铝锂合金T型自冲铆接头疲劳特性及失效机理

为研究T型自冲铆接头的疲劳特性,以铝锂合金(AL1420)同种及其与铜合金(H62)和不锈钢(410)异种组合制备的AA、HA和SA三组T型自冲铆接头为研究对象,基于其静力学试验采用正弦波进行拉-拉加载疲劳试验,用二参数威布尔分布对所得疲劳数据的有效性进行验证,采用最小二乘拟合直线得到拟合的F-N曲线,并采用SEM扫描电镜对各组典型疲劳失效断口进行微观分析。结果表明:SA组疲劳寿命对载荷变化最敏感,并且疲劳寿命随相对滑移量的增加而减小;由三组T型接头失效形式可看出接头薄弱环节有由铆钉向板材转移的趋势;从微观分析疲劳断口可知裂纹萌生区多为形貌特征较为平坦的准解理,裂纹瞬断区形貌更加突出,多为沿晶断裂或韧窝形貌。     

A Walker exponent corrected model for estimating fatigue life of metallic materials in loading with mean stress

Mean stress significantly influence the fatigue life predictions of metallic materials. The Walker mean stress equation with its additional material parameter w provides good predictions for a wide range of materials. Unfortunately, additional tests are necessary to determine the Walker exponent w. In order to overcome this shortcoming, for aluminum alloys, the Walker exponent w was correlated linearly with the sum of ultimate tensile strength and true fracture strength. Then, a Walker exponent corrected effective strain energy density criterion was developed by incorporating the Walker mean stress equation into the strain life curve. The capability of fatigue life prediction for the developed model was checked against the tested data of 304 L stainless steel, SAE 1045 steel, 7075‐T651 aluminum alloy, and Incoloy 901 superalloy, and comparisons were also performed by using the Lv's Walker exponent corrected model. The developed model provides more satisfactory results, especially for the considered materials in loading with mean stress.     

Al-7Si-0.3Mg铸造合金的旋转弯曲疲劳性能研究

研究了M-7Si-0.3Mg铸造铝合金的室温旋转弯曲疲劳性能。结果表明,A1-7Si-0.3Mg铸造铝合金在10~7次疲劳寿命下的极限应力为88 MPa;结合对疲劳试样断口的分析发现,在高应力低寿命区,A1-7Si-0.3Mg合金试样的表面划痕和缺陷造成了材料的表面破坏模式;而在低应力高寿命区,试样内部近表面区的铸造孔洞则是引起材料疲劳破坏的主要原因。使用Paris公式推导出孔洞尺寸和疲劳寿命之间的关系,并计算出不同应力水平条件下的临界孔洞尺寸。     

Influence of inclusion size on S-N curve characteristics of high-strength steels in the giga-cycle fatigue regime

Fatigue fracture of high-strength steels often occurs from small defect on the surface of a material or from non-metallic inclusion in the subsurface zone of a material. Under rotating bending loading, the S-N curve of high-strength steels consists of two curves corresponding to surface defect-induced fracture and internal inclusion-induced fracture. The surface defect-induced fracture occurs at high stress amplitude levels and low cycles. However, the subsurface inclusion-induced fracture occurs at low stress amplitude levels and high-cycle region of more than 10⁶ cycles (giga-cycle fatigue life). There is a definite stress range in the S-N curve obtained from the rotating bending, where the crack initiation site changes from surface to subsurface, giving a stepwise S-N curve or a duplex S-N curve. On the other hand, under cyclic axial loading, the S-N curve of high-strength steels displays a continuous decline and surface defect-induced or internal inclusion-induced fracture occur in the whole range of amplitudes. In this paper, influence factors on S-N curve characteristics of high-strength steels, including size of inclusions and the stress gradient of bending fatigue, were investigated for rotating bending and cyclic axial loading in the giga-cycle fatigue regime. Then, based on the estimated subsurface crack growth rate from the S-N data, effect of inclusion size on the dispersion of fatigue life was explained, and it was clarified that the shape of S-N curve for subsurface inclusion-induced fracture depends on the inclusion size.     

渗碳Cr-Ni高强硬度合金钢超高周疲劳寿命预测

为了给渗碳合金钢提供一种有效可行的超高周疲劳寿命预测方法,在应力比为0和0.3两种情况下,对渗碳Cr-Ni高强硬度合金钢展开疲劳试验研究.通过对试样断口的微观组织观测,发现渗碳层与基体材料中均有非金属夹杂的存在;通过对裂纹萌生位置和疲劳断口形貌的观察,将疲劳失效分为带有细晶粒区(Fine Granular Area,F...     

