Cyclic creep behaviour described in terms of a one-parameter kinetic model

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 (σ/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.

Fatigue and cyclic deformation behavior of iron aluminide

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

Cyclic creep behaviour of modified 9Cr–1Mo steel was investigated by a series of cyclic creep (CC) tests at 600°C, which were performed under controlled tension–tension loading cycles with the magnitude of stress ranges in a constant stress ratio (R = 0·1). Hold time was applied for a 10 min hold at the maximum stress (σmax) and minimum stress (σmin). The CC properties were compared with the static creep (SC) using Norton’s power law, Larson–Miller plot, and Monkman–Grant relation, and the microstructure was examined. For the test conditions employed in the present investigation, retardation in the CC behaviour in terms of a lower creep rate and longer rupture time compared to those in the SC was obtained. The retardation was ascribed to the effects associated with anelastic recovery during the 10 min hold time at the minimum load of the cyclic loading. The creep rupture ductility decreased with a general decrease in stress, and there was no difference in the creep ductility between the CC and SC. The steel displayed a transgranular fracture characterised by the presence of dimples resulting from micro-void coalescence. Carbide precipitation was more coarsened with increasing in exposure time in the CC tests.

Influence of anelastic recovery on the cyclic creep behavior of AlCoCrFeNi2.1 eutectic high-entropy alloy

Deformation and fracture are examined for a cyclic loading condition in which a 24 h period of forward creep in IN 597 at 850°C is reversed (a) by a similar period of creep and (b) by fast monotonic strain-limited plasticity with and without a subsequent 1 h period of relaxation. When ratchetting is evident, it is shown that a life fraction rule based upon the Monkman—Grant law describes the observed behaviour and that, for a given creep stress, the accumulated creep strain—time plot correlates with a simple creep curve. At lower-magnitude creep dwell stresses a cyclic equilibrium state is achieved. The absence of ratchetting during equilibrium results in a net increase in rupture life and fracture strain when compared with those for simple creep under the same stress. Time-limited cyclic behaviour is compared with further cyclic tests in which the reversal occurs at fixed limits of total strain. These did not show an equilibrium condition but a progressive cyclic softening in which the creep time and strain successively diminished. Predictions to individual creep curves are made from the mechanical equation of state and the time hardening law. However, empiricism is preferred to describe the softening behaviour observed for the alloy more precisely. A theoretical assessment is made of the effect of imposing a period of relaxation at peak reversed strain upon cyclic creep deformation and life.

Cyclic creep deceleration relative to static creep was observed in oxide dispersion strengthened alloy Inconel MA 754 at 760 °C and cyclic stresses of 221 MPa-41 MPa, 231 MPa-41 MPa, and 241 MPa-41 MPa. Tests were run over the range of frequency from 0.05 cycles per hour to six cycles per hour. The maximum cyclic deceleration, which was manifested as a reduced net creep rate and increased rupture life, was observed at the highest cyclic frequency. Considerable anelastic strain, having a magnitude of ea/ee∼ 1/3, is stored in MA 754 at 760 °C under these creep loads, and this strain may be recovered in the off-load half cycle of a cyclic creep test. During the higher frequency tests, the effect of an incomplete storage of anelastic strain on the accompanying nonrecoverable creep rate provides a mechanism for the frequency dependent cyclic creep deceleration. The proposed mechanism is in agreement with mixed-mode test results and with TEM examination of interrupted-test specimens.

Mechanism-oriented characterization of the load direction-dependent cyclic creep behavior of the magnesium alloys Mg-4Al-2Ba-2Ca and AE42 at room temperature

