Fracture Mechanics

Abstract

In the course of this thesis, a research has been conducted to the application of fracture mechanics on an othotropic steel deck (OSD). Obtaining accurate predictions of crack path and fatigue life assessment were the main objectives for this thesis. As 3D crack propagation modelling in orthotropic steel decks is an arduous task, simpler studies were examined first in order to obtain confidence for the task at hand. These studies are comprised of a through thickness crack in an asymmetrical four point bending specimen and a semi-elliptical crack in a centre cracked plate. Furthermore, studies on computational fracture mechanics and the integration of Python scripting language with Abaqus were conducted for numerical analysis of crack propagation in complex structures. The extended finite element method was used for evaluation of contour integrals of a crack with a constant mesh throughout the propagation analysis. Post-treatment of the results was applied by smoothing stress intensity factors and the shape of the crack front. Moreover, inaccurate contour integral results were excluded from analysis through extrapolation. A Python code was developed for updating the crack front for a given number of loading cycles integrated in Paris' law. The investigated crack was located at the rib-to-crossbeam joint of an OSD. Here, crack propagation occurs along the lower weld toe and in the longitudinal direction of the rib. Validation of results was obtained through experimental studies conducted at the TU Delft laboratory. The weld geometry was incorporated in the model from measurements in order to identify local stresses. Finally, results of strain gauges were used to validate the model. Measurements of the experimental crack growth were obtained at the surface of the crack only. Consequently, assumptions were made of both the depth and shape of the crack in the thickness direction of the rib. A semi-elliptical crack shape was selected with an initial crack depth of 1 mm. Initially, Paris law parameters C and m were selected as recommended by BS7910. The numerical results of crack path complied with those of experimental results, with deviations of no more than 1 mm. Both data sets show that the propagation rate remained somewhat constant. On the other hand, fatigue life assessment results showed stronger deviation. A parametric study was performed to investigate the effect of the initial crack depth. Three different cases were examined for which the initial crack depths were selected as 1, 2 and 3 mm. Varying the initial crack depth did not lead to a significant change in crack path at the surface of the rib, while the depth in the thickness direction was affected. Regarding the fatigue life assessment, the results with respect to an initial crack depth of 1 mm, complied best with experimental results. Given the strong correlation between experimental and numerical results, the methodology followed in this thesis has proven valuable regarding fatigue crack modelling. As the methodology is based on fracture mechanics, an initial crack is required prior to evaluation. Consequently, the application in engineering practices lies in re-evaluation of structures. Moreover, as the parameters in crack propagation relations have a significant influence on the fatigue life assessment, these should be carefully chosen. Therefore, more insights must be obtained regarding the selection of such parameters.