Characterization and Modeling of the Performance of Advanced Alloys for the Transportation Industry -- Bridging the Data Gap II: Evaluating the Performance of Advanced Lightweight Alloys I
Sponsored by: MS&T Organization
Program Organizers: Mark Stoudt, National Institute of Standards and Technology; Adam Creuziger, National Institute of Standards and Technology; John Carsley, General Motors Research & Development; Michael Miles, BYU; Kip Findley, Colorado School of Mines
Wednesday 8:00 AM
October 19, 2011
Room: D131
Location: Greater Columbus Convention Center
Session Chair: John Carsley, General Motors R&D
8:00 AM Invited
The Shear Fracture of Dual-Phase Steels: Robert Wagoner1; Ji Hoon Kim2; Ji Hyun Sung3; 1Ohio State University; 2Korea Institute of Materials Science; 3Korea Institute of Industrial Technology
The wide-spread adoption of dual-phase (DP) steels in the automotive industry has been limited by unexpected “shear fracture” in regions of high curvature during sheet forming. Fundamental advances in understanding this phenomenon led to the conclusion that shear fracture occurs by temperature increases in sharp bending regions related to the high ductility and strength of these alloys. Microstructural details and brittle fracture are not usually a determining factor, but can be important in rare cases. The following specific advances leading to this conclusion will be outlined: 1) An accurate temperature-sensitive constitutive model (“H/V Model”) was developed and verified; 2) New draw-bend tests reproducing industrial conditions for formability and springback were devised and used extensively; 3) Thermo-mechanical simulations of the draw-bend tests using the H/V Model were implemented in Abaqus Standard, with predicted formabilities comparing well with experiments.
8:40 AM
The Characterization of Unloading after Plastic Deformation: Chester Van Tyne1; Bernard Levy2; 1Colorado School of Mines; 2BS Levy Consultants
One of the key factors in modeling springback after sheet metal forming is the "elastic" unloading. There have been a number of papers in the literature that report that the elastic modulus or Young's modulus decreases after the sheet has undergone some plastic deformation. This interpretation of a lower modulus upon unloading is incorrect. Because of the plastic deformation there is a relaxation process involving some of the dislocations causing the unloading line to have a slope that is smaller than the elastic modulus of the material. Having the correct understanding of the underlying mechanism is important in designing appropriate lab experiments, in interpreting the experimental results, and in developing appropriate models for analyzing springback.
9:00 AM
Modeling the Deformation Texture in Transformation-Induced Plasticity (TRIP) Steels under Multiaxial Loads: Lin Hu1; Adam Creuziger2; Thomas Gnaeupel-Herold2; Anthony Rollett1; 1Carnegie Mellon University; 2National Institute of Standards and Technology
The stress-induced transformation of retained austenite in TRIP steels under multiaxial loading was modeled using a modified visco-plastic self-consistent (VPSC) plasticity model. The model uses an available work criterion to predict the austenite to martensite phase transformation. A twinned martensite structure was assumed. The orientation relationship between the austenite and martensite were calculated with the crystallographic theory of martensite. The comparison between the simulated texture and the experiment showed a good qualitative prediction of the transformation texture. The evolution of austenite volume fraction with the strain was also compared to the experiment. This work was supported by NIST.
9:20 AM
Time-Dependent Springback of Advanced High Strength Steels: Hojun Lim1; Myoung-Gyu Lee2; Ji Hyun Sung3; Ji Hoon Kim4; Robert Wagoner1; 1The Ohio State University; 2Pohang University of Science and Technology; 3Korea Institute of Industrial Technology; 4Korea Institute of Materials Science
Advanced high strength steels (AHSS) are now widely used in automotive industries for their impressive combinations of strength and formability. Strength/Modulus ratios of typical AHSS are larger than for autobody steels of the 1990’s by a factor of 2-3 and are comparable to aluminum. Draw-bend springback tests were performed for four AHSS (DP 600, DP 800, DP 980, and TRIP 780) to investigate time-dependent springback, which has been reported for aluminum alloys. In contrast to conventional steel alloys (AKDQ, DQSK and HSLA), which do not exhibit time-dependent springback, all tested AHSS showed a linear increase of the springback angle with log time for the first few days to weeks. The total time-dependent shape change for AHSS was approximately half of that observed for aluminum alloys under similar conditions. Finite element simulations of time-dependent springback based on creep modeling were in good agreement with the experiments.
