Stress Relaxation Conditions Research Articles

Study of mechanical durability of heat resistant polymers under conditions of stress relaxation (using polybenzoxazole)

A study was made of relaxation properties of aromatic polybenzoxazole over a wide range of temperature and deformation. Detailed analysis of the relaxation behaviour of this polymer revealed substates distinguished by different rates of relaxation processes in the range of glass-like states. It was established that linearity of mechanical behaviour of polybenzoxazole is observed over a fairly wide range of temperature and deformation. It was shown that the principle of temperature-time analogy holds good fairly satisfactory both at low and high deformations, although the temperature dependence of the shear factor does not conform to the Williams-Lundell-Ferry equation in the entire range glass temperature.

Study of three-dimensional polystyrene networks containing linear chains: 2. Relaxation spectrum of linear chains

A study of the relaxation phenomenon in polystyrene networks containing linear polystyrene chains of high molecular weight has been carried out using measurements in methylcyclohexane and in the dry state by dynamic sinusoidal deformation and static stress relaxation. It is shown that the relaxation of the free chains contained in the network is (i) proportional to (1 − v 2 2) where v 2 is the volume fraction of the crosslinked network in the dry state, and (ii) depends very little on their concentration in the network. The distribution of relaxation time shows a peak related to the glass transition of the networks and a second peak corresponding to the spectrum of the linear chains of high molecular weight.

Effect of vacuum on various stages of fatigue at high and low strain amplitudes

1. Vacuum has no effect on cyclic strain-hardening of copper at small or large strain amplitudes. A difference in the strain-hardening of the specimens tested in the intermediate strain region is determined by a shift in the vacuum of the point of transition from large-amplitude facture to small-amplitude fracture. 2. At small strains vacuum affects the crack initiation as well as its propagtion. This effect is particularly great during the crack growth stage with the result that in vacuum this stage becomes comparable in duration to the crack propagation stage. 3. At large strain amplitudes vacuum affects most the length of the crack propagation stage. 4. Two different mechanisms of the effect of vacuum on the fatigue fracture process in vacuum are observed for small-and large-amplitude strain; they are due to the homogenization of slip and to the improvement of stress relaxation conditions at the crack tip.

Effect of long-term storage of polycarbonates on stress relaxation

The effect of long-term storage (three-five years) on the mechanical properties of polycarbonates and Makrolon has been experimentally investigated under conditions of stress relaxation at normal and elevated temperatures.

Investigation of the mechanical properties of acrylics under conditions of creep, stress relaxation, and harmonic vibration

The dynamic and static (creep, stress relaxation) mechanical properties of various acrylics have been investigated. A relation is established between the structure and the macroscopic mechanical properties. The previous thermal history and the characteristics of the state of stress are shown to affect the nature of the relaxation processes and the mechanical properties of the acrylics. Generalized stress relaxation curves are constructed. The mechanical glass transition temperatures of the acrylics investigated are determined.

Long-term strength of molybdenum alloys during cyclic heating

1. The long-term strength of molybdenum alloys subjected to cyclic changes of temperature is considerably lower than the long-term strength at constant temperature. The long-term strength of each alloy decreases in a certain temperature range if the holding time at the maximum temperature of the cycle is shorter than some critical time. 2. The temperature range in which the long-term strength declines is 1500–2300° for TsM-2A and 1000–1900 for TsM-3. The critical holding time at the maximum temperature of the cycle is 50 sec for both alloys. Only at temperatures that are characterized by the largest reduction of the long-term strength (1700–1750° for TsM-2A and 1250° for TsM-3) is the critical holding time over 50 sec. Reducing the holding time at these temperatures leads also to a sharp reduction of the long-term strength during thermal cycling. 3. It can be considered that for alloys of the solid solution type (TsM-2A) the drop of the long-term strength depends mainly on the cyclic thermal stresses that occur in rapid heating and cooling and the stress relaxation conditions. The long-term strength of the two-phase alloy hardened with carbides (TsM-3) may be substantially affected by internal stresses of structural origin that occur during thermal cycling and lead to larger thermal cycling effects.

Failure of austenitic steel under conditions of cyclic stress relaxation at 750�C

1. Failure under conditions of cyclic stress relaxation of parts (or specimens) of austenitic steel, possessing a considerable reserve of plasticity at a given temperature, occurs after the value of the mean stress σ mean attains or exceeds the level of stress-rupture strength. 2. Failure in cyclic relaxation may be observed not only at the peak stress of a cycle but also in the period of stress reduction if the plasticity reserve of the material is exhausted.

