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Technical Paper

Effect of Gap on AHSS RSW Weldability

2009-04-20
2009-01-0030
When a gap exists in a resistance spot welded (RSW) joint, the lack of intimate contact between the members at the faying interface can have a significant influence on sheet steel spot weldability, especially for advanced high strength steels. Several test joints simulating gap conditions observed in typical auto body structures have been designed for study of the effect of gap magnitude on the resistance spot weldability of such AHSS as DP980, DP780 and DP600. Tests show that an existing gap reduces the confinement of the molten zone during the welding process, increasing the probability of expulsion and therefore decreasing the current range. Increasing the electrode force helps reduce the gap influence and broadens the current range. Employing a longer weld time also has a beneficial effect when used in combination with an appropriate electrode force.
Technical Paper

A Novel Approach for Generating a Full-Range Tensile Stress-Strain Curve

2009-04-20
2009-01-0470
A new method has been developed to measure full stress-strain curves using Digital Image Correlation (DIC) for Advanced High Strength Steels (AHSS). With the post-necking strain measured by the built in-house DIC system during tensile tests, stress-strain data for AHSS beyond uniform elongation up to fracture can be determined. In this paper, the technique to generate full stress-strain curves by DIC is introduced. The measured stress-strain curves are compared with those obtained by extrapolation methods. The measured stress-strain data generated by the new method is validated by finite element analysis (FEA).
Journal Article

Experiences with Experimental Determination of the Yield Locus and its Evolution for Advanced High Strength Steels

2010-04-12
2010-01-0976
Accurate description of the plastic yield locus is important for accurate prediction of sheet metal formability and springback using FEM. This paper presents experimental results obtained for the initial plastic yield locus and its evolution for some selected Advanced High Strength Steels (AHSS). A review of available experimental methods was conducted to select appropriate techniques for testing. For loading in tension-shear, the Arcan test was selected, however because of lack of uniformity of the stress distribution, the test was not included in the final series of tests. Shear testing, uniaxial tensile testing, plane strain testing and stacked compression testing were used to determine the yield locus. From the test results and analysis for the selected AHSS, it seems that the onset of initial yielding and its isotropic evolution to 4% plastic strain is best described by the von Mises yield function.
Journal Article

Advanced Material Characterizations and Constitutive Modeling for AHSS Springback Predictions

2010-04-12
2010-01-0980
Springback prediction is one of the roadblocks for using advanced high strength steel in the automotive industry. Accurate characterization and modeling of the mechanical behavior of AHSS is recognized as one of the critical factors for successful prediction of springback. Conventional tensile test based material characterization and constitutive modeling may lead to poor springback simulation accuracy. Aiming to accurately predict springback, a series of advanced material characterizations including bi-axial material testing, large-strain loading path reversal testing, unloading tests at large strain, stress-strain behavior beyond uniform elongation, were performed for selected AHSS and associated constitutive models were developed to incorporate these characterizations. Validations through lab samples and industrial parts show that the AHSS springback prediction accuracy is significantly improved with these improved material models.
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