Institute of Fundamental Technological Research

The experimental study of evolution rules of two state variables: back street αij and yield stress R are first discussed. It is shown that back stress evolution is affected by the maximal prestress, and asymptotic value of yield stress depends on strain amplitude. The constitutive model is presented next, and its prediction for cyclic loading program is compared with experimental data.

The creep hardening constitutive model based on the concept of kinematic and isotropic hardening, and the memory of maximal prestress, developed in ́ [1984], is modified and extended in order to increase its simplicity and to describe creep-plasticity interaction programs. Next, the model is applied to the case of cyclic creep deformation in pure copper tubes subjected to combined tension and torsion at 300°C and to cyclic creep of 304 stainless steel at 600°C for varying complex stress levels.

The creep-hardening constitutive model based on the concept of kinematic hardening is discussed. The distinction is made between active and unloading creep processes and the hardening is assumed to depend on two scalar parameters αm and αn the first representing the maximal prestress measure, the other corresponding to accumulated measure of unloading events. The constitutive model is applied to simulate monotonic and cyclic creep deformation in pure copper tubes subjected to combined tension and torsion at 300°C. The interaction between monotonic and cyclic hardening and the effect of initial prestress are studied and model predictions are compared with experimental data.

Plastic pre-strain induces directional material response. It has been observed that uni-directional pre-strain results in anisotropic creep. Creep tests in various directions with respect to the direction of pre-strain have shown different steady creep rates and times to rupture.

A theory of anisotropic creep is proposed employing the properties of tensor functions. The creep rate in the steady creep range is expressed as a tensor valued tensor function of the stress and pre-strain. In comparison with the Odqvist theory additional material constants appear in the law proposed. The constants are established and predictions from the theory advanced are compared with the results of experiments concerning two pre-strain magnitudes and seven inclinations of the uniaxial creep tests with respect to the pre-strain direction.