Effect of strain rate on high-temperature low-cycle fatigue of 17-4 PH stainless steels

The effect of strain rate (10⁻², 10⁻³ and 10⁻⁴ s⁻¹) on the low-cycle fatigue (LCF) behavior was investigated for 17-4 PH stainless steels in three different conditions at temperatures of 300–500 °C. The cyclic stress response (CSR) for Condition A tested at 300 and 400 °C showed cyclic hardening due to an influence of dynamic strain aging (DSA). An in situ precipitation-hardening effect was found to be partially responsible for the cyclic hardening in Condition A at 400 °C. For H900 and H1150 conditions tested at 300 and 400 °C, the CSR exhibited a stable stress level before a fast drop in load indicating no cyclic hardening or softening. At 500 °C, cyclic softening was observed for all given material conditions because of a thermal dislocation recovery mechanism. The cyclic softening behavior in Conditions A and H900 tested at 500 °C is attributed partially to coarsening of the Cu-rich precipitates. The LCF life for each material condition, tested at a given temperature, decreased with decreasing strain rate as a result of an enhanced DSA effect. At all given testing conditions, transgranular cracking was the common fatigue fracture mode.     

Effect of grain size on austenite stability and room temperature low cycle fatigue behaviour of solution annealed AISI 316LN austenitic stainless steel

Abstract

The present paper investigates completely reversed room temperature low cycle fatigue (LCF) behaviour of solution annealed austenitic stainless steel AISI 316L with two different grain sizes of 90 and 139 μm developed by solution annealing treatment at 1050 and 1150°C respectively and at six strain amplitudes ranging between ± 0·375 and ± 1·00%. Complete cyclic hardening has been observed for both the grain sizes. While fine grained steel shows an improvement in cyclic life compared with that of coarse grained steel for strain amplitudes ± 0·375 and ± 0·50%, and perfectly follows the Coffin–Manson (C–M) behaviour within the experimental domain, higher cyclic life with bilinear C–M behaviour is observed in the case of coarse grained steel at ± 0·625% strain amplitude and above. Optical microscopy of fatigue fracture surfaces reveals the formation of martensite on cyclic straining predominantly at higher strain amplitudes.     

Low cycle fatigue and creep–fatigue interaction behavior of 316L(N) stainless steel and life prediction by artificial neural network approach

Low cycle fatigue (LCF) behavior of solutionized 316L(N) stainless steel (SS) has been studied at various temperatures, strain amplitudes, strain rates, hold times and in 20% prior cold worked condition. The alloy in general showed a reduction in fatigue life with, increase in temperature, increase in strain amplitude, decrease in strain rate, an increase in duration of hold time in tension and with prior cold work. The LCF and creep–fatigue interaction (CFI) behavior of the alloy was explained on the basis of several operative mechanisms such as dynamic strain ageing, creep, oxidation and substructural recovery. The capability of artificial neural network (ANN) approach to life prediction under LCF and CFI conditions has been assessed by using the data generated in the present investigation. It is demonstrated that the prediction is within a factor of 2.     

Mean stress effects on low‐cycle fatigue for a precipitation‐hardening martensitic stainless steel in different tempers

The objective of this study is to investigate the effects of mean stress and ageing treatment on the low‐cycle fatigue (LCF) behaviour of a precipitation‐hardening martensitic stainless steel (PHMSS). Uniaxial LCF tests were conducted under strain control with three strain ratios, R = ?1, 0 and 0.5 on specimens heat‐treated to three different tempers, i.e. solution‐annealed (SA), peak‐aged (H900) and overaged (H1150) conditions. Experimental results indicated that under a strain ratio of R = ?1, specimens in H900 temper exhibited longer LCF lifetimes than those in SA and H1150 tempers. However, this advantage for H900 over SA and H1150 tempers disappeared at higher strain ratios (R = 0 and 0.5) due to the greater sensitivity to mean stress effects in H900 temper. For a given temper at high strain amplitudes, the LCF lifetimes under the three applied strain ratios did not show significant differences as a result of the mean stress relaxation effect. However, at low strain amplitudes, cyclic loading at R = ?1 generated longer LCF lifetimes in comparison to R = 0 and 0.5 due to the absence of detrimental tensile mean stress. LCF lifetime data obtained for the given PHMSSs under various combinations of strain ratio and heat treatment were well correlated with a strength‐normalized Smith–Watson–Topper (SWT) parameter in a log–log linear model.     

