The interpretation of cyclic creep deformation mechanism of copper in terms of the anelastic recovery

The cyclic creep deformation behaviour of copper has been studied in the temperature range of 0.4 to 0.5T _m and under the constant stress range of (/E)=4×10^–4 to 10×10^–4. To see the effect of cyclic stress frequency, stress amplitude and the duration of the unloading time in a fixed frequency on cyclic creep behaviour, static and cyclic creep tests were conducted under the conditions mentioned above. The measured activation energies for static and cyclic creep were analysed in terms of the various experimental parameters. Anelastic behaviour during the unloading period was also studied to find out the possible assistance for the positive creep deformation in cyclic creep. Using the concept of anelastic recovery and the activation energy for the anelastic it is hypothesized that the accelerated cyclic creep deformation is controlled by the anelastic recovery during the unloading period.     

The cyclic creep behaviour of Type 316 stainless steel

The cyclic creep behaviour of a Type 316 stainless steel at 625° C has been examined as a function of the maximum applied stress and frequency using trapezoidal loading cycling between zero and a maximum stress. The so-called static-to-dynamic creep transition observed is interpreted in terms of recoverable anelastic strain behaviour without using an internal stress argument. Over the range of experimental conditions examined, failure occurs by static creep modes, namely wedge crack nucleation and growth. The loading strain increments appear to be damaging to about the same extent as the much slower strain occurring at constant load, such that it is the overall strain rate that determines the rate of damage. A cursory examination of square wave load cycling shows that the behaviour is very similar to that observed during trapezoidal loading and suggests that the rate of loading and unloading does not play an important part in determining the creep and rupture behaviour.     

Static and cyclic creep behaviour of SiC whisker reinforced aluminium composite

Abstract

Static and cyclic creep tests were carried out in tension at 573–673 K on a 20 vol.-%SiC whisker reinforced aluminium (Al/SiC_w ) composite. The Al/SiC_w composite exhibited an apparent stress exponent of 18·1–19·0 at 573–673 K and an apparent activation energy of 325 kJ mol^-1 for static creep, whereas an apparent stress exponent of 19·6 at 623 K and an apparent activation energy of 376 kJ mol^-1 were observed for cyclic creep. A cyclic creep retardation (CCR) behaviour was observed for the Al/SiC_w composite. The steady state creep rate for cyclic creep was three orders of magnitude lower than that for static creep. Furthermore, the steady state creep rates of the composite tended to decrease continuously with increasing percentage unloading amount. The static creep data of the Al/SiC_w composite were rationalised by the substructure invariant model with a true stress exponent of 8 together with a threshold stress. The CCR behaviour can be explained by the storage of anelastic strain delaying non-recoverable creep during the onload cycles.     

Elastic and anelastic behaviour of icosahedral quasicrystals

A theory is developed, based on theoretical-group analysis, to describe the linear, reversible, time-dependent response of an icosahedral quasicrystal, containing point defects, to a stress field and known as anelastic relaxation. We obtain also anelastic relaxation relationships for the practical Young’s, shear and Poisson’s moduli.     

The creep behaviour of isotropic polyethylene

Dead loading creep and constant strain rate yield experiments have been used to study the tensile creep behaviour of three grades of isotropic polyethylene. This has provided further evidence for the existence of two yield points in isotropic polyethylene. Two different models have been used to attempt to describe this behaviour. Although the results can be described by to both the two process model of Wilding and Ward and the co-operative jump model of Fotheringham and Cherry, it appears that the two process model provides a more convincing quantitative fit to the data. This revised version was published online in September 2006 with corrections to the Cover Date.     

The recovery of mechano-sorptive creep strains

Mechano-sorptive strain was generated in strips of rectangular section stressed in torsion with the humidity cycled between 38% and 84%. After a number of humidity cycles the specimen was unloaded, the applied stress was reversed and further humidity cycles were imposed. All the initial strain was recovered and reversed strain was developed. Tests were carried out at two maximum shear stress levels, 1.12 MPa and 5.05 MPa. After seven humidity cycles at the higher stress a large mechano-sorptive strain of about 0.06 was developed, which was nearly nine times the initial elastic strain. During the development of this strain no significant change of the shear modulus occurred, from which it is concluded that the large MS strain produces no permanent change of the features of the cell wall structure that determine the shear modulus. There are close parallels between the mechanism generating the MS strains and that responsible for the large plastic strains reported by Keckes et al. in wet compression wood loaded in tension. In both cases it is suggested that the shear strains arise from the breaking and reforming of hydrogen bonds between parallel polymer chains.     