S–N curve characteristics and subsurface crack initiation behaviour in ultra-long life fatigue of a high carbon-chromium bearing steel

The S – N curve obtained from cantilever-type rotary bending fatigue tests using hour-glass-shaped specimens of high carbon-chromium bearing steel clearly distinguished the fracture modes into two groups each having a different crack origin. One was governed by crystal slip on the specimen surface, which occurred in the region of short fatigue life and a high stress amplitude level. The other was governed by a non-metallic inclusion at a subsurface level which occurred in the region of long fatigue life and low stress amplitude. The inclusion developed a fish-eye fracture mode that was distributed over a wide range of stress amplitude not only below the fatigue limit defined as the threshold for fracture due to the surface slip mode but also above the fatigue limit. This remarkable shape of the S – N curve was different from the step-wise one reported in previous literature and is characterized as a duplex S – N curve composed of two different S – N curves corresponding to the respective fracture modes. From detailed observations of the fracture surface and the fatigue crack origin, the mechanisms for the internal fracture mode and the characteristics of the S – N curve are discussed.     

The cyclic strain resistance, fatigue life and final fracture behavior of magnesium alloys

This paper summarizes the results of a comprehensive study on the cyclic strain resistance, low-cycle fatigue life and fracture behavior of three rapidly solidification processed magnesium alloys. Test specimens of the magnesium alloy were cyclically deformed under fully-reversed total strain amplitude control straining, over a range of strain amplitudes, giving less than 10⁴ cycles to failure. The cyclic stress response characteristics, strain resistance and low-cycle fatigue life of the alloys are discussed in light of alloy composition. All three alloys follow the Basquin and Coffin-Manson strain relationships, and exhibit a single slope for the variation of cyclic elastic and cyclic plastic strain amplitude with reversals-to-fatigue failure. The cyclic stress response characteristics, fatigue life and final fracture behavior of the alloy are discussed in light of competing and synergistic influences of cyclic total strain amplitude, response stress, intrinsic microstructural effects and dislocation-microstructural feature interactions during fully-reserved strain cycling.     

Evaluation of fatigue life of AZ31 magnesium alloy fabricated by squeeze casting

Cyclic deformation behavior and fatigue life of squeeze-cast AZ31 magnesium alloy was studied under stress amplitude-control at room temperature. Low and high cycle fatigue tests with engineering stress amplitudes in the range from 40 to 110 MPa were conducted. Analysis of hysteresis curves was performed. Tension–compression asymmetry of hysteresis loops was not observed; the alloy exhibited cyclic hardening in tension and compression. The fatigue life in the low cycle fatigue region was expressed by Wöhler and derived Manson–Coffin curves. Experimental data in both, the low and high cycle fatigue regions were fitted by means of regression functions. S–N curves exhibited a smooth transition from the low to the high cycle fatigue regions and significant scattering of experimental points was observed. Furthermore, metallographic and fractographic analyses were performed. Crack initiation occurred from the specimen surface or on clusters of secondary particles; the region of final fracture was characterized by a transgranular ductile fracture.It can be concluded that the fatigue properties of squeeze cast magnesium alloy AZ31 are significantly improved comparing to materials prepared by common methods of casting. Squeeze casting also enables the cost-effective fabrication of complicatedly shaped parts.     

The influence of low‐plasticity burnishing process on the fatigue life of friction‐stir‐processed Al 7075‐T6 samples

The effects of low‐plasticity burnishing (LPB) on the fatigue life of friction‐stir‐processed (FSP) Al 7075‐T6 plates were examined experimentally and numerically. Aluminum samples were taken from plates to test fatigue response in the presence of heat‐affected zone (HAZ) at different loading magnitudes. Finite element method was employed to numerically evaluate fatigue life of FSPed samples by means of the Smith–Watson–Topper (SWT) model. Through numerical analysis, the FSP and its cooling procedure were modelled on the basis of the arbitrary Lagrangian–Eulerian technique, and then, the effect of the LPB to assess fatigue response of samples was examined. Aluminum samples undergoing friction‐stir process presented lower‐fatigue life as stresses were highly concentrated within FSP regions. Involvement of LPB regained fatigue durability through compressive residual stress induced on the aluminum samples. The higher applied force over the LPB promoted compressive residual stress on the sample surface and improved fatigue life of samples. The predicted life results were found twice more in magnitude than those of experimentally obtained.     

