch load-displacement were recorded during the tests. The GISSMO damage based failure criteria was implemented in a finite element model to predict the load-displacement of the punch and the initial crack formation. This failure criteria was calibrated using the effective plastic failure strain versus triaxiality data obtained from a set of uniaxial and notched tensile samples. The predicted punch load-displacement curve was found to be in agreement with observations including the load drop at the onset of failure. The predicted peak load of 23.4kN compared favorably to the observed peak of 24.4kN. The model predicted a hole expansion ratio of 53.4% and compared well to the experimentally observed ratio of 51.0% ± 10.0%.The edge failure of a dual phase steel, DP780, under hole expansion (stretch flange) deformation conditions is examined. In order to assess the effect of damage processes without the presence of notches due to shearing, machined holes with a smooth edge condition were expanded with a conical punch. Images of the expansion as well as punch load-displacement were recorded during the tests. The GISSMO damage based failure criteria was implemented in a finite element model to predict the load-displacement of the punch and the initial crack formation. This failure criteria was calibrated using the effective plastic failure strain versus triaxiality data obtained from a set of uniaxial and notched tensile samples. The predicted punch load-displacement curve was found to be in agreement with observations including the load drop at the onset of failure. The predicted peak load of 23.4kN compared favorably to the observed peak of 24.4kN. The model predicted a hole expansion ratio of 53.4% and compared well to the ...

Constitutive Response of AA7075-T6 Aluminum Alloy Sheet in Tensile and Shear Loading

Tensile and shear experiments were performed on AA7075-T6 sheet at strain rates ranging from quasi-static (0.001 s−1) to high (1000 s−1) in three sheet orientations (0°, 45° and 90°) with respect to the rolling direction. Digital image correlation (DIC) techniques were employed to measure the strains in the experiments. The AA7075-T6 alloy showed mild rate sensitivity over the range of strain rates tested. The level of plastic anisotropy was characterized and was shown to be rate-insensitive. The quasi-static experimental data was used to calibrate the eight-parameter Barlat YLD2000 anisotropic yield criterion to describe the anisotropic behaviour of the sheet material. The quasi-static hardening behaviour to large strains was also experimentally determined by converting the shear stress to an equivalent uniaxial stress and fit using a Hockett–Sherby model. The calibrated anisotropic yield criterion was able to capture the material anisotropy providing good agreement with the experiment data.

Dynamic and Quasi-Static Testing and Modeling of Hot Stamped Tailor-Welded Axial Crush Rails

In the current research, the use of tailor-welded blanks (TWBs) comprising Usibor® 1500-AS laser welded to more ductile Ductibor® 500-AS is considered. The TWBs were hot stamped to form top-hat cross-section channels with axially tailored properties. Axial crush rails were assembled by spot welding together two of these hot stamped channels along their flanges. The tailored rails were crush tested under dynamic (crash) and quasi-static conditions using an 855 kg crash sled facility at 10.6 m/s impact speed, and a 670 kN servo-hydraulic press at 0.5 mm/s, respectively. Non-tailored channels composed entirely of Ductibor® 500-AS were also tested for base material characterization and as a comparison to the tailored conditions. Numerical models of the crash experiments were developed. The material models include measured fracture loci using the generalized incremental stress state dependent damage model (GISSMO), with rate sensitive constitutive behavior. Spot weld failure was also considered based on tests of spot welded coupons. The accuracy of the predicted force-displacement and energy absorption response, extent of parent metal cracking, and extent of weld failure are evaluated in comparison to the experiments. The difference in response between quasi-static and dynamic testing is also evaluated.

Finite element simulation of non-isothermal warm forming of high-strength aluminum alloy sheet

This paper examines non-isothermal warm forming experiments on AA7xxx-series channel sections at two different initial blank temperatures (187 and 253°C) with room temperature tooling. Supporting non-linear thermo-mechanical finite element simulations are performed, considering multiple stages comprising heating of the blank, transfer to the press, forming, dwell in die and cooling in air to room temperature. Elevated Twist Compression Tests are performed for the lubricant Forge Ease AL278 which allows characterization of friction coefficient as a function of temperature. Based on attached thermocouples during forming, thickness measurements and recorded force-stroke curves, the simulation model is validated. Furthermore, micro hardness measurements on the formed rail are performed to study the effect of heat exposure on the material strength.

Effects of die quench forming on sheet thinning and 3-point bend testing of AA7075-T6

Lab-scaled AA7075 aluminum side impact beams were manufactured using the die quenching technique in which the sheet was solutionized and then quenched in-die during forming to a super saturated solid state. Sheet thinning measurements were taken at various locations throughout the length of the part and the effect of lubricant on surface scoring and material pick-up on the die was evaluated. The as-formed beams were subjected to a T6 aging treatment and then tested in three-point bending. Simulations were performed of the forming and mechanical testing experiments using the LS-DYNA finite element code. The thinning and mechanical response was predicted well.

Calculation and validation of heat transfer coefficient for warm forming operations

In an effort to reduce the weight of their products, the automotive industry is exploring various hot forming and warm forming technologies. One critical aspect in these technologies is understanding and quantifying the heat transfer between the blank and the tooling. The purpose of the current study is twofold. First, an experimental procedure to obtain the heat transfer coefficient (HTC) as a function of pressure for the purposes of a metal forming simulation is devised. The experimental approach was used in conjunction with finite element models to obtain HTC values as a function of die pressure. The materials that were characterized were AA5182-O and AA7075-T6. Both the heating operation and warm forming deep draw were modelled using the LS-DYNA commercial finite element code. Temperature-time measurements were obtained from both applications. The results of the finite element model showed that the experimentally derived HTC values were able to predict the temperature-time history to within a 2% of th...

Microstructural and Numerical Investigation on the Shear Response of a Rare-Earth Magnesium Alloy Sheet

Shear tests were performed on an anisotropic rare-earth magnesium alloy rolled sheet (ZEK100-O) to study the constitutive plastic behaviour of the material under quasi-static conditions at room temperature. The shear response was characterized in three orientations by setting the shear loading direction at 45°, 90°, and 135° with respect to the rolling direction. Each orientation displayed unique trends in terms of yielding and the work hardening rates. Electron Backscattered Diffraction (EBSD) analysis was used to analyze the microstructure of the deformed samples and to determine the twinning modes and twin fraction in each specimen. The results suggest that these parameters have a significant impact on the constitutive behavior of the material. Furthermore, the viscoplastic self-consistent twinning-detwinning (VPSC-TDT) model was used to understand the activity of the various deformation mechanisms and validate the experimental observations.