焊缝匹配对力学性能影响的综合研究

对几种钢在静载、动载和疲劳载荷条件下的焊缝强度匹配分别进行了研究,并对有关接头进行了残余应力的测试。得出的结论是恰当地提高焊缝金属强度无论是对抗开裂性能还是止裂性能均是有利的;采用同种焊条LB52NS施焊A131和A537所构成的两种接头其强度匹配系数的差别对ΔK (th)无影响,对da/dN无论是在常规疲劳还是海水腐蚀疲劳也均无显著影响;强度匹配系数较高A131接头焊缝残余应力略低于强度匹配系数较低的A537接头焊缝的残余应力。     

Two-dimensional finite element simulation of fracture and fatigue behaviours of alumina microstructures for hip prosthesis

This paper describes a two-dimensional