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Title: The incremental collapse and high strain fatigue of mild steel and aluminium alloy tubes subjected to biaxial stresses
Author: Crosby, John Richard
Awarding Body: University of London
Current Institution: Imperial College London
Date of Award: 1968
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A machine has been designed and developed to subject thin walled tubes to static or repeated biaxial stress by applying internal pressure together with an end load which is in phase with and a linear function of that pressure. A furnace has been designed to enable such tests to be carried out at temperatures up to 400°C. Data has been obtained for lifetimes ranging from the static burst or half cycle to 105 cycles at frequencies of 4-16 cycles per minute. The possible hoop/longitudinal stress ratios obtainable ranging from pure hoop stress to pure axial stress were 1:0, 2:1, 2:-1, 1:1, 1:-1, 2:3, 1:2, 0:1 and 0:-1. Materials tested were a typical pressure vessel mild steel at both room temperature and 400°C and three aluminium alloys - namely 4% Copper L65c/L63 (duralumin in bar and tubular form respectively), 2.8% Magnesium ASA 5454 and Zinc-Magnesium alloy DTD 683 at room temperature. Two modes of failure were observed, the normal high strain fatigue fracture and the necked out fracture of ductile appearance resulting from a cyclic creep action leading to incremental collapse. This has led to the cyclic test results being presented in two parts, the mild steel incremental collapse tests on the one hand and the aluminium alloy low endurance fatigue tests on the other. The static burst behaviour is shown to be predictable from the materials' generalised stress-strain relationship and all such results have been presented together. Repeated torsion tests on solid duralumin specimens were carried out on a Schenck Torsion Machine in an attempt to confirm the biaxial fatigue test findings that failure is governed by the maximum principal stress theory for the cyclic range investigated. Repeated tension tests were undertaken on an Avery Linescale Fatigue Machine with solid duralumin specimens for comparison with the thin walled tube repeated axial tests and with published repeated tension test results on solid specimens of the same material carried out on a 6 ton Schenck Push-Pull Machine. Fracture Mechanics Theory has been adopted to differentiate between catastrophic and leak-before-break type fatigue fractures in the biaxial stress field.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available