Extending Tensile Curves beyond Uniform Elongation Using Digital Image Correlation: Capability Analysis

A uniaxial stress-strain curve obtained from a conventional tensile test is only valid up to the point of uniform elongation, beyond which a diffuse neck begins to develop, followed by localized necking and eventual fracture. However Finite Element Analysis for sheet metal forming requires an effective stress-strain curve that extends well beyond the diffuse necking point. Such an extension is usually accomplished by analytical curve fitting and extrapolation. Recent advancement in Digital Image Correlation (DIC) techniques allows direct measurement of full-range stress-strain curves by continuously analyzing the deformation within the diffuse neck zone until the material ruptures. However the stress-strain curve obtained this way is still approximate in nature. Its accuracy depends on the specimen size, the gage size for analysis, and the material response itself. An extensive numerical study is presented in this paper to evaluate the capability of extending tensile curves based on DIC measurement data. A numerical model for uniaxial tensile test is established and used to simulate the experimental tests. The results are used as the input for DIC analysis. A wide range of conditions are considered, and the “full-range stress-strain curve” obtained from the DIC analysis is compared with the actual curve in the numerical model to assess the measurement error. It is found that, within 5% error range, DIC analysis can provide sufficient stress-strain curve extension for FEA analysis. The study also concludes that narrower test specimens give longer-range extensions of the tensile curve and closer to the actual material response.