Large-deformation measurement based on reliable initial guess in digital image correlation method

In order to measure large deformation using digital image correlation method, a new initial guess technique is presented. The proposed technique obtains the reliable initial deformation guess of the first calculation point by manually selecting three or more points with distinct characteristics surrounding the reference subset center in the reference image and their corresponding locations in the deformed image. Then, reliable initial deformation guess can be determined according to the coordinate correspondence and is used as the iteration initial value of Newton-Rapshon method to further improve measurement accuracy. The proposed technique overcomes the shortcoming of the commonly used digital image correlation method which cannot work under the condition that the tested object has a rotation angle larger than seven degrees or large deformation. The measured displacement fields from image pairs with relative large rigid body rotation and large deformation compression test of polypropylene foam obviously demonstrate the validity of this technique.