The Effects of Programme Loading on Fatigue Crack Propagation


The object of the present research was to find and evaluate the effects of programme loading on fatigue crack propagation in EN24 steel. A potential drop technique was developed to measure very small changes in the crack length. The constant load amplitude test results, analysed on the basis of fracture mechanics, served as a basis for comparison with the results of the programme loading tests. The block programme loading schedules were designed to find the effects of load interaction and stress levels on the crack propagation rate. Microscopic and fractographic examinations were conducted to understand the influence of micro-structures and strength levels on the fatigue fracture process. The results showed that the micro-structural features of the three heat-treatments considered exerted a secondary influence on the crack growth rate. The capacity of the low strength ductile steels to accommodate large amount of strain at the crack-tip by plastic deformation and their ability to dissipate energy by numerous crack-branching and island formation were associated with the higher crack growth resistance of these steels as compared with the high strength martensitic steel. At any ΔK level, the crack growth rate was initially constant followed by a gradual slowing down which depended on the loading variables. With a growing fatigue crack at a constant ΔK, a decrease in the maximum stress caused a transient slow growth period whereas an increase in σmax did not show any appreciable acceleration of the crack growth rate. Based on a proposed mechanism of fatigue crack growth, an equation of the type da/dn= A(ΔK)M/R' , where R' = σys/σmax , was found to unify the results of different loading conditions seat This equation does not account for the slow growth periods and would, therefore, yield a conservative estimate of fatigue life.

Divisions: College of Engineering & Physical Sciences > Aston Institute of Materials Research (AIMR)
Additional Information: Copyright © Khairuzzaman, 1973. M. Khairuzzaman asserts their moral right to be identified as the author of this thesis. This copy of the thesis has been supplied on condition that anyone who consults it is understood to recognise that its copyright rests with its author and that no quotation from the thesis and no information derived from it may be published without appropriate permission or acknowledgement. If you have discovered material in Aston Publications Explorer which is unlawful e.g. breaches copyright, (either yours or that of a third party) or any other law, including but not limited to those relating to patent, trademark, confidentiality, data protection, obscenity, defamation, libel, then please read our Takedown Policy and contact the service immediately
Institution: Aston University
Uncontrolled Keywords: programme loading,fatigue crack propagation
Last Modified: 28 Jun 2024 07:06
Date Deposited: 22 Feb 2011 14:03
Completed Date: 1973
Authors: Khairuzzaman, Mirza

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