Creep crack growth simulations by a modified damage model

摘要

Researchers have proven that both environmental effects and the formation of defects such as cavities due to plastic flow and creep control the high temperature fracture behavior of nickel base superalloys under quasi-static load. Creep crack growth experiments are well suited to investigate these effects. Modeling of such tests can help to better understand the occurring mechanisms. Waspaloy is an example for a material with numerous cavities ahead of the crack tip after creep crack growth testing at 973 K. In this case, the model must be able to combine damage caused by plastic and viscoplastic flow. The complicated stress state caused by the geometry, the crack propagation and the material's behavior require the use of FEM to predict creep crack growth. Thus, several modifications to the classic damage models become necessary. Independent uniaxial creep tests provide the required model parameters. However, the stress state at a crack tip is multiaxial. Therefore, the parameters describing the interdependence between the hydrostatic pressure and the damage evolution must be determined separately. One single creep crack growth test is used for their determination. An application of the developed model to creep crack growth produces a good consistency with the experimental data. Some details of the tests which could not be explained before are now better understood. For example, the calculations indicate a pre-damage in the specimen which may be produced by inserting the pre-crack and by preparing the specimen. Taking into account all these facts, damage evolution due to ductile deformation is probably the decisive mechanism in the investigated case. The results also support the assumed dependency of damage on the hydrostatic pressure.