Showing papers on "Stress relaxation published in 1972"

Grain Growth and Stress Relief in Thin Films

Abstract: The effect of grain growth on stresses in films is discussed. Grain growth generally leads to stress relaxation when the film is under a compressive stress. When the initial grain size is below a critical value, grain growth can occur by normal boundary migration and generates a tensile stress in the plane of the film. Grain growth proceeds until a particular grain size is reached, at which the sum of the strain energy and surface energy is minimized. Further grain growth can occur if plastic flow relieves some of the strain energy. When the initial grain size is above the critical size, there is no minimum in the energy vs grain size relationship, and grain growth to infinite size can occur by normal boundary migration. In the presence of impurities, apparently anomalous intrinsic stresses in films may be produced.

The influence of oxygen concentration on the internal stress and dislocation arrangements in α titanium

Abstract: The temperature dependence of the long range (internal) and thermally activated components of the flow stress have been measured by a stress relaxation technique over the temperature range 200 to 550 K in α titanium containing five different levels of oxygen. In addition, the dislocation arrangements have been studied using thin foil electron microscopy techniques. In the higher oxygen materials it has been found that a transition from wavy to planar slip occurs towards lower temperatures. The internal stress varies more strongly with temperature than would be predicted by the temperature dependence of the elastic modulus; simultaneously, the thermally activated component of the flow stress (t*) obtained as the difference between the flow and internal stresses, goes through a maximum at the temperature where the internal stress (Tint) becomes strongly temperature dependent. An increase in t* and rise of Tint accompany the onset of planar slip.

The Dynamic Stress-Strain Behavior in Torsion of 1100-0 Aluminum Subjected to a Sharp Increase in Strain Rate

Abstract: : A modification of the torsional split Hopkinson bar is described which superimposes a high rate of shear strain on a slower 'static' rate. The 'static' rate of 0.00005/sec is increased to 850/sec at a predetermined value of plastic strain by the detonation of small explosive charges; the rise time of the strain rate increment is about 10 microseconds. During deformation at the dynamic rate, direct measurement is made of the excess stress above the maximum static stress attained. Results for 1100-0 aluminum show that the initial response to the strain rate increment is elastic, followed by yielding behavior reminiscent in appearance to an upper yield point. The magnitude of the stress measured at this yield point is always less than the stress obtained at the same strain in a wholly dynamic test; as the stress-strain curve asymptotically. It is concluded that the material behavior is a function of strain, strain rate and strain rate history. (Author)

The Theory of Stress Wave Radiation from Explosions in Prestressed Media

Abstract: Stress wave radiation from underground explosions has been observed to contain an anomalous shear wave contribution which is most likely of tectonic origin. In this paper the theoretical radiation field to be expected from an explosion in a prestressed medium is given under the assumption that no secondary low symmetry faulting on a large scale occurs and that the total tectonic component of the field is due to stress relaxation around the roughly spherical fracture zone created by the explosive shock wave. Evidence for the occurrence of this simple kind of tectonic source is considered, and it is concluded that this model is appropriate in many, if not most, instances involving underground explosions. Expressions for the spectrum of the radiation field and its spatial radiation pattern are given in terms of multipole expansions for the components of the rotation potential and the dilatation potential. Several possible rupture formation models are treated. All models show that the tectonic radiation is of simple quadrupole form, as has been observed. The energy radiated due to stress relaxation is considered in detail, and it is also shown that, in terms of the energy released, a dislocation source can be used as an equivalent for the stress relaxation effects. The theoretical energy partition between compressional and shear waves for the tectonic field is in the ratio of (approximately) 1 to 10, so that tectonic stress release does not affect the direct compressional body wave particularly, but gives rise to totally anomalous SH polarized waves (e. g. Love waves) and affects Rayleigh type surface waves significantly, as is also observed. The theory can be applied to obtain estimates of source dimensions and the orientation and magnitude of the initial prestress field in the region of the explosion. In addition, application of this particular form of the general tectonic source theory to deep earthquakes and volcanic earthquakes also appears to be reasonable in view of the probable high symmetry of the failure or phase transition regions for such events.

A Modified Recovery-Creep Model and Its Evaluation

Abstract: Based upon previous treatments of the recovery-creep theory a model. has been formulated that takes into account the separation of the applied stress into an effective stress and an internal stress and the experimentally determined stress-dependence of the activation area. The model describes the time- and stress-dependence of the creep rate in the primary and the secondary stages. It is shown that the theory adequately simulates the experimental creep curves. The attainment of constant true and apparent activation energies, and of a constant exponent in the creep rate/stress power law, are interpreted as different consequences of the internal stress being approximately equal to the applied stress at low stresses and high temperatures. The physical significance of the apparent activation energy is discussed in terms of the model.

