椭圆形路径载荷下的微动疲劳失效特征

为了解微动疲劳失效机理,通过柱面对柱面的接触方式,研究了60Si2Mn钢在椭圆形路径、拉扭耦合作用下的多轴低周微动疲劳特性,深入分析讨论了不同轴向循环拉伸应力幅值对摩擦磨损表面和断口形貌的影响.结果认为：磨损区产生的氧化物磨屑对微动区磨擦损伤行为具有显著影响;微动摩擦磨损对试样表面的影响深度只有数十微米;微动疲劳裂纹源...     

Metallurgical failure analysis of fasteners in an impeller assembly

This paper examines a failure analysis of the bolts from a failed joint between an impeller blade and a rotating assembly unit. The bolts failed due to poor thread manufacture and installation. Sharpened thread roots led to high stress concentrations that favored crack initiation. An oddly shaped thread profile allowed friction between mismatched thread surfaces. Poor installation procedures allowed for the possibility of overtightening to nucleate cracks in the head-to-shank interface (which had a smaller radius and therefore a higher stress concentration) and possibly also in the thread roots. Each of these influences contributed to crack initiation in the bolts. After cracks had formed, bending fatigue then propagated the nucleated cracks to final fracture. The failure analysis also recommended using bolts with rolled threads, which allow a more complete fit between mating male and female threads, and assuring that an appropriate preload is placed on bolts during installation.     

A plausible strategy for modeling fatigue life variation

In this paper stochastic modeling is used to predict fatigue life uncertainty by simulating small variations in strain-life material constants. The Monte Carlo method,^[1] using either known cumulative distribution functions (CDFs) for the material constants and/or postulated CDFs, provides the mechanism to generate a set of failure data. These data are analyzed, using ordinary statistical techniques, to develop the Weibull CDF for cycles to failure. It is seen that small variations (∼±15%) in values of the strain-life constants result in large variations (∼600%) in predicted fatigue life at moderate strain. The often-observed phenomenon that spread in fatigue data is greater at lower strain is also described by the stochastic model.     

Crack growth in the gigacycle fatigue regime for helicopter gears

Investigations were performed on helicopters' gears that had failed in service, using quantitative fractographic analyses. It was shown that a combination of LCF and HCF creates damage in gears on a flight-by-flight basis which produces beach marks on the fatigue surface of the gears that correlate one-to-one with flight cycles. The crack increment between beach marks occurs under HCF at 7000–8000  r.p.m. The ratio of the crack growth period to durability is smaller in the gigacycle fatigue area (more than 10⁸ cycles) than in the high cycle fatigue area (10⁶–10⁷ cycles) for those gears investigated that have stress raisers. This relationship for damaged gears depends not only on durability but also on the type of stress raiser and its location.     

Multiaxial fatigue and failure analysis of helical compression springs

Multiaxial fatigue criteria are applied to the analysis of helical compression springs. The critical plane approaches, Fatemi–Socie and Wang–Brown, and the Coffin–Manson method based on shear deformation, were used to predict fatigue lives of the springs under constant amplitude loading. Experimental fatigue lives are compared with the multiaxial fatigue criteria predictions. The stress analysis was carried out in the finite element code ANSYS, and the multiaxial fatigue study was performed using the fatigue software nCode. A failure analysis was conducted in order to determine the fatigue crack initiation point and a comparison of that location with the most damaged zone predicted by the numerical analysis is made. The Fatemi–Socie critical plane approach gives a good prediction of fatigue life. While the Wang–Brown criterion overestimates spring fatigue life, the Coffin–Mason model gives conservative results.     

Mobile harbor crane wheel hub fatigue failure

Failure analysis of a mobile harbor crane wheel hub demonstrated that the mechanism of failure was fatigue. The wheel hub was a ductile cast iron component that had been subjected to cyclic loading during a ten-year service period. The fracture surface of the fatigue failure also contained corrosion deposit, suggesting that cracking occurred over a period of time sufficient to allow corrosion of the cracked surfaces. Replacement and alignment of the failed wheel hub is recommended along with inspection of the nonfailed wheel hubs that remain on the crane.     

Experimental study on fatigue failure and damage of sandwich structure with PMI foam core

Sandwich structures consisting of aluminium skin sheets and polymethacrylimide foam core have been gradually used in the high‐speed trains. The static mechanical properties and fatigue damage of the sandwich structures with polymethacrylimide foam core were experimented in three‐point bending and were discussed. The failure mode is identified as local indentation. The static strength was obtained, and it showed good consistency with the forecasting formula. The fatigue property and damage evolution were also researched under cyclic loading. The fatigue life curve and the fitting formula were submitted. The fatigue damage evolution started from the skin sheet fracture and then the foam core indentation. The displacement at the midpoint as the damage parameter was discussed, and the evolution prediction formula was submitted, which showed great agreement with the experimental results.     

