Fatigue Strength Of Smooth Specimens Research Articles

Influence of small defects and nonmetallic inclusions on the high and very high cycle fatigue strength of an ultrahigh‐strength steel

AbstractThe high and very high cycle fatigue (VHCF) properties of ultrahigh‐strength Ck45M steel processed by thermomechanical rolling integrated direct quenching were investigated. S–N tests with smooth and small drilled holes containing specimens as well as near‐threshold fatigue crack growth measurements were performed up to 2 × 1010 cycles using ultrasonic‐fatigue testing technique. The fatigue strength of smooth specimens is mainly determined by the size of nonmetallic inclusions. For surface defects larger than 80 μm, the fatigue limit can be correlated with a constant threshold‐stress intensity factor. The ‐parameter model adequately predicts the fatigue limit for internal defects and for surface defects with sizes between 30 and 80 μm. VHCF failures from smaller surface defects occur at stress amplitudes below the predicted fatigue limit. The long‐crack threshold in ambient air is close to the effective threshold stress intensity factor. In optically dark areas at interior inclusions, cracks grow at mean propagation rates of 10−15 m/cycles.

Fretting Fatigue Behaviour of Alloy Steel in the Very High Cycle Region

It is well known that fretting fatigue strength is much lower than the fatigue strength of smooth specimens and the fatigue limit disappears. Many studies on fretting fatigue have been reported but most of the studies have not cover fatigue properties in the very high cycle regime more than 107 cycles. In this study, an accelerated fretting fatigue testing method was developed by using an ultrasonic torsional fatigue testing machine with a clamping fretting pad. Fretting fatigue tests of CrMo steel were conducted by using the developed method. Test results showed that fretting fatigue failure occurs in the very high cycle region.

Effect of Static and Intermittent Shear Stress on the Fatigue Strength of Notched Components under Combined Rotating Bending and Torsion

In power transmission machinery or crane industries, shafts are designed using specific standards that may not correctly cover the large panel of materials and loadings involved in these sectors, making it necessary to use safety factors leading to over-conservative life predictions and non-optimized designs.Shafts are key components, usually experiencing alternated normal stresses combined with static or fluctuating (intermittent) shear stresses. In high cycle multiaxial fatigue, it is admitted that mean shear stress has no significant effect on the fatigue strength of smooth specimens if the maximum shear stress does not exceed about 80% of the material shear yield limit. Under combined rotating bending and torsion the situation is unclear, especially in presence of notches.In this paper the effect of a static and intermittent shear stress on the fatigue strength of two quenched and tempered steel grades, 30NiCrMo8 and 42CrMo4, is studied on notched specimens with bending theoretical stress concentration factors Kt = 1.7 and Kt=2.7. It is shown that mean shear stress has little effect on the median rotating bending endurance limit at 3×106 cycles, the maximum decrease is about 5%, whereas this decrease is more pronounced for intermittent shear stress (varying in blocks), reaching up to 30% for the sharpest notch.

Fatigue strength and failure mechanisms of wrought aluminium alloys in the VHCF-region considering material and component relevant influencing factors

Investigations at room temperature were carried out with smooth specimens made of the aluminium alloys EN AW 6056 and EN AW 6082. Different failure mechanisms were investigated for instance according to material and number of cycles. The influence of mean stress on fatigue strength of smooth specimens made of EN AW 6056 and transition of failure mechanism from the high cycle fatigue (HCF) to the very high cycle fatigue regime (VHCF) are discussed. Fracture surface and microstructure of the crack initiation sites were observed by means of scanning electron microscope (SEM), electron back scatter diffraction analysis (EBSD analysis) and metallographic methods. Transmission electron microscopy (TEM) was used to find differences between unstressed and tested material of EN AW 6082. The results reveal for both aluminium alloys that the fatigue strength decreases with increasing number of cycles, which is known for materials with face-centred cubic lattice. In the very high cycle regime the crack initiation sites of the smooth specimens were internally. The crack initiation sites of the notched specimens were always at the root of the notch independent of the number of cycles. The microstructures of fracture surfaces were different for both aluminium alloys. Slightly notched and sharp notched (screws M10, thread rolled before heat treatment) specimens made of EN AW 6056 were investigated. Fatigue tests were done with a servohydraulic testing machine and a resonant frequency machine. Tests run up to a maximum number of N = 2 × 10 8 cycles (EN AW 6056) or N = 10 9 cycles (EN AW 6082). A comparison of the experimental results with estimated fatigue strength of a design code (FKM-Guideline, 2003 [1]) is presented.