Biaxial stress relaxation in glassy polymers - Polymethylmethacrylate.

Abstract: Biaxial stress relaxation studies were performed on glassy polymethylmethacrylate in combined torsion-tension strain fields using a specially designed apparatus with exceptionally high stiffness and low cross talk between the torsional and tensile load measuring transducers. It was found that at low strain levels uniaxial tension relaxation is slower than pure torsion relaxation; tensile-component relaxation rates are unaffected by the level of torsional strain; torsional-component relaxation rates decrease as tensile strain is increased; uniaxial tension relaxation rates approach the pure torsion rates at higher strains (about 2%). A phenomenological treatment is presented which shows that relaxation rates can be coupled to the strain fields in which they are observed and yet be consistent with the concepts of linear viscoelasticity and the Boltzmann superposition integral.

An interpretation of steady state creep

Abstract: Effective stress in steady state creep in aluminum was measured by the strain transient dip test technique with the aim to analyse the creep in terms of two separate kinetic processes driven by different components of the applied stress [sgrave]: the effective stress [sgrave]∗ and the internal stress [sgrave]i (Ahlquist, Gasca-Neri and Nix 1970). The analysis has led to values of’ the apparent activation energy of slip ‘, Q∗, and ‘the apparent activation energy of recovery’, Qi, which generally differ from the apparent activation energy of creep and, therefore, also from the activation enthalpy of lattice self-diffusion, and cannot be correlated with the process controlling dislocation glide velocity and recovery rate, respectively. This supports the contention that if [sgrave]∗ is measured by a dip test technique [sgrave]i, obtained as [sgrave]—[sgrave]∗, cannot be identified with the internal stress taking part in processes other than the dislocation glide. Assuming that the recovery is driven ...

Dynamic mechanical and rheo-optical studies of collagen and gelatin.

Abstract: The frequency dependence of dynamic mechanical properties of rat tail tendon (RTT), enzyme-solubilized collagen membranes (ESC), AKM-23 dialysis membranes, and gelatin film have been measured at 110, 11, and 3.5 Hz from - 160 to 220°C. RTT and AKM-23 are devoid of a rubbery region; there are as many as six mechanical loss transitions. Gelatin and ESC membranes behave as rubbery materials above room temperature; only three tan δE peaks can be resolved for these materials. Strain birefringence was used to measure the crystalline and amorphous contribution of orientation induced by strain. Both the birefringence and the strain optical coefficient are sensitive to the amount of water in a sample. The effect of chemical swelling agents and of annealing on birefringence are described. Stress relaxation data on gelatin film were analyzed with the rubber elasticity theory to give the average number of chains per unit volume, the specific polarizability, the stress-birefringence ratio and the average molecular weight between hydrogen bonds were calculated. The intrinsic amorphous birefringence for “wet” gelatin film is 1.25 × 10−2; it is estimated to be about 6 × 10−2 for “dry” gelatin film.

Fracture behavior in nylon 6 fibers

Abstract: Electron paramagnetic resonance (EPR) techniques are used to determine the number of free radicals produced during deformation leading to fracture of nylon 6 fibers. A reaction rate molecular model is proposed to explain some of the deformation and bond rupture behavior leading to fracture. High-strength polymer fibers are assumed to consist of a sandwich structure of disordered and ordered regions along the fiber axis. In the disordered or critical flaw regions, tie chains connecting the ordered or crystalline block regions are assumed to have a statistical distribution in length. These chains are, therefore, subjected to different stresses. The effective length distribution was determined by EPR. The probability of bond rupture was assumed to be controlled by reaction-rate theory with a stress-aided activation energy and behavior of various loadings determined by numerical techniques. The model is successfully correlated with experimental stress, strain, and bond rupture results for creep, constant rate loadings, cyclic stress, stress relaxation and step strain tests at room temperature.

The low stress creep of aluminium near to the melting point: The influence of oxidation and substructural changes.

Abstract: Creep rates of pure metals measured at high temperatures and low stresses often, agree well with those predicted by the Nabarro-Herring theory. Aluminium has been regarded as an exception since reported creep rates are about three orders of magnitude faster than predicted and other observations could not be accounted for by the theory. Various explanations of these discrepancies have been attempted in the literature involving the effects of oxidation and of substructural changes. In this paper experiments are described in which the low stress creep of aluminium is re-examined with special reference to the influence of these variables. The first part describes experiments on the creep of aluminium in vacuum at 913°K and in the stress range 0-1 MN/m2. At a stress level of ∼ 650 kN/m2a transition occurred in creep behaviour. Above this transition creep rate varied with applied stress σ raised to the power of ∼ 5 typical of a process controlled by dislocation climb and results were in good agreement ...

