Cyclic Creep Acceleration Research Articles

Effect of Cyclic Stress Reduction on the Creep Characteristics of AZ91 Magnesium Alloy

The creep deformation behavior of aged AZ91 magnesium alloy under the cyclic stress reduction has been investigated in the temperature range 353–383 K. The microstructural evolution of the AZ91 magnesium alloy at different heat treatment conditions has been studied using optical microscopy and scanning electron microscopy. The minimum strain rate, $$ \dot{\varepsilon}_{\hbox{min} } $$ , was found to be higher under cyclic stress reduction condition than that under static creep condition for the same maximum stress. Cyclic creep acceleration is interpreted using the interaction of moving dislocations with the existing β-phase (Mg17Al12) precipitates. The mean value of the activation energy of secondary creep stage was found to be equal to that quoted for lattice self-diffusion of magnesium mechanism.

Dynamic response of the piezoelectric polymer PVDF

The paper concerns the dynamic response of polyvinylidene fluoride (PVDF), a commercially available piezoelectric polymer currently used in numerous applications. The paper consists of two parts. The first part presents a summary review of recent experimental results regarding the time- dependent behavior of PVDF tested under combined static and cyclic loading conditions. It has been observed that the polymer has demonstrated accelerated creep rates due to cyclic loading effects. Cyclic creep acceleration in PVDF has been significant even in the range of stresses well below the viscoelastic linearity limit. This phenomenon, defined as "vibrocreep", appears to be essentially nonlinear, since the material response under combined static and cyclic loads did not represent a simple superposition of the responses to static and fully reversed cyclic loads applied separately. The second part of the paper provides a formulation of a nonlinear constitutive model, which describes the synergistic interaction between creep and cyclic damage evolution. The proposed constitutive model combines the concepts of linear hereditary viscoelasticity with damage characterization through a cyclic damage function. The latter is formulated using the basic principles of continuum damage mechanics. The potentials and limitations of the proposed constitutive model are discussed, and experimental results providing model validation are presented.

Effect of grain boundary orientation and cyclic load on creep and fracture behaviour in directionally solidified nickel base superalloy

The effects of grain boundary orientation and cyclic load on the creep and fracture behaviour of directionally solidified superalloy DZ17G have been investigated. The results show that grain boundary orientation has no effect on the creep deformation mechanism, but has a marked effect on the fracture mechanism. The creep fracture mechanism changes from a transgranular mode in longitudinal specimens to intergranular failure in transverse and inclined (45° to applied stress axis) specimens. Compared with constant load, cyclic load remarkably reduces creep life with the increment of cyclic frequency, but has no effect on the creep ductility. After careful examination of the transient response following the load changes, it is suggested that an intrinsic softening occurred during unloading, which accelerates the creep deformation rate on subsequent reloading, and the accumulation of this softening effect finally results in cyclic creep acceleration. The contribution of anelasticity has also been discussed.

Cyclic stress-strain behavior of 9Cr1Mo steel at positive mean stress

Cyclic plastic behavior of a ferritic martensite 9%Cr–1%Mo steel was studied under load controlled cycling with positive mean stresses. The results were compared with data determined under symmetrical cycling. Cyclic hardening followed by softening was found to be inherent to cycling at positive mean stress contrary to the symmetrical loading where only continuous softening was observed. For a given stress amplitude, higher mean stress results in higher plastic strain amplitude. The onset of cyclic softening and the onset of cyclic creep acceleration occur simultaneously. In the region of cyclic softening a unique relation between the instantaneous cyclic creep rate and the instantaneous plastic strain amplitude was found for cycling at constant mean stress.

Effect of Cyclic Stress Reduction on the Creep Behaviour of Al-Ag and Al-Ag-Zr Alloys Containing γ′ and γ Precipitates

Creep tests under the effect of cyclic stress reduction were conducted to Al-16 wt%Ag and Al-16 wt%Ag-0.1 wt%Zr alloys aged at 548 and 673 K. The steady state creep rate was found to be higher under cyclic stress reduction condition than that under static creep condition for the same maximum stress. An expression for the temperature dependence of the threshold stress for cyclic creep acceleration is offered. Cyclic creep acceleration is interpreted using the dislocation cross-slip model. TEM investigations confirmed that the addition of Zr to Al-Ag alloy accelerates the formation and coarsening of γ′ and γ-phases. The mean value of activation energy for both alloys aged at 548 K was found to be 35±0.3 kJ/mol, and 58±0.4 kJ/mol for alloys aged at 673 K.

