Influence Of Stress Amplitude Research Articles

Investigation on the Effect of Asymmetric Cyclic Loading on Ratcheting Deformation and Bulk Texture Development in HSLA Steel

Ratcheting deformation in association with bulk texture evolution in HSLA steel is studied in this investigation. The cyclic deformation behaviors and damage modes of the specimens are obtained at room temperature, employing asymmetric stress-controlled tests under various combinations of mean stress and stress amplitudes. Bulk texture measurements of the specimens are done adopting an X-ray diffraction analysis. The results exhibit an increase in strain accumulation from 0.23 to 30.61% associated with a sharp reduction in ratcheting life from 3320 to 880 cycles by the increment in mean stress/stress amplitude. A continuous cyclic softening until the final failure is observed for all the ratcheting tests. It is noticed that variation of mean stress is more detrimental than stress amplitude in controlling strain accumulation while the effect of stress amplitude is more pronounced than mean stress for the reduction of ratcheting life. The fracture surface morphology indicates typical striation formation in the crack propagation region along with overload failure. The bulk texture studies indicate that there is a transition between ‘ϒ’ fiber and ‘ζ’ fiber during all loading conditions. The texture intensity is increased from 5.71 to 11.68 with progressive ratcheting strain accumulation. Here also, influence of stress amplitude is more significant than the mean stress.

Influence of stress amplitude on the dynamic characteristics of phyllite samples under triaxial multi-stage cyclic loading

The generation of a rockslide in an earthquake event is strongly dependent on the dynamic characteristics of the rock materials constituting the rock slope. In this study, phyllite samples were extracted from the Donghekou landslide area of Qingchuan County. The landslide occurred during the 2008 Wenchuan earthquake and the samples were made to undergo multi-stage cyclic loading and unloading tests under triaxial compression conditions. The confining pressure during the tests was 10 MPa, the loading frequency was 2 Hz, the loading waveform was sine wave, and 60 loading cycles were carried out at each level of stress. From the dynamic axial stress–strain curves, the effect of stress amplitude on the hysteresis loop area, the dynamic elastic modulus, the dynamic damping ratio, and the damping coefficient of the phyllite samples was analyzed. The results are as follows. (1) The strain–stress hysteresis loop area experienced a rapid increase as the stress amplitude exceeded a certain value, indicating that the rock samples have entered an accelerating crack propagation stage under high levels of cyclic loading. (2) As the stress amplitude increases, the dynamic elastic modulus, the dynamic damping ratio, and the damping coefficient of the rock sample followed a similar two-phase evolution law. There was a rapid reduction in the three parameters relative to the initial value in Phase I. Moreover, there was a minor change in Phase II. (3) The effect of the number of loading cycles number on dynamic elastic modulus and damping parameters was much more obvious under the lower levels of cyclic loading than under the higher levels.

Uniaxial Ratcheting Behavior of a Weather-Resistant Rail Steel: Effect of Mean Stress and Stress Amplitude

Uniaxial ratcheting behavior of a weather-resistant rail steel belonging to 60 kg/m 90-UTS variety was studied in true stress control mode with varying mean stress (σm) and stress amplitude (σa) at constant σa and σm, respectively. In all the loading conditions, the ratcheting strain (er) increased almost monotonically and accumulation of ratcheting strain was found to increase at higher magnitude of σm or σa, perhaps due to the increase in remnant dislocation density in the substructure of the investigated steel. After the initial sharp fall in ratcheting strain rate with elapsed cycles in all the cases, a constant ratcheting rate was attained very soon; thereafter, the ratcheting strain kept on accumulating in the specimen at this constant rate till the failure of the specimen occurred. It was quite interesting to see that on increasing the mean stress level at a fixed stress amplitude, after the initial decrease, fatigue life of the steel improved marginally owing to the effect of mean stress-induced hardening. In absolute contrast, under the influence of stress amplitude at a fixed mean stress, fatigue life of the rail steel deteriorated gradually but to a significant extent. Finally, stress amplitude was found to have more severe effect than that offered by the mean stress in terms of` ratcheting strain accumulation and fatigue life of the investigated rail steel.

