Short-term flexural creep deformation in synroc-C

The flexural creep behaviour of synroc-C in an inert atmosphere was studied at temperatures of 860°C, 900°C and 940°C under constant-load conditions in four-point bending. Applied stresses ranged from 100 to 160 MPa. Individual creep curves show primary and secondary creep but little or no tertiary creep stage. The log of the creep rate was found to increase linearly with log of the applied stress at each temperature over the entire stress range. Analysis of the creep data using the Norton power-law function revealed that the stress exponent decreased from 3.3 ± 0.6 for the 860°C and 900°C data to 2.0 ± 0.2 for the 940°C data, and an activation energy of 440 ± 40 kJ/mol was obtained over the entire temperature and stress range. Comparative analysis with the theta-projection equation was found to adequately represent the data yielding an activation energy of 464 kJ/mol while also showing a trend for the stress exponent to decrease with increasing temperature. Microstructural examination revealed extensive cavitation on the tensile surface of the creep specimens subjected to higher stresses at 900°C and 940°C. Dynamic high temperature X-ray diffraction analysis indicated little change in the phase assemblage apart from a slight reduction in the amount of the hollandite phase at higher temperatures which was attributed to a minor amount of oxidation. The possible creep damage mechanism was explored with reference to creep test results and microstructural modifications and the implications of the observations are discussed.     

High temperature creep in polycrystalline AIN-SiC ceramics

Polycrystalline dense ceramic specimens containing 75 mol % AIN-25 mol % SiC and 60 mol % AIN-40 mol % SiC were subjected to creep deformation in bending at elevated temperatures. Over the range of temperatures and stresses investigated, the creep rate was found to vary linearly with stress indicative of diffusional creep. Creep was found to be thermally activated with activation energy in the range from 175 kcal mol⁻¹ to 219 kcal mol⁻¹. Electron microscopic observation indicated that crack like cavities formed near the tensile surfaces during creep.     

Estimating creep deformation of glass-fiber-reinforced polycarbonate

Thermoplastic resin and fiber-reinforced thermo-plastics (FRTPs) were used without post-cure treatment as “molded material.” For such materials, creep behavior and physical aging occur simultaneously. This study examined the creep behavior of polycarbonate (PC) and glass-fiber-reinforced polycarbonate (GFRPC) injection moldings, including the effect of physical aging and fiber content, and determined that the time–temperature superposition principle could be applied to the creep behavior for different fiber contents. The effects of physical aging on creep behavior were evaluated quantitatively on pure resin and with various fiber contents without heat treatment. We found that the effect of physical aging could be evaluated with the proposed factor, “aging shift rate.” To discuss the linearity of viscoelasticity in FRTPs, this study used two shift factors: time and modulus shift factors. The fiber content affected creep behavior by both retarding and restraining it through changing the elastic modulus. This was shown by generating a grand master curve of creep compliance, which included the effects of time, temperature, and fiber content. Using the grand master curve of creep compliance and shift factors, it was possible to estimate the creep deformation of molded materials under varying conditions and fiber contents. The estimated creep deformation gave a very good fit to the experimental creep deformation.     

The activation areas for creep deformation

The activation areas for creep deformation are collected and examined in the light of many material and deformation variables. The activation area is A ^*= (kT/b) ( In /^*) _T where k is Boltzmann's constant, T the absolute temperature, b the Burgers vector, the steady state creep rate, and ^* the effective shear stress. It is found that within a factor of 5, there is a general correlation between activation area and stress for all metals, alloys, semiconductors and ionic crystals. A jog-limited dislocation motion with a distribution of jog spacings is suggested as a possible mechanism for this behaviour. Some limitations for the jog mechanism are discussed.     

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.     

A micromechanics model of creep deformation in dispersion-strengthened metals

A micromechanics model, in which work-hardening caused by second-phase particles and a recovery process by diffusion of atoms were taken into account, has been proposed for explaining the creep deformation of dispersion-strengthened metals in high-temperature creep. A constitutive equation of the projection was employed to describe the whole creep curves from the onset of loading to rupture. The results of the calculations based on the present model have been compared with those of experiments on the carbon steels containing spherical cementite particles. There was a correlation between the experimental creep curves and the calculated ones. The changes in the calculated creep strain and creep rate with time have also been compared with the experimental results on carbon steels. The micromechanics model was found to be applicable to any kind of two-phase material, if the constitutive equation was appropriately chosen.     

