Repeated Relaxation Tests Research Articles

On the plasticity mechanisms of lath martensitic steel

The plasticity mechanisms of press hardening steel with a fully lath martensite microstructure were examined experimentally by strain rate sensitivity measurements, repeated relaxation tests and internal friction measurements. The analysis of relaxation tests suggests that the micro-plasticity could be due to the motion of mobile non-screw dislocations, based on mobile dislocation exhaustion observed in the micro-plastic range. In the macro-plastic range, the plasticity is thought to be due to the generation of mobile screw dislocations. The solute carbon-dislocation interaction results in a negative strain rate sensitivity and a Snoek-Köster-Kê peak in the internal friction spectrum of the lath martensitic press hardening steel. The magnitude of the effective activation volume and its stress dependence indicate that plastic deformation is most likely controlled by screw dislocation motion by formation and lateral movement of kink pairs dragging solute carbon atom atmospheres. Both isotropic and kinematical hardening seem to play a role in the strain hardening behavior of lath martensitic steel.

High temperature deformation mechanisms in the 14% Cr ODS alloy

The stress relaxation behavior of the ODM401 ODS steel is tested at the temperature range 400–800°C in order to investigate the deformation mechanisms. The repeated relaxation tests are performed on the tensile specimens within the plastic strain range 10−4–10−1. The deformation mechanisms are analyzed in terms of an activation volume, dislocation density and velocity, a thermal stress component and a strain rate hardening. The deformation is driven by the dislocation glide and climb. Dispersed particle-dislocation interactions is a dominant effect at temperatures 400–600°C. Short-range interactions between dislocations and a lattice are most significant at 600°C. The non-conservative motion of dislocations becomes active at temperatures above 600°C. The majority of the mobile dislocations is not trapped and is able to move freely in the microstructure.

Deformation and fracture behavior of the P91 martensitic steel at high temperatures

The stress relaxation behavior of the P91 martensitic steel is tested for the temperature range 400–600°C in order to investigate the plastic deformation and fracture processes. The repeated relaxation tests are performed on the tensile specimens at the stress level of 95% of the ultimate tensile stress. The deformation mechanisms are analyzed in terms of an activation volume, dislocation density and velocity, a thermal stress component and a strain rate hardening. The deformation is driven by the dislocation glide. Short range interactions between dislocations and a lattice become dominant at the studied temperature range. The dislocations are able to overcome a pinning effect by the thermal activation at 600°C.

Repeated load relaxation testing of pure polycrystalline nickel at room temperature using nanoindentation

We present the results of repeated relaxation tests using nanoindentation to derive the activation volume of the dislocation velocity and the ratios of the dislocation density and dislocation velocity. An experimental technique, based on classical uniaxial relaxation experiments, was developed to establish a constant strain during repeated load relaxation transients and then to calculate the stiffness of unloading, and therefore the hardness, across the transients with acceptable results. We found that the activation volume of the dislocation velocity from our nanoindentation methodology was in good agreement when compared to the same reported for uniaxial experiments.

Plastic deformation of icosahedral Al-Pd-Mn single quasicrystals I. Experimental results

Abstract In order to compare plastic deformation along different loading axes, icosahedral Al—Pd—Mn single quasicrystals were deformed in uniaxial compression along twofold and fivefold axes, between the lowest temperatures where deformation at strain rates of 10−5 and 10−4s−1 was possible and 820°C. For the first time, experiments were performed below 680°C. The strain rate and temperature sensitivity of the flow stress were studied by stress relaxation and temperature change tests. In a range of strain of steady state deformation following a yield drop, the flow stress and its strain rate sensitivity increase strongly with decreasing temperature. The temperature sensitivity shows a maximum at about 700°C. The data hardly depend on the specimen orientation. Repeated relaxation tests indicate changes in the microstructure during the relaxations. Transient phenomena also occurred when the relaxation tests had not been started from steady state deformation. e.g. before the upper yield point, after long relaxations or after unloading. The results will be discussed in Part II of this paper.

Triaxial stress relaxation tests on Saskatchewan potash

Relative to the study of creep, the stress relaxation phenomenon has been overlooked in research on salt rocks, especially with respect to the behavior of salt in deep underground mining environments. In the laboratory tests described in this paper, deformational properties of potash were first studied by performing reverse-loading tests that approximately simulate in situ loading paths during excavation of underground cavities. Potash relaxation properties were then investigated by multiple-stage, repeated relaxation tests along the reverse-loading path. Results show that potash behavior along a reverse-loading path is highly stress rate dependent and is reflected in the dependence of the initial axial deformation modulus, ductile point, and post-yield behavior on the rate at which the deviatoric stress is applied to the specimen. The inserted relaxation phases did not significantly affect the subsequent behavior of the potash, and therefore inserted relaxation tests are useful for investigating time-dependent behavior at various stages of deformation. A linear relationship exists between normalized stress drop and logarithm of time, which might imply that potash has no creep limit. Finally, an attempt was made to extend a uniaxial strain hardening constitutive law postulated by another researcher to include confining pressure, and the experimental data were found to approximately fit the proposed constitutive model. Key words : potash, stress relaxation, triaxial tests, constitutive model.

THE OVERSTRESS DEPENDENCE OF INELASTIC RATE OF DEFORMATION INFERRED FROM TRANSIENT TESTS

Transient tests are used to demonstrate unusual or even paradoxical deformation behaviors in metallic alloys and polymers at ambient and elevated temperatures. Included are repeated relaxation tests and creep tests of short duration. It is shown that creep rate need not increase with a stress increase and that, at the same stress level, creep rate can be different on loading and unloading. Also, a stress magnitude increase can be found during relaxation. Once steady inelastic flow is reached, the relaxation rate is nearly independent of the stress and strain at which relaxation starts; it depends mainly on the preceding prior inelastic strain rate. These phenomena can be modeled using an overstress (effective stress) dependence of inelastic rate of deformation and a proper evolution law for a single state variable, the equilibrium stress (back stress) within the context of a “unified” state variable theory. The author and his students have developed the viscoplasticity theory based on overstress (VBO) that can model these unusual behaviors.