Evaluation of Press Work on Sheet Metal

Significance of the topic

The evaluation of press workability for lightweight sheet materials such as high-strength steel, aluminum and magnesium alloys is critical for reducing vehicle weight and improving fuel efficiency. Accurate forming simulations guided by reliable material data can greatly decrease die development cycles, lower costs and shorten time-to-market.

Objectives and overview

This study reviews a suite of testing methods required to generate precise input parameters for press-forming simulations. It covers:

• r-value measurement as an indicator of thickness change during drawing
• Assessment of elastic modulus variation with plastic strain
• Quantification of the Bauschinger effect under reversing loads
• Yield surface characterization via biaxial tensile testing

The goal is to demonstrate how integrating these evaluations improves the predictive accuracy of forming analyses.

Methodology and instrumentation

Key test procedures include:

• Uniaxial tensile tests with automatic and non-contact extensometry to determine r-value
• Cyclic tensile-bending sequences to capture elastic modulus evolution
• Tensile-compression reversing tests using a comb-tooth jig to measure Bauschinger behavior
• Biaxial tensile tests on small cruciform specimens compliant with ISO 16842 to map yield surfaces under multi-axial stress

Used Instrumentation

The main equipment and fixtures used are:

• Shimadzu Autograph AG-X plus universal testing machine
• Automatic contact extensometer SIE-560/560S
• Non-contact optical extensometer TRViewX
• TRAPEZIUM X control and analysis software
• Bauschinger effect measurement jig with anti-buckling guide
• Biaxial tensile test jig meeting ISO 16842

Main results and discussion

r-Value evaluation showed high reproducibility when using automated extensometer attachment and optical measurement. Real-time plots from TRAPEZIUM X revealed that the elastic modulus decreases progressively with increasing plastic strain, a phenomenon directly tied to springback predictions. Bauschinger tests with the specialized jig produced true stress–strain loops demonstrating yield stress reduction after prior bending. Biaxial testing at stress ratios of 0:1, 1:1, 1:1.5 and 1:2 generated distinct yield loci, indicating that multi-axial stress states must be accounted for to avoid inaccurate forming predictions.

Benefits and practical application of the method

Integrating these advanced evaluations into forming simulations offers:

• Enhanced accuracy of springback and defect forecasts
• Reduced need for iterative die trials
• Ability to characterize new alloys before plant trials
• Improved die design safety margins and material utilization

Future trends and possibilities for use

Emerging directions include:

• Incorporation of sophisticated constitutive models such as the Yoshida-Uemori formulation
• Digital twins and machine learning to further optimize forming predictions
• High-throughput multi-axial testers for rapid material screening
• In-line sensor integration for real-time process feedback

Conclusion

Comprehensive experimental characterization of r-value, plastic strain-dependent elasticity, Bauschinger effects and biaxial yield surfaces delivers essential data for high-fidelity press-forming simulations. The use of specialized fixtures and advanced control software enables efficient data acquisition, reducing die development time and improving production quality.

Reference

1) Vol. 54 (2013) No. 4 SOKEIZAI 16–19