Berechnung von Kriechverformungen unter strikter Einhaltung kontinuums-mechanischer Gleichgewichtsbedingungen

摘要

The creep deformations of a body under mechanical load is usually being calculated by an incremental process causing stress redistribution. Creep strains calculated from a given creep law are imposed onto the body in a restraining manner. Although the restraining is corrected for after each time step in order to restore the equilibrium of forces, correction cannot be achieved completely. In the present paper it is shown that the well-known instability problems of Finite-Element creep calculations are caused by residual restraining. A numerical method to impose creep strains without restraints is described and examined. The two methods have been used to calculate stress relaxation in a thick walled tube under both thermal load and internal as well as external pressure. The results for stress relaxation which in the case of unrestraining creep strains is treated as an independent relaxation process proves to be different for the two methods. In a second application the creep properties of a circumferentially notched bar under uniform tension has been investigated. The calculations show that in the state of stationary creep the minimum cross section retains its diameter, which is a statement depending on the very circumstances chosen. Experimental results confirm this property as far as the initial change of the specimens shape is concerned. However, a long-time extrapolation of the results on a steady-state creep basis would underestimate the creep deformations of the specimen at the notch.