It is noted that calculations of the rated strength for bearing elements of extremely loaded structures, including nuclear power plants allow inelastic deformation of the materials of these elements. At the same time calculations of the low cycle fatigue require taking into account factors that are not observed under single loading including, i.e., kinetics of cyclic strains, cyclic creep, change of the mode of inelastic cyclic deformation upon normal operation. Moreover, the materials can be of different types: cyclically hardening, softened or stable. For the first type of materials at a soft loading with constant amplitude of stresses in cycles, the range of strains decreases with an increase in the number of cycles, but increases for the second one. Under a hard mode of loading with constant amplitude of strains the maximum stresses in a cycle for the hardening material increase, and, on the contrary, decrease for softened material. Moreover, the soft loading of softened material results in one-sided accumulation of plastic strains as the number of loading cycles increases. These circumstances must be taken into account both in the analytical description of the kinetics of deformation diagrams and in the corresponding calculation equations used in the strength standards. It is noted that at early stages of forming computation methods developed for these conditions, calculation of stresses was carried out in the assumption of ideal elasticity of the material. The use of such approach was attributed to the lack of available methods for addressing the problem of an inelastic cyclic deformation, complicated on the statement. The subsequent evolution of the theory of cyclic elastoplastic deformation, analytical and numerical solutions of cyclic boundary-value problems, developing of numerical methods of computation and powerful computer packages fundamentally changed the situation providing the possibility of analysis and modeling of physically and geometrically nonlinear deformation processes. It is shown that transition from the elastic adaptability (with an elastic deformation of the structure in a stable cycle) to a sign-variable flow is smooth and continuous, similar to the transition from the elastic to plastic deformation under a single loading. Such a mechanism is similar to conditional boundary of the transition from low cycle to high-cycle fatigue under a cyclic strain. At the same time, we offer to use in calculations the existing rather simple models and experimentally determined parameters of cyclic deformation diagrams of materials. In the modern statement of the problems under consideration, taking into account both the kinetics of cyclic and unilaterally accumulated deformations, with allowance for the manifestation of creep effects in cycles is of fundamental importance. This approach also makes it possible to take into account the acceleration of unsteady cyclic creep due to previous plastic deformation of a different sign, which can be rather significant.

Cyclic hardening behavior and deformation mechanisms of friction-stir-welded dissimilar AA5083-to-AA2024 joints with heterogeneous microstructures

The strength in low cycle fatigue of a high speed rotating disc, caused by the repeated start and stop of rotation, was experimentally studied by using aluminium and brass discs. Results of the disc-test were compared with those of the uniaxial push-pull fatigue test conducted under the same loading condition as that in the most highly stressed region of the disc. There was no good coincidence between them. However, the prediction of the fatigue life of the rotating disc based on the push-pull test values falls on a safety side. Results oft he disc-test were also examined with a theory of cyclic creep deformation of a disc presented by the authors. The critical number of cycles corresponding to the start of unstable cyclic creep deformation was found to be pretty larger than the number of cycles to fracture of the disc obtained by the experiment.

Recent fatigue studies of Sn-rich Pb-free solder alloys are reviewed to provide an overview of the current understanding of cyclic deformation, cyclic softening, fatigue crack initiation, fatigue crack growth, and fatigue life behavior in these alloys. Because of their low melting temperatures, these alloys demonstrated extensive cyclic creep deformation at room temperature. Limited amount of data have shown that the cyclic creep rate is strongly dependent on stress amplitude, peak stress, stress ratio and cyclic frequency. At constant cyclic strain amplitudes, most Sn-rich alloys exhibit cycle-dependent and cyclic softening. The softening is more pronounced at larger strain amplitudes and higher temperatures, and in fine grain structures. Characteristic of these alloys, fatigue cracks tend to initiate at grain and phase boundaries very early in the fatigue life, involving considerable amount of grain boundary cavitation and sliding. The growth of fatigue cracks in these alloys may follow both transgranular and intergranular paths, depending on the stress ratio and frequency of the cyclic loading. At low stress ratios and high frequencies, fatigue crack growth rate correlates well with the range of stress intensities or J-integrals but the time-dependent C* integral provides a better correlation with the crack velocity at high stress ratios and low frequencies. The fatigue life of the alloys is a strong function of the strain amplitude, cyclic frequency, temperature, and microstructure. While a few sets of fatigue life data are available, these data, when analyzed in terms of the Coffin–Mason equation, showed large variations, with the fatigue ductility exponent ranging from −0.43 to −1.14 and the fatigue ductility from 0.04 to 20.9. Several approaches have been suggested to explain the differences in the fatigue life behavior, including revision of the Coffin–Mason analysis and use of alternative fatigue life models.

Dislocation structure evolution in 304L stainless steel and weld joint during cyclic plastic deformation

Implicit material modeling for temperature-dependent viscoplaticity using multi-layer neural networks

Cyclic deformation behavior of ultra-fine grained copper processed by accumulative roll-bonding