HIGH STRAIN BIAXIAL FATIGUE OF A STRUCTURAL STEEL

Abstract— The biaxial fatigue behaviour of two batches of a structural steel, termed isotropic and anisotropic batches, is examined. Tests were carried out using thin-walled hollow specimens subjected to axial load and internal and external pressure. The fatigue life of the isotropic batch is found to be about two to three times greater than the fatigue life of the anisotropic material. The ratio however is found to be dependent on the degree of biaxiality. Three equivalent strain parameters were used to correlate the fatigue lives, octahedral strain, Brown-Miller and Lohr-Ellison parameters. The difference between the theories are compared and the relation between the shear strain orientation and failure is discussed.     

BIAXIAL THERMAL FATIGUE TESTING WITH NON-UNIFORM STRAIN DISTRIBUTIONS

Abstract— Methods of producing a preset thermal stress state are presented. A specimen design for providing such a state under the action of heating or cooling, without external loading by mechanical forces, is suggested. The methods and some thermal fatigue test results are described. The influence of a non-stationary biaxial state on the lifetime of a steel is shown.     

HIGH STRAIN FATIGUE OF A TYPE 316 STAINLESS STEEL

Abstract— High strain fatigue tests of Type 316 stainless steel have been carried out at room temperature and 600°C. Cold work reduces the life relative to the annealed condition when considered in terms of plastic strain per cycle but not when the total strain range is used. It also renders the material far more resistant to cyclically induced changes in strength than the annealed state in which the strength is more than doubled at 600°C. Strain ageing causes further instability and enhances the hardening and loss of ductility in the annealed material. Because of the large number of variables involved, comparison with other work is unable to identify the parameters most influential in determining endurance. It is argued that where a fatigue–creep interaction occurs, then cyclic hardening will promote a higher creep resistance in a sequential interaction and favour a fatigue dominated failure in a simultaneous interaction.     

BIAXIAL HIGH CYCLE FATIGUE TESTS ON A GAS TRANSMISSION PIPELINE STEEL

Abstract— Gas transmission pipes are sometimes subject to external damage due to interference by excavators. Optimized grinding of gouges may offer a solution in repairing pipes. Since pipelines may be subjected to internal pressure variations, the Dang Van criterion has been used to size the allowable grinding depth. The criterion's boundary has been determined from uniaxial fatigue testing and extrapolated to the higher hydrostatic pressure which occurs in practice. The aim of this paper is to check by means of biaxial fatigue tests on specimens, that this extrapolation ensures a lifetime of 10⁵ cycles for the ground pipes. The state of stresses in the central area of the biaxial specimen was calculated from an elastic finite element simulation. The test machine had independent motions of the two perpendicular axes of loading. The fatigue testing was limited to 10⁵ cycles and was carried out at a hydrostatic pressure of 200 MPa. The results validated the extrapolation technique for the Dang Van criterion.     

HIGH TEMPERATURE LOW CYCLE BIAXIAL FATIGUE OF TWO STEELS

Abstract— Biaxial low cycle fatigue tests at various temperatures and strain rates were performed on 1% Cr-Mo-V steel and AISI 316 stainless steel under combined torsional and axial loads. A correlation for fatigue strength has been derived, and it is also shown that if the Gough ellipse quadrant criterion is rephrased in terms of strain amplitudes, it may be used as a safe design rule for ductile metals in both the low and high cycle fatigue regimes.     

EFFECT OF BIAXIAL MEAN STRESS ON CYCLIC STRESS-STRAIN RESPONSE AND BEHAVIOUR OF SHORT FATIGUE CRACKS IN A HIGH STRENGTH SPRING STEEL

Abstract— Biaxial fatigue tests were conducted on a high strength spring steel using hour-glass shaped smooth specimens. Four types of loading system were employed, i.e. (a) fully reversed cyclic torsion, (b) uniaxial push—pull, (c) fully reversed torsion with a superimposed axial static tension or compression stress, and (d) uniaxial push—pull with a superimposed static torque, to evaluate the effects of mean stress on the cyclic stress—strain response and short fatigue crack growth behaviour. Experimental results indicate that a biaxial mean stress has no apparent influence on the stress—strain response in torsion, however a superimposed tensile mean stress was detrimental to torsional fatigue strength. Similarly a superimposed static shear stress reduced the push—pull fatigue lifetime. A compressive mean stress was seen to be beneficial to torsion fatigue life. The role of mean stress on fatigue lifetime, under mixed mode loading, was investigated through experimental observations and theoretical analyses of short crack initiation and propagation. Using a plastic replication technique the effects of biaxial mean stress on both Stage I (mode II) and Stage II (mode I) short cracks were evaluated and analysed in detail. A two stage biaxial short fatigue crack growth model incorporating the influence of mean stress was subsequently developed and applied to correlate data of crack growth rate and fatigue life.     

