Pure Fatigue Tests Research Articles

Investigation of the damage and fracture of Ti-6Al-4V titanium alloy under dwell-fatigue loadings

Ti-6Al-4V alloy turbine components for aeronautical applications operate at a steady state after reaching the maximum stress during each duty cycle. The load holds combined with the fatigue process causes a phenomenon called the dwell effect. The dwell periods reduce the fatigue strength of disks and blades of aeronautical turbine engines made of titanium alloys. In this work, trapezoidal dwell-fatigue and triangular fatigue tests were performed with a fatigue ratio of 0.1 at room temperature. The trapezoidal waveforms (dwell-fatigue tests) had a 10-second dwell period and 1-second loading and unloading rates for each fatigue cycle. The triangular waveforms (fatigue tests) were equivalent to trapezoidal waveforms with zero seconds at the maximum stress. The Weibull distribution was used to analyze the dwell-fatigue data statistically. The experimental results showed that the dwell-fatigue life debits for dwell periods of 10 seconds at stress levels of 1.0, 0.975, and 0.95 of the material yield strength were 10.2, 10.0, and 4.5, respectively. The results suggest that the dwell sensitivity of Ti-6Al-4V alloy increases at high-stress levels. The fracture of dwell-fatigue specimens occurred at a high-cumulated plastic strain and after a significantly lower fatigue life than at pure fatigue tests, indicating a substantial dwell-life debit of the Ti-6Al-4V alloy. The damage mechanism responsible for reducing the fatigue life when the dwell time was introduced was the plastic deformation accumulation observed in dwell-fatigue tests, possibly as a result of the stress redistribution mechanism in the α phase grains that leads to slip of dislocations and, consequently, early plastic deformation processes that instigate crack nucleation.

Effect of Alternate Corrosion and Fatigue on Fatigue Crack Growth Characterization of 2024-T4 Aluminum Alloy

To investigate the effect of alternate corrosion and fatigue on fatigue crack growth characterization of 2024-T4 aluminum alloy, the pure fatigue test, “corrosion + fatigue” test (commonly known as pre-corrosion fatigue), and the “fatigue + corrosion + fatigue” alternating test of single-sided notched specimen of 2024-T4 aluminum alloy were carried out. The fatigue crack life of alternate corrosion and fatigue is longer than that of the pre-corrosion fatigue when the corrosion time is the same. And the more the times of alternating, the longer the crack growth life will be. This is mainly because the fatigue crack growth rate will change suddenly, which will have a certain hysteresis effect on the fatigue crack growth. The corrosion and fracture morphology characteristics of the alternate test were examined by scanning electron microscopy (SEM), and the composition of corrosion products was examined by the energy dispersion spectrum (EDS). The prediction model of fatigue crack growth rate considering the alternating hysteresis effect of corrosion and fatigue based on the Paris formula was established, and the parameters and precision of the model were calculated and verified. The results show that the model has certain engineering application value.

On the dwell-fatigue crack propagation behavior of a high strength superalloy manufactured by electron beam melting

To demonstrate the reliability of additively manufactured superalloys for critical turbine engine components, dynamic tests simulating in-service condition are required. The present study aims to study the dwell-fatigue crack propagation behaviors of IN718 manufactured via electron beam melting (EBM). The textured and columnar-grained microstructure of EBM IN718 shows anisotropic dwell-fatigue cracking resistance when loading axis is aligned parallel and perpendicular to the columnar grains. High and low angle grain boundaries interact differently with the dwell-fatigue cracking path. The effect of different heat treatments on the cracking behavior is also discussed. The dwell-fatigue crack propagation rate of EBM IN718 is compared with forged IN718 under both dwell-fatigue test condition and pure fatigue test condition. The superiority of dwell-fatigue cracking resistance of EBM IN718 to forged IN718 is shown and discussed.