Analysis of high temperature fatigue lifetime of GH4133B superalloy used in turbine disk of aero-engine

Based on the S-H cavity model theory and the thermodynamic diffusion equation, the high temperature fatigue lifetime equation is deduced, and the influence of stress amplitude and mean stress on fatigue lifetime is quantitatively analyzed. At high temperature of 650°C, according to the test data of fatigue lifetime of GH4133B superalloy under different stress ratios or alternatively at various maximum stress levels, the nonlinear regression analysis method is applied to identify the material parameters in the fatigue lifetime equation, and a 3D Nf-σm-σa curve surface is plotted. The comparison between theoretic fatigue lifetime Nfp and test one Nft indicates that the fatigue lifetime equation derived from the microstructure evolution of metallic materials can accurately predict the fatigue lifetime of GH4133B superalloy under different cyclic loading parameters. Finally, a parameter γ is introduced to characterize the effect of mean stress σm and stress amplitude σa on fatigue lifetime Nf of GH4133B superalloy. It is suggested that the effect of mean stress σm on Nf is larger than that of stress amplitude σa on Nf under the condition of tensile-tensile fatigue loading.

Fatigue Behavior of Critical Current Degradation for YBCO Tapes at 77 K

In the application of high-temperature superconducting (HTS) tapes, they are prone to subject fatigue loadings such as repeated thermal cycles, periodic electromagnetic force, and so on, which will affect the performance of superconductors and lead to the degradation of superconducting magnets and power systems. To optimize the design of a superconducting device, it is important to fully understand the fatigue properties of REBCO conductors. Recently, fatigue loading that influences critical current $(I_{{\text{c}}})$ of HTS materials has been studied by several research teams. However, the influence of mechanical fatigue stress on REBCO tapes under background magnetic field has not been reported. In this paper, the $I_{{\text{c}}}$ as a function of external magnetic field under different loading cycles was investigated at 77 K. The results showed that for fatigue damage of a YBCO-coated conductor, the influence of stress amplitude on $I_{{\text{c}}}$ degradation is more significant than the maximum stress applied to the coated conductor. Under higher fatigue load cycles, the YBCO-coated conductor was insensitive to the external magnetic field.

Damage morphology study of high cycle fatigued as-cast Mg–3.0Nd–0.2Zn–Zr (wt.%) alloy

Laser scanning confocal microscopy (LSCM) and Electron back-scattered diffraction (EBSD) were applied to the study of surface morphology variation of as-cast Mg–3.0Nd–0.2Zn–Zr (NZ30K) (wt.%) alloy under tension-compression fatigue test at room temperature. Two kinds of typical damage morphologies were observed in fatigued NZ30K alloy: One was parallel lines on basal planes led by the cumulation of basal slips, called persistent slip markings (PSMs), and the other was lens shaped, thicker and in less density, led by the formation of twinning. The surface fatigue damage morphology evolution was analyzed in a statistical way. The influences of stress amplitude and grain orientation on fatigue deformation mechanisms were discussed and the non-uniform deformation among grains and the PSMs, within twinning were described quantitatively.

Effect of Stress Amplitude on the Damping of Recycled Aggregate Concrete.

Damping characterizes the energy dissipation capacity of materials and structures, and it is affected by several external factors such as vibrating frequency, stress history, temperature, and stress amplitude. This study investigates the relationship between the damping and the stress amplitude of environment-friendly recycled aggregate concrete (RAC). First, a function model of a member’s loss factor and stress amplitude was derived based on Lazan’s damping-stress function. Then, the influence of stress amplitude on the loss tangent of RAC was experimentally investigated. Finally, parameters used to determine the newly derived function were obtained by numerical fitting. It is shown that the member’s loss factor is affected not only by the stress amplitude but also by factors such as the cross section shapes, boundary conditions, load types, and loading positions. The loss tangent of RAC increases with the stress amplitude, even at low stress amplitude. The damping energy exponent of RAC is not identically equal to 2.0, indicating that the damping is nonlinear. It is also found that the energy dissipation capacity of RAC is superior to that of natural aggregate concrete (NAC), and the energy dissipation capacity can be further improved by adding modified admixtures.

