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Title: The behaviour of short fatigue cracks in near-titanium alloy IMI 829 at room temperature
Author: Hardy, M. C.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1994
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The behaviour of short and LEFM cracks has been examined in β heat treated near-α titanium alloy IMI 829 at room temperature. Short fatigue crack initiation and growth result from slip activity in the α phase. Crack nucleation occurs at the boundaries of specific α-colonies due to inhomogeneous plastic deformation and microscopic stress redistribution. Cracks then develop along α basal plane slip bands inclined at approximately 45^o to the loading axis. Specimen surface finish influenced the location of crack initiation and the specimen fatigue life. These effects are attributed to surface residual stresses produced in specimen manufacture. Crack propagation is predominantly perpendicular to α-plates due to basal plane slip band cracking. Cracks were temporarily arrested at misorientated α-colonies and coarse α-platelets, or prior β grain boundaries. The ability of microstructural barriers to obstruct crack growth is reduced on increasing the product of stress and crack size. The influence of stress ratio on short crack growth has been studied. It was found that crack closure reduced the stress range for crack growth in tests where the stress ratio is below 0.5. Crack opening stress values were determined using pseudo S-N curves constructed from crack propagation data. Power law growth rate expressions do not characterise the discontinuous behaviour of short cracks. Growth rate data from LEFM and short cracks coincided over a limited range of stress intensity factor range values. Below this range short cracks propagated faster than LEFM cracks. This behaviour is not due to differences in crack closure. A two stage empirical model has been formulated to describe short crack behaviour. It is shown to give an excellent correlation of crack growth rate versus crack length data. Procedures have been established to determine a critical crack size above which continuum fracture mechanics applies.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available