Single overload fatigue crack growth retardation : an implementation of plasticity induced closure
Date
2017-05-11
Authors
Kirmani, Ghulam Ashraf-Ul-Harmain
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Abstract
Fatigue life prediction following overloads is required in such applications as, aerospace,
automobile, and pressure vessels industries for damage tolerant design. Modelling of
life prediction in fatigue is complicated by a host of variables which includes loading
history. The characteristic features which result as a posteriori evidence of the
loading history include overload plasticity zone, crack closure with a special trend
with respect to crack length, spike-dip in fatigue crack growth rate and retardation
in fatigue growth. This research focuses on life prediction of components subjected
to variable magnitude single overloads, in a cyclic loading situation.
This thesis introduces the plasticity range interaction, and closure effects for variable
magnitude single overload problems. A simple model is presented which captures
these characteristic features following overloads. A detailed study on the crack-tip
plasticity is conducted to identify the dimensions of the plasticity zone.
A new approach is presented which is useful in obtaining suppression factor for fatigue growth retardation. This factor is required in fatigue crack growth models to account for retardation effect following overloads. The model for fatigue crack growth is tested for constant amplitude loads. A detailed study is presented on fatigue
crack closure based constant amplitude calculations. Two different approaches to
fatigue growth calculations are presented. An assessment of the errors that occur in
assumed-crack extension method is also presented. Several examples have shown a
good agreement between experimental and theoretical results.
The study is extended to variable magnitude single overload problem for determination
of fatigue growth calculations. Two different approaches have been adopted,
one based on plasticity range interaction, and the other on closure. It has been shown that the two approaches are equivalent. There is an excellent agreement between predictions and theory for fatigue life calculations and fatigue growth rate.
This research directly contributes to life prediction under single overloads without
reliance on data fitting. It has tremendous potential for fatigue life prediction under
programmed block loads, multiple overloads and finally for random loads, which can
be investigated in further studies.
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Keywords
Plasticity, Metals