Influence of temperature on the low cycle fatigue behaviour of a modified 9Cr–1Mo ferritic steel

Low cycle fatigue (LCF) behaviour of a modified 9Cr–1Mo steel under normalized and tempered conditions is reported. The alloy was normalized at 1313K for 1 h followed by tempering at 1033K for 1 h, which resulted in a tempered martensitic structure. Total axial strain controlled LCF tests were conducted at a constant strain rate of 3×10⁻³ s⁻¹ at different strain amplitudes varying from ±0.25 to ±1.0% in the temperature range of 300–873K. The cyclic stress response behaviour, in general, showed an initial brief hardening for the first few cycles, followed by a continuous and gradual softening regime that ended in a stress plateau that continued up to the specimen failure. The fatigue life decreased as the temperature increased. The temperature effect on life was more pronounced at low strain amplitudes. The metallography of the failed samples revealed that the fatigue failure at high amplitudes of testing was marked by extensive crack branching and the formation of secondary cracks. Oxidation was found to exert major influence on LCF life reduction at 873K.     

Assessment of fatigue response of thermally aged reduced activation ferritic-martensitic steel based on finite element analysis

Abstract

In the present investigation, effect of thermal ageing on low cycle fatigue (LCF) behaviour of Reduced Activation Ferritic Martensitic steel has been assessed by finite element analysis. The steel was thermally aged at 873 K for 3000 hour. Low cycle fatigue tests were carried out on both the as-received and thermally aged material at strain rate of 3×10^?3 s^?1 at 823 K, over strain amplitudes in the range of ± 0.25 to ± 0.8%. Continuous cyclic softening till final failure, except for initial few cycles especially at relatively lower strain amplitudes, was observed in both the material conditions. Thermal ageing resulted in marginally higher cyclic stress response accompanied by lower fatigue life. The differences in fatigue responses have been attributed to the coarsening of precipitates on thermal ageing. Finite element analysis has been carried out considering combined isotropic and kinematic hardening as material model to estimate the effect of thermal ageing on the response of material under LCF loading. Thermal ageing was found to decrease both the isotropic and kinematic hardening with appreciable effect on isotropic hardening. The predicted cyclic stress response and hysteresis loops were found to be in good agreement with the experimental data. The LCF life of the steel has been estimated based on the hysteresis energy approach.     

Low-cycle fatigue of austempered ductile irons

The effects of austempering temperature and isothermal transformation time on the low-cycle fatigue (LCF) behaviour in ductile irons have been studied. The fracture surfaces were observed by a scanning electron microscope in order to understand the fracture mechanism of LCF. From the results, it can be concluded that the best LCF behaviour is for the irons austenitized at 950 °C and there is very good cyclic stability at the lower strain amplitude irrespective of the austempering condition. However, there is a little cyclic softening at higher strain amplitudes for all the austempering conditions. Under a larger strain amplitude, the best LCF behaviour is for the specimen that has undergone austempering at a higher temperature, but under a smaller strain amplitude, the best LCF behaviour is for the specimen austempered at 350 °C.     

Effects of temperature on the low cycle fatigue behaviour of nitrogen alloyed type 316L stainless steel

Strain-controlled low cycle fatigue tests have been conducted in air between 298–873 K to ascertain the influence of temperature on LCF behaviour of nitrogen-alloyed type 316L stainless steel. A strain amplitude of ± 0.60% and a symmetrical triangular waveform at a constant strain rate of 3 × 10⁻³ s⁻¹ were employed for all tests. Crack initiation and propagation modes were evaluated, and the deformation and damage mechanisms which influence the cyclic stress response and fatigue life identified. The cyclic stress response at all temperatures was characterized by an initial hardening to the maximum stress, followed by gradual softening prior to attaining saturation. Temperature dependence of fatigue life showed a maximum in the intermediate temperature range. The drastic reduction in fatigue life at elevated temperatures has been ascribed primarily to the combined influence of dynamic strain ageing effects and oxidation-enhanced crack initiation, while the lower life at room temperature is attributed to detrimental effects associated with deformation-induced martensite.     