Endurance limit of die-cast magnesium alloys AM50hp and AZ91hp depending on type and size of internal cavities

The aim of this work was to investigate the influence of internal cavities on the fatigue properties of two of the technical most common die-cast magnesium alloys, AM50hp and AZ91hp. For this purpose the endurance limits of altogether three batches of S–N specimens, two conventional cast and one vacural cast, with varying internal defects have been measured. After fatigue failure the fracture surface of each sample has been analysed with respect to the site of crack initiation and, where appropriate, the size of the crack initiating cavity or pore. Moreover, on both alloys crack growth tests have been carried out and the thresholds ΔK_th of the stress intensity factor have been measured.Finally, the experimental data from both, the S–N tests and the crack propagation measurements, were depicted in a modified Kitagawa–Takahashi diagram. Using El Haddad’s and Topper’s approach the distribution function of the endurance limit has been proposed, whose parameters could be determined by fitting them to the experimental results. The knowledge of these parameters allows the calculation of the fracture probability as a function of an equivalent crack length and the stress amplitude.     

Experimental characterisation of a CuAg alloy for thermo‐mechanical applications. Part 2: Design strain‐life curves estimated via statistical analysis

Strain‐life fatigue data on copper alloys, especially type CuAg, are seldom available in the literature. This work fills this gap by estimating the strain‐life curves of a CuAg alloy used for thermo‐mechanical applications, from isothermal low‐cycle fatigue tests at 3 temperatures (room temperature, 250°C, 300°C). Regression analysis is used to estimate the median fatigue curves at 50% survival probability. The comparison of median curves with the Universal Slopes Equation model, calibrated on monotonic tensile properties, shows a fairly good agreement. Design strain‐life curves with a lower failure probability and given confidence are estimated by several approximate statistical methods (“Equivalent Prediction Interval,” univariate tolerance interval, Owen's tolerance interval for regression). When higher survival probabilities are considered, the results show a marked decrease in the allowable design strain at a prescribed fatigue life. The suggested procedure thus improves the durability analysis of components loaded thermo‐mechanically.     

Microstructures of modified RR2072 single crystal superalloys and their effects on LCF response at 950 8°C

Effects of microstructural modifications, that are a consequence of adding minor grain boundary strengthening elements (C, Hf), on the fatigue response of an experimental single crystal superalloy have been studied. Investigations show that in the modified alloys, as a consequence of the casting process, the population of pores is reduced, but the average pore size increases and larger pores occur close to the fatigue sample surface. Such porosity changes in the modified alloys are an important contributor to their low cycle fatigue (LCF) properties. At the high stress range of LCF tests undertaken, the growth of cracks initiates from pores and their location in the modified alloys are responsible for decreased fatigue lives. At lower stress ranges, crack initiation consumes a considerable proportion of fatigue life. Since crack initiation involves strain localisation at or near the specimen surface where there is interaction with the environment, the population of pores near the specimen surface and the increased length of the tests plays a significant role in governing fatigue lives. The modified alloys benefit from their lower density of pores in the vicinity of the free surface, and tend to have comparable fatigue lives to that of the base alloy at the lower stress level.     

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.     

Microstructure and fatigue crack growth of EA4T steel in laser cladding remanufacturing

EA4T steel has been used widely in railway axles. Considering the failure behavior of railway axles and other solving methods, laser cladding remanufacturing was used for repairing the railway axles. The microstructure and fatigue fracture mechanism of EA4T steel in laser cladding remanufacturing were investigated by optical microscope (OM), scanning electron microscope (SEM) and X-ray diffraction (XRD). The fatigue life and fatigue crack growth were measured by three-point bending experiments. The microstructure of cladding layers was composed of columnar crystal and fine dendrites, and the main phases of cladding layers were γ and Fe₂B shown by XRD results, which lead to the highest micro-hardness value in the cladding layer. The da/dN-ΔK curves and fracture morphologies indicate that the cladding layers could delay the fatigue crack initiation and improve the fatigue life of EA4T steel.     

Fatigue and corrosion-fatigue strength of hot rolled Ti35Nb2.5Sn alloy

The Ti₃₅Nb_2.5Sn alloy was obtained by vacuum arc melting. The alloy was homogenized and solubilized. It was then hot rolled with 40% reduction and quenched in water. Samples were also solubilized and quenched in water after hot rolling to check the procedure's effect on the formation of metastable phases. The microstructures were analyzed by microscopy and X-ray diffraction. Tensile tests were performed. Fatigue tests were also done to obtain the S–N curves in air and in environment containing 0.9% NaCl dissolved in water. The fracture micromechanics were analyzed in scanning electron microscope. The results were compared with published data from other beta titanium alloys. The fatigue limit is associated with critical stress by dislocations. The fatigue limit is higher than the stress required to drive the formation of the martensitic α″ phase. The study showed that the corrosion effect is higher in low cycle fatigue.     

