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Title: Computer modelling of fatigue in titanium alloys
Author: Dandre, C. A.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1995
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A computer package has been developed that models the inter-granular stress distributions that are considered to be responsible for the fatigue crack initiation and short crack growth stages in near-α titanium alloys. The computer package incorporates the finite element method, and by modelling stress distributions at the microstructural level, this research is placed at the forefront of the field. A computer program generates at hypothetical uniform grain structure consisting of hexagonal grains. However, in order to model the anisotropic nature that is inherent in titanium alloys, a texture is developed for the computer generated structure. The directional variations for elastic and plastic properties are incorporated into the model by allocating crystallographic orientations to each grain individually. Since failure in near-α titanium alloys has been attributed to slip on the basal plane, the grain orientations describe the inclination of the basal plane to the direction of applied stress. The computer package models the principal inter-granular stress redistributions that occur at grain discontinuities, where 'weak' grains off-load stress onto adjacent 'strong' grains. Certain grains that are suitably orientated for slip experience an increased stress. When resolved onto the basal plane, this is evident as a unique combination of tensile and shear stresses that are considered to activate the separation of slip bands that have formed. In order to support the theoretical model, a limited material testing programme was devised which was considered to provide important information regarding the failure mechanisms. Four-point bend tests were performed on IMI 829 barstock material which was heat-treated to produce a coarse grain structure consisting of colonies of aligned α-platelets. SEM measurements were taken to determine the texture at crack initiation sites. The computer package was implemented to model the inter-granular stresses for these local textiles. Stress contour plots indicated that significant inter-granular stress distributions existed which were unique to each initiation site. These results were supported by the fatigue observations in the bend specimen.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available