Effect of Shot Peening on Torsional Fatigue Strength of High Strength Spring Steel in Air and Corrosive Environments

Torsional fatigue tests were conducted for a spring steel, SUP7, with Vickers hardness of 480. The effect of shot peening on the torsional fatigue strength of smooth and artificially notched specimens in the corrosive environment and air was studied. The increase of the fatigue strength of smooth specimens in air by shot peening was relatively small, and the surface roughness due to peening did not reduced the fatigue strength. On the other hand, the fatigue strength of notched specimens increased greatly by shot peening and the notch sensitivity was reduced. In corrosion fatigue, the fatigue strength increased by shot peening in the medium life range. The strength of shot-peened specimens was reduced in the long life range around 107cycles and was approached to that of not-peened specimens. A similar improvement was observed for notched specimens in the medium life range, and the notch sensitivity was reduced by shot peening. The compression zone made by shot peening extended about 0.3mm below the surface, and relaxed by fatigue loading. The observation results on the cracking behavior and residual stress distribution were used to interpret the effect of shot peening on the fatigue strength of spring steel.

Effects of Artificial Small Defect and Corrosive Environment on Torsional Fatigue Strength of High Strength Spring Steel

Torsional fatigue tests were conducted for spring steels whose Vickers hardness were 430, 480, 550, and 620. The effects of corrosive environment and surface artificial pits on the fatigue strength were studied. The fatigue strength of smooth specimens was maximum at 550HV for fatigue in air and 480HV for corrosion fatigue. Intergranular fracture surfaces were observed on the fatigue fracture surface corresponded to the decrease in the fatigue strength of the high-hardness materials. The fatigue fracture of smooth specimens was generated from shear crack for fatigue in air and from corrosion pit in corrosion fatigue. The stress intensity factor for crack initiation from corrosion pit was nearly the same for all materials. The fatigue strength reduction factor was constant under fatigue loading in air, while decreased with increasing fatigue life and hardness in corrosion fatigue. For the case of long of the material with 620HV in corrosive environment, a significant influence of the artificial pit on the fatigue strength was not seen. Many small cracks were observed at the sites except for artificial pit on the surface of the 620HV specimens fatigued at low stress levels.

High cycle fatigue behavior of a single crystal superalloy at elevated temperatures

Smooth and notched specimens of single crystal (SC) superalloy SRR99 with [0 0 1] orientation were subjected to high-cycle fatigue (HCF) loading at temperatures of 700 °C, 760 °C, 850 °C and 900 °C in ambient atmosphere. The results demonstrate that the fatigue strength of smooth specimens reached the maximum at 760 °C and decreased with increasing temperature. The alloy became more notching sensitive with increase of temperature while the notch sensitivity declined at 900 °C. Analysis on fracture surfaces of both smooth and notched specimen shows that a transition from ductile fracture at lower temperatures to cleavage mode at higher temperatures were observed. Evolution of the microstructure was investigated by SEM and TEM observation. With the process of cyclic plastic deformation at elevated temperatures, the primary cuboidal γ′ precipitates tended to dissolve into the matrix channels, meanwhile a larger number of secondary γ′ particles were formed in the γ matrix. In addition, different types of dislocation structures were developed during the cyclic deformation, which would have a significant impact on the fatigue life of the material.

