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Theory of Fatigue for Brittle Flaws Originating from Residual Stress Concentrations
Author(s) -
FULLER EDWIN R.,
LAWN BRIAN R.,
COOK ROBERT F.
Publication year - 1983
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.1983.tb10039.x
Subject(s) - residual stress , indentation , stress intensity factor , brittleness , fracture mechanics , materials science , mechanics , equivalence (formal languages) , residual , power law , stress (linguistics) , structural engineering , mathematical analysis , mathematics , physics , engineering , composite material , statistics , linguistics , philosophy , discrete mathematics , algorithm
A theory is formulated for the general fatigue response of brittle flaws which experience residual stress concentrations. The indentation crack is taken as a model flaw system for the purpose of setting up the basic fracture mechanics equations, but the essential results are expected to have a wider range of applicability in the strength characterization of ceramics. A starting fatigue differential equation is first set up by combining an appropriate stress intensity factor for point‐ or line‐contact flaws with a power‐law crack velocity function. Analytical solutions are then obtained for the case of static fatigue. The resulting relation between lifetime and failure stress is shown to have exactly the same power‐law form as the conventional solution for Griffith (residual‐stress‐free) flaws. This “equivalence” is used as a basis for extending the results to dynamic fatigue. A comparison of these analytical solutions with numerical counterparts defines the limits of accuracy of the theoretical procedure. However, while the form of the lifetime relation remains invariant, the values of the exponent and coefficient differ significantly for flaws with and without residual stress. Accordingly, the application of conventional fatigue theory to evaluate crack velocity parameters, without due regard for the nature of the critical flaw, can lead to serious errors. Explicit conversion formulas are given for transforming “apparent” velocity parameters for indentation flaws directly into “true” parameters. The implications of these results concerning the use of the indentation method for materials evaluation are discussed.