THE EFFECTS OF MEAN STRESS AND STRESS CONCENTRATION ON FATIGUE UNDER COMBINED BENDING AND TWISTING

The Gough test data on fatigue under combined bending and twisting with superimposed mean bending and torsion stresses with and without stress raisers has been re-investigated in terms of the stresses acting on the plane of maximum range of shear stress. It has been shown that the allowable amplitude of shear stress on this plane can be predicted from an equation of the form τ_a= [t - c₁ (K_t×σ_a)^1.5?c₂σ²_m]/K_t where σ_a and σ_m are the normal stress amplitude and mean normal stress respectively on the plane of maximum range of shear stress, c₁ and c₂ are defined material constants and K_t is the theoretical stress concentration factor.     

THE CREEP/FATIGUE BEHAVIOUR OF TYPE 316 STEEL UNDER REVERSE BENDING CONDITIONS

Abstract— Reverse bending creep/fatigue tests have been performed on Type 316 steel at temperatures of 550 and 600°C for test durations of up to 12,000 h. It is shown that endurances obtained are comparable to those observed under push-pull conditions and that the introduction of a hold period can significantly reduce the endurance. Detailed fractography indicates that creep/fatigue in tension is more damaging than creep/fatigue in compression. The crack initiation behaviour is shown to be temperature dependent and may result from the combination of tensile and shear stresses, the relative importance of which will depend on the precise conditions of temperature, stress level and possibly microstructure.     

A NEW MULTIPARAMETER APPROACH TO THE PREDICTION OF FATIGUE CRACK GROWTH

A new multiparameter approach is proposed for the prediction of the combined effects of multiple variables on fatigue crack growth. The method, which is based on multiple linear regression analysis, involves the statistical formulation of mathematical expressions for the crack growth rate, da/dN, as a function of multiple variables, e.g. stress intensity factor range, ΔK, crack closure stress intensity factor, K_cl , and stress ratio, R. A general empirical approach is proposed for the estimation of the fatigue crack growth rate as a function of the above variables. The predictive capability of the empirical approach is then verified by comparing predicted and measured fatigue crack growth and crack growth rate data obtained from tests on a quenched and tempered Q1N (HY80) pressure vessel steel. Error ranges and reliability functions are presented within a probabilistic mechanics framework, and the implications of the results are discussed for the development of generalized fatigue life prediction methods.     

FATIGUE CRACK INITIATION LIFE PREDICTION FOR WELDED JOINTS BY LOW CYCLE FATIGUE APPROACH

Abstract— Analytical procedures based on low cycle fatigue theory are used to estimate the fatigue crack initiation life (N_i) for a cruciform welded joint in mild steel under constant amplitude tensile cyclic loading; the fatigue crack initiating at the weld toe. Effects due to welding such as residual stresses, geometrical variability and changes in material properties are handled. It is shown that for high mean stresses the discrepancies observed between the N _i estimates provided by commonly used analytical procedures exceed an order of magnitude. For the base metal (BM) the discrepancies become negligible if cyclic relaxation of notch mean stress is taken into consideration. The differences betwen the N _i estimates for heat affected zone (HAZ) material (where fatigue cracks at the weld toe usually initiate) and for BM are quantified. The applicability of HAZ material properties, estimated from hardness, to N _i prediction is evaluated.     

INFLUENCE OF STRESS RELIEVING BY VIBRATION ON THE FATIGUE BEHAVIOUR OF WELDED JOINTS IN COMPARISON TO POST-WELD HEAT TREATMENT

Abstract— Constant amplitude fatigue tests with welded specimens under fully reversed four-point bending as well as under axial loading have shown that vibration stress relieving does not lead to a fatigue life improvement of welded parts when compared to the as-welded state. Thus, a substitution of thermal stress relieving by a vibration treatment is not successful. This was also proved by residual stress measurements in the welded parts studied in this paper.     

THE INITIATION AND PROPAGATION BEHAVIOUR OF SHORT FATIGUE CRACKS IN WASPALOY SUBJECTED TO BENDING

The fatigue crack growth behaviour of a nickel base superalloy, Waspaloy, has been studied using four point bending specimens in laboratory air. Two groups of tests, for which the span/width ratios were 1 and 2.5, were conducted and the results compared. Surface crack lengths were measured from plastic replicas of the surface. Equations which describe both short and long cracks have been derived and used to predict the fatigue life for the Waspaloy specimens. From plastic replication studies and scanning electron microscope examinations, a physical understanding of the relationship between crack growth and microstructural features was enhanced.     

