Very high cycle fatigue properties of high‐strength spring steel 60SiCrV7

The very high cycle fatigue properties of spring steel 60SiCrV7 for automotive suspension system with different hydrogen contents were studied by using ultrasonic fatigue testing and fatigue crack growth testing. The results show that the S–N curves exhibit continuous drop of fatigue lives and no obvious horizontal line exists. Similar fracture surface features were observed for all the specimens that failed mainly from internal inclusions with surrounding granular bright facet (GBF). Fatigue strength decreases remarkably with increasing hydrogen content. The applied stress intensity factor range at the periphery of GBF ΔK_GBF is approximately proportional to 1/3 power of the square of GBF area. The average values of ΔK_GBF for uncharged specimens are close to crack growth threshold ΔK_th, which indicates that ΔK_GBF could be regarded as the threshold value governing the beginning of stable fatigue crack propagation. The increase of hydrogen content tends to reduce ΔK_GBF.     

A cumulative damage model for fatigue life estimation of high‐strength steels in high‐cycle and very‐high‐cycle fatigue regimes

A cumulative fatigue damage model is presented to estimate fatigue life for high‐strength steels in high‐cycle and very‐high‐cycle fatigue regimes with fish‐eye mode failure, and a simple formula is obtained. The model takes into account the inclusion size, fine granular area (FGA) size, and tensile strength of materials. Then, the ‘equivalent crack growth rate’ of FGA is proposed. The model is used to estimate the fatigue life and equivalent crack growth rate for a bearing steel (GCr15) of present investigation and four high‐strength steels in the literature. The equivalent crack growth rate of FGA is calculated to be of the order of magnitude of 10^?14–10^?11 m/cycle. The estimated results accord well with the present experimental results and prior predictions and experimental results in the literature. Moreover, the effect of inclusion size on fatigue life is discussed. It is indicated that the inclusion size has an important influence on the fatigue life, and the effect is related to the relative size of inclusion for FGA. For the inclusion size close to the FGA size, the former has a substantial effect on the fatigue life. While for the relatively large value of FGA size to inclusion size, it has little effect on the fatigue life.     

Low‐cycle fatigue/high‐cycle fatigue interactions in notched Ti‐6Al‐4V*

Combined low‐cycle fatigue/high‐cycle fatigue (LCF/HCF) loadings were investigated for smooth and circumferentially V‐notched cylindrical Ti–6Al–4V fatigue specimens. Smooth specimens were first cycled under LCF loading conditions for a fraction of the previously established fatigue life. The HCF 10⁷ cycle fatigue limit stress after LCF cycling was established using a step loading technique. Specimens with two notch sizes, both having elastic stress concentration factors of K_t = 2.7, were cycled under LCF loading conditions at a nominal stress ratio of R = 0.1. The subsequent 10⁶ cycle HCF fatigue limit stress at both R = 0.1 and 0.8 was determined. The combined loading LCF/HCF fatigue limit stresses for all specimens were compared to the baseline HCF fatigue limit stresses. After LCF cycling and prior to HCF cycling, the notched specimens were heat tinted, and final fracture surfaces examined for cracks formed during the initial LCF loading. Fatigue test results indicate that the LCF loading, applied for 75% of total LCF life for the smooth specimens and 25% for the notched specimens, resulted in only small reductions in the subsequent HCF fatigue limit stress. Under certain loading conditions, plasticity‐induced stress redistribution at the notch root during LCF cycling appears responsible for an observed increase in HCF fatigue limit stress, in terms of net section stress.     

Crack growth rates and microstructure feature of initiation region for very‐high‐cycle fatigue of a high‐strength steel

The S‐N data up to very‐high‐cycle fatigue (VHCF) regime for a high‐strength steel were obtained by fatigue tests under constant amplitude and variable amplitude (VA) via rotating bending and electromagnetic resonance cycling. Crack initiation for VHCF was from the interior of specimens, and the initiation region was carefully examined by scanning electron microscopy and transmission electron microscopy. Crack growth traces in the initiation region of fine‐granular‐area (FGA) were the first time captured for the specimens under VA cycling by rotating bending. The obtained crack growth rates in FGA were upwards to connect well with those in fish‐eye region available in the literature and were associated well with the calculated equivalent crack growth rates in FGA. The observations of profile samples revealed that FGA is a nanograin layer for the specimens under VA cycling, which is a new evidence to support the previously proposed “numerous cyclic pressing” model.     