Estimation of the fatigue strength distribution in high-cycle multiaxial fatigue taking into account the stress–strain gradient effect

Based on the weakest link concept, a probabilistic approach in high-cycle multiaxial fatigue is developed to predict, for a given number of cycles, the probability distribution of the fatigue strength for metallic structural components. A three-parameter Weibull distribution is combined with the energy-based and volumetric high-cycle multiaxial fatigue criterion proposed by Banvillet et al. in 2002 [Banvillet A, Palin-Luc T, Lasserre S. A volumetric energy based high cycle multiaxial fatigue criterion. Int J Fatigue 2003;26(8):755–69; Banvillet A, Palin-Luc T, Lasserre S, Vittori JF. Energy based high cycle multiaxial fatigue criterion depending on stress–strain distribution. In: Blom AF, editor. Fatigue 2002: eighth international fatigue congress, vol. 1, EMAS: Stockholm; 2002, p. 283–90]. Whatever the stress state and the loading type are, the corresponding fatigue strength probability distribution can be deduced from three usual experimental fatigue limits. The scale effect is also predicted. Experimental probability distributions and theoretical predictions of the fatigue strength of smooth specimens are in good agreement for the five materials investigated: the 30NiCrMo16 and 35CrMo4 quenched and tempered steels, the C20 annealed steel, the EN-GJS800-2 nodular cast iron and the Ti–6Al–4V titanium alloy.

Influence of sea water on the fatigue strength and notch sensitivity of a plasma nitrided B–Mn steel

Notched and smooth cylindrical plasma nitrided (PN) and quench and tempered (Q&T) steel specimens made of a B–Mn SS2131(≈AISI 15B21H) steel have been exposed to constant amplitude plane reversed bending corrosion fatigue tests ( R=−1) at 47 Hz in sea water. S– N curves show that sea water suppresses the fatigue limit and reduces fatigue strength (especially at long lives) of smooth and notched Q&T and PN specimens. Plasma nitriding improves the corrosion fatigue resistance of Q&T specimens; this is associated with the good corrosion resistance of ε and γ′-phases, the enhancement of corrosion and fatigue by compressive residual stresses, and the consumption of H + ions during reduction of nitrogen. This improvement is more significant for smooth specimens and for long lives. Notch sensitivity of Q&T and PN specimens decreases with fatigue life. Pitting corrosion, cyclic applied stress and residual stresses due to plasma nitriding are the main topics of this work. An equation to predict corrosion fatigue strength from air fatigue strength of smooth specimens, has been established.

Influence of plasma nitriding on fatigue strength and fracture of a B-Mn steel

The first part of a systematic investigation is presented of surface treatments affecting the fatigue behavior of smooth and notched quenched and tempered (Q&T) specimens made of a B-Mn SS2131 (≈AISI 15B21H) steel. In this part, the effects of plasma nitriding (nitriding temperature=480°C, time=24 h) on the fatigue strength and notch sensitivity were investigated. Constant stress amplitude plane reversed bending fatigue tests ( R=−1) at 47 Hz were conducted using cylindrical plasma nitriding (PN) and Q&T steel specimens with K t =1.05 and 1.7. The compound layer was found to consists of ϵ-phase and γ′-phase. S-N curves show that plasma nitriding improves the fatigue limit by 53 and 115% of Q&T smooth and notched specimens, respectively. The fatigue strength of smooth specimens is improved through the whole fatigue life but only for long fatigue lives for notched specimens. Plasma nitriding reverses the low notch sensitivity (at short lives) and high notch sensitivity (at long lives) exhibited by Q&T specimens. It is shown that compressive stresses introduced by plasma nitriding play the principal role on improvement of fatigue strength, subsurface crack nucleation, fish-eye asymmetry and reduction of stress intensity factors. Fracture toughness, evaluated as a function of depth from the surface, decreases rapidly when approaching the surface. A simple model based on crack retardation and reduction of stress intensity factors produced by the net stress distribution is used to explain fish-eye formation and subsurface fatigue failure.

