Measurement of Three-Dimensional Concentric and Angular Misalignment in Static and Fatigue Testing of Materials by Stereo-Digital Image Correlation

Background For the mechanical testing of materials, the electromechanical or hydraulic testing facility should be periodically aligned. The ASTM standard E1012 prescribes some procedures for aligning the test facility, after which alignment is taken for granted during the subsequent mechanical testing of the actual specimen. Objective The goal of this paper is to develop methods based on stereo-Digital Image Correlation (stereo-DIC) to quantitatively measure the misalignment during static and fatigue testing. Methods Two methods, a measurement device and a novel linear decomposition approach were implemented in this study, both relying on stereo-DIC data. The measurement device can be mounted on two sides of the tested specimen and moves together with the corresponding specimen ends as a rigid body. The rigid translation and rotation can be extracted by rigid body transformation from the stereo-DIC measurement, without any numerical model linked to it. Linear decomposition is based on minimizing the discrepancy in displacement between simulation and experiment. By assuming linear elastic material behavior and small deformation, linear decomposition will result in a group of linear equations with variables quantifying misalignment in boundary conditions. Results Both the measurement device and linear decomposition have been successfully utilized in high-cycle fatigue testing. Although both the measurement device and linear decomposition gave close values in the three-dimensional concentric and angular misalignment, the numerical simulation with misaligned boundary conditions given by linear decomposition reproduced more accurate displacement fields on the gauge section of the specimen than the measurement device. Conclusions The reason why the measurement device has relatively lower precision than linear decomposition approach is due to the shallow depth of field around the measurement device in stereo-DIC. The measurement device can be applied in broad testing conditions following the same procedure while linear decomposition is limited to cases where linear superposition holds in identifying the misaligned boundary conditions, meaning that geometric and material nonlinearity are excluded.