Novel Non-Contact Evaluation of Strain Distribution Using Digital Image Correlation with Laser Speckle Pattern of Low Carbon Steel Sheet

An uni-axial tensile test is one of the general methods to obtain plastic properties for estimating deformation behavior of materials in the simulation of practical metal forming process. Non-contact measurement is generally used because of determination of accurate plastic properties excepting for contact effects. In the conventional method, strain distribution of specimen sprayed with random pattern is measured by digital image correlation (DIC). However, sprayed treatments on the specimen surface is unsuitable under various testing conditions, for example an elevated temperature, micro scale, and large deformation. Thus, non-sprayed specimen is desirable under such test conditions. In this study, we propose perfectly non-contact strain distribution measurement by using DIC with laser speckle pattern generated on specimen surface by irradiation of laser source. True strain in the un-axial tensile test is measured by following methods (1) laser speckle pattern by DIC, and (2) conventional measurement method with tracking gage marks. As a result, strain level measured by DIC with laser speckle pattern has good agreement with that of conventional extensometer. Furthermore, the strain distribution can be measured successfully under uniform and non-uniform deformation during necking by proposed method. Thus, we can verify the validity of proposed measurement method for determination of plastic properties.