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Abstract

Specimens which represent structural features have been tested under Gaussian narrow band random fatigue loading and an experimental study has been made of the local stresses at stress concentrations under prescribed sequences of variable amplitude loading. From the results of these tests and from relevant results obtained by other authors, the design of structural components subjected to variable amplitude loading is discussed in the context of fatigue life prediction and material evaluation. The thesis is divided into four parts.

In Part 1, factors which influence the validity for service conditions of cumulative damage calculations by the Palmgren-Miner hypothesis are discussed. The term ‘elastic interaction’ is introduced to cover various interrelated sources of inaccuracy in damage calculations for plain specimens. In specimens containing geometric stress concentrations, particularly in tests with a static mean stress, the local plastic deformation introduces a ‘geometric residual stress' effect which is superposed on the effects visible in plain specimens, so that the influence of elastic interaction can be deduced when the geometric residual stresses are separately accounted for. The development of geometric residual stresses is investigated in tests on pinned-lug and notched specimens subjected to 'quasi-static' and continuous random fatigue loading. Palmgren-Miner estimates of endurance, from which the influence of geometric residual stresses has been eliminated - by linear strain approximations combined with use of the alternating/mean stress diagrams for the materials concerned - reveal a distinction between the elastic interaction which occurs in the crack initiation phase (Stage 1 of fatigue) and that which occurs in crack propagation (Stage 2). For a wide range of service-type loadings, elastic interaction in Stage 1 is found to lead to values of (*) between about 0.2 and 0.6, whereas in Stage 2 elastic interaction generally leads to values between about 0.8 and 2. Thus, if under service conditions the problems of Stage 1 and Stage 2 fatigue are treated separately, the range of values for (*) is narrowed sufficiently for a meaningful application to material evaluation and design.

On the basis of this research, suggestions are made in Parts 2 and 3 for improvements to the procedures used in material evaluation and the prediction, Part 4 gives independently an appraisal of the research which must be done to evaluate the structural fatigue performance of carbon fibre reinforced plastics.

Relevant reports prepared by the author at various stages in the evolution of the thesis, are reproduced (in order of preparation) in Appendices A to G as part of the work submitted.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TA Engineering (General). Civil engineering (General) T Technology > TJ Mechanical engineering and machinery T Technology > TN Mining engineering. Metallurgy
Departments:	School of Science & Technology > Department of Engineering School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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