Fracture in wood of Norway spruce. Experimental and numerical study.

Abstract

In this thesis, the fracture mechanical properties of wood from Norway spruce at the scale of the structural component were investigated. For that purpose, a detailed analysis of the displacement fields in the zone of the crack propagation was performed. The experimental measurement and numerical analyses were utilized to validate non-linear fracture finite element models. The experimental basis of the thesis contains a series of wedge split test specimens with a different angle to the grain for the opening mode and mixed-mode fracture investigation. The experimental displacement fields were obtained in tests with stable crack propagation in different configurations using the Digital Image Correlation technique. Series expansion of the Kolosov-Muskhelishvili solution to a 2D crack problem was utilized to quantify the displacement field around the crack tip. An optimization technique was utilized to find crack tip location and mode decomposition of the displacement field as well as the separation of the crack lips during crack development and propagation. Initial crack lengths were measured for all tests and are highest for mode I and drop with the increase in grain angle. The crack length histories obtained with the developed technique have adequate precision to distinguish between the crack growth during fracture process zone formation and subsequent crack propagation. Collected data were processed to provide fracture energy characterization in the form of R-curves. The fracture process zone formation was captured and quantified by measuring separation history. The experimental input was directly utilized in the traction-separation law in the finite element model. The numerical parametric study was performed to find optimal parameters for modelling fracture in Norway spruce with the use of cohesive zone models and eXtended Finite Element Method. The crack lengths residuals between experimental and numerical tests were minimized. The extrinsic exponential traction-separation laws for numerical fracture models of structural wood components in mode I and mixed-mode I and II were established. Shear test specimens with brittle failure were designed, tested and 3D scanned post-failure to characterize shear stiffness, shear strength, and fracture surface along the grain on the structural component scale. The 1000 MPa value of shear stiffness was found consistently for all series with different material orientation and notch detail. However, shear strength was highly dependent on the notch geometry and mode decomposition of the displacement fields around the notch tip indicated the presence of mode I.