A method for determining the strain-stress curve of larger-strain is proposed when plastic instability occurs in standard tension tests. Thin tested steel sheet is subjected to tension loading until fracture occurs. The deformation process is captured with a digital camera. Displacement and strain field of material deformation can be calculated by a mesh-free PIM method. A tensile experiment is simulated to verify that local measuring stress-strain curve by PIM method near the center of the specimen can describe a full stress-strain curve clearly. Numerical simulation results, at different location along the specimen axial, present that different parts of specimen have different deformation distribution in tensile and the center fracture part of tensile specimen is the only region which can experience full strain. The true stress- true strain curves, based on the estimated parameters, are validated in all strain regions by comparison with curves from standard tension tests. The measured curves by PIM method are very stabilization. Compared with several material constitutive equations, The Swift’s equation is very close to experiment curve at plastic deformation.

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