Homogeneous shear stress field of wood in an Arcan shear test configuration measured by means of electronic speckle pattern interferometry: description of the test setup

Several studies have dealt with the problem of how to measure the shear modulus of small clear wood specimens, avoiding bias from normal compression, bending, tension or torsional stresses. Unbiased results can be used to estimate realistic shear modulus values relevant for timber construction. However, a stress field that contains only uniformly distributed shear stress cannot be achieved. The Arcan shear test is one of the test setups that allows the measurement of more or less homogeneous shear stress and, therefore, shear deformation in a small volume of the specimen at macroscopic material level. The shear deformation of Arcan shear test samples was measured between the two notches by means of electronic speckle pattern interferometry (ESPI). Shear tests were performed in a tension mode, which provided a homogeneous strain field in the reduced midsection. Twisting forces and inhomogeneous shear deformation of the sample volume were assumed, which resulted in different shear deformation on the front and back sides of the sample. In this way, both surfaces of the samples were measured in parallel. Using the mean value of both deformation fields allowed a significant reduction in the coefficient of variation of shear modulus measurements to be achieved in comparison with values gained from a single ESPI measurement.