Fatigue Strength of Austempered Ductile Cast Iron in a Long Life Regime.

This paper describes the effects of shot peening and artificial small defects on the fatigue strength of austempered ductile cast iron (ADI). The fatigue strength of smooth specimens was increased by shot peening in a short life regime and the increments reached about 33 and 29% at 105 and 106 cycles, respectively. The improvement of fatigue strength, however, almost disappeared in a long life regime of 108 cycles. Small drilling holes were used as artificial defects. The diameters of the holes were 0.1, 0.2, 0.3 and 0.4mm and their depths were almost the same as the diameter. The fatigue strength of the specimens with a 0.1mm drilling hole did not decrease when compared with that of the smooth specimens, but the fatigue strengths of the specimens with 0.2∼0.4mm drilling holes decreased. The experimental results of the specimens with 0.2∼0.4mm drilling holes were consistent with the predlction by Murakami's √(area) parameter model. The maximum size, (√(area))max, of the pre-existent defects in ADI was equivalent to the defect of the specimen with a 0.1mm drilling hole and was estimated to be about 90 μm by the above model.

Notch Sensitivity in Fatigue of Austempered Ductile Iron.

Tensile fatigue tests were performed on smooth and notched specimens of a ferritic (FDI) and an austempered ductile iron (ADI). Stress to number of cycles to failure curves were measured, and a notch factor, Kf, was determined from the static strength to fatigue limit region at 5×106 cycles. In FDI, Kf is much lower than the form factor, αe, of the notches, but in ADI, it was close to αe. The notch sensitivity factor η in AID is 0.6-0.9 while it is 0.2 in FDI, indicating very high notch sensitivity of ADI. The results were discussed in terms of the over sressed depth (δ) concept estimated from the stress distribution at the notch root and the fatigue strength of smooth specimens. The δ is 0.1-0.2 mm in ADI while it is 0.5-1 mm in FDI. Macroscopic and SEM fractography were also performed, and the high notch sensitivity was attributed to the brittle and defect-sensitive ausferrite matrix.

Effect of loading cycle asymmetry, stress concentration, and fretting corrosion on the fatigue resistance of alloy D16T

Experimental data are presented for the fatigue strength of smooth specimens, specimens with a stress concentrator, and specimens under fretting corrosion conditions for alloy D16AT at three levels of loading cycle asymmetry coefficient R = −1, 0, and −0.5. It is established that fatigue resistance for alloy D16AT under fretting corrosion conditions is governed by the amplitude of operating stresses but it is practically independent of changes in loading cycle asymmetry coefficient. The possibility is demonstrated of calculating the fatigue limit for specimens with a stress concentrator with different loading cycle asymmetry from known values of the threshold stress intensity factor, the theoretical stress concentration factor, and the length of an nondeveloping crack at fatigue limit. Limiting stress amplitude diagrams for smooth specimens under fretting corrosion conditions have a different nature for the relationship between amplitude and average stresses of the loading cycle and therefore they cannot be described by a general analytical expression.

炭素鋼の圧入軸の疲労強度とき裂進展挙動

An Experimental study has been carried out to determine fatigue strength and crack propagation behavior of press-fitted axles of a S45C carbon steel. Six kinds of specimens were prepared by means of heat treatments. That is, material A and B were normalized and annealed after heating at 850°C for 30min, C and D were normalized and annealed after heating at 1000°C for 4h, H was quenched and tempered after heating at 850°C for 30min, and R is as-received condition. For rotating bending fatigue tests, the press-fitted specimens, inserted two axles of 8mm in diameter into a joint with an interference of 10μm, were used to evaluate the fatigue strength, and the taper-fitted specimens, jointed the same axle with a tapered boss, were used to observe intermittently the crack propagation behavior.Although the fatigue strength of smooth specimens for all material conditions varied from 220 to 435MPa, that of the press-fitted specimen was in the narrow range from 140 to 190MPa. The fatigue strength of press-fitted axles was independent of the static strength or the plain fatigue strength of the material. From the observations of the crack propagation behavior at stress amplitude of 200MPa with the taper-fitted specimens, it was found that crack initiation was earlier in high strength materials, such as H and R, than in low strength materials, such as B and D. And, the crack propagation rate in the material R was higher than that in the material B or D, and that in the material H was lowest. Consequently, the fatigue strength of press-fitted axles was found to be lowest for the material R and highest for the material H.

