The utility of low-cost photogrammetry for stiffness analysis and finite-element validation of wood with knots in bending

This article presents the capabilities of low-cost photogrammetry as a tool for stiffness analysis and finite-element (FE) validation in conventional bending tests of structural timber that contains knots. The accuracy offered by three consumer-grade digital cameras with a resolution of 10 megapixels was 1/6200, which did not allow for the evaluation of the stresses or strains in a multiaxial stress field, but provided a thorough contrast of the 3D nodal displacements, showing the distortion of knots in the modulus of elasticity (MOE), so that the heterogeneous stiffness of this natural material could be measured and accounted for. This allowed a more precise determination of the wood stiffness in bending, and a novel procedure for the estimation of a Clear MOE related to the defect-free areas, an MOE in Knots related to the distorted regions, and a Global MOE which encompasses both was developed. The proposed method can be applied to conventional mechanical tests. In addition, other interesting possibilities for FE validation and monitoring were obtained. The case study demonstrates how, with a very small investment, an FE model that simulates the influence of knots in bending could be accurately validated with many fewer specimens, and how the predictions of displacements could be improved by up to 49% using the Clear MOE measured by means of the photogrammetry rather than the conventional MOE without shearing strain.