Fatigue lifetime of AZ91 magnesium alloy subjected to cyclic thermal and mechanical loadings

In the present paper, thermo-mechanical fatigue (TMF) and low cycle fatigue (LCF) or isothermal fatigue (IF) lifetimes of a cast magnesium alloy (the AZ91 alloy) were studied. In addition to a heat treatment process (T6), several rare elements were added to the alloy to improve the material strength in the first step. Then, the cyclic behavior of the AZ91 was investigated. For this objective, strain-controlled tension–compression fatigue tests were carried out. The temperature varied between 50 and 200 °C in the out-of-phase (OP) TMF tests. The constraint factor which was defined as the ratio of the mechanical strain to the thermal strain, was set to 75%, 100% and 125%. For LCF tests, mechanical strain amplitudes of 0.20%, 0.25% and 0.30% were considered at constant temperatures of 25 and 200 °C. Experimental fatigue results showed that the cyclic hardening behavior occurred at the room temperature in the AZ91 alloy. At higher temperatures, this alloy had a brittle fracture. But also, it was not significantly clear that the cyclic hardening or the cyclic softening behavior would be occurred in the material. Then, the high temperature LCF lifetime was more than that at the room temperature. The OP-TMF lifetime was the least value in comparison to that of LCF tests. At the end of this article, two energy-based models were applied to predict the fatigue lifetime of this magnesium alloy.     

Understanding the influence of particle size on strain versus fatigue life,and fracture behavior of aluminum alloy composites produced by spray deposition

The strain versus fatigue life and fracture behavior of spray-formed Al–Si composites reinforced with SiC particles of two different sizes were studied under total strain amplitudes. Both composites exhibit short low-cycle fatigue (LCF) which follows a Coffin-Manson relationship, and display cyclic hardening at all strain amplitudes. The LCF endurance of the composite with large particles is higher than that of composite containing small particles in the high strain amplitudes, however, at low strains the difference in fatigue endurance between the two composites decreased. Moreover, the decrease in particle size results in a higher degree of hardening at low and middle strains, but reduces the magnitude of hardening at highest strain. Fractographic analysis reveals that particle/matrix debonding is the main mechanism of failure in composite with small particles, while fracture and debonding of SiC particle are predominant in the large particle reinforced composite.     

Room-temperature low-cycle fatigue behaviour of Ti-27Al-15Nb alloy

Low-cycle fatigue (LCF) behaviour of the alloy Ti-27Al-15Nb, in (α ₂+B2) heat-treated condition was studied in total axial strain control mode at different total strain amplitudes (Δɛ ₁/2) from ±0·65 to ±1·0% and room temperature. While there was little hardening of the material at the lowest strain amplitude (Δɛ _t/2: ±0·65%), pronounced hardening was observed at the higher strain amplitudes (Δɛ _t/2⩾0·83%). The cyclic stress, at high strain amplitudes, continuously increased from the beginning till fracture of the specimens. The LCF resistance of the material was found to be low and this was due to its poor ductility at room temperature. Dual slope was observed in the Coffin-Manson plot, with less slope of the upper segment than that of the lower one, as observed in several alloys. The fracture behaviour pointed to brittleness and faceted features were observed.     

Low-cyclic fatigue behaviour of an Al–Cu–Mg–Ag alloy under T6 and T840 conditions

The low-cyclic fatigue (LCF) behaviour of an AA2139 alloy belonging to the Al–Cu–Mg–Ag system was investigated under T6 and T840 conditions. The T840 treatment involves cold rolling with a 40% reduction prior to ageing, and this was effective in increasing the tensile strength of the alloy. Under cyclic loading at total strain amplitudes (ε_ac) of ±0.4 to ±1.0%, the mechanical behaviour is defined as the prevalence of elastic over plastic deformation processes under both the T6/T840 conditions. The initial weak hardening during one to two cycles of loading at ε_ac?>?0.55% and an insignificant softening upon following the cyclic loading to fracture was observed for the T6/T840 conditions. The LCF behaviour of the alloy under the T6/T840 conditions is described by the Basquin–Manson–Coffin relationship.

