Assessing the Validity of Original and Modified Failure Criteria to Predict the Forming Limit of Unwelded and Tailor Welded Blanks with Longitudinal Weld

Abstract

The main objective of the work is to assess the validity of modified failure criteria proposed by Naik et al. [1] and original failure criteria in predicting the forming limit of unwelded and tailor welded blanks (TWBs) with longitudinal weld, during stretching and drawing operations. The four different failure criteria, namely, effective strain rate-based, major strain rate-based, thickness strain rate-based, and thickness gradient-based failure criteria, both in original and modified forms, are used to predict the forming limit strains of unwelded and welded blanks. In the case of unwelded sheet, the forming limit predictions are consistent with the experimental result in the drawing side, but considerable difference is observed in the stretching side of forming limit diagram (FLD). In the case of tailor welded blanks (TWB), whenever failure is seen near the weld region, the strain rate-based criteria are modified as RC₁ ≥ 25, RC₂ ≥ 32, RC₃ ≥ 32 for failure to occur. TWB forming limit curve (FLC) predicted using the modified criteria show better accuracy than the original failure criteria in the stretching side of the FLD. In the drawing side, FLC predicted by original criteria is the same as that from modified criteria, where in strain path change is witnessed instead of limit strain improvement. Through this work, it is demonstrated that using modified failure criteria, forming limit of TWBs can be predicted with better accuracy during stretching operation, while original failure criteria are sufficient for a drawing operation or in the drawing side of FLD

Keywords:

• Failure
• Prediction
• Steel
• Stretching
• Weld

ACKNOWLEDGMENTS

R. Ganesh Narayanan would like to thank Prof. K. Narasimhan, IIT Bombay, India for permitting us to utilize the experimental facilities. The Department of Science and Technology, India is also acknowledged for funding a project through which the computational facilities were developed.

Notes

σ _YS  = yield strength; UTS = ultimate tensile strength; ϵ _u  = uniform elongation; ϵ _t  = total elongation; n = strain hardening exponent; K = strength coefficient; R = plastic strain ratio; R = 1.72 (normal anisotropy); ΔR = 0.034 (planar anisotropy); sheet thickness = 1.2 mm.

R = 1.26 (normal anisotropy) and ΔR = 0.36 (planar anisotropy); sheet thickness = 0.97.

*Approximate values, as it is difficult to obtain accurate values from less σ_weld − ϵ_weld data pair; weld width = 1 mm.