Effects of elastoplastic deformation and creep in threaded connections

The specificity of loading conditions for critical threaded connections of power equipment is shown. Approaches to determination of their stress-strain states, which are dictated by redistribution of stresses and strains in turns of the thread under the static and low-cycle loading, are considered. The technique for experimental investigations of plastic deformations and creep deformations of bolts at the normal temperature is represented. This makes it possible to determine the tightening reduction in bolts.     

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.     

Dislocation network models for recovery creep deformation

The development of dislocation network models for recovery creep and the important results which arise from the application of the models are discussed. These models are basically aimed at describing the two simultaneous processes, namely strain hardening and recovery, which occur during high-temperature creep deformation. These processes are modelled using detailed dislocation mechanisms which occur in the deforming crystalline materials. The present models, although still being approximations, are reasonably well able to describe high-temperature recovery creep deformation of crystalline materials.     

Tensile creep and deformation mechanisms in oriented low-density polyethylene

Low density polyethylene (LDPE) which had been uniaxially oriented by cold drawing was stressed parallel to the fibre axis at 20? C. In most of the experiments static tensile loads were applied. Changes in the internal texture and molecular orientation during tensile deformation and recovery were determined by low- and wide-angle X-ray diffraction. Instantaneous elasticity, both in tension and recovery, occurred by extension of the macrolattice formed of layered crystalline and amorphous regions, of which the volume of the material was found to be mainly comprised. Tensile creep was observed having a retardation time for the main process of about 70 h, and was not produced by simple changes in the geometry of the macrolattice. Relaxation mechanisms which could account for slow viscoelasticity in this fibrous material are discussed, and the results are analysed with reference to earlier work on these subjects.     

Defects activation in CoFe-based metallic glasses during creep deformation

Maxwell-Voigt model with two Kelvin units and one Maxwell unit was utilized to analyze the microalloying effect of Cu on the creep behavior of CoFe-based metallic glasses at different loading rates. The defect activation during creep deformation was detected by the relaxation time spectrum based on this model. The defect, with respect to a short relaxation time in relaxation spectra, intends to be activated at a quasi-static loading mode in the alloy with 0.5 at.% Cu addition. With further increasing loading rates,more defects with a large size were provoked activated at both hard and soft regions in the Cu-containing sample. A softening with the reduction of elastic modulus and hardness about 10 % and 15 %, respectively,was also observed in the Cu-doped sample. It is consistent with the pronounced viscoplastic deformation of this alloy along with the decrease of viscosity. Our work provides a microscopic insight into structural evolution during creep deformation in a Cu-doped metallic glass, which might help for understanding the plastic deformation of metallic glasses upon a minor addition.     

The importance of multiaxial stress in creep deformation and rupture

This paper investigates the importance of multiaxial stress states by considering several distinct testing techniques used in assessing both creep deformation and creep damage accumulation. The requirements of testing programmes to determine the necessary data are discussed in respect of sensitivity and interdependence of the principal and hydrostatic stress ratios.     

Finite element modelling of creep deformation in fibre-reinforced ceramic composites

The tensile creep and creep-recovery behaviour of a unidirectional SiC fibre-Si₃N₄ matrix composite was analysed using finite element techniques. The analysis, based on the elastic and creep properties of each constituent, considered the influence of fibre-matrix bonding and processing-related residual stresses on creep and creep-recovery behaviour. Both two- and three-dimensional finite element models were used. Although both analyses predicted similar overall creep rates, three-dimensional stress analysis was required to obtain detailed information about the stress state in the vicinity of the fibre-matrix interface. The results of the analysis indicate that the tensile radial stress, which develops in the vicinity of the fibre-matrix interface after processing, rapidly decreases during the initial stages of creep. Both the predicted and experimental results for the composite show that 50% of the total creep strain which accumulated after 200 h at a stress of 200 MPa and temperature of 1200°C is recovered within 25 h of unloading.     

Study on creep deformation mechanism of 316LN stainless steel from impression creep tests

Abstract

Impression creep tests were carried out on 316LN stainless steel (SS) at various temperatures in the range 898–973 K. The stress dependence of the steady state impression velocity followed the power-law with stress exponent n?=?6. The temperature dependence of the steady state impression velocity obeyed Arrhenius type rate equation. The apparent activation energy for creep deformation (Q_c) was estimated to be 500 kJ mol^?1. Based on the n and Q_c values, it is concluded that the rate controlling mechanism is dislocation creep.