Towards improved ODS steels: A comparative high‐temperature low‐cycle fatigue study

Within the frame of this work, the mechanical behaviour of a bimodal ferritic 12Cr‐ODS steel as well as of a ferritic‐martensitic 9Cr‐ODS steel under alternating load conditions was investigated. In general, strain‐controlled low‐cycle fatigue tests at 550°C and 650°C revealed similar cyclic stress response. At elevated temperatures, the two steels manifest transitional stages, ie, cyclic softening and/or hardening corresponding to the small fraction of the cyclic life, which is followed by a linear cyclic softening stage that occupies the major fraction of the cyclic life until failure. However, it is clearly seen that the presence of the nano‐sized oxide particles is certainly beneficial, as the degree of cyclic softening is significantly reduced compared with non‐ODS steels. Besides, it is found that both applied strain amplitude and testing temperature show a strong influence on the cyclic stress response. It is observed that the degree of linear cyclic softening in both steels increases with increasing strain amplitude and decreasing test temperature. The effect of temperature on inelastic strain and hence lifetime becomes more pronounced with decreasing applied strain amplitude. When analysing the lifetime behaviour of both ODS steels in terms of inelastic strain energy calculations, it is found that comparable inelastic strain energies lead to similar lifetimes at 550°C. At 650°C, however, the higher inelastic strain energies of 12Cr‐ODS steel result in significant lower lifetimes compared with those of the 9Cr‐ODS steel. The strong degradation of the cyclic properties of the 12Cr‐ODS steel is obviously linked to the fact that the initial hardening response appears significantly more pronounced at 650°C than at 550°C. Finally, the obtained results depict that the 9Cr‐ODS steel offers higher number of cycles to failure at 650°C, compared with other novel ODS steels described in literature.     

Fatigue crack propagation behavior of multiphase microstructure in a quenched and partitioned medium-carbon bainitic steel

A medium-carbon steel was treated by the bainitic isothermal transformation plus quenching and partitioning (B-QP) process to obtain bainite/martensite/retained austenite multiphase microstructure, and its fatigue crack propagation (FCP) behavior was evaluated in contrast with BAT (bainite austempering) sample with fully bainite microstructure. Results show that B-QP sample exhibits a lower FCP rate and higher fatigue threshold ΔK_th (12.6 MPa·m^1/2). Moreover, the FCP path of B-QP sample displays a strongly tortuosity and more crack branching due to more filmy retained austenite (7.2%) and higher percentage of high angle misoriented boundaries (68%). The larger crack tortuosity and the secondary cracks as result of crack branching are primarily responsible for the lower FCP rate of B-QP sample. In addition, the FCP rate curve of B-QP sample shows a pronounced small plateauing at the near-threshold zone, which can be ascribed to the mechanical twinning that occurred in the filmy retained austenite.     

1010‐cycle fatigue properties of 1800 MPa‐class JIS‐SUP7 spring steel

10¹⁰‐cycle fatigue tests were conducted at 100 Hz for three years and at 20 kHz for one week on 1800 MPa‐class JIS‐SUP7 spring steel. Uniaxial tests up to 10⁸ cycles were also conducted at 120 and 600 Hz. The 120 Hz tests had larger control volumes. The fatigue limit at 10¹⁰ cycles was lower than at 10⁸ cycles, and any frequency effect was shown to be negligible. A size effect was found; the tests with larger control volumes showed results of lower fatigue strength.     

A convex hull‐based approach for multiaxial high‐cycle fatigue criteria

Design methods against multiaxial high‐cycle fatigue require the formulation of appropriate criteria that differ in the definition of critical measures introduced to quantify damage, as the amplitude of shear stress. The present paper proposes a novel approach to compute the amplitude of shear stress in multiaxial high‐cycle fatigue. The approach is based on the computation of the convex hull enclosing the stress history under investigation and is validated on proportional and non‐proportional paths in several dimensions and for different materials. High accuracy is achieved when compared with alternative methods from the literature.     

An improved multiaxial high‐cycle fatigue criterion based on critical plane approach

Several groups of fatigue damage parameters are discussed and then an improved multiaxial high‐cycle fatigue criterion based on critical plane defined by the plane of maximum shear stress range is presented in this paper. A compromising solution to consider the mean normal stress acting on the critical plane is also proposed. The new fatigue criterion extends the range of metallic materials which is valid for the ratio 1.25 < f_?1/t_?1 < 2. The predictions based on the presented model show a good agreement with test data.     

