Abstract
A model of fatigue crack propagation has been proposed. The model is essentially based on the following four assumptions:
(1) The displacement along the crack line follows the BCS-model solution.
(2) Every point along the crack line has a maximum amount of residual plastic deformation that it received in its history.
(3) The rate of the fatigue crack propagation is given by the equation da/dN=C(ΔKeff)m, where ΔKeff is the effective range of the stress intensity factor and C and m are the material constants.
(4) The effective stress range is evaluated by the stress at which the point rp* behind the crack tip opens or closes. The parameter rp* denotes the size of the cyclic plastic zone.
Based on this model, a computer simulation of fatigue crack propagation has been carried out. The summary of the results is as follows:
(1) When the applied stress range and the stress ratio R are held constant, the effective stress range ratio U has a constant value and it is independent of the crack length. The relation between U and R is in good agreement with Elber's equation, i. e., U=0.5+0.4R.
(2) In the case of the decreasing ΔK test, the model can describe well the effect of R. The threshold stress intensity factor is given by the assumption that the crack will stop propagating when the COD at the crack tip is less than a certain characteristic value.
(3) When there is a sudden change in the applied stress range, an accelerating or retarding effect occurs for a certain period.
In these three cases, the model can express the essential features observed in the experiments. Then the simulation technique is considered to be useful for the estimation of fatigue lives.