FATIGUE LIFE PREDICTION IN BENDING FROM AXIAL FATIGUE INFORMATION

Bending fatigue in the low cyclic life range differs from axial fatigue due to the plastic flow which alters the linear stress-strain relation normally used to determine the nominal stresses. An approach is presented in this report to take into account the plastic flow in calculating nominal bending stress S_bending based on true surface stress. For a given surface strain ε_S, hence the surface life (N_S), an equation is derived to express S_bending in terms of axial fatigue stress (S_axial), involving material constants c, b, N_T which are obtained from axial fatigue information and f₁ and f₂ which are functions of strain hardening exponent and depend on the geometry of cross-section. These functions are derived in closed form for rectangular and circular cross-sections. The nominal bending stress and the axial fatigue stress are plotted as a function of life (N_S) and these curves are shown for some materials of engineering interest.     

THE FATIGUE BEHAVIOUR OF CRACKED CONCRETE

Abstract— Cracks are among the most common flaws in civil engineering structures and, considering the cyclic nature of the variable loading patterns, it is evident that the evaluation of the structure safety factor should consider the stability of cracks when fatigue loading occurs.
In the present paper, the behaviour of cracked concrete subjected to cyclic loading is experimentally investigated by means of direct tension tests. In particular, the experiments concern cycles that stay below the critical concrete softening curve.
The results show that, depending on the maximum load, damage could occur and that the cyclic envelope curve is very close to the monotonic quasi-static curve. On the basis of the experimental results, an empirical model for subcritical crack growth for cracked concrete under low-cycle fatigue loading is proposed.     

THE TWO THRESHOLDS OF FATIGUE BEHAVIOUR

Two limiting thresholds to fatigue crack propagation are discussed. The first threshold is related to the microstructural texture and this threshold may therefore be deemed a material-based threshold. The second threshold is mechanically-based, and is related only to the stress state at the tip of a substantial defect. The material-based threshold is characterized in terms of Microstructural Fracture Mechanics (MFM) and the mechanically-based threshold is characterized in terms of Linear Elastic Fracture Mechanics (LEFM). The former condition is important when considering the fatigue limit of materials and components, while the latter is more applicable to the fatigue limit of structures. The different factors which affect the two threshold conditions are briefly presented. Finally, this paper discusses aspects of MFM relevant to the fatigue resistance of metals and components.     

THE EFFECT OF BIAXIAL STRESS STATE AND CHANGES OF STATE ON FATIGUE CRACK GROWTH BEHAVIOUR

Abstract— The effect of biaxial loading on the fatigue crack growth properties for a stainless steel has been examined. From comprehensive experiments, a significant biaxial stress effect on crack growth was found when the stress level was high and the crack was short. In this paper, the critical region where the effect of biaxial stress appears was clarified quantitatively. Moreover, the effect of changing the biaxial stress condition on fatigue crack growth behaviour was investigated. Significant acceleration of crack growth was observed just after the uniaxial or equibiaxial stress condition was changed to the shear stress condition. This acceleration seems to be due to the change of plastic zone shape at the crack tip.     

THE PROGRESSION OF BENDING FATIGUE IN NICKEL

Abstract— The progression of fully reversed bending fatigue, in a 99.6% pure polycrystalline nickel (NO2200) at room temperature, was monitored utilizing Nomarski interference contrast microscopy in conjunction with microhardness measurements. It was found that a gradual hardening, without early saturation, occurred from about 95 to 160 HV. Similar data were obtained with indenter weights of 15–500 g, from which it was concluded that a different behaving surface layer did not develop during fatigue. Cracks initiated at coarse slip bands within the first 1% of the life, when the hardness reached 140 HV. The cracking of these bands initiated away from grain boundaries. The slow growth of these slip band cracks dominated the life subsequently. Growth of the cracks both across and beyond the initial grain was difficult. The significant hardening prior to crack initiation and the confinement of the cracking mainly to coarse slip bands, contributed to a good inherent fatigue resistence of nickel. with a fatigue limit above the yield strength. Non-propagating cracks were not observed below the fatigue limit.     

受横向分布载荷直梁弯曲应力分析的分离变量解法

考虑端部约束效应,并利用分离变量法,对受横向分布载荷作用的直梁弯曲应力问题进行分析,得到了直梁横截面上正应力和切应力沿梁高度方向以及沿梁轴线方向的分布规律,并将推导结果与经典结果进行比较。结果表明:在靠近约束端附近横截面上的切应力分布与用材料力学公式的计算结果相差较大,但应力最大值比较接近,正应力分布规律与用材料力学公式的计算结果相差较小,但应力最大值的差别不可忽略。稍微远离约束端处横截面上的正应力和切应力的分布均与材料力学计算结果一致。     

