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Title: Small fatigue crack growth in a near alpha titanium alloy : crack closure, stress gradient and temperature considerations
Author: Goulding, A.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1993
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The effect of fatigue crack closure in IMI 829 has been addressed for cracks growing from plain surfaces and under the influence of a stress concentration. Several test-piece configurations were employed, incuding thick and thin section double edge notches and standard corner crack (CC) geometries. All types incorporated an ultra fine corner slit to bias the crack initiation site. A thin double edge notch (DEN) specimen with a through section starter slit was also tested. Crack closure loads were measured using direct current potential drop (PD) and replica techniques. The primary closure mechanism was found to be a plasticity induced closure. The results indicate the dominance of surface effects. They also permit near tip and wake related closure effects to be resolved. At the higher stresses, notch root plasticity in the thick DEN dominates closure. At lower stresses where elastic conditions prevail, the results were comparable to those found in the plain CC specimen. Much work was carried out to characterise closure of part-through and through thickness cracks in the thin section notch. The transition between the two crack types invoked a complex closure response. A secondary closure mechanism was also identified, for all specimen types. This was roughness induced closure. On the basis of the PD measurements, an effective ΔK was derived which improved correlation of data over the range of stress levels and R values tested. At room temperature, crack lengths were measured using the above PD system and an existing photomicroscopic arrangement. A study of crack shape morphology was carried out using optical and SEM techniques. The observed complex stress and crack length dependency of shape development in thick notch specimens at higher stresses, was explained on the basis of enhanced plasticity induced closure in the notch root. Other deviations from expected shape characteristics, could be rationalised in terms of microstructural interactions.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available