精细陶瓷高温疲劳性能测试方法研究

对平面弯曲型的三点弯曲试样，采用共振法研究并测定了两类Ｓｉ３Ｎ４陶瓷的高温疲劳性能曲线。研究结果表明：所用陶瓷材料的高温疲劳数据位于两个极端；即：起始阶段就破断或超过指定寿命不破坏。经疲劳试验未破断的样品在高温下的弯曲强度明显地高于原邕样品的高温弯曲强度。     

FATIGUE DAMAGE IN 1045 STEEL UNDER VARIABLE AMPLITUDE BIAXIAL LOADING

Abstract— During constant amplitude loading, two different types of crack systems have been reported In the high cycle fatigue (HCF) region, cracks nucleate on a small number of maxium shear strain amplitude planes One of these cracks becomes a dominant crack and leads to failure of the specimen In the low cycle fatigue (LCF) region, equally developed microcracks are observed over the entire gage section and grow during the majority of the life. The failure is due to a linking in which the microcracks join up during the last few cycles of the fatigue life.
To investigate the interaction of these two types of crack systems in biaxial fatigue, experiments were performed on thin-wall tubular specimens in tension, torsion and combined tension-torsion loading The test program included step loading and block loading in which two equivalent strain amplitudes were employed. One of the equivalent strain amplitudes is in the HCF region and the other was in the LCF region
Fatigue lives were predicted from constant amplitude damage curves when a single crack system dominated the fatigue process Two competitive crack systems were sometimes developed on the maximum shear strain amplitude planes in a single specimen under block loading This resulted in a conservative prediction of the fatigue life.     

FATIGUE CRACK GROWTH FROM A CIRCULAR NOTCH UNDER HIGH LEVELS OF BIAXIAL STRESS

Abstract— A series of experiments have been conducted on cruciform specimens to investigate fatigue crack growth from circular notches under high levels of biaxial stress. Two stress levels (Δσ₁= 380 and 560 MPa) and five stress biaxialities (λ=+1.0, +0.5, 0, −0.5 and −1.0; where λ=σ₂/σ₁ were adopted in the fatigue tests in type 316 stainless steel having a monotonic yield strength of 243 MPa. The results reveal that fatigue crack growth rates are markedly influenced by both the stress amplitude and the stress biaxiality. A modified model has been developed to describe fatigue crack growth under high levels of biaxial stress.     

FATIGUE DAMAGE IN 1045 STEEL UNDER CONSTANT AMPLITUDE BIAXIAL LOADING

The progressive nature of fatigue damage under multiaxial stress states has been investigated. Experiments were performed on thin-wall tubular specimens of 1045 steel in tension, torsion and combined tension-torsion loading. Two equivalent strain amplitudes, one in the high cycle fatigue (HCF) region and one in low cycle fatigue (LCF) region were employed in this study. Four recently proposed damage theories were evaluated. Crack depth was used as a damage parameter in comparing damage curves under different loading modes.
Different types of crack systems were observed in the HCF and LCF regions. The damage curve obtained in tension loading can be used to evaluate the damage behavior under combined tension—torsion loading. The results of torsion loading show that torsional damage behavior is different from the above two loading modes.     

CYCLIC DEFORMATION OF 1% Cr-Mo-V STEEL UNDER OUT-OF-PHASE LOADS

Abstract The stable cyclic stress strain response of 1% Cr Mo V steel has been studied for combined axial and torsional loads, where the respective sinusoidal strains were applied both in and out of phase. Typical hysteresis loops are presented, and a correlation is proposed for the cyclic stress-strain curve.     

高应变动测技术在海洋石油平台桩基工程中的应用研究

针对目前国内海洋石油平台桩基工程中打入大直径超长桩时经常出现的拒锤、应力过大造成坏桩等工程问题,海洋工程中急需填补在桩基打入的同时缺少必要的监控手段和在打桩结束后缺少检验桩基设计承载力方法的空白.在海洋桩基工程很难做静载试验的情况下,在国内首次使用高应变动测技术对海洋石油平台桩基打桩过程进行全程监控,并且进行了评估其桩...     

EFFECT OF RELATIVE SLIP AMPLITUDE ON FRETTING FATIGUE OF HIGH STRENGTH STEEL

Abstract— The effect of relative slip amplitude on fretting fatigue in high strength steel was studied at various contact pressures using fretting pads of various lengths. Under a given contact pressure, the fretting fatigue life showed a minimum at a certain relative slip amplitude. Under a fixed pad length, the life also showed a minimum at a certain contact pressure. A map of fretting fatigue life versus contact pressure and relative slip amplitude was obtained using the data of this study. The map indicated that both the phenomena which showed a minimum life in relation to slip dependence and contact pressure dependence were the same, as were the underlying mechanisms. The minimum life was interpreted in terms of local stress concentration at the fretted area.     

EFFECTS OF DRAWING STRAIN AND BLUING ON THE FATIGUE STRENGTH OF EUTECTOID STEEL WIRES

Abstract— The effects of bluing, associated with drawing strain, on the fatigue strength of eutectoid steel wires have been investigated. The fatigue limit increases by bluing and the increase is more significant with higher drawing strain. The peak in the fatigue limit with regard to the drawing strain in the wires, at a strain of 2.5, disappears after bluing. On the other hand, in the ferritic steel wires investigated for comparison, the fatigue limit gradually increases with the drawing strain up to 7.7. Furthermore, no appreciable change in the fatigue limit due to bluing is found. Based on the results of hardness tests on fatigue specimens with- and without-bluing, it is deduced that the decrease of the fatigue limit beyond the peak drawing strain in the eutectoid steel wire can partly be attributed to insufficient locking of the high-density dislocations by solute atoms. The effect of relaxation of residual stress during bluing is also briefly discussed.     