Probabilistic fatigue S–N curves including the super-long life regime of a railway axle steel

A concurrent probability method is proposed for estimating probabilistic fatigue S–N curves including the super-long life regime. This work is based on experimental investigation of LZ50 railway axle steel. Test results reveal that fatigue cracks are initiated from the weakest surface phase and the damage preferentially follows a competition mechanism between the surface quality and subsurface inclusions. The curves are estimated by the test data in the mid-long life regime and the fatigue limits, which were connected together in concurrent probability levels. The accuracy of the curves was verified by the test data in the super-long life regime.     

Development of a stochastic approach for fatigue life prediction of AlSi12 alloy processed by selective laser melting

Parts manufactured by selective laser melting (SLM) process possess unique features in terms of surface roughness, microstructure, residual stresses as well as defect distribution. These defects are responsible for failure of the parts in functional applications. When fatigue loading is applied, these defects are the dominant cause of crack initiation, resulting in scatter of fatigue properties. This scatter occurs due to interacting phenomena like defect size, location as well as the magnitude and type of load. For the purpose of investigating the effect of defects on fatigue life performance of AlSi12 manufactured by selective laser melting, a procedure was developed based on the weakest-link theory and Weibull's probability density function. Using various destructive and non-destructive techniques, defects, including remnant porosity and surface roughness, have been characterized in amount, size and location. Therefore fatigue life prediction, relying on equations constituted from crack propagation properties, was carried out. Predicted fatigue life and Weibull's statistical parameters were used to compare the effect of both defect types on fatigue reliability of AlSi12 produced by SLM. The most probable fatigue life for a sample was interpreted based on Weibull probability density function with respect to maximum probability of occurrence. The prediction of numerous possible values enabled an estimation of fatigue scatter to be made. Thus, the findings of this novel approach enabled conclusions about strength and reliability of different SLM AlSi12 configurations and gave a prelude towards application-oriented design of SLM components.     

高周疲劳过程中45钢表面显微硬度变化规律的研究

应用显微硬度计对高周疲劳不同循环周次下正火45钢光滑圆试样进行了表面显微硬度测量。使用数理统计的方法对显微硬度测试值进行了处理。研究结果表明，不同疲劳阶段，试样表面显微硬度测试值呈正态分布，显微硬度方差在整个疲劳过程中始终呈现较大的离散性。在三级应力水平下显微硬度统计均值随循环周数的不同变化规律基本相同，表现出明显的阶段性，即显微硬度先升高，后下降，再上升并逐渐趋于稳定，其中在寿命的20％～30％阶段是一个活跃时期。随着应力水平的提高，表面硬度达到稳定状态所需时间有缩短趋势。疲劳后期，随着应力水平的提高，试样表面显微硬度有明显降低。     

Fatigue and fracture behavior of laser clad repair of AerMet® 100 ultra-high strength steel

The effect of laser cladding on the fatigue and fracture behavior under variable amplitude loading is a major consideration for the development of laser cladding process to repair high value complex fatigue critical aerospace military components, that otherwise would be replaced. The selected material, AerMet®100, is a widely used ultra-high strength steel in current and next generation aerospace components, such as landing gears. Laser cladding was performed using AerMet® 100 powder on AerMet® 100 fatigue substrate specimens. No micro-cracking and very little porosity were observed in the clad layer. The fatigue tests were performed under variable amplitude loading with a maximum stress of 1000 MPa. Residual stress, microstructure, and hardness, was also evaluated. Both the as-clad and post-heat treated (PHT) samples were compared to a baseline sample with an artificial notch to simulate damaged condition. Results show that laser cladding significantly improves fatigue life, as compared to the baseline sample with a notch. However, the fatigue life of the as-clad sample is lower as compared to a baseline sample without a notch. A compressive residual stress of 300–500 MPa was observed in the clad region and HAZ. The fracture modes in the as-clad specimen consisted mainly of tearing topology surface and some regions of decohesive rupture through the columnar austenite grains. The PHT condition however was not effective in improving the fatigue life. The fracture modes showed mainly decohesive rupture, and as a consequence, reduced the fatigue life.     

A remaining life assessment of a cracked attemperator steam line

The present paper attempts to describe a series of detailed remaining life assessments of a cracked reducer weld location in a attemperator steam line in a 120 MW fossil-fired unit. Crack growth occurred by either pure fatigue or by creep-fatigue depending upon the combinations of stress and crack depth, i.e. maximum stress intensity level, K_MAX. From consideration of certain fatigue and creep threshold details and measured fatigue striation spacings on the failure surface a stress of 80 MPa was estimated to be driving the cracking process. Also high temperature fatigue crack growth data was treated in terms of a series of constant probability crack growth curves which allowed real probability values to be assigned to any particular remaining life assessment. It was shown that for probability levels of 10⁻⁴ and an estimated active stress of around 80 MPa the remaining life of initial 4 mm and 7 mm deep cracks was 231 and 87 starts respectively.