THE BEHAVIOUR OF FATIGUE CRACKS SUBJECT TO APPLIED BIAXIAL STRESS: A REVIEW OF EXPERIMENTAL EVIDENCE

Abstract— Experimental studies aimed at understanding the fatigue process in metals and polymers have usually been performed under uniaxial stress. Only in the last two decades or so has much experimentation been carried out on fatigue crack propagation under biaxial stress. This paper reviews the available published data. Crack propagation behaviour under biaxial stress is dictated by 3 parameters: stress biaxiality itself, which is defined here as the ratio of the in-plane principal stresses, crack angle with respect to the applied principal stress directions and stress intensity factor range. Depending on the first two parameters, cracks may grow in Mode I, Mode II or Mixed-Mode. Crack growth data have been presented using these three divisions. Two short sections have been included on initiation and cyclic stress/strain behaviour under biaxial stress to emphasise the fact that crack growth cannot be fully understood without knowing something of them. The accumulated data do not lead to adequate conclusions on either the qualitative or quantitative behaviour of cracks subject to cyclic in-plane biaxial stress. Reasons for the confusion and even contradiction of independent results are put forward and some discussion given to the possible directions of future experimental work.     

THE EFFECTS OF STRESS RATIO, COMPRESSIVE PEAK STRESS AND MAXIMUM STRESS LEVEL ON FATIGUE BEHAVIOUR OF 2024-T3 ALUMINIUM ALLOY

Abstract— Fatigue crack growth behaviour of 2024-T3 aluminium alloy is investigated as a function of stress ratio, compressive peak stress and maximum stress level. It is found that as the stress ratio and the magnitude of the compressive peak stress are increased, the threshold stress intensity range decreased linearly. Intermediate and near threshold growth rate data are analysed with different formulae for effective stress intensity range. The data covered different values of stress ratio, compressive peak stress and maximum stress level. A formula for the crack opening stress level is introduced as a function of stress ratio, compressive peak stress and maximum stress level. The formula permitted a good correlation between crack growth data for both positive stress ratio and negative compressive peak stress values. Using the new formula, intermediate and near threshold crack growth data for the 2024-T3 aluminium alloy yielded a unique crack growth rate vs effective stress intensity range curve for all stress ratio and compressive peak stress values investigated. This suggests that for the 2024-T3 aluminium alloy the crack growth rate vs effective stress intensity range curve does not depend on stress ratio, compressive peak stress, or maximum stress level. The significance of the new equation and the crack growth rate versus effective stress intensity range curve is that they allow a designer to find crack growth rate vs stress intensity range data for the 2024-T3 aluminium alloy in both intermediate and near threshold regions for the particular stress ratio, compressive peak stress and maximum stress level conditions of the component under investigation.     

钢轨各部位的弯曲疲劳性能

针对实物疲劳试验不能得到钢轨各截面上的疲劳性能参数,采用材料疲劳试验方法测钢轨不同部位的疲劳性能。按GB/T 4337-1984的方法在钢轨截面上取不同部位的试样进行旋转弯曲疲劳试验,在给定的1 000×104次循环基数上来测定弯曲疲劳极限。结果表明,轨底中心部位的弯曲疲劳性能最好,而轨头中心部位的弯曲疲劳性能最低。     

PREDICTION OF FATIGUE LIFE BY X-RAY DIFFRACTION METHODS

Abstract— X-ray double crystal diffractometry and reflection topography were employed to examine the defect structure induced by fatigue of A1 2024 specimens. Analysis after various amounts of tension compression cycling revealed that the excess dislocation density in the surface layer increased rapidly early in the fatigue life, and maintained virtually a plateau value from 20 to 90% of the life. Beyond 90%, the excess dislocation density increased rapidly again to a critical value at failure. Investigation of the defect distribution in depth showed that the excess dislocation density in the bulk material, by contrast to the surface layer, increased more gradually during the life. Using deeply penetrating molybdenum radiation, the grains in the bulk could be analyzed, and thus the fatigue life and onset of failure could be predicted nondestructively. It was also shown that, after removal of the surface layer, the defect structure induced in the bulk by prior cycling was unstable. The resultant extension of fatigue life by surface layer removal was explained on the basis of this structural instability of the bulk material.     

THE PROGRESSION OF BENDING FATIGUE IN INCONEL 625

Abstract— Inconel 625 (N06625) was fatigued in reversed bending and the sequence of surface damage observed. A comparison was made with nickel and Monel K500 studied previously. It was found that, as with nickel, slip markings were present at one cycle even in high life samples. In the Inconel 625, the slip was always planar, multiple slip was common and a plethora of secondary slip was associated with most crack fronts. A noticeable distinction, from nickel and Monel, was the absence of any intergranular cracking. Microhardness measurements revealed that, as with Monel and in contrast to nickel, strain amplitudes high enough to give surface zone hardening led to short lives. from which the inference was made that the inherent fatigue properties of Inconel 625 are inferior to nickel, for advantage cannot be taken of its high strain hardening exponent. Nevertheless, the fatigue limit, as in nickel remains above the yield stress because cracks only initiate when the grains are replete with slip and a rumpled topography has developed.