Effect of pitting corrosion on fatigue crack initiation and fatigue life

Finite element modelling of fatigue crack growth of surface cracked plates: Part I: The numerical technique

R-curves for a sinter/HIPed SiC(whisker)-reinforced alumina and a sintered silicon nitride were assessed by direct measurements of lengths of cracks associated with Vickers indentation flaws. The fracture toughness measurements based on (a) initial (as-indented) crack lengths, (b) equilibrium growth of cracks during increasing far-field loading, and (c) crack lengths corresponding to unstable fracture showed definitive trends of R-curves for both materials. The fracture mechanics analyses employed an indenter-material constant that was independently estimated using a physical model for the residual driving force and a free surface correction factor that accounted for the effects of size and shape of the cracks on stress intensity. It is shown that R-curve estimations based on crack length measurements have the intrinsic advantage that crack length dependence of fracture toughness is not assumed a priori as is done in conventional analysis based on strength. The measured fracture toughness of SiC(whisker)-reinforced alumina was in agreement with the prediction of a toughening model based on crack bridging by partially debonded whiskers.

Rising Crack-Growth-Resistance (R-Curve) Behavior of Toughened Alumina and Silicon Nitride

Fatigue crack propagation: In situ visualization using X-ray microtomography and 3D simulation using the extended finite element method

Fatigue crack growth rates have been determined on standard specimens containing long cracks (∼5–10mm) and on specimens containing two-dimensional short cracks (∼0.10–0.50mm). Large differences have been observed indicating that at a given stress intensity factor short cracks propagate much faster than long cracks. Mouth opening displacement measurements for both specimen geometries have shown that the crack closure effect is largely responsible for the observed effect. These results are used to rationalize the behaviour of short cracks initiated from natural sites which were either graphite nodules or microshrinkage pores. The three-dimensional aspect of these natural small cracks is analysed and discussed in detail.

Short crack behaviour in nodular cast iron

Several crack tip stress intensity factor solutions have been published for semi-elliptic, surface breaking cracks in plates subjected to tension or bending forces. These solutions do not agree with each other particularly well and the basis for choosing which one is the best has not been established. In this paper, the development of fatigue crack shape is used as a diagnostic tool to test the accuracy of these theoretical stress intensity solutions in predictive fatigue crack growth calculations. Those solutions giving the best engineering estimate of crack tip stress intensity factors are identified. Single equations are also given for each loading case at the deepest point or surface intersection point of semi-elliptic cracks in order to facilitate calculations on programmable calculators. A rational basis for calculating the progress of a crack which snaps through the thickness and continues to propagate in a stable way by fatigue is suggested.

P. M. Scott

Papers

A critical review of crack tip stress intensity factors for semi-elliptic cracks*