Simulation of the long-term cyclic creep behaviour of a low alloyed ferritic chromium steel

Low alloyed ferritic chromium steels of the 21/4 %Cr 1%Mo-type (German designation: 10 CrMo 9 10) have been used in electric power plants for many years. During service startand stop-operations cause cyclic creep. Long-term deformation tests showed that cyclic creep is often accelerated compared to monotonie creep (at constant load and constant temperature). The aim of the present work is to simulate the long-term cyclic creep behaviour on a microstructural basis. The simulation was carried out with the composite model of plastic deformation, which has been applied in the past to simulate the monotonie and cyclic creep behaviour of martensitic 9–12% chromium steels. The model was adapted to the present case by specifying the microstructural development of the 21/4 %Cr 1%Mo-heat through transmission electron microscopic investigations of the initial state prior to creep. The simulation is able to predict the creep behaviour for constant as well as cyclic loading and thus to quantify the degree of cyclic creep acceleration.

Microstructure and deformation rate during long‐term cyclic creep of the martensitic steel X22CrMoV12‐1

The microstructure of three specimens of the martensitic steel X22CrMoV12‐1 which had been subjected to long‐term cyclic creep at 873 K with intermittent phases of unloading (stress ratio R = 0) and compression (R = −1) was quantified by electron microscopy with regard to carbides, dislocations and pores. The laws of time dependent coarsening of carbides and strain controlled growth of subgrains found for monotonic creep hold also for cyclic creep. The longer time it takes cyclic creep to reach a given strain leads to a growth advantage of carbides compared to monotonic creep. The microstructural model of plastic deformation previously developed for monotonic creep on X20(22)CrMoV12‐1 allows to calculate the cyclic creep acceleration due to this advantage in carbide growth.

Effect of unloading time on interrupted creep in copper

The effect of unloading time on the interrupted creep behaviour of polycrystalline copper specimens was investigated over the temperature range 298–773 K. Up to 553 K, cyclic creep acceleration could be explained in terms of deformation and hardening using a dislocation glide model with recovery during unloading being due to dislocation climb. At higher temperatures, recrystallisation effects probably influence behaviour.

Effect of frequency on cyclic creep of polycrystalline aluminium at room temperature

A study was made of the effect of cyclic frequencies from zero to 1 Hz on the cyclic creep behaviour with different extents of unloading in commercial purity aluminium. There exists an optimum frequency corresponding to the lowest steady state creep rate, the value of which exhibits the trend of decreasing with attenuating stress amplitude at a given cyclic peak stress. Both the cyclic frequency and the stress amplitude strongly affect the threshold stress for the transition from cyclic creep acceleration to retardation but do not change the average internal stress. Expressions for the threshold stress were derived as a function of thermal activation parameters. When the value of V c is near that of V s ( V c and V s are the apparent activation volume of cyclic and static creep, respectively) the threshold stress varies inversely as ( V c/ V s −1); otherwise, it is inversely proportional to 1 − V c/ V c. The effect of grain size on cyclic creep is also discussed.

Effects of cyclic stress on the creep behaviour and dislocation microstructure of pure copper in the temperature range 0.4 to 0.5T m

The creep deformation behaviour of polycrystalline pure copper under static and cyclic stress was studied in the temperature range 0.4 to 0.5Tm. Both cyclic creep acceleration and retardation occurred depending on the condition of peak stress and temperature combination. The comparison of dislocation microstructures, developed during steady state static and cyclic creep deformation, has also been performed to determine the effect of cyclic stress on the dislocation microstructure and evidence for the enhanced recovery of the cell wall under cyclic stress was found. These effects of cyclic stress on the creep rate and dislocation microstructure were interpreted on the basis of diffusion-controlled recovery creep theory and the cyclic creep acceleration mechanism is suggested as the enhanced recovery of the cell wall with the help of athermally generated excess vacancies.