Complex stress relaxation of metals at elevated temperatures

AbstractFour engineering metals have been investigated under complex stress relaxation conditions at elevated temperatures. These are a 0·24%C steel tested at 450°C, a commercially pure copper at 250° C, an aluminium alloy at 200°C, and a magnesium alloy at 50° and 20°C. The objects of the investigation were to determine whether simple tensile relaxation tests could provide an acceptable prediction of complex stress relaxation behaviour, and whether simple tensile creep data couldform the basis of a similar prediction of relaxation under complex stresses. Combined tension-torsion relaxation tests were made on thin-walled tubular specimens of each metal; these tests could be considered representative of complex stress conditions in general. The study has shown that tensile relaxation tests can provide a satisfactory prediction of complex stress relaxation. Alternatively, where only tensile primary creep data are available, the mechanical equations of state in association with tensile creep data lead to rea...

Comparison between the rates of plastic deformation under creep and stress-relaxation conditions

Comparison between the rates of plastic deformation under creep and stress-relaxation conditions

Degradation mechanisms of mechanical connectors on aluminium conductors

The factors which affect the performance of mechanical connectors for use on aluminium power cables have been investigated. The initial resistance depends upon surface roughness, mechanical load and degree of plastic deformation. The stability of resistance has been studied under conditions of mechanical-load cycling, stress relaxation, interface oxidation, temperature cycling and current loading. The conditions for designing stable mechanical connectors for use on aluminium conductors are given, and the stability of connectors satisfying these conditions has been illustrated by current-cycling measurements.

On the softening of heat-resistant alloys under stress-relaxation conditions at high temperatures

On the softening of heat-resistant alloys under stress-relaxation conditions at high temperatures

Stress Relaxation in Combined Torsion-Tension

In this paper we briefly examine the form of the isothermal constitutive relation of an isotropic simple material under stress-relaxation conditions. We then compare the predictions of the Signorini form of the Bernstein, Kearsley, Zapas elastic fluid theory and the Lianis constitutive equation for single-step stress-relaxation histories and show that they are identical. Using these theories, we develop theoretical expressions for the torque and axial force in combined torsion-tension stress relaxation. Experiments were performed on samples of an SBR polymer, and the observed response is compared with theoretical predictions.

Zero point oscillation energy effect on plastic deformation in solidified gases

AbstractAn investigation is made of the deformation of molecular crystals of methane and argon under the conditions of tension and stress relaxation. The Peierls barriers are shown to be the main obstacles for dislocation movement in these crystals in a wide low‐temperature range. The result obtained is in good agreement with data determined for hydrogen, deuterium, and ammonium and is discussed in terms of the peculiarities of intermolecular interactions in molecular crystals. In particular, it is shown, that the contribution of zeropoint oscillation energy to the bonding energy increases with decreasing temperature at which the transition from the Peierls mechanism to higher energy mechanism occurs.

Typical Lubricating Greases as Linear Viscoelastic Materials

The results of an experimental study of rheological properties of typical lubricating greases are reported in this paper. Viscosity-shear rate relations are compared for steady-state shear and for transient shear conditions of stress relaxation and creep. In steady-state shear the systems are non-Newtonian, but under transient conditions of shear stress relaxation, they are linear viscoelastic for strains approaching their ultimate yield strain. The linear viscoelastic nature of two greases was confirmed by studies of the damping of a torsion pendulum for small strains. A sodium soap grease is shown to be linear viscoelastic by an analysis of creep data from the literature. Recognition of the wide range of strains for linear viscoelastic behavior simplifies the study of the flow and deformation of greases.

Stress Relaxation in Solids

THE purpose of this communication is to direct attention to a similarity in the shape of stress–log time graphs relating to static stress relaxation in solid materials. For a number of high polymers and metals the following relation between the inflexion slope F of the stress–log time curves and the total decrease in stress Δσ was found: F is defined as (dσ/d ln t)inflexion. A condition for the applicability of equation (1) seemed to be absence of internal stresses, that is, careful annealing. The substances investigated were rubber hydrochloride1, polyethylene2,3, cellulose and paper4, polycrystalline and single crystal cadmium, indium, lead and tin5, and polycrystalline molybdenum6. Experiments have also been carried out on a number of other substances including polyisobutylene, polyvinyl acetate, beryllium, aluminium, Lipowitz's alloy, cetyl alcohol, and single crystals of lithium fluoride7.