Accuracy of digital image correlation system for compressive behaviour of wood

Abstract

The digital image correlation (DIC) system is a powerful tool for measuring distributions of displacement and strain on the surface of a specimen. DIC systems are employed not only for homogeneous materials such as metals but also for heterogeneous materials such as wood. Although numerous validations of DIC system accuracy for metallic materials exist, accuracy verification for wood, especially under multiaxial stress conditions, is less common. This study investigated the accuracy of a DIC system equipped with a bilateral telecentric lens on wood (Douglas fir). The accuracy verification in uniaxial stress fields was conducted through full compression testing, while verification in multiaxial stress fields was performed through partial compression testing. Additionally, compression tests on A6063 (aluminium alloy) were conducted to examine the differences in DIC system accuracy between homogeneous and heterogeneous materials. The accuracy of the DIC system was assessed by comparing the results with those obtained from strain gauges. The results from the full compression tests indicate that the accuracy of axial strain measured by the DIC system was comparable for the specimens of A6063 and Douglas fir in the longitudinal (L) direction but was inferior for Douglas fir in the radial (R) direction. Furthermore, the differences in Young’s modulus obtained from the DIC system and strain gauge for the specimens of A6063, Douglas fir (L), and Douglas fir (R) were 2.3%, 2.4%, and 13.7%, respectively. In the partial compression tests, the accuracy of equivalent strain measured by the DIC system in the specimens of Douglas fir (R) was lower than that in A6063, attributable to a high strain gradient in the attachment region of the strain gauge. A novel variable was proposed using the standard deviation of strain to evaluate this strain gradient within the region. This variable demonstrated correspondence to the difference in equivalent strain obtained from the DIC system and the triaxial strain gauge, regardless of the material’s homogeneity or heterogeneity.