Test and Research on Ratchet Effect of 16 Mn Steel under Cyclic Loading

The uniaxial and multiaxial non-proportional cyclic experiments of 16Mn steel under stress-control were carried out using MTS809 Series machine and Teststar control system .The influence of stress amplitude, mean stress and their loading history on uniaxial and multiaxial ratcheting is studied.

Fatigue and ratcheting behaviors of CP-Ti at room temperature

Based on the careful analyses of the stress controlled fatigue behavior of commercially pure titanium (CP-Ti) at room temperature, two stress regions can be evidently identified by cycle strain range, ratcheting strain, min ratcheting strain rate, ratcheting failure strain and fracture morphologies. In the high stress region, the ratcheting behavior is significant, and the failure is caused by the large ratcheting deformation. However, in the low stress region, the ratcheting behavior is weak, and the failure is the typical fatigue fracture, with fatigue crack initiation, fatigue extension, and instant rupture. In the high stress region, for both ratcheting and fatigue behaviors, the influence of max stress is greater than stress amplitude. On the contrary, in the low stress region, the influence of stress amplitude on the fatigue behavior is greater than max stress. But the ratcheting behavior in the low stress region is still max stress controlled. Based on the discussion on the fatigue life prediction model, traditional Basquin model and energy based model cannot supply precise fatigue life in the wide stress range, since the ratcheting behavior is not considered. The coupled fatigue damage and ratcheting damage model is proved to make a robust prediction.

Test and Research on Ratchet Effect of 16 Mn Steel under Cyclic Loading

The uniaxial and multiaxial non-proportional cyclic experiments of 16Mn steel under stress-control were carried out using MTS809 Series machine and Teststar control system .The influence of stress amplitude, mean stress and their loading history on uniaxial and multiaxial ratcheting is studied.

Fatigue rates of monocrystalline silicon thin films in harsh environments: Influence of stress amplitude, relative humidity, and temperature

This study investigates the separate influence of stress, temperature, and relative humidity (RH) on the fatigue behavior of 10-μm-thick, monocrystalline silicon (Si) films at 40 kHz, under fully reversed loading. The fatigue rates are most sensitive to stress, with four orders of magnitude decrease from 3.2 to 1.5–2 GPa, confirming a size effect associated with the fatigue behavior of Si under bending load. The fatigue rates are also much more sensitive to RH than temperature or partial pressure of water, indicating that the effective environmental parameter is the adsorbed water layer. The implications on the relevant fatigue process(es) are discussed.

Fatigue behaviour of carbon fibre/PEEK laminate composites

The fatigue behaviour of continuous carbon fibre-reinforced poly(ether ether ketone) (PEEK) composites under tension-tension (T-T) and tension-compression (T-C) loadings was investigated. A quasi-isotropic lay-up was used, namely [0/45/90/-45] 2s AS-4/PEEK. The experimental results for fatigue life and residual tensile strength under different stress amplitudes are analysed by the median rank method. The influences of stress amplitude on the fatigue life and the residual tensile strength were investigated. The S-N curve is presented by the pooled Weibull distribution function. Two segments of the S-N curves show a different failure mode in the fatigue failure of the thermoplastic composites. Moreover, a residual strength degradation model is used to predict the residual strength for the composites subjected to a number of fatigue cycles and to simulate the effects of the stress amplitude on the fatigue life. According to the obtained results, the correlation between the experimental data and the theoretical prediction is in good agreement.

Fatigue behavior of quasi‐isotropic carbon fiber/PEEK laminates under tension‐tension loading

AbstractFatigue behavior of carbon fiber reinforced poly(etheretherketone)(PEEK) laminates was investigated. The static tensile measurement, tension‐tension fatigue loading tests, and residual tensile strength measurement of the [0/45/90/‐45]2s AS‐4/PEEK laminates were performed at various levels of stress amplitudes. The influences of stress amplitude on the fatigue life and the residual tensile strength were investigated. The experimental results for fatigue life and residual tensile strength under different stress amplitudes are analyzed by the median rank method. The S‐N curves at various survival probabilities are also presented by the pooled Weibull distribution function. Furthermore, a residual strength degradation model is used to predict the residual strength for the composites subjected to a number of fatigue cycles and to simulate the effects of the stress amplitude on the fatigue life. The agreement between experiment and theory is good.