高强轨道钢双轴压-扭棘轮行为的实验和模拟

在室温下对高强轨道钢进行了单轴和非比例双轴压-扭循环变形行为的实验,讨论了不同加载路径对轨道钢棘轮变形行为的影响。结果显示：该轨道钢呈现出明显的循环软化效应和压缩方向的棘轮行为,且棘轮行为的演化表现出强烈的加载路径相关性;在椭圆路径下,棘轮应变较其他四种路径更小。进而建立了基于Abdel Karim-Ohno非线性随动硬化律的非比例多轴循环棘轮本构模型,并通过在随动硬化和各向同性软化律中引入非比例因子来考虑非比例路径对双轴压-扭棘轮行为的影响。实验结果和模拟结果的对比表明：该本构模型能够较好地模拟高强度轨道钢的非比例双轴压-扭棘轮行为。     

STRAIN AGEING AND THE FATIGUE LIMIT IN CARBON STEEL

Abstract The fatigue phenomenon of mild steel is characterized by the existence of a fatigue limit and coaxing effects. Although many studies have been made on strain ageing due to interstitial solute atoms of carbon and nitrogen, the effect of strain ageing on the fatigue limit is not yet fully understood. The objective of this work is to clarify the effect of strain ageing on the fatigue limit. Experiments were undertaken from the view point that fatigue is a competitive process of two mechanisms of damage and strengthening of the material. Two techniques were applied for this purpose. One was the measurement of the change in solute contents of C and N atoms, and the other the reversion of the Cottrell atmosphere.
The experimental results recognized that strain ageing occurred in the fatigue process and that by removing the strain ageing effect, specimens could fail below the fatigue limit. The fatigue limit could be increased above the original value by suppressing dislocation multiplication during fatigue. It was also observed that, as the time for strain ageing increased at higher temperatures, the knee of the S–N curve shifted to smaller number of cycles.     

高强度钢动态冲击疲劳性能的研究与应用

对高强度钢ＨＱ－６０和ＨＱ－８０的动态冲击疲劳性能作了全面的试验研究，测定了两种材料的冲击疲劳性能曲线，即能量－寿命曲和和冲击疲劳下的裂纹扩展速度ｄａ／ｄＮ，以及动态断裂韧性ＫⅠｄ等。结合１５４ｔ电动轮自卸车的车架强度和寿命校核，提出了一种新的应用模型，为受冲击疲劳型载荷的零部件找到了一种实用的校核计算方法，具有普遍的推广应用价值。     

A METHOD OF ESTIMATING BIAXIAL FATIGUE GROWTH RATES

Abstract —Conventionally, fatigue growth rate data are correlated in terms of the range Δ Kof the applied stress intensity factor. It is argued that a correlation involving both Δ Kand the constant stress T close to the tip of the equivalent elastic crack should be adequate for moderate stress, biaxial fatigue data. Practical application of these ideas would involve the preparation of " T -term" compendia similar to those already available for K -calibrations. " T -term" data are presented for the case of an edge-cracked strip in tension.     

FRACTURE MECHANICS APPROACH FOR PREDICTION OF FATIGUE LIFE OF HIGH STRENGTH STEEL COMPONENTS

Abstract— A fatigue life estimation procedure based on a fracture mechanics approach is analytically developed for components made of a 4140 grade high strength steel. The maximum allowable flaw sizes for both static and cyclic loads are computed. For different stress levels and shapes of the defects, initially-existing, allowable defect size versus number of operating cycles, as well as the thickness versus tolerable defect sizes curves, are presented. The maximum allowable operating cycles of components for non-destructively detectable minimum defects are also estimated. From these estimations, a more realistic prediction of the safe service life of components is possible.     

THE PREDICTION OF CRACK GROWTH RATES FROM TOTAL ENDURANCES IN HIGH STRAIN FATIGUE

Abstract— A method of calculating crack growth rates in high strain fatigue from total endurance data is presented. Endurances are known to be affected by dwell periods and environment and the expression derived takes these into account. The basic argument is that crack propagation may be regarded as increments of successive reinitiation and so the deeper a crack grows, with an accompanying increase in strain concentration at the tip, the fewer fatigue cycles are required for the next step. Previously derived expressions for strain concentration are employed which require a knowledge of the cyclic strain hardening properties of the material. These are produced in detail for a cast 1/2 Cr-Mo-V steel at 550°C. Two constants in the resulting propagation expression are semi-empirical, but if the surface (plastic or total) strain range is known then cyclic crack growth rates may be calculated for any crack depth. It is also shown that in the limiting case of zero surface plastic strain, the expression reduces to that observed for crack growth under linear elastic conditions.