Load relaxation studies of zircaloy-4

Load relaxation studies of a metallic glass

An attempt has been made to determine if the material parameters in constitutive equations for inelastic deformation which fully accommodate the deformation history dependence of inelastic properties can be evaluated with sufficient accuracy at elevated confining pressures for such equations to be of interest in characterizing geological materials. Accordingly, a large number of constant displacement rate and load relaxation experiments (or variants thereof) have been conducted on Carrara marble at 200 MPa confining pressure in the temperature range 120 to 400°C, and the results have been interpreted in terms of Hart's state variable description of inelastic deformation. The observed mechanical behavior conforms to Hart's description, with a changeover at about 250°C from deformation rate controlled by dislocation glide controlled processes (as represented by concave upward () load relaxation curves), to rate control by strongly thermally activated processes (as represented by concave downward relaxation curves). All relaxation curves generated at a given temperature may be superposed by a translation in fixed direction in the () plane. This defines a master relaxation curve which represents what any individual relaxation curve would look like if it could be determined over a sufficiently wide range of strain rates. For Concave downward relaxation curves the master curve extends over 9 orders of magnitude of strain rate and, in consequence, substantially eases the problem of extrapolating the laboratory data to geological strain rates. The success of Hart's description in accounting for the mechanical behavior means that it is possible to avoid the steady state approximation both in the experimental evaluation of inelastic flow laws and in the application of those flow laws in geophysical modeling problems. Moreover, it provides a simple method of quantifying the mechanical state of a material and thereby is of potential significance for the problem of correlating microstructural observations with mechanical properties.

The application of Hart's state variable description of inelastic deformation to Carrara marble at T < 450°C

The time-dependent deformation of materials or creep governs the useful life of many engineering structures It assumes even higher significance in the case of structures constituting a nuclear reactor, wherein materials bombarded with neutrons develop defects that assist faster diffusion leading to greater plastic deformation As a result, an understanding of the creep deformation process and factors controlling it is necessary for gauging the usefulness of materials in a nuclear reactor as well as for predicting life-times of various structures Thus in this work we discuss the various mechanisms of creep, the rate controlling factors, deformation mechanism maps and useful life prediction methodologies We also identify a few cases where direct application of simple creep correlations might not be feasible Finally, we discuss the various factors that control the creep behavior of materials in light water reactors

Creep deformation of materials in light water reactors (LWRs)

The significance of a scaling behaviour in the creep and stress-relaxation log σ-log ġe curves is analysed. It is shown that such a property imposes some restrictions on the parameters of the theoretical models. Finally, the formalism is applied to some constitutive equations used in the literature to describe creep and stress-relaxation data.

On the scaling behaviour in a logσ-log ġe diagram

At low homologous temperature the plastic strain rate seems to be controlled largely by dislocation glide friction. However, since a sizeable fraction of the applied stress σ is dissipated in overcoming the strong barriers due to dislocation tangles generated by strain hardening, only a portion of the applied stress is actually expended against the frictional resistance. A recent model for this process, proposed by Hart, includes the role of dislocation pile-ups at the strong barriers. The pile-ups provide a mechanism for producing the internal back stress that reflects the barrier penetration stress. They also appear in the deformation as a stored anelastic strain component. The resultant behavior at low temperature and high stress is similar to that proposed by Gupta and Li. The same model also predicts an anelastic behavior at low stress. Measurements at both high and low stress levels on 316 Stainless Steel have now shown that the predictions of the model are quantitatively consistent at both stress levels.

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N. Nir

Papers

Relationship between anelastic and nonlinear viscoplastic behavior of 316 stainless steel at low homologous temperature