Numerical Simulation Of Fatigue Crack Growth Research Articles

Fatigue crack growth in AA6082-T6/AA7050-T6 bi-materials: Effect of plastic zone ahead of crack tip

The main objective here is the understanding of the effect of plastic zone ahead of crack tip on fatigue crack growth (FCG). For that, a numerical simulation of FCG was made, based on cumulative plastic strain, considering a bi-material composed by the 6082-T6 and 7050-T6 aluminium alloys. The first one has a lower yield stress but a higher critical cumulative plastic strain. The values of da/dN are higher for the 6082-T6 which means that the effect of the easier plastic deformation dominates over the effect of the higher critical cumulative plastic strain. The bi-material 6082-7050 showed a pronounced decrease of da/dN near the interface, which starts when the monotonic plastic zone ahead of crack tip reaches the material transition. This indicates that the crack tip deformation is affected by the material inside the monotonic plastic zone. On the other hand, the bi-material 7050-6082 shows an accentuated increase of da/dN, which starts when plastic deformation is observed ahead of transition. The elimination of the contact of crack flanks reduces drastically the transient effects, which indicates that these are linked with plasticity induced crack closure effects. Crack tip blunting was proposed to be the mechanism of plasticity induced crack closure explaining the trends observed.

Evaluation of fracture strength and residual lifetime of materials damaged by simulated plasma disruption

In this study, the surface damage of type 304 stainless steel, which is one of the candidates for the first-wall structural material in the fusion reactor, at plasma-disruption loading is simulated by high heat flux NBI. Influences of the surface damage on the fracture strength and the residual lifetime are studied. The results obtained are summarized as follows: (1) the present surface damage gives qualitatively a good simulation for plasma-disruption loading; (2) the fracture strength of the damaged material is improved by the existence of a melted layer, which has a higher hardness. There is no effect of microcracks in the melted layer on the fracture strength, and the plastic collapse criterion still stands; (3) the fatigue strength of the damaged material is reduced considerably due to the existence of microcracks in the melted layer; (4) numerical simulations of fatigue-crack growth are successfully attempted. It is shown that the residual lifetime can be predicted quantitatively by the present method.