Some aspects of fatigue and fracture in a C-Mn structural steel

Aboutorabi, Seyed Ali Akbar (1985) Some aspects of fatigue and fracture in a C-Mn structural steel. PhD thesis, University of Glasgow.

Abstract

The work described in this thesis comprised an investigation into a number of concepts associated with the failure of a C-Mn steel (BS4360 Grade 50D) which is frequently used as a prime structural material in offshore structures. The initial investigation concerned the temperature dependence of the stress state sensitivity of the ductile failure strain of the material. For the limited range of the stress state studied, it was found that this sensitivity is independent of temperature. Failure initiation characterizing parameters deltai and Ji were also found to be sensitive to stress state in the crack tip region. This sensitivity however depends on the temperature and decreases with decreasing temperature in the ductile-brittle transition region. This behaviour was shown to be associated with the interruption of the ductile failure process by the lower temperature brittle fracture mechanism. The investigation into the development of part through surface cracks under tensile fatigue loading indicated that the crack profile develops towards an equilibrium shape of a/c = 0.8. The distribution of stress intensity factor KI around the periphery of part through surface cracks under remote tensile loading was determined for a variety of crack geometries. This distribution was found to be a function of the crack profile. Various solutions were examined and it was shown that the numerical solution of Newman and Raju correlates relatively well with the experimental results for fractional depth in the range 0.22≤a / t≤0.6. It was observed that subsequent to the adoption of an equilibrium shape, further fatigue crack growth produced bul ging near the surface intersections. This behaviour was modelled by considering the variation of stress state and its effect on the plastic zone ahead of the crack tip. The initiation and subsequent propagation due to post yield failure around the periphery of a part through surface crack of a/c = 0.69 and a/t = 0.7 subjected to montonic tensile loading was investigated. The distribution of initiation COD around the crack front was determined. It was found that this distribution is different from that for KI. Initiation of ductile failure at ambient temperature occurs first in to regions of high constraint at locations close but under the plate surface. The subsequent crack propagation however is in a manner in which crack front progresses towards regions of lower constraint. It was shown that post yield failure from the part through crack geometry studied may be correlated with failure parameters measured in various types of standard laboratory through crack test pieces. Experimental limit loads were compared with empirical prediction procedures which were found to be in general conservative for the defect geometry studied.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Mechanical engineering, Materials science
• Date of Award: 1985
• Depositing User: Enlighten Team
• Unique ID: glathesis:1985-76590
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Nov 2019 14:05
• Last Modified: 19 Nov 2019 14:05
• URI: https://theses.gla.ac.uk/id/eprint/76590