The effect of prior cyclic loading variables on the creep behaviour of ex-service Type 316H stainless steel

Abstract

Initial tests have been conducted for a systematic survey of the effect of prior cyclic loading on subsequent creep properties. Samples of 316H stainless steel were subjected to prior cyclic loading at 550°C at different combinations of strain range and cycles experienced. These samples were then remachined into uniaxial creep specimens and tested under a constant load at 250 MPa at 550°C.

The initial results from specimens subject to prior cyclic loading show significant decreases in the minimum true creep strain rate of between 30 and 94%. A consistent decrease in the minimum creep strain rate was found with increases in both the strain range of the prior cyclic loading and the number of cycles experienced by the sample. In addition, the prior-cyclic loading has significantly changed the shape of the creep curve to varying degrees depending upon the applied cyclic loading.     

Effect of prestrain on cyclic creep behaviour of a high strength spring steel

Cyclic creep and fracture behaviour of SAE spring steel 5160 under two prestrain conditions were studied. The cyclic creep rate, cyclic plastic strain range, total plastic strain range and cyclic creep life of the material were all found to depend strongly on the prestrain condition. It was also found that, after a certain number of cycles, cyclic hardening would reach a saturation state, followed by cyclic softening which would last until the final failure. However, a higher prestrain delayed the appearance of cyclic softening. Prestraining was also found to extend considerably the third stage of cyclic creep and thus increase the elongation to fracture of the material to the extent observed in a uniaxial tension test. Fracture surface study exhibited a mixed mode failure process and also a transition from quasi-brittle fracture at the lower prestrain, consisting of both intergranular and transgranular features, to ductile fracture at the higher prestrain, which is mainly made of dimples and is similar to that found in a uniaxial tension test. However, in the ductile fracture area, some transgranular facets and fatigue striation regions could still be observed. Finally, when the prestrain reached a certain value, compressive cyclic creep was detected and this is attributed to a strong Bauschinger effect.     

Transitions in creep behaviour

Attempts to identify the mechanisms operating during creep are often made by examining plots which yield apparent activation energies, or the stress or grain size-dependences of creep-rate. The forms of such plots are here examined and the ambiguities which arise near transitions from one regime to another are noted. The ranges of temperature, stress and grain size commonly used are inadequate and serious errors in interpreting the results of creep tests will continue to be made until a better understanding of the interaction of the basic processes is developed, so as to enable the positions of transitions to be predicted.At Dept. of Metallurgy, University of British Columbia, Vancouver 8, BC, Canada until 31 March 1970.     

The mechanism of creep behaviour in glassy polymers

A model is presented to describe the creep behaviour of glassy polymers below the glass transition temperature. It consists of a Hookean spring in series with a non-Newtonian dashpot having an entropy spring in parallel. The shape of the response of this spring is deduced from a master curve, giving the extension as a function of logarithm of time, built from creep data, reported here and obtained on polycarbonate over a wide range of times and temperatures. The model takes into account a number of aspects of creep behaviour and predicts a threshold stress beneath which delayed yielding no longer occurs. Torsional creep data, obtained on Polyvinylchloride by Mallon and Benham are found to be in excellent agreement with the proposed model.     

Negative creep and recovery during high-temperature creep of MgO single crystals at low stresses

High-temperature creep equipment with very high precision has been used to measure the creep of MgO single crystals above 1948 K and stresses lower than 4 MPa. A transition in exponent,n, from 3 at stresses higher than 2 MPa to almost unity at lower stress region was observed. Since in a single crystal deformation can only occur by the generation and movement of dislocations, the transition in stress exponent from high to low stress region cannot be interpreted in terms of a change from dislocation to diffusional creep processes. Decreasing the stress by a small amount during steady-state creep resulted in an incubation period of zero creep rate before creep commenced at lower stress. However, large stress reduction led to a period of negative creep during which the dislocation substructure coarsens and the subgrain cell boundaries straighten. On the basis of dislocation substructure studies, it is proposed that the kinetics of backflow are thought to be based on the local network refinement caused by the reverse movement of dislocations and that recovery is necessary before further movement of dislocation can occur. It is shown that the network theory proposed by Davis and Wilshire can satisfactorily account for all stress reduction observed during forward creep.     

The creep behaviour of ultra-high modulus polypropylene

The creep and recovery behaviour of highly drawn polypropylene monofilaments has been studied over the temperature range 20 to 50° C. A range of samples was examined to identify the influence of draw ratio and molecular weight. It is concluded that the permanent flow creep arises from the presence of two thermally activated processes, one of which relates to the -relaxation process and is associated with the crystalline regions of the polymer, and the second with the molecular network.     

