Papers by Keyword: Crack Shielding

Abstract: Pressure casting dies are exposed to harsh service conditions consisting of cyclic thermal and mechanical loading and thus undergo thermo-mechanical fatigue. Due to cyclic plastic deformation of the material near the surface of the dies the loading conditions gradually change because of the formation of tensile residual stresses which add to the stress field from external loading. This change in the stress field influences the nucleation and the growth of cracks. Typically after a few thousand casting cycles a network of heat checks forms. In such a network crack shielding has a big influence on the evolution of the crack array. Firstly, it influences the propagation rates of the cracks and secondly it may change the propagation direction compared to the case where no neighbors are present. The crack growth rate and the length at which the cracks stop growing are also influenced by the thermo-physical and mechanical properties of the die material. It was found that the shielding effects of neighboring cracks are of equal importance. Crack deflection caused by the presence of neighboring cracks can lead to break-outs at the surface ensued by fast degradation eventually necessitating the replacement of the die. Consequently, the focus in this work is put on the investigation of the interaction of cracks in a network and their effect on the fatigue life. The problem is tackled by means of an automated strategy based on the finite element method.

Abstract: This paper presents a very brief overview of the philosophy underlying a plastic inclusion approach to defining the boundary stresses imposed on the applied elastic stress or displacement field by the plastic deformation attendant on crack growth in a ductile material. It leads to two new fracture mechanics parameters, KR and KS. KR defines a retardation component arising from wake contact and the Poisson’s contraction associated with the plastic zone, whilst KS describes a compatibility-induced component arising from shear at the elastic-plastic interface. These additional components imply that KF is not directly comparable with KI, as it describes the net driving force on the crack from the applied load.

Abstract: This paper presents an outline of the development, verification and application of a new model of crack tip stress fields in the presence of a plastic enclave around a growing fatigue crack. The approach taken rests on capturing the effects of this ‘plastic inclusion’, comprising the crack tip and crack wake plastic zones, via elastic stress distributions applied at the elastic-plastic boundary. The model is therefore independent of the mechanisms of plastic deformation and potentially applicable to a variety of materials. A Muskhelishvili complex potential extension to the Williams crack tip stress field is found for four stress parameters representing a K-stress, a T-stress, a crack retardation stress and a compatibility-induced shear stress at the elastic-plastic boundary. This model is validated via full field fitting to photoelastic stress fringe patterns, obtained from epoxy resin and polycarbonate specimens. It has also been extended to the strain fields measured in digital image correlation techniques, which allows its application to metallic alloys.