Fatigue strength of high strength dual-phase steel sheet

Fatigue tests have been carried out on lean-alloyed dual-phase steels with tensile strengths ranging from 300–800 MPa. Smooth specimens and specimens with punched holes were tested. The fatigue strength of dual-phase steel was found to be similar to that of other types of steel ( eg solution hardened or microalloyed steels) of equal tensile strength. The fatigue strength increases with increasing yield strength. For notched specimens it is also related to the yield ratio. Work and bake hardening increase the fatigue strength of smooth specimens in proportion to the increase in yield strength. For notched specimens this effect is less and is dependent on the yield ratio. Bake hardening of material which was not work hardened also increased the fatigue strength. The notch sensitivity of low yield ratio dual-phase steel is found to be low. The notch sensitivity seems to increase with increasing yield ratio.

２種の鋼における確率疲労特性のチャージ間比較

The comprehension of the statistical fatigue property of structural steels is increasingly considered to be important as the basic data in designing with high reliability.The authors had formerly reported the results of statistical fatigue tests performed on several structural alloys. The distribution characteristics in fatigue strength of the material were discussed by examining the distribution of the deviations in stress of individual test data from the median S-N curve. The present paper deals with heat-to-heat change of the statistical fatigue property analysed by the same method as before, for each 3 heats of 0.45%C carbon steel and 1%Cr-0.2% Mo-0.35%C low alloy steel. The effects of heat treatments and sharp notch were studied for 12 test conditions in total, each comprising more than 120 test specimens.It was revealed that the Vickers hardness of heat treated materials coming from one heat obeyed the normal distribution law, and the fatigue strength of smooth specimens could be predicted by the same distribution law; while the fatigue strength of notched specimens showed an asymmetric distribution having a larger scatter in the weaker side and being approximated by Weibull's function. The variation coefficient of fatigue strength increases under the effect of notch, in varying degree depending upon heat treatment condition, probably reflecting microstructural differences at the fatigue crack tips. The heat-to-heat change in variation coefficient of fatigue strength for smooth specimens was explained by the variation of material's homogeneity, e. g. chemical composition, and by the substantial fatigue variation which increases with increasing fatigue strength of the material.

Scatter of Fatigue Strength in Medium Carbon Steels

A series of fatigue tests were conducted, from a statistical viewpoint to obtain foundamental data on the scattering characteristics of fatigue strength, on 3 kinds of commercial plain carbon steels of 0.45% C type and 2 kinds of 0.25% C type. The analysis of the results revealed that the scatter in fatigue strength of smooth specimens may generally be fitted to the normal distribution, and it is possible to obtain P-S-N curves by shifting laterally on S-N diagram the plot of median fatigue strengths estimated by Probit analysis from the data obtained at different numbers of cycles.The scatter in fatigue strength of smooth specimens made of the same steel subjected to a constant condition is affected by the difference in the sample rods from which the specimens were made. For the specimens subjected to different heat treatment conditions, the scatter tends to increase as the fatigue strength becomes higher. The variation coefficient of fatigue strength at 107 cycles is estimated to be approximately 2.5% for all the cases studied, except for the case of 0.45% C steel tempered at 350°C to achieve high brittleness, where it is 4.9%.For notched specimens of the same steel, on the other hand, the variation coefficients are 6.3% and 5.3% for the specimens tempered at 600°C and 350°C, respectively. From the fact that the notch causes a large increase in the variation coefficient for ductile materials but not so much for brittle materials, the process of crack propagation is thought to have great influence on the scattering phenomenon of fatigue strength.