9:40 AM Break
10:00 AM Invited
Measuring Evolving Multiaxial Yield Surfaces - Is There a Best Way to Go at It?: Tim Foecke1; 1NIST
The design of automotive components formed from sheet metal has generally occurred in strain space. For mild steel, this has worked fairly well, but moving to advanced materials has exposed shortcomings. Modelers have grappled with a fundamental problem with this approach - strain is not an intrinsic state variable. The safe region of the forming limit diagram can move dramatically depending on the strain path, and allowing for evolution of the constitutive relation for the material during the forming process is very difficult. In the past 10 years, there has been a gradual push to move toward a stress based forming methodology, where failure is determined by a single intrinsic material failure stress. However, moving from strain to stress space requires detailed knowledge of how the yield surface in stress space evolves with plastic strain along a random path, and in particular how kinematic and isotropic hardening combine to produce these changes. There are several methods being used currently to generate this data, and they will be discussed and contrasted in detail. The ultimate question is whether any of these methods can be understood sufficiently to be standardized and accepted into general use as a design tool for the automotive industry.
10:40 AM
Yield Surface Evolution in HSLA Steel: Adam Creuziger1; Mark Iadicola1; Thomas Gnaeupel-Herold1; 1National Institute of Standards and Technology
Experimental measurements of yield surface evolution in high strength low alloy (HSLA) steel and 1008 mild steel under multi-axial linear strain paths is the focus of this presentation. New materials for automotive lightweighting need to be rapidly integrated into vehicle design. This rapid integration requires accurate constitutive relations for accurate FEA modeling. Unfortunately most constitutive relationships are based on uniaxial tensile tests and extrapolated to multi-axial strain states. By simultaneous measurement of the plastic strain fields using digital image correlation (DIC) and elastic strains from x-ray diffraction (XRD), the stresses under balanced biaxial, plane strain and uniaxial strain states have been measured for HSLA and 1008 steels.
11:00 AM
Instrumented Hole Expansion Testing of Advanced High Strength Steels: Christian Krempaszky1; Andreas Mackensen2; Hartmut Hoffmann2; Ewald Werner3; Erich Spindler4; 1CD Laboratory of Material Mechanics of High Performance Alloys, TU-Munich; 2Institute of Metal Forming and Casting; 3Institute of Materials Science and Mechanics of Materials, TU München; 4TU-München
An instrumented hole expansion device was designed and constructed in order to enhance the standard hole expansion testing method. A camera system enables the exact detection of the first through thickness crack occurring in the hole edge during the expansion which is the determining factor in the description of material formability. Additionally an optical 3D deformation analysis system was integrated for recording the development of the deformation field of the specimens free surface. Based on this information a detailed evaluation of the materials response gets feasible.This is shown on the hand of a study including four different AHSS grades, each providing a unique material design concept. Additionally, a single phase low carbon steel was investigated for comparison. To investigate the influence of the hole processing method on the stretch-flangeability which strongly on the hole edge condition, two different hole processing methods were applied: hole punching and wire cutting.
11:20 AM
Pneumatic Stretching through Open Die Inserts for Constructing the FLDs of Lightweight Alloys at Elevated Temperatures: Fadi Abu-Farha1; Louis Hector2; John Curtin1; Nickolas Snyder1; 1Penn State Erie; 2General Motors R&D Center
As uniaxial tensile testing fails to closely represent the actual loading case in sheet metal forming operations, evaluating limiting strains has been largely based on forming limit curves and diagrams, primarily produced by the Marciniak and the Nakajima methods. However, the accuracy of these methods is subject to uncertainties in the actual deformation state and friction conditions; they are also impractical at elevated temperatures, which is of prime importance in forming several lightweight alloys. In this work, a comprehensive methodology for evaluating elevated temperature limiting strains of metallic sheets by pneumatic stretching is presented. The approach is used to establish the forming limit diagrams for the 5083 aluminium, AZ31 magnesium and the Ti6Al4V titanium alloy, at conditions that best resemble actual Quick Plastic Forming (QPF) and Superplastic Forming (SPF) operations. The results show accurate tracks of the formability curves, with clear distinction between the safe and unsafe deformation zones.