Kinetics Of Stress Relaxation Research Articles

Kinetics of Stress Relaxation in Normal and Superconducting States

AbstractAn investigation is described of the kinetics of stress relaxation and postrelaxational effects in normal (N) and superconducting (S) states of polycrystalline 99.9995% pure lead at 4.2 °K. The work‐hardening coeificient θ(r) during stress relaxation has been found to be much larger than that θ(r) during dynamic deformation. It has also been revealed that θ(r)depends on the state of the sample (N or S state) where θ>θ.

Stress relaxation kinetics and the possibility of separating the deforming stress components at low temperatures

Stress relaxation kinetics and the possibility of separating the deforming stress components at low temperatures

Mechanism of polymethylmethacrylate cracking in various working media

The authors studied the growth of rupture cracks in polymethylmethacrylate (PMM) specimens, the nucleation and growth of a pencil of cracks at the tip of a rupture crack under the influence of various working media, the kinetics of stress relaxation in bent specimens and the dependence of the critical breaking stress of PMM specimens with stress raisers on the time during which this polymer is held under stress in various working media. The results obtained were interpreted on the basis of an assumption that the adsorption-effect is mainly manifested in the initial stages of the action of a working medium; the variation in the above-listed properties due to protracted action of the working media is produced by penetration of these media into the supermolecular polymer structures. Dipole moment, polarizability, and the degree of association of medium molecules were chosen as the criteria of the influence of working media on PMM.

Effect of temperature on the dynamic properties of polyethylene

When the temperature is reduced and the loading rate increased, parts made of low-density polyethylene exhibit brittle fracture. The brittle point depends on the nature of the supermolecular structure formed during processing. A method is proposed for determining brittleness by means of impact tests that also gives qualitative information about the internal stress relaxation kinetics.

MECHANICAL HYSTERESIS IN INDIUM-THALLIUM ALLOYS

Alloys near the composition In-20 at. % Tl are examples of a class of material that has a twinned non-cubic structure and possesses high mechanical hysteresis. The present investigation is concerned with the mechanism by which this high damping is generated. It is well established that the application of stress to the twinned tetragonal structure moves the twin boundaries in the sense that gives partial relief of the applied strain. It is further observed that the boundary movement lags behind the stress during cyclic straining. The strain corresponding to the boundary hysteresis can be deduced from the twin crystallography and the unit cell parameters and agrees with the measured hysteresis. So it is clear that the hysteresis arises from the irreversible boundary movements. The purpose of this investigation was to establish the cause of this irreversible movement. The torsional hysteresis of 2 mm diameter cylinders of polycrystalline In-Tl alloys was measured by plotting torque-twist curves during cycles of stress between constant strain limits over the frequency range 0.004 to 3.0 C/s and for temperatures from - 65 to + 45 °C. The hysteresis loops for 18,20 and 22 at. % Tl reached a maximum area at temperatures which depended on frequency through an activation energy of about 18,000 cal. mole. This is close to the published value for the tracer diffusion of Tl in In of 15,500 cal.mole, which is not a particularly reliable figure because of doubts in the interpretation of the experimental data. The relationship between these two energies suggests that the mechanical hysteresis might be explained by the existence of a force on the twin boundaries caused by an ordering of Tl atoms relative to the orientation of the C axis of the tetragonal cell. If boundaries are suddenly moved at low temperatures the orientation of the C axis will change in the swept-out region and the Tl atoms, which will not have time to jump to the new preferred sites, find themselves in high energy positions. This generates a force which will return the boundaries to their original position when the stress is removed. The rubber-like behaviour at low temperatures is thus explained. At higher temperatures the atoms rapidly move to the newly preferred sites, the boundary is stabilised in the new position, and a permanent deformation results. These mechanical characteristics are reproduced by the model of a Maxwell Solid, comprising a spring and dash-pot in series, and so the possibility that this model reproduces the behaviour of In-Tl was explored. This was done by comparing the hysteresis and the stress relaxation behaviour. When the twin boundaries in the In-Tl alloy are suddenly displaced, the stress required to keep the strain constant steadily relaxes ultimately to zero, as is the case with the Maxwell Solid. It did not prove to be possible to relate the kinetics of stress relaxation, however, to the hysteresis behaviour. The reasons for this will be discussed, in terms of the likely existence of several mechanisms contributing to the deformation behaviour.

Mechanisms of Stress Relief in Polycrystalline Films

The stress required to operate dislocation sources within a grain, at a grain boundary, and at surfaces is found to be larger than the intrinsics tresses observed in polycrystalline films. It is therefore unlikely that a dislocation flow mechanism can relieve stresses in films. Grain boundary sliding and diffusional creep can, however, relieve stresses in films and equations describing the kinetics of stress relaxation are derived. It is suggested that stress relief occurs primarily by a diffusion-creep mechanism. Growth of hillocks during annealing of a film is briefly discussed in terms of the diffusion-creep mechanism.

Kinetics of Stress Relaxation in Metals

The phenomenon of stress relaxation, the time decrease of internal stress due to localized flow in a solid under constant uniaxial strain, is viewed in terms of present kinetic theories and available data for metals. Rate equations for shear flow, derived from molecular models of liquids and gases, are reviewed briefly. Combined with the theory of elasticity, these equations are shown to be identical to the classical Maxwell model and to the empirical logarithmic form in limiting cases for stress relaxation in solids. Without reference to the specific details of the localized flow, the general equations are found to correlate uniaxial data for many types of steels, copper, aluminum, magnesium, and lead for various temperatures and stress levels. In all of the data, one characteristic controlling reaction is found, and in some cases a secondary reaction dependent upon the level of the uniaxial prestress is also found. For nonaging metals, the data fit equations in which the relaxation stress approaches zero asymptotically in time. A simple modification of the rate equations is made so that an age-hardening metal can approach a nonzero stress during relaxation. Prestress is used as a controlling variable in correlating data for uniaxial stress relaxation. Apparent heats and entropies of activation are calculated for Inconel and copper at elevated temperatures. For nearly all metals, the apparent free energy of activation for the controlling reaction in the relaxation process decreases slightly as the prestress is increased.