Metallurgical failure analysis of titanium wing attachment bolts

Two titanium alloy wing attachment bolts from a commercial jetliner failed during the course of a routine service operation. Failure of the bolts occurred during the re-torque process as the wing was being reattached. Metallurgical failure analysis indicated that the fracture mechanism was ductile overload and that the mechanical properties of the bolts were consistent with exemplar bolts that had been supplied. After eliminating other sources of excessive load application, the most probable cause of failure was ascribed to variances between the frictional characteristics of the bolt at the time of re-torque and at the time of initial torque application several years earlier.     

Corrosion fatigue behaviour of brass in aqueous solutions

The corrosion fatigue behaviour of α-brass was investigated in NaCl, NaNO₂, (NH₄)₂SO₄ and (NH₄)₂SO₄ + CuSO₄ solutions by applying the reverse bending technique at 60 cycles/min. With the exception of NaCl, all electrolytes reduced the fatigue life compared with that in air or water. Anodic polarization caused further reduction in the fatigue life in all solutions. In NaCl the failure was mainly due to electrochemical dissolution of the brass, while in ammoniacal solution both thinning and corrosion fatigue occurred. In NaNO₂ solution corrosion was inhibited but severe corrosion fatigue occurred. The fracture mode changed from transgranular in air to mixed mode, with different percentages of the intergranular type depending on the electrolyte.     

Cumulative fatigue damage and life prediction of elastomeric components

Elastomeric components are widely used in many applications due to their good damping and energy absorption characteristics. The type of loading normally encountered by these components in service is variable amplitude cyclic loading. Therefore, fatigue failure is a major consideration in their design. In this work capabilities of Rainflow cycle counting procedure, maximum principal strain as a damage criterion, and Miner's linear cumulative damage rule are evaluated with both specimen and component tests. An automotive cradle mount is used as an illustrative component. Comparison of predicted and experimental fatigue lives in both specimen and cradle mount variable amplitude load tests indicate satisfactory predictions in both cases.     

Investigation of mechanical properties and fatigue life of ECARed AA5083 aluminium alloy

Equal‐channel angular rolling (ECAR) is a continuous severe plastic deformation process. In this process, severe shear strains apply to the sheet. This strain increases the yield or ultimate strength of sheet without significant change in sheet dimension. In this paper, the effect of ECAR process on mechanical properties and fatigue life of manufactured sheets will be studied. Four AA5083 samples have been prepared and annealed for obtaining stress‐free samples. Three samples have been rolled by the ECAR process with one, two and three passes of rolling, respectively. Mechanical tests including tensile test, hardness and axial fatigue tests have been carried out on prepared samples. Fatigue tests have been implemented according to a strain‐based approach with a constant strain ratio equal to 0.75 and 0.5 Hz frequency of loading. All of the tests have been carried out in controlled laboratory conditions. Results show that the ultimate tensile strength of samples increases with increasing the pass of rolling. Also, the maximum elongation of samples decreases. Maximum elongation was 17% in annealed samples, while it decreases to 10% in samples with three passes of rolling. The hardness of samples has been measured, and the results show an increase in hardness for a higher pass of the ECAR process. Fatigue test results show that fatigue life of AA5083 samples decreases in manufactured sheets of the ECAR process. Also, cyclic softening has been observed in the ECARed sample. The fracture surfaces of samples after fatigue test have been observed with a scanning electron microscope. A comparison of fracture surfaces confirms that the crack growth was intergranular in annealed samples while it changes in ECARed samples to transgranular.     

Statistical prediction of fatigue failure of fibre reinforced composite materials

A statistical approach is proposed to evaluate the residual strength and life of unidirectional and angle-ply composite laminates subjected to in-plane tensile cyclic stresses. The method is based on the extension of previous static failure criteria describing independently the fibre failure and matrix failure modes, combined with the statistical nature of fatigue failure of fibre-reinforced composites. The static and fatigue strengths of composite laminates at any off-axis angle are evaluated using the fatigue failure functions for the three principal failure modes, which are determined from the fatigue behaviour of unidirectional composites subjected to longitudinal and transverse tension as well as in-plane shear stresses. The evaluations of the fatigue strength of unidirectional E-glass/epoxy laminates under off-axis fatigue loading and angle-ply S-glass/epoxy laminates under in-plane fatigue loading show good agreement between theoretical predictions and experimental results.     