O65] Optimization of Mechanical Properties of High Strength TRIP Steels

The boiler tubes and pipes in the present day coal fired power plants are designed against damage arising from the interaction of creep and fatigue. The present investigation reports the microstructural evolution in P92 grade martensitic steel during pure fatigue and hold time fatigue tests conducted at 600 °C. Fatigue life significantly dropped for hold time fatigue tests in comparison with pure fatigue tests. The drop in fatigue life was more for hold time fatigue tests conducted with compressive hold. Grain boundary oxidation and cracking was identified as the major cause for the decrease in fatigue life under compressive hold. Annihilation of dislocations and pinning of dislocations by MX precipitates were observed to be the microstructural changes during cyclic deformation.

Designing P92 grade martensitic steel header pipes against creep–fatigue interaction loading condition: Damage micromechanisms

The boiler tubes and pipes in the present day coal fired power plants are designed against damage arising from the interaction of creep and fatigue. The present investigation reports the microstructural evolution in P92 grade martensitic steel during pure fatigue and hold time fatigue tests conducted at 600 °C. Fatigue life significantly dropped for hold time fatigue tests in comparison with pure fatigue tests. The drop in fatigue life was more for hold time fatigue tests conducted with compressive hold. Grain boundary oxidation and cracking was identified as the major cause for the decrease in fatigue life under compressive hold. Annihilation of dislocations and pinning of dislocations by MX precipitates were observed to be the microstructural changes during cyclic deformation.

Experimental Study on the Effect of Strain Cycles on Mechanical Properties of AISI 1022 Steel

Abstract: The effect of cyclic loading on various mechanical properties of AISI 1022 steel was investigated in this study using laboratory‐based experimental method. Groups of specimens were tested in push‐pull strain‐controlled cyclic loading. Some of these specimens were tested to failure in pure fatigue tests. For the remaining specimens, cyclic loading tests were terminated at a specific number of cycles and these specimens were then tested to failure in quasi‐static tension. It was found that the strength increased, while the ductility and toughness decreased because of applications of strain cycles. As reduction in ductility weakens plastic strain resistance, and drop in toughness reduces resistance to fracture of this steel, the quasi‐static mechanical behaviour is expected to change as a consequence of application a certain number of strain cycles. This study therefore suggests that these changes in mechanical properties have to be contemplated in the associated design processes.

Effect of temperature and hold time on internal hardening behavior of a near α titanium alloy under cyclic deformation

In the present work, the study of dynamic strain aging (DSA) in near α titanium alloy Timetal 834 is reported in terms of internal hardening variables (kinematic and isotropic hardening variable). Total strain controlled low cycle fatigue tests have been conducted in air at 300 °C and from 400 °C to 500 °C at a temperature interval of 25 °C at nominal strain rates of 6.67 × 10 −3 s −1. The alloy exhibits gradual cyclic softening till failure at 300 °C, whereas, it exhibits initial cyclic softening followed by marked cyclic hardening from 400 °C to 500 °C. The cyclic hardening is attributed to DSA phenomena, resulting due to increase in isotropic stress component. The observed maximum peak stress ratio, lower fatigue life and minimum half-life plastic strain range at 450 °C indicates the maximum effect of DSA at that temperature. The fatigue life of tensile and compressive hold at 450 °C was observed to be inferior as compared to pure fatigue tests.

Creep–fatigue–oxidation interactions in a 9Cr–1Mo martensitic steel. Part I: Effect of tensile holding period on fatigue lifetime

Cyclic tests with or without tensile holding periods were conducted in air at 823 K on a modified 9Cr–1Mo martensitic steel. In addition to stress–relaxation fatigue (RF) tests with a hold time at maximum load, creep–fatigue (CF) experiments were carried out. These CF tests were strain-controlled during the cyclic part of the stress–strain hysteresis loop and then load controlled when the stress was maintained at its maximum value to produce a prescribed value of the creep strain before cyclic deformation was returned under strain-controlled conditions. This unusual testing procedure enabled larger viscoplastic strains to be reached during the holding period than during usual relaxation–fatigue (RF) tests. The relationship between the number of cycles to failure of pure fatigue tests and the cyclic strain range is established for pure fatigue tests. The lifetime reduction due to holding periods is highlighted and quantified. The fatigue lifetime reduction due to holding periods is all the more pronounced as the cyclic strain amplitude is low. No creep cavitation is visible by microscopic observations, while the environment is found to play a key role in damage accumulation and interaction. Two main failure mechanisms are observed depending both on the fatigue strain range and on the duration of the holding period. An attempt is made to explain the existence of these two domains in relation with oxidation effect.