Uniaxial extension of polystyrene at true constant stress

Abstract: An apparatus for extension of polymers in the fluid and high-elastic states under constant true stresses is described. Determinations are made of the total, high-elastic and flow deformations and their rates in the course of extension of polystyrene having a viscosity-average molecular weight of 3 X 105 at 130 °C under various constant true stresses. The stress relaxation and elastic recoil after the cessation of extension are investigated at various values of deformation. Refore the steadystate flow is attained the viscosity and relaxation characteristics of polystyrene passes through a maximum. As was to be expected, the values of longitudinal viscosity under steady flow conditions correspond to the viscosity values determined during the extension at a constant deformation rate.

Constant stress creep and constant true strain-rate tensile tests of the superplastic alloy PbSn

Abstract: Strain-time curves and stress-strain curves have been obtained for the superplastically deformed PbSn eutectic tested in creep under constant stress and in tension under true strain-rate conditions at temperatures ranging between −44 and 30° C. It is shown that the flow stress does not depend on strain and time and is only a function of the true strain rate, of the temperature and of the initial grain size. The result is that this superplastic alloy does not strain-harden and that the grain size is constant. The apparent activation energy does not depend on stress and temperature and is equal to 11.5±0.5 kcal/mole.

Thermal Instability in a Viscoelastic Fluid Layer. I

Abstract: The stability of a horizontal layer of a viscoelastic fluid (Oldroyd fluid) heated from below is considered. Linear stability theory is used to derive an eigenvalue equation system of sixth order. Consideration is given to the two cases ( a ) both bounding surfaces free and ( b ) both bounding surfaces rigid, the former being solved exactly and the latter appoximately by the Galerkin method. Tables of the various critical values for the onset of instability are provided for various assigned values of parameters. It is shown that the principle of exchange of stabilities does not hold for moderately elastic fluids; and that the critical Rayleigh number is lowered by the presence of a stress relaxation time and raised by the presence of a strain retardation time.

Time Dependent Viscoelastic Properties of Concentrated Polymer Solutions

Abstract: Measurements of stress growth and stress relaxation after onset and cessation of steady simple shear flow in concentrated polymer solutions were carried out with a Weissenberg rheogoniometer R‐17 having a gap servo system. By using monodisperse polymers and their blends, the effect of molecular weight distribution on those transient phenomena is discussed. The so‐called stress‐overshoot was observed in both experiments of shear and normal stress growths. Ratio of the time of the maximum normal stress difference and that of the maximum shear stress is close to 2 at the limit of low shear rate for both monodisperse and polydisperse polymers. In the range of finite shear rate, the ratio is remarkably dependent on shear rate for polydisperse samples, whereas it is almost independent of shear rate for monodisperse polymers. Shear rate dependence of the ratio of the apparent relaxation time of normal stress difference and that of shear stress in stress relaxation experiments is also found to be remarkably affected by molecular weight distribution. From a comparison between these experiments and theories so far published, it is concluded that these transient phenomena may be explained by assuming that the relaxation spectrum is a function of shear rate at least if the shear rates are not too high. The comparison between theory and experiments is carried out without assuming explicit forms for relaxation spectrum.

Stress Relaxation of Solutions of Flexible Macromolecules

Abstract: For stress relaxation after cessation of steady shearing flow with vx=κ0y, the following relation has been proposed for connecting the normal stress difference τxx − − τyy − during steady state flow and the shear stress τyx + during the relaxation process after t=0: τxx − − τyy −=2κ0 ∫ 0∞ τyx +dt It is shown here that this relation can be derived for a dilute suspension of flexible macromolecules represented as a set of N beads joined by N — 1 connectors which may be nonlinear springs; equilibrium‐averaged hydrodynamic interaction is included in the theory A generalization of the above formula may be derived for the stress relaxation following any steady homogeneous flow In the derivation use is made of an expression for the stress tensor which differs from that of Giesekus in that hydrodynamic interaction has been included From the latter formula it is particularly easy to rederive the Lodge‐Wu constitutive equation for the Zimm model with Gaussian springs

Magneto-Plastic Effect in Nickel and Nickel-Cobalt Alloy Single Crystals

Abstract: It is shown that the flow stress and the stress relaxation of Ni and Ni-Co alloy single crystals in a plastic region are affected significantly by applying an alternating magnetic field This “magneto-plastic effect” can be interpreted in terms of a concept that oscillating magnetic domain walls give a force on the dislocation The magnitude of the shear stress drop caused by this effect depends upon the plastic strain, total strain rate, magnetostriction constants, total volume swept out by the oscillating domain walls and frequency of the alternating magnetic field The activation volume of the glide dislocation in thermally activated process is reduced to about one half of its original value by this effect Further, on the basis of the experimental results obtained through this effect, a role of the short range stress to the flow stress is discussed in some detail

