Browsing by Author "Marquis, Gary, Prof., Aalto University, Department of Applied Mechanics, Finland"

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Fatigue crack growth model for arbitrary planar cracks subjected to non-uniform stress field induced by variable amplitude loading
(2014) Lindroth, Pasi
School of Engineering | Doctoral dissertation (monograph)
Engineering components that are subjected to cyclic loads frequently contain imperfections which can rationally be assessed using linear elastic fracture mechanics. Fatigue crack growth models make use of the stress intensity factor (SIF) range and the maximum SIF to describe the driving force for crack propagation. Determining the stress intensity factor in many practical cases is difficult because the stress field is complex due to the structural geometry of the component, the crack geometry and the residual stresses caused by the manufacturing process. Weight function (WF) solutions for numerous crack geometries have been published, e.g., for through cracks, edge cracks, surface cracks and corner cracks. However, the use of the WF for cracks of arbitrary shape is less developed. The point-load WF method for arbitrary planar cracks gives sufficiently accurate estimates of stress intensity factors for embedded cracks but the results for the surface and edge cracks are less accurate. A new form of the general point-load WF more suitable for surface breaking cracks is proposed and is combined with a crack tip plasticity based crack growth model. During numerical simulations cracks are normally modelled using a discrete number of linear segments. A new method for shifting these segments as a crack advances is also proposed. The important aspect of the crack tip plasticity based crack growth model is the blunted crack tip with the radius of ρ* that is assumed to be a material constant. A new method to approximate the ρ* parameter based on the theoretical strength and fracture toughness of material is presented. The theoretical strength can be approximated based on the modulus of elasticity, and therefore, only two well established material properties are needed. The method is verified based on comparison with published experimental results involving complex two dimensional stress fields and geometries. The experiments are related to the engineering problems such as welded structures and inclusions in a valve spring wires. The method is used to study the effect of the initial crack geometry and residual stresses. The analyses help to understand how much fatigue life estimations depend on initial parameters and when these effects must be taken into account.