Analysis of the hysteresis loops of a martensitic steel: Part II: Study of the influence of creep and stress relaxation holding times on cyclic behaviour

The second part of this work is devoted to the study of holding time effects on the cyclic plastic behaviour of a martensitic steel tested at 823 K. Both relaxation and creep holding times of various durations were applied. The enhanced stress partitioning method presented in the first part [B. Fournier, M. Sauzay, C. Cae's, M. Mottot, Mater. Sci. Eng., submitted for publication] is used to evaluate the kinematic, isotropic and viscous parts of the cyclic stress. The bulk Young's modulus is found to vary significantly during cycling for creep-fatigue tests, which might be correlated to specific environmental interaction. The viscous stress measured at the end of the holding period tends to vanish as the holding time increases. The introduction of creep holding times enabled higher viscoplastic strains per cycle to be reached and allowed a larger range of strain rates to be studied. In all the cases tested, the observed softening effect is mainly due to the kinematic stress decrease. Nevertheless, even though the kinematic stress is always found to decrease with increasing accumulated viscoplastic strain, the initial magnitude of the creep-fatigue kinematic stress (measured at the end of the first holding period) can be either higher or lower than that of the corresponding pure-fatigue test. These effects of the holding period on the kinematic stress value can be related to the viscoplastic strain rate (and to the nature of the holding time: creep or relaxation). This dependency presents a maximum at intermediate strain rate, suggesting that two competing microstructural mechanisms control the magnitude of the kinematic stress. The enhanced stress partitioning method also enables the activation volume of both the creep and fatigue deformation mechanisms to be evaluated. The observed values are compared to those found in the literature.

Creep-fatigue behaviour of an AISI stainless steel at 550 °C

In the past, a lot of experimental studies have been devoted to creep-fatigue interactions in austenitic stainless steels. Tests have been carried mainly at temperatures of at least 600 °C and at high applied strains, which are supposed to be the most damaging. The present work is dedicated to mechanical tests, TEM observations and lifetime predictions at 550 °C which corresponds to the real industrial temperature in Liquid Metal Fast Breeder Reactors. It is shown that if pure fatigue test results are close to those performed at 600 °C, some of the creep-fatigue results are different, particularly for small applied strains which correspond once more to the industrial conditions. In the 0.25–0.3% strain amplitude range, the stress is larger with hold time than without whatever is the hold time up to 5 h. The numbers of cycles to failure are greatly reduced and no saturation with the hold time is observed, contrary to higher temperature results. The stress–strain behaviour is discussed considering several high temperature mechanisms such as ageing, recovery and viscoplasticity and using TEM observations and stress partitioning into kinematic, isotropic and thermal stresses. Finally, a simple linear damage accumulation model is applied to the 550 °C and 600 °C tests, using the measured stresses. The stress dependence on hold time can partly explain the observed failure results on fatigue life.

CREEP, FATIGUE AND CREEP‐FATIGUE CRACK GROWTH RATES IN PARENT AND SIMULATED HAZ TYPE 321 STAINLESS STEEL

Abstract— Crack growth rate data are presented from a range of fully reversed displacement‐controlled fatigue and creep‐Fatigue tests and from static load‐controlled creep crack growth tests on aged 321 stainless steel (parent and simulated HAZ) at 650 ° C. In the creep fatigue tests, constant displacement tensile hold periods of 12–192 h were used. Crack growth rates comprised both cyclic and dwell period contributions. Cyclic growth contributions are described by a Paris‐type law and give faster crack growth rates than those associated with pure fatigue tests. Dwell period contributions are described by the C* parameter. The total cyclic crack growth rates are given by summing the cyclic and dwell period contributions. Estimates of C* using a reference stress approach together with the appropriate stress relaxation creep data are shown to correlate well with experimentally measured C* values. Crack growth rates during static load‐controlled tests correlate well with C*. Good agreement is obtained between crack growth rates during the static tests and those produced during the hold period of the creep‐fatigue tests.