E-GFRP单向板疲劳性能及失效机制的实验研究

对铺设角分别为0°、30°、45°、90°的玻璃纤维增强不饱和聚酯树脂单向板进行拉-拉疲劳试验, 得到不同铺设角玻璃纤维增强不饱和聚酯树脂(GFRP)单向板疲劳数据, 通过分析不同铺设角单向板中值S-N曲线及正则应力-寿命曲线特点, 得到了单向板疲劳性能随铺设角变化的关系。并且基于对不同铺设角单向板疲劳断口的宏观及微观形貌的分析, 研究了铺设角对单向板的疲劳损伤机制、失效模式的影响。分析表明, 对应于不同的铺设角, 单向板存在不同的失效机制, 从而导致不同形式的损伤模式。     

High cycle fatigue characteristics of 2124-T851 aluminum alloy

The fatigue crack growth rate, fracture toughness and fatigue S-N curve of 2124-T851 aluminum alloy at high cycle fatigue condition were measured and fatigue fracture process and fractography were studied using optical microscopy (OM), X-ray diffraction (XRD) technique, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The results show that at room temperature and R = 0.1 conditions, the characteristics of fatigue fracture could be observed. Under those conditions, the fatigue strength and the fracture toughness of a 2124-T851 thick plate is 243 MPa and 29.64 MPa · m^1/2, respectively. At high cycle fatigue condition, the higher the stress amplitude, the wider the space between fatigue striations, the faster the rate of fatigue crack developing and going into the intermittent fracture area, and the greater the ratio between the intermittent fracture area and the whole fracture area.     

Retrieving Effective Stress Spectrum of Fatigued Components from Fractographic Quantitative Analysis

Under the constant amplitude and pro-grammed loading conditions the fatigued fracturesurfaces of 2024-T3 sheet are examined by SEMand TEM.A corrected method,in which the effectsof loading sequence are taken into account,hasbeen developed for the retrieval of effective stressspectrum.The parameters including d,a,C,n,R,αand σ_y are evaluated according to their influenceson the retrieved results.Several computer programshave been produced for the fractographic quantita-tive analysis,and the application of image pro-cessing technique in the fractographic analysis is al-so interpreted.     

Interior failure mechanism and life prediction of surface‐treated 17Cr‐Ni steel under high and very high cycle fatigue

The understanding of very high cycle fatigue (VHCF) mechanisms is critical to the development of life prediction approach. For this purpose, high cycle fatigue (HCF) and VHCF properties of a surface‐treated 17Cr‐Ni steel were investigated under axial loading with stress ratio of 0. This steel exhibits the constantly decreasing S‐N characteristics associated with the inclusion‐fisheye induced failure under the HCF and the inclusion‐FGA (fine granular area)‐fisheye induced failure under the VHCF. The cyclic pressing plays an important role in the FGA formation process, but the FGA still can be observed for the stress ratio of zero due to the slight crack closure effect. Two life modelling approaches associated with related failure mechanisms in the HCF and VHCF regimes are proposed based on the agreement between experimental and predicted results.     

Finite-element analysis and experimental study on tensile fatigue behaviour of bitumen–stone adhesion

Tensile fatigue behaviours of bitumen–stone adhesion were investigated using a dynamic mechanics analyser under stress‐controlled mode at two temperatures of 5 and 25°C and various controlled‐stress levels. Failure characteristics including interfacial failure and cohesive failure were examined using image analysis of fracture surfaces. Finite‐element analysis on stress distributions was conducted under different temperatures, film thickness and interfacial bonding conditions. A Coulomb–Mohr like criterion in combination with shear and normal stresses is proposed to deal with the extreme thin adhesive layer, which can be further simplified into an adhesive zone without significant loss of accuracy for stress analysis.     

A unified statistical model for S‐N fatigue curves: probabilistic definition

In recent years, experimental tests exploring the gigacycle fatigue properties of materials suggest the introduction of modifications in well‐known statistical fatigue life models. Usual fatigue life models, characterized by a single failure mechanism and by the presence of the fatigue limit, have been integrated by models that can take into account the occurrence of two failure mechanisms and do not consider the presence of the fatigue limit. The general case, in which more than two failure mechanisms coexist with the fatigue limit, has not been proposed yet. The paper presents a unified statistical model which can take into account any number of failure mechanisms and the possible presence of the fatigue limit. The case of S‐N curves with different fatigue life distributions coexisting for the entire stress range covered by fatigue tests is also considered. The adaptability of the statistical model to the S‐N curves proposed in the open literature is demonstrated by qualitative numerical examples.     

某型航空发动机压气机四级转子叶片失效分析

某型航空发动机压气机四级转子叶片是故障多发叶片.对叶片典型失效件进行了断口分析,发现叶片故障是由于叶片表面发生腐蚀导致疲劳强度降低,使得叶片在振动应力下发生疲劳失效.进一步对显微组织进行分析,发现叶片材质合乎要求,叶片腐蚀主要是由于使用环境因素造成的.研究结果对于叶片的故障分析及预防具有重要的意义.