Effect of grain size on cyclic creep behaviour in a commercially pure aluminium at room temperature

Both static creep behaviour and cyclic creep behaviour of a commercially pure aluminium with different grain sizes have been investigated at room temperature. The creep rate of both stage I and II static creep and stage I cyclic creep were drastically affected by grain size. It was found that a critical grain size existed above which the cyclic creep retardation gave way to acceleration. The threshold stress at which the cyclic creep retardation is replaced by acceleration, or vice versa, was found to be affected by impurities in the material. An intermediate amount of unloading which gave rise to the lowest cyclic creep rate was observed in the present study. The behaviour of cyclic creep acceleration and retardation was explained satisfactorily in terms of thermally activated deformation processes.

Threshold stress for cyclic creep acceleration in copper

The phenomenon of cyclic creep acceleration (CCA) is well known. Also, it has been observed experimentally that there exists a peak stress level above which CCA begins to occur. For copper the value of this peak stress is known to be independent of cyclic stress conditions such as stress amplitude and frequency. This stress level or the threshold stress was observed to increase with temperature in the range 0.4 to 0.5T m. A model is suggested to predict the threshold stress level and to explain the temperature dependence of the stress. This model is based on the dislocation core diffusion process by the athermally generated excess vacancies. The concept of critical length of dislocation core is used for core diffusion fast enough to enhance dislocation climb. This critical length is expressed as a function of stress and temperature. From these concepts, the threshold stress is expressed asτ/G - A exp (-ΔQ/RT). The experimentally observed values of the threshold stress are in good agreement with this model, and the temperature dependence is also experimentally proved.

Microstructure, creep, and tensile deformation in Ti-6Al-2Nb-1Ta-0.8Mo

The effects of microstructure, temperature, and stress level on the creep response of Ti-6211 have been investigated. A variety of microstructures simulating the heat affected zone of a weld (HAZ), as well as the as-received structure, were tested in a temperature range of 298 K to 873 K. At stress levels below the tensile yield strength, creep curves saturate in the ambient temperature regime. The colony type Widmanstatten alpha + beta as-received structure exhibited the highest creep strains at ambient temperatures. Long slip lengths associated with the large colony size, and sliding along various interfaces account for the relatively high creep strains to saturation. Planar arrays of straight dislocations operating on a single slip system were observed for samples crept at 298 K while thermally activated cross slip was observed for samples crept at 453 K. Beta-annealed martensitic micro-structures displayed enhanced creep resistance, out-performing other recrystallized HAZ structures. Above 778 K the activation energy of creep is close to that for self-diffusion in titanium, suggesting that diffusion-controlled dislocation mechanisms are the rate-controlling processes at elevated temperatures. Creep rupture at elevated temperatures occurred by microvoid nucleation and growth. Fracture occurred along colony boundaries in the as-received structure but appear to be intergranular with the crack propagating along G. B. alpha/matrix interfaces in the equiaxed microstructures. Sliding along alpha/beta interfaces, colony boundaries, prior beta grain boundaries, and slip traces contributed to the creep strain and rupture process. Cyclic creep with a loading-unloading sequence was also performed at room temperature and cyclic creep acceleration was observed.

CYCLIC CREEP ACCELERATION AND RETARDATION IN Cr‐Mo‐V ROTOR STEEL AT AMBIENT AND ELEVATED TEMPERATURE RESPECTIVELY

AbstractThe cyclic creep properties of Cr‐Mo‐V rotor steel have been explored at room temperature and elevated temperature by multiple‐step tests with increasing maximum stress (R= 0), and with subsequent tests in partial unloading (0 ≤R < 1). Cyclic creep acceleration is observed at room temperature and cyclic creep retardation at elevated temperature. This behavior is explained in terms of cyclic softening at low temperature and by creep‐dominated deformation at high temperature with a ferritic material considered especially prone to retardation because of the high diffusivity of b.c.c. material. Other interesting effects, such as strain burst phenomena and the observation of anomalously high values of the ratio of diametral strain to axial strain, are also reported.

Cyclic creep acceleration and retardation in polycrystalline copper tested at ambient temperature

In order to understand the mechanisms of cyclic creep retardation and acceleration, tests in both static and cyclic creep have been made on copper polycrystals at ambient temperature. Cyclic retardation occurs at low stress and acceleration at high stress. An expression for the temperature dependence of the threshold stress for acceleration is offered. The dislocation structures of the cyclically-crept copper have also been explored both in the presence and absence of strain bursts, and as a function of stress amplitude and grain orientation. Cyclic creep acceleration is interpreted by a cyclic softening process involving dislocations on secondary systems, and the role of point defects is discounted.