This paper is part of a Themed Issue on Aluminium-based materials: processing, microstructure, properties, and recycling.     

Effect of Sc addition on low-cycle fatigue properties of extruded Al-Zn-Mg-Cu-Zr alloy

ABSTRACT

The influence of minor Sc addition on the low-cycle fatigue (LCF) properties of hot-extruded Al-Zn-Mg-Cu-Zr alloy with T6 state was investigated through performing the LCF tests at room temperature and air environment. The results indicate that two alloys show cyclic stabilisation, cyclic hardening and cyclic softening during fatigue deformation. The addition of Sc can significantly enhance the cyclic stress amplitude of the alloy. Al-Zn-Mg-Cu-Zr-Sc alloy shows higher fatigue lives at lower strain amplitudes, while has lower fatigue lives at higher strain amplitudes. For the two alloys, the density and movability of dislocations are related to the change of cyclic stress amplitudes. The existence of Al₃(Sc,Zr) phase can inhibit the appearance of cyclic softening phenomenon in the Al-Zn-Mg-Cu-Zr-Sc alloy.     

The effect of thermal ageing on low cycle fatigue behaviour of 316 stainless steel welds

It is well known that welds are the weak links in any structure. Therefore, it is of out most importance to characterize the mechanical properties of welds. Moreover, the changes in the microstructure that occur in welds on exposure to high temperatures affect the mechanical properties and must be studied by ageing the welds at high temperature. In this paper the low cycle fatigue behaviour of thermally aged 316 stainless steel weld metal is presented. Weld pads with single V configuration were prepared by the shielded metal arc welding process using 316 electrodes. Thermal ageing was done for 10,000 h at 823 and 873 K. Total strain controlled low cycle fatigue tests were conducted at a constant strain rate of 3 × 10^?3 s^?1 with strain amplitudes in the range ±0.25% to ±0.6% at 823 and 873 K. Weld metal exhibited initial hardening followed by cyclic softening prior to failure. The aged samples exhibited higher stress response as compared to the unaged samples. At both the temperatures and all strain amplitudes fatigue life was inferior to that of unaged samples. The metallography of the aged and tested material was studied through optical, scanning and transmission electron microscopy. The effect of transformation of δ-ferrite to sigma phase and carbides in the weld metal on low cycle fatigue behaviour was evaluated.     

Damage mechanisms of single and polycrystalline nickel base superalloys SC16 and IN738LC under high temperature LCF loading

The mechanical behaviour of polycrystalline and single crystal nickel base superalloys, IN738LC and SC16, respectively, has been investigated under strain controlled LCF loading at 1223 K. The alloys showed a very similar stress response: a long stable stress amplitude period preceded by either cyclic hardening at higher strain amplitudes or by cyclic softening at lower strain amplitudes. The fatigue life of the single crystal alloy was, however, measured to be about 10 times higher than that of the polycrystalline variant. This can be attributed to the differences in deformation and damage processes observed in the two alloys.     

Cyclic strain rate effect on martensitic transformation and fatigue behaviour of an austenitic stainless steel

In this study, the effect of strain rate on the cyclic behaviour of 304L stainless steel is investigated to unveil the complex interrelationship between martensitic phase transformation, secondary hardening, cyclic deformation and fatigue behaviour of this alloy. A series of uniaxial strain controlled fatigue tests with varying cyclic strain rates were conducted at zero and non‐zero mean strain conditions. Secondary hardening was found to be closely related to the volume fraction of strain‐induced martensite which was affected by adiabatic heating due to increasing cyclic strain rates. Tests with lower secondary hardening rates maintained lower stress amplitudes during cyclic loading which resulted in longer fatigue lives for similar strain amplitudes. Fatigue resistance of 304L stainless steel was found to be more sensitive to changes in strain rate than the presence of mean strain. The mean strain effect was minimal due to the significant mean stress relaxation in this material.     

