Fatigue crack growth under variable amplitude loading and negative stress ratios

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In order to prevent unexpected failures and to assure a safe life, regular non-destructive inspections are required in cases of a damage tolerant design or safety-relevant components. The estimation of the inspection intervals is done by the calculation of the residual lifetime based on a crack, which can be detected with non-destructive inspection methods. A service loading leads to load interaction effects, which either increase or decrease the residual lifetime. In contrast to variable amplitude loadings at positive stress ratios the effect of service loadings at negative stress ratios is only poorly investigated. Furthermore, the analytical concepts for the calculation of residual lifetimes are almost exclusively based on investigations at positive stress ratios. This means that predictions, which are based on the available lifetime concepts include a high potential of uncertainty. Therefore, the global objective of this research project is to investigate the effect of variable amplitude loading on the crack growth rates at negative stress ratios as basis for further developments of load interaction models. Therewith, a safe design of components, machines and means of transport can be assured. In the framework of this research work the influence of compressive loads on the crack growth rate at variable amplitude loadings under the consideration of sigmatip as measure for the crack tip loading at compression is experimentally and numerically investigated. The emphasis of the investigations is on two main influencing factors: a) the load history und the intensity of compressive loads as well as b) the material properties and the constraint effect. For the investigation of the influence of the load history and the load level different over-/ underloads, over-/underload sequences, block loads and standard load sequences will be investigated. The investigated materials are chosen on the basis on the one hand of the amount of kinematic hardening and on the other hand of the cyclic hardening and softening. For the explanation of experimentally observed phenomena numerical simulations are performed by 2D and 3D finite element analyses. The simulations are performed to investigate the influence of crack closure, residual stresses and contact on the crack tip deformation and load. The results of the simulation are validated with experimental measurements of the crack flanks. The experimental and numerical results are used both for further developments and for modifications of the available analytical approaches, especially the strip yield model.