Method for predicting prestress losses in a prestressed concrete structure

Abstract: A METHOD IS PESENTED FOR PREDICTING ACTUAL PRESTRESS LOSSES IN A PRESTRESSED CONCRETE STRUCTURE. IT USES AVAILABLE INFORMATION ON CONCRETE CREEP AND SHRINKAGE AND STEEL STRESS RELAXATION, AND INCLUDES THE EFFECT OF INTERACTION OF THE VARIOUS FACTORS CONTRIBUTING TO PRESTRESS LOSS. THE REQUIRED ITERATIVE PROCEDURE AND CURVE TRANSFER CONCEPTS ARE EXPLAINED. /AUTHOR/

Comparison of optical and mechanical limits of linear relaxation behavior in glassy polycarbonate

Abstract: The decay in birefringence of glassy polycarbonate held at constant extension has been studied at 23°C, in the time-scale range 10–103 sec, up to about 6% strain. The results show that, under these conditions, the birefringence can be validly expressed as a linear hereditary integral of the strain history up to a relatively high strain level which is about 3.4% for an experimental time-scale of 100 sec. Comparison with previously obtained data on the stress relaxation behavior of the same polymer shows that, other factors remaining constant, mechanical relaxation is linear only up to about 1.1% strain. The earlier onset of mechanical nonlinearity is discussed and it is suggested that the mechanical relaxation spectrum is richer than the optical spectrum in relatively long relaxation times, corresponding to relatively slow molecular motions. It is further suggested that these slow molecular motions are accelerated first as the polymer is extended beyond the limit of linear viscoelastic behavior. The observed nonidentity between strain limits for linear mechanical and linear optical behavior is discussed in the light of current practices in photomechanical stress analysis.

Polydisperse polymers: Relations among molecular weight distribution, stress relaxation, and distribution of relaxation times

Abstract: Relaxation times in polydisperse polymers were calculated on the basis of more realistic viscosity mixing rules than have previously been used. These relaxation times and mixing rules are in turn used to calculate viscoelastic functions such as stress relaxation following sudden straining or steady shearing. Inversion of these functions provides an accurate way to estimate the molecular weight distribution. This method is useful for insoluble or otherwise intractable polymers.

Effect of Additional Cross-linking of Continuous Chemical Stress Relaxation of cis-Polybutadiene

Abstract: : Continuous and intermittent stress relaxation experiments were carried out on dicumyl peroxide cured cis-polybutadiene in vacuum at several temperatures. An extension of the two-network theory is proposed whereby the effectiveness of a crosslink at various extension ratios is experimentally defined. The effectiveness varies from unity at an extension ratio of unity to zero at very high extension ratios. (Author)

A combined strain and time-hardening non-linear creep law

Abstract: A modified non-linear creep law is proposed, in which each component of creep strain for constant stress is generalized to the case of variable stress by a combined strain-hardening and time-hardening procedure. The resultant constitutive law is represented by a non-linear viscoelastic model containing aging elements. Strain solutions are presented for various simple loadings, and are given graphical interpretations. As an application of the proposed theory, material parameters are determined for two aluminium alloys. An extremely good fit with experimental data is obtained in both cases.

Interpretation of yield drops induced by stress-aging treatments of polymers

Abstract: A formula relating the upper yield stress δ y, following stress aging, in a constant strain rate test to the delay time for yield td([sgrave]r) at a constant nominal reference stress δ r has been derived, viz., where ∊ is the imposed strain rate, E is Young's modulus of the polymer, V∗ is a shear activation volume, kB is the Boltzmann constant, and T is the absolute temperature. Measurements of the upper yield stress following stress aging are reported, which, together with previous measurements of delay times, confirm the form of this equation. It is also demonstrated that stress aging cannot be due to a change in the shape of the shoulder of the neck. Ringing and oscillation of the stress after stress aging are shown to be the result of alternate cycles of thermal runaway of the neck, which causes rapid stress drops, and of further stress aging during reloading, which causes yield points.

Extensional Wave‐Propagation Characteristics in Striated Muscle

Abstract: The real part of the complex dynamic modulus for frequencies 50–10 000 Hz was derived from viscoelastic wave‐propagation constants in whole frog skeletal muscle, in order to provide a material function characteristic of the dynamic longitudinal viscoelastic behavior of resting skeletal muscle in the higher‐frequency range. The method of measuring the phase velocity and attenuation coefficient of longitudinal waves propagated along the longitudinal axis of thin muscles is described. The results showed that there were significant viscoelastic effects between 50 and 5000 Hz with some leveling off above 5000 Hz, as indicated by the frequency dependence of the wave propagation constants. The relaxation time spectrum derived from the dynamic modulus‐frequency function showed that about half of the relaxation times were less than 1 msec. The results were generally consistent with other studies based on the time course of stress relaxation and on the relationship between sinusoidal stress and strain.