Characterization of cyclic plastic bending of austenitic AISI 304 stainless steel

Steels used in chemical, nuclear and power industries are usually subjected to repeated loads that may induce significant inelastic deformations. Successful assessment of a component life places a great demand on understanding of material behaviour under strain histories involving cyclic plasticity under bending conditions.

This work is concerned with an experimental study of low-cycle fatigue behaviour under both load and deflection controlled conditions. The tests were conducted on AISI 304 stainless steels with the objective of determining strain cycling properties under cyclic plastic bending. These properties are characterized by the cyclic stress-strain hysteresis responses obtained.

The cyclic bending plasticity behaviour is investigated for beams with circular cross-section at two conditions of the material, under constant strain and under constant stress. Hysteresis loops for five different strain ranges of ±0.4%, ±0.82%, ±1.1%, ±1.33% and ±1.55% were obtained and also for different stress ranges of 730, 800, 870 and 940 MPa. The material exhibits cyclic hardening in its virgin state, the rate of hardening is very rapid at first but diminishes quickly as the stabilized condition is approached. The cyclic hardening for the constant strain condition is higher than the constant load condition.

The results also confirm the applicability of the reference stress method for predicting the cyclic moment-curvature curve for beams with circular cross-sections. The observed strain values are higher in the lower strain range and lower in the higher strain. However, the correlation obtained is considered quite satisfactory in view of the nonlinearity of the problem.     

Experimental and numerical characterization of low cycle fatigue and creep fatigue behaviour of P92 steel welded joint

Low cycle fatigue (LCF) and creep fatigue interaction (CFI) behaviour of P92 steel welded joint were investigated experimentally and numerically. Strain‐controlled LCF tests at different strain amplitudes and CFI tests at different peak strain holding time were conducted. Evolutions of cyclic stress response, mean stress, and creep strain during cycling were described, in which the influence of strain amplitude and holding time were investigated. A specific heat treatment process was proposed to get the homogenous simulated material of fine grain region and coarse grain region in the heat affected zone. Material parameters of parent material, fine grain heat affected zone, coarse grain heat affected zone, and weld metal in the unified viscoplasticity model were then determined and validated. To predict the LCF and CFI behaviour of welded joint, 3‐dimensional unified viscoplasticity model with a modified isotropic variable was compiled into ABAQUS UMAT. The comparison between the predicted and experimental result under LCF and CFI loadings showed that the simulation results were reasonable and agreed with the experimental data well.     

Role of dynamic strain ageing in low cycle fatigue

Low cycle fatigue (LCF) at elevated temperatures is known to be influenced by time-dependent processes like creep, oxidation and metallurgical instabilities. Another time-dependent phenomenon namely, dynamic strain ageing (DSA) has been found to exert an influence on LCF behaviour at high temperatures. Research activities carried out in the present author’s laboratory with a view to understanding the effects of DSA on LCF are highlighted in this paper. Occurrence of DSA manifests during total strain-controlled fatigue tests in the form of serrated plastic flow in stress-strain hysteresis loops, increased cyclic work hardening and reduced plastic strain range. Further, DSA causes localization of plastic flow leading to enhanced planarity of slip and widely-spaced slip bands. Impingement of slip bands on grain boundaries causes increased grain boundary decohesion, leading to reduced fatigue life. The influence of prior microstructure such as second phase particles and grain size on the effects of DSA on LCF is also discussed.     

超低碳奥氏体不锈钢在低周疲劳中的组织结构研究

研究了超低碳３１６Ｌ、３１６ＬＮ奥氏体不锈钢在常温下的低周疲劳行为及组织结构变化规律。结果表明，材料在循环变形过程中呈现不同程度的硬化、软化，应变幅的增加促进硬化，缩短低周疲劳寿命。透射电镜分析表明，奥氏体内的组织结构的演变过程与循环应力特性曲线的变化规律相呼应。