Effect of microstructure of simulated heat‐affected zone on low‐ to high‐cycle fatigue properties of low‐carbon steels

To clarify the effect of microstructural changes on the fatigue property of the weld heat‐affected zone (HAZ), low‐ to high‐cycle fatigue tests were conducted on 16 types of simulated HAZ specimens that had been prepared using thermal processes. The results showed the fatigue S‐N curves of the HAZ to be widely scattered as a function of strength level. These fatigue data were divided into two groups: coarse grain (CG) and fine grain (FG) HAZ, when strain amplitude was used to represent S‐N curves. The fatigue data for the CGHAZ group showed a relatively short fatigue life. Based on surface observations, the initiated fatigue crack size of CGHAZ was larger than that of FGHAZ as a function of microstructural unit size. Hence, fatigue crack growth life, which is almost the same as total fatigue life of CGHAZ, decreased.     

A general scenario of fish‐eye crack initiation on the life of high‐strength steels in the very high‐cycle fatigue regime

When high‐strength steels are subjected to very high‐cycle fatigue loading, crack initiation site shifts from surfaces to the interior, and a fish‐eye forms on the fracture surface. Majority of the fatigue life is estimated to be associated with the formation of this internal crack morphology. In the present work, features of such internal cracks in two high‐strength steels are studied. Specifically, three initiation patterns are investigated. A general internal crack initiating scenario is proposed base on an understanding of dislocation slip in the materials. A simplified threshold is calculated from Young's modulus and interatomic spacing, defining the transition from the initiation stage to the crack propagation. The relationship between internal crack initiation and slower descending S‐N curves is discussed.     

热轧TRIP钢残余奥氏体及其稳定性研究

采用X射线衍射(XRD)、透射电镜(TEM)和拉伸实验等方法,研究三种工艺制备的热轧TRIP钢残余奥氏体及其碳含量和稳定性.结果显示:贝氏体区停留时间对残余奥氏体量影响较大,当在贝氏体区模拟卷取时,残余奥氏体量最多;适当的增加弛豫时间,会增加最终组织中残余奥氏体的碳含量;残奥碳含量,还有残余奥氏体的形状和晶粒大小及周围...     

Low cycle fatigue life enhancement of 316 L stainless steel by nitrogen alloying

Tests carried out at room temperature on 316 L stainless steels with different nitrogen contents show that nitrogen improves the low cycle fatigue resistance of the materials. However, saturation occurs when nitrogen content is above 0.12 weight per cent. The microstructural aspect is also studied; the deformation is more difficult and more planar when nitrogen is present. Moreover, nitrogen delays the formation of cells. A single relation, derived from the Manson-Coffin formula, describes the low cycle fatigue behaviour of these steels by taking into account plastic strain range and nitrogen content.     

High‐cycle and very‐high‐cycle fatigue behaviour of a titanium alloy with equiaxed microstructure under different mean stresses

The fatigue behaviour of a titanium alloy Ti‐6Al‐4V with equiaxed microstructure (EM) under different values of tensile mean stress or stress ratio (R) was investigated from high‐cycle fatigue (HCF) to very‐high‐cycle fatigue (VHCF) regimes via ultrasonic axial cycling. The effect of mean stress or R on the fatigue strength of HCF and VHCF was addressed by Goodman, Gerber, and Authors' formula. Three types of crack initiation, namely, surface‐with‐RA (rough area), surface‐without‐RA, and interior‐with‐RA, were classified. The maximum value of stress intensity factor (SIF) at RA boundary for R < 0 keeps constant regardless of R in HCF and VHCF regimes. The SIF range at RA boundary for R > 0 also keeps constant regardless of R in VHCF regime, but this value decreases linearly with the increase of R for surface RA cases. The microstructure observation at RA regions gives a new result of nanograin formation only in the cases of negative stress ratios for the titanium alloy with EM, which is explained by the mechanism of numerous cyclic pressing.     

不同强度中碳TRIP钢的高周疲劳破坏行为

对比研究了不同强度中碳TRIP钢的旋转弯曲疲劳性能和疲劳裂纹扩展速率特征.结果表明,对于1100 MPa和1300 MPa两种强度级别,等温淬火(AT)处理试样的旋转弯曲疲劳强度均高于淬火回火(QT)处理的试样.两种强度级别的AT样的疲劳极限与抗拉强度之比均高达0.56,明显高于QT样的0.51-0.52,同时,AT样的疲劳裂纹扩展速率均明显低于QT样.此外,实验钢的抗拉强度从1100 MPa级提高到1300 MPa级,AT样与QT样的疲劳强度和疲劳裂纹扩展速率之间的差异均缩小.     