Strain bursts in the cyclic creep of copper single crystals at ambient temperature

In order to understand the mechanism of strain bursts previously observed in polycrystalline metal tested in cyclic creep, studies have been carried out on single crystals, variously oriented. Regular stage I and stage II creep were observed under pulsating tension and a saturated hysteresis loop developed during stage II creep. For stresses above a threshold of 35 MPa in maximum stress, strain bursts occurred frequently in the single crystals. The bursts proceeded through the crystals in the manner of Luders bands. Studies of the dislocation structure by TEM showed the bursts to be accompanied by very large increases in secondary dislocation densities and by the formation of cell structures. The strain bursts are interpreted by cyclic latent softening and dislocation avalanches originating from secondary-rich dislocation clusters. Cyclic creep acceleration is interpreted as a gentler and more widely-pervading form of strain bursts.

The influence of fatigue stress on the creep behaviour of metals

A theoretical model, which considers the nature of crystallographic slip in the grain and the heterogeneous stress distribution arised from grain orientations, is proposed for the calculation of cyclic creep behavior of metals. Here the hardening mechanisms which cause the creep rate of slip systems to decrease are generally classified into two broad groups: isotropic, and directional or kinematic; under cyclic deformation the influence of isotropic mechanisms was found to gradually give way to the kinematic mechanisms. An implication of this transition is a softer substructure under cyclic loading, and with the heterogeneous stress field it can also assist the backward dislocation motion, or strain recovery, upon unloading. Indeed, backward dislocation motion and the transition of hardening mechanisms were found to be the major driving forces for cyclic creep acceleration. If this transition takes place too slowly cyclic creep retardation may result. This micromechanical model was applied to study the cyclic creep behavior of a 99.999% pure aluminum. The results, including the effect of fractional unloading, were seen to be in accord with the experimental data.

Cyclic creep of type 304 stainless steel during unbalanced tension-compression loading at elevated temperature

Samples of Type 304 stainless steel were subjected to cyclic stresses with a positive mean stress at 300 and 560°C. Very rapid net elongation was observed whenever the stress limits were such as to produce a plastic strain amplitude of the same order of magnitude as the elastic strain at the peak stress. The maximum mean strain-rate, or cyclic creep rate, for a given peak tensile stress was achieved when the mean stress was just slightly above zero. Increasing the mean stress caused the mean strain rate to de-crease. The sensitive dependence of the mean strain-rate on the plastic strain ampli-tude and inverse dependence on the mean stress indicates that remobilization of disloca-tions by the reverse strain is an important mechanism for cyclic-creep acceleration. Although rapid cyclic creep was observed at both temperatures, a measurable mean strain rate was found for a much narrower range of stress conditions at 560 than at 300°C. The strain accumulated during cyclic creep did not produce any strain hardening, but did influence the shape of the stress-strain curve in a subsequent tensile test.

Influence of carbon on the static and cyclic creep behaviors of alpha iron

The plastic deformation of both ZrH 2-treated iron and iron with a controlled amount of carbon is examined under constant load and repeated loading. The pure iron exhibits cyclic creep acceleration under cyclic loading but the magnitude of the creep enhancement is small and is relatively insensitive to applied stress, a result in contrast to the behavior of f.c.c. metals. The presence of carbon, even in small concentration (65 at. ppm), enhances the relative cyclic creep acceleration when the specimens are aged after carburization. In carburized and quenched specimens, however, both static and cyclic creep are suppressed. The observed difference in cyclic creep behavior is correlated to the difference in the nature and distribution of the carbon and the relative ease of pinning of dislocations by the carbon atoms.

A mathematical model of cyclic creep acceleration

A mathematical model is proposed to explain qualitatively the creep acceleration of materials under cyclic loading. Under constant load, the non-uniformity of the distribution of plastic strains, which increases with loading time, introduces an internal back stress field in the material. Since the net stress in the matrix is reduced, the rate of creep is retarded with the increase of loading time. When the load is cycled, the internal stresses homogenize the distribution of the plastic strains during unloading, resulting in the partial recovery of work-hardening. The material gains a higher creep rate for the next loading. The cyclic loading under this condition accelerates the creep rate.