Masamichi Kawai

Constitutive Model for Coupled Inelasticity and Damage.

Constitutive models to describe a coupling between deformation and damage due to creep of polycrystalline metallic materials are discussed from phenomenological and continuum mechanics points of view. The constitutive modeling is based on the irreversible thermodynamics for internal state variable theories, where the thermodynamic potentials, i. e., free energy and dissipation energy functions, are defined using hardening and damage variables. The material damage is assumed to be isotropic. First, a new damage-coupled kinematic-hardening model is developed in the invariant form on the basis of the Malinin-Khadjinsky model. The evolution equation of the hardening variable is prescribed by the Bailey-Orowan format which includes the effects of isotropic damage. Then, an isotropic-hardening model taking damage into consideration is formulated by assuming a particular representation of the kinematic hardening variable. The evolution equation of the isotropic creep damage is analogous to that developed by Kachanov and Rabotnov. However, it takes into account a coupling with creep hardening and softening. The present models can describe primary, secondary and tertiary creep behavior, and they are applicable to variable loading conditions. The creep rupture time predicted, even in the simplest case, depends on the time and degree of damage at which the hardening variable reaches its saturation state under the applied stress conditions.

Alternative Form of the Auxiliary Hardening Rule for Multiaxial Repeated Creep.

An alternative form of the auxiliary hardening rule is developed to describe the anisotropic creep behavior under multiaxial nonproportional repeated stress changes. The auxiliary hardening rule controls the combined isotropic and kinematic hardening with a memory surface in which isotropic hardening is suppressed and kinematic promoted. Differences from the previous studies are found in the alternative definitions of the memory surface and its evolution equations. A method for the enhancement of the creep recovery is also proposed within the present constitutive framework. The applicability and insufficiency of the creep model are elucidated by comparison with the typical experimental results on multiaxial nonproportional repeated creep. The simulated results of the isotropic and kinematic hardening models, which are derived as special cases of the present model, are included.

Effects of cyclic plasticity on subsequent creep for type 316 stainless steel at elevated temperature.

Multiaxialhistory effects of cyclic plasticity on creep were studied experimentally for type 316 stainless steel at an elevated temperature. Firstly, after a cyclic stabilization of prior cyclic tension-compression of constant total strain amplitude, constant stress creep tests were conducted under simple tension, simple torsion, and combined tension-torsion (√(3)τ/σ = 0, oo, 1). Three levels of creep stress were chosen which corresponded to the saturated cyclic stress amplitude and to certain values larger or smaller than that. Secondly, in order to elucidate the path shape effect of the prior strain cycle, a non-proportional strain cycle along a circular path was followed by a constant stress creep in tension where the strain amplitude was specified so as to result in the same saturated stress amplitude as in the tension-compression cycle compared. It was observed that the prior tension-compression cycle induced an anisotropic hardening for creep ; creep resistance was enhanced in the torsional direction, while somewhat decreased in the tensile direction. This is similar to the cross hardening effect reported in experiments on non-proportional cyclic strain hardening. The circular cycle, as expected, showed a much more significant hardening effect on creep.

Effects of strain amplitude history and temperature history on multiaxial cyclic hardening of type 316 stainless steel.

The title problem was discussed by performing a series of total-strain controlled cyclic tests under uniaxial tension-compressoin and circular strain paths. Constant strain rate of 0.2% /min was specified throughout the tests. The effects of strain amplitude history were examined by changing the strain amplitude between 0.2% and 0.4% (step-up and step-down test) at room temperature, 400°C and 600°C. For temperature history dependence tests, the temperature was changed between 200°C and 600°C, 400°C and 600°C, 500°C and 600°C, respectively by specifying a constant strain amplitude of 0.3%. It was observed that though the history of the preceding cycles with smaller strain amplitude or preceding cycles at lower temperatures was erased by the subsequent cycles with larger strain amplitude or at higher temperatures, the preceding cycles with larger strain amplitudes or those at higher temperatures had considerable effects on the subsequent cycles.