A modified digital phase-shift moiré technique for impact deformation measurements

Abstract

Due to the short loading times and high deformation rates inherent to impact experiments, measurement of the occurring deformations is not straightforward. Presented in this paper is a technique to obtain the displacements and deformations of a specimen subjected to a uni-axial impact load. During the experiment the deformation of a line grating attached to the specimen is captured using a streak camera. From the recorded deforming grating the specimen displacements are automatically derived using an advanced numerical algorithm, based on the interference between the specimen grating and a virtual reference grating. Numerical interference is considered because it allows that the pitch of the reference grating is adapted to the changing amplitude of the deformation. Indeed, at each moment of the deformation process the pitch of the reference grating is chosen such that the highest possible accuracy and sensitivity is guaranteed. Because of this, large changes in deformation amplitude are allowed, and the technique is applicable to a wide range of materials. Eventual imperfections of the specimen grating and temperature effects are taken into account. Specimen displacements are extracted automatically by means of a phase-shifting technique. The non-contact measurement technique yields high resolution, quantitative information on the specimen deformation, along the entire length of the specimen and during the full duration of the experiment. Interaction by the operator is excluded. Results are presented of a high strain rate tensile test on a steel sheet specimen showing local deformations up to about 170%.