The creep behaviour of Synroc and alumina

The creep of Synroc C and alumina in four-point bending in argon was investigated in terms of the relaxed symmetric stress and the reference asymmetric stress; the alumina being used as a reference material. The creep tests were undertaken in the temperature range from 850°C to 1300°C. The rupture behaviour of Synroc at 950°C indicated a high stress exponent, and that the creep ductility was unusual in that the strain increased with increasing test time. A scanning electron miscroscopy examination of Synroc after creep revealed the development of defect-free oxidised surface layers. For Synroc, neither prior exposure to pre-heating in air, nor prior indentation affected the creep rate behaviour. This was attributed to the formation of the oxidised surface layers and the associated healing effects of the damage produced by the indentations.     

Work hardening and recovery during compressive creep of polycrystalline MgO

High precision equipment has been used to study the effects of small stress changes during steady state creep of magnesia at 1596 K. When the stress,σ ₁, is reduced by a small amount,Δσ, the creep rate decreased to zero for a period,Δt, before accelerating to a new steady value. Calculating the rate of recovery,r(=?δσ/δt) asΔσ/Δt and the coefficient of strain hardeningh(=δσ/δε) asΔσ/Δε (whereΔε is the instantaneous strain recorded on increasing the stress byΔσ) gave the ratio,r/h, which predicts accurately the observed steady creep rate, \(\dot \in _s \) . It is proposed that when \(\dot \in _s \) ∝ σ ³, creep is recovery controlled. The results are explained in terms of a model for creep in which the rate controlling process is the growth of the 3-D dislocation network within subgrains, to form dislocation sources allowing slip to occur.     

On the role of grain-boundary migration during the creep of zinc

Constant engineering strain-rate tensile tests have been carried out in the temperature range 20 to 150C on high purity Zn, Zn-0.14 at. % Cu (alloy C) and Zn-0.16 at. % Al (alloy A) alloys. Measurements of angular distribution of orientations of grain boundaries have been used to study grain-boundary migration during deformation. Significant cavitation, with increasing propensity at higher test temperatures, occurred in the two alloys but not in pure Zn. A striking feature of the observations in pure Zn and alloy C, as the test temperature was raised, was the formation and subsequent decay of a diamond pattern of uncavitated grain boundaries, a majority of which were preferentially aligned at 45 to the stress axis. By comparison the changes in the angular distribution of grain boundaries was least marked in alloy A. Cavitation was observed in alloy C to maintain grain boundaries in the 45 orientation. At the test temperature of 150C alloy C, which was prone to the formation of diamond grain-boundary configuration, developed much larger volume fraction of cavities than alloy A. These results are discussed in terms of the different distribution coefficients of Cu and Al in Zn, the different rates of grain-boundary migration in pure Zn and the two alloys and the differences in the substructural features (cells) formed during high-temperature deformation.     

The effect of cyclic creep on grain-boundary cavitation in ceramics

Small-angle neutron scattering has been used to characterize the cavity distribution in crept samples of a hot-pressed silicon carbide, which contained a thin continuous amorphous phase, and a sintered alumina, which contained no amorphous phase. The compression creep experiments were performed under cyclic loading at 1600° C and a frequency of 0.33 Hz. Comparison of the cavitation rates under cyclic loading with previously measured rates under static loading indicates that cavitation in the silicon carbide was unaffected by the cyclic loading, while the cavity volume fraction and the cavity size in the alumina were slightly increased by the cyclic loading. The results suggest that 0.33 Hz is too slow a frequency to affect the stress distribution and thus cavitation in the glassy phase containing silicon carbide, but it is rapid enough to accelerate cavitation in the absence of a glassy phase. This hypothesis is supported both by experimental results from other ceramic and metal systems and by calculations of characteristic stress relaxation times.     

The compression creep behaviour of silicon nitride ceramics

A comparison has been made of the compression creep characteristics of samples of reaction-bonded and hot-pressed silicon nitride, a sialon and silicon carbide. In addition, the effects of factors such as oxide additions and fabrication variables on the creep resistance of reaction-bonded material and the influence of dispersions of SiC particles on the creep properties of hot-pressed silicon nitride have been considered. For the entire range of materials examined, the creep behaviour appears to be determined primarily by the rate at which the development of grain boundary microcracks allows relative movement of the crystals to take place. Now with the BNF Metals Technology Centre, Wantage.