Low cycle fatigue behaviour of a 1Cr-Mo-V steel with bainite and ferrite microstructure

A 1Cr-Mo-V turbine rotor steel forging, heat treated to obtain a bainite-20% ferrite microstructure, has been investigated for low cycle fatigue behaviour at room temperature and 535°C. In addition to establishing life expectancy curves, cyclic stress response and cyclic stress/strain curves were derived and the influence of time-dependent effects on high temperature low cycle fatigue life was also determined by introducing varying hold times at the peak tensile strain level of the fatigue cycle. The life expectancy curve obtained is comparable to that reported for the bainitic structure in this steel. Introduction of a dwell period of 5 min is found to reduce the low cycle fatigue life by a factor of 1.6.     

低硅TRIP钢的力学性能及残余奥氏体稳定性研究

通过常温拉伸实验研究低SiTRIP钢的力学性能及其加工硬化特点,利用扫描电镜(SEM)、X射线衍射(XRD)、电子背散射衍射(EBSD)、透射电镜(TEM)等手段研究低SiTRIP钢在拉伸变形前后组织变化特别是残余奥氏体特性的变化。结果表明:低SiTRIP钢表现出良好的力学特性,没有屈服平台,低的屈强比,同时又拥有比双相钢更好的延伸率和均匀应变。TRIP钢的残余奥氏体的分布也影响着其稳定性,分布在铁素体晶内的残余奥氏体在拉伸变形后仍然存在。     

The influence of partial surface shot peening on fatigue crack growth behaviour of a high‐strength ferritic steel

The effects of partial surface shot peening on the fatigue crack growth behaviour of a ferritic steel have been experimentally investigated in this paper. Dog‐bone specimens fabricated from Optim700QL were tested under tension‐tension fatigue loads. Three distinct extents of partial shot peening, with respect to the crack tip and specimen symmetry line, were tested. The fatigue crack growth results from these experiments have been compared with those obtained from the same specimen geometry but with no peening. The results show that the residual stress fields formed ahead of the initial notch tip due to the partial peening process play a significant role in the fatigue crack growth behaviour of the material and effectively result in accelerated crack propagation at the midwidth of the specimens. It has been shown in this study that partial peening can lead to a fatigue crack growth rate around twice as fast as that of the unpeened specimen.     

High‐cycle fatigue and fracture behaviours of Cu‐Be alloy with a wide strength range

The high‐cycle fatigue and fracture behaviours of Cu‐Be alloy with tensile strength ranging from 500 to 1300 MPa acquired by different treatments were studied. Fatigue crack initiation, fracture surface morphologies, S‐N curves and fatigue strength show obvious differences due to the change of microstructure. At relatively low‐strength level, some fatigue cracks originated from defects; while at high‐strength level, all the fatigue cracks initiated from cleavage facets. It was found that the fatigue ratio increases linearly and fatigue strength changes quadratically with increasing tensile strength, only considering one strengthening mechanism. Finally, the fatigue strengths of various Cu‐Be alloys were summarized.     

The effect of PVD coatings on the tensile strength and low‐cycle fatigue resistance of stainless steel and titanium alloys

PVD coatings applied to components form hard, stronger layers and generate high residual compressive stresses that limit the plastic deformation in surface layers of the base metal thus increasing its tensile strength and resistance to fatigue loading. The purpose of this paper is to experimentally determine the influence of the deposition of 2 to 16.5‐μm‐thick PVD coatings of TiN, Cr, (Cr+TiN), (TiC)N, (TiAl)N onto specimens of stainless steel 321 and titanium alloys of types MILT‐81556A and (10‐2‐3; 4966) on their tensile strength and low‐cycle fatigue resistance when the development of large elastic–plastic strains takes place. The tensile and low‐cycle fatigue tests were conducted under conditions of axial zero‐to‐tension cycle of the stress‐controlled loading on flat 1‐ to 1.5‐mm‐thick specimens in the initial state (uncoated specimens) and after application of a PVD coating, including those after pretensioning or after cyclic prestraining in the low‐cycle fatigue range. The deposition of PVD coatings is found to enhance the characteristics of tensile strength and low‐cycle fatigue resistance in the quasi‐static fracture range. The deposition of PVD coatings on specimens cyclically prestrained to the values of 53–86% of the number of cycles to fracture, changes the cyclic properties of the material and predetermines the fatigue fracture mode only.