Determination of chip speed and shear strain rate in primary shear zone using digital image correlation (DIC) in linear-orthogonal cutting experiments

Strategies to investigate the primary shear zone in metal cutting are largely based on chip root examinations. This type of analysis is limited to providing only indirect results, because it is a post-process measurement. It is significantly influenced by the metallographic preparation of the chip root as well as the analysis method of the micrographs. Some applications require precise determination of the load conditions in the workpiece material during chip formation. Therefore, it appears promising to use experimental procedures that allow in situ measurement during chip formation to obtain strain, strain rate and chip speed data directly out of the deformation zones. This study exhibits a newly developed experimental test setup that is based on optical measurements using a coupled system comprising of a microscope and a high-speed camera in conjunction with a laser light source. The system allows the capturing of high-resolution pictures with a frame rate of more than 100 kHz based on a double-frame technique. The quality of the pictures is suitable to be examined by the digital image correlation (DIC) approach. Typically, double-frame approaches lack of a suitable amount of picture pairs at higher cutting speeds because the light sources need to recover after each image pair. This leads to a time delay and limits the amount of data in dependence of the specimen length and cutting speed. The test setup in this work performs an orthogonal cut with stationary tool. Together with a specimen length of 750 mm and cutting speeds up to 240 m/min this setup allows the acquisition of up to 18 image pairs with a resolution of 2560 x 1600 pixel in one cutting test. In the present study this method is used to determine the chip speed and the maximum shear strain rate of AISI 1045 steel at 18 different cutting conditions, varying the undeformed chip thickness, the rake angle and the cutting speed. The results are discussed qualitatively and quantitatively to evaluate the method in comparison to alternative measurement methods.