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Title: Low temperature dwell sensitive fatigue in near alpha titanium alloys
Author: Davies, H. M.
Awarding Body: University of Wales Swansea
Current Institution: Swansea University
Date of Award: 1998
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The ambient temperature fatigue response of the near alpha titanium alloy Timetal 834 has been evaluated using specimens manufactured from two material sources:- hot-rolled barstock and an isothermally forged compressor disc. The effects of dwell periods imposed at peak stress and different R values were assessed. Significant differences were observed between the two alloy variants particularly with respect to their sensitivity to dwell loading. In particular it was found that dwell sensitivity is enhanced by high R values (R = 0.5) and longer dwell times but is reduced by compressive minimum loads (R = -0.5). Variations in microstructural morphology together with differences in localised texture were found to control the contrasting dwell performance in the two materials. Sub-surface fatigue crack initiation has been observed for both coarse grained Timetal 685 and the two Timetal 834 forms. With typical as received hydrogen concentrations of 40-60ppm these initiation sites are characterised by quasi-cleavage facetting. EBSD analysis has demonstrated that the facets are of a near basal orientation with respect to the hexagonal crystal lattice. At high hydrogen concentration in Timetal 685, fractographic and EBSD studies combined with mechanical data from a previous investigation, show that premature fatigue failures are associated with the operation of alternative failure mechanisms. These include strain induced alpha/beta interface cracking and the shear related separation of basal planes. Fatigue crack propagation in the disc material, under dwell and cyclic loading, was explored using three specimen designs: CC, DEN and TDEN. The CC and DEN geometries allowed the behaviour of part through cracks to be evaluated in a uniform stress field and within a plastically deformed stress gradient. The TDEN enabled through section crack behaviour to be quantified. Growth rates were dependent on R-ratio but unaffected by dwell periods at peak load. Increased growth rates at higher R values were attributed to the fact that a high mean stress encourages enhanced strain accumulation and facet formation. The lack of a dwell effect on crack propagation is due to stress relaxation at the crack tip. It could also be associated with compression in the crack tip plastic zone on unloading.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available