Micromechanical modelling of size effects in crack initiation with application to fretting fatigue and cold dwell fatigue
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This thesis presents the development of a micromechanical computational framework for microstructure sensitivity of crack initiation in metals, with application to fretting fatigue and cold-dwell facet fatigue. It is well-known that the mechanical and fatigue behaviour of metals is sensitive to a number of size effects, which can occur when critical component length-scales are directly comparable to key microstructural dimensions (e.g. grain size). For example, in fretting contacts, the key length-scales (e.g. contact width, relative slip) are typically of the order of micro-meters, and therefore fretting fatigue life is sensitive to size effects. This phenomenon is a concern for engineering designers across a broad range of applications as popular continuum mechanics-based mechanical analysis and fatigue prediction techniques are not representative of real material behaviour at small length-scales. However, micromechanical modelling techniques that explicitly model the material microstructure are capable of capturing size effects. Three dimensional crystal plasticity (CP) modelling is employed to simulate the micromechanical response of three microstructurally distinct metals: CoCr alloy for biomedical applications, dual-phase alpha-beta Ti alloy for aerospace applications and ferritic-pearlitic steel for marine structural applications. A microstructure basis for a well-known contact size effect in fretting is identified through modelling of statistical and strain gradient size effects. A strain-gradient, length-scale dependent model has been successfully implemented in a crystal plasticity finite element (CPFE) fretting model. The model was thus able to demonstrate, for the first time, the beneficial effects of reducing length-scale (viz. smaller contact and microstructure length-scales) on resistance to fretting crack initiation. A 3D cylinder-on-flat finite element fretting contact model is developed to incorporate realistic microstructure geometries and a CP material model in the fretting contact zone. The micromechanical fretting model predicts that a key consideration in the design of metallic fretting contacts is the ratio of contact semi-width to average grain size. A critical contact semi-width to average grain size ratio of approximately 1 is identified, vii below which an increase in fatigue scatter and increase in average number of cycles to crack initiation is observed. It is shown that values greater than 1 should be chosen in the design of fretting contacts to reduce uncertainty in fatigue life predictions A physically-based, length-scale dependent material model is employed to determine the role of the beta phase on the micromechanical response of a dual-phase alpha-beta Ti alloy. Beta lath width and relative orientation is shown to significantly affect the micro-mechanical response of the material. The role of the beta phase in cold-dwell facet fatigue is investigated; the presence of beta laths in a rogue grain combination is predicted to increase dwell fatigue resistance. The key driving force in “faceting”, the rogue grain local normal stress, is shown to reduce by as much as 12% with explicit inclusion of beta laths. A CPFE fretting modelling framework is developed to study the role of length-scale effects and crystallographic texture in fretting crack initiation of ferritic-pearlitic steel for marine risers. A program of experimental testing has facilitated the calibration and validation of the material model and fatigue prediction model. Strain gradient length-scale effects and crystallographic texture are shown to play a key role in the predicted number of cycles to crack initiation. By decreasing the length-scale of the fretting contact by a factor of 10, an increase of up to 24% in number of cycles to crack initiation is predicted. The texture induced by cold forming of the marine riser material is shown to be beneficial in fretting crack initiation.
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