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Title: Fracture under primary and secondary stresses
Author: James, Peter Michael
ISNI:       0000 0004 2740 171X
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2013
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Components found within many industries contain crack like defects. The work detailed here considers such a component under the combined influence of primary and secondary stresses; where primary stresses contribute to plastic collapse and secondary stresses are redistributed under plastic deformation. A number of approaches are available to detail the combined loading on the crack tip parameter J, or KJ, which is used to assess proximity to failure from crack extension. However, these approaches are recognised to be conservative and can lead to the unnecessary replacement of components, stricter surveillance and inspection regulations, and further costs associated with downtime.The aim of the work presented is to investigate these conservatisms and develop a further approach to quantify the interaction of primary and secondary stresses on fracture. A large matrix of cracked body finite element analyses of a circumferentially cracked cylinder has been performed under a range of loadings. This is then used to detail the interaction of primary and secondary stresses on fracture by providing a function to describe a scaling term, g, that multiplies the secondary crack driving force contribution. This term has been shown to be relatively independent on the magnitude of secondary stresses and is also dependent on the material stress strain relation. This relation for g has also been shown to be compatible with the R6 defect assessment procedures V factor approach, through the Vg plasticity interaction term, that provides a scaling term to the secondary contribution in R6. A review of experiments considering combined loading has indicated that the number of tests that cover a range of primary stress induced plasticity levels is limited. Further experiments were therefore considered within this research to provide added experimental fracture toughness data by which to compare the R6 V factor and Vg approaches. These experiments introduced a compressive pre-load to the ends of three-point bend specimens so that a tensile residual stress resulted on unloading. A crack was introduced and the specimens tested at one of three temperatures so that changes in the materials fracture toughness with temperature ensured different levels of plasticity at failure; so that crack growth occurred over three sets of load normalised to the load for plastic collapse. Tests were also conducted that did not include the residual stress so that the effect of residual stress could be shown under different levels of plastic redistribution. The Vg Approach and the existing Complex R6 V Approach have then been applied to all available experimental data for validation. The results show that both approaches conservatively predict the failure of all tests and that the Vg Approach can reduce the level of conservatism.
Supervisor: Sherry, Andrew Sponsor: AMEC Clean Energy ; EDF Energy
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Combined Loading ; Fracture Assessment ; R6