Conclusions

The results from the analyses presented here show that the effect of surface roughness and sub-surface porosity is summative. If either the roughness or the porosity is neglected, then the development over multiple load cycles of equivalent plastic strain in the sub-surface will be under-represented. There is significant coupling between the porosity and the roughness pits, meaning that two pits that are reasonably well-spaced can actually work together in combination with sub-surface porosity to weaken a wedge of material in the sub-surface. This is also suggestive as a mechanism for stress related corrosion.
Greater levels of porosity in the sub-surface imply greater equivalent plastic strain penetration into the bulk material; however, for larger numbers of reasonably homogenously arranged smaller pores the plastic strain creates a network. This seems to be an emergent phenomenon arising from the random distribution.
The results presented here for the larger pore examples are not an ideal fit with the other data generated. This is because of the small numbers of the pores, and the relatively poorly defined domain over which the pores could be defined. The problem could be resolved by increasing the problem size, increasing the number of pores within the model, and having greater control over the porosity distribution.
Each of the models presented shows some similar features. In each case, under first loading, the presence of stress raiser features – surface roughness and porosity – leads to local stresses exceeding the yield stress of the material, and the formation of localised zones of equivalent plastic strain. The shape of each of these zones is similar, but varies in size with the pore size. As the number of load cycles is increased, the area of these zones of plastic strain remains the same, or does not grow significantly, but the level of plastic strain increases within that area. High plastic strain could indicate local failure, so this is suggestive of a failure initiation mechanism.
Overall, the “PEEQ penetration” – the greatest depth into the sub-surface where there is plastically strained material – increases with increasing pore size, and reducing spacing between pores. A more significant effect is that the combined effect of porosity and surface roughness is to generate “<” shaped networks of PEEQ into the material, defined on the surface by nearby deeper furrows in the surface. Characterization of the PEEQ penetration would depend most critically on the analysis of the surface roughness data to identify these potentially damaging furrow spacings.
The data presented here is indicative only, and further modelling work will be required to describe the trends with more precision. It is proposed that the “PEEQ penetration” depth be used as a proxy for the EIDS to be used in the Hartman-Schijve variant of the NASGRO equation, and life predictions be made on that basis. These computed lives must then be compared with test data, to test whether this can provide a conservative basis for life prediction.