A method of the direct measurement of the true stress-strain curve over a large strain range using multi-camera digital image correlation

A quick method to directly determine the true stress-strain curve over a large strain range is proposed. The true stress-strain curve includes the post-necking strain, is directly measured via a special multi-camera digital image correlation (DIC) system. A tension test with the standard dog-bone shape specimens is used in the system. The cross-section area of the specimen throughout the test is measured by the DIC, as well as the entire field true strain. The true stress is calculated based on the measured cross-section area and the load. A dual-phase steel with a minimum 780 MPa tensile strength (DP780), as a typical advanced high-strength steel (AHSS), is used in the test to demonstrate the method and to validate the result. The true stress-strain curve including the post-necking data of DP780 is presented. The test result is validated with the theoretical calculation as well as the experimental method. Furthermore, the measured true stress-strain curve is approximated by the power law equation to identify the material constants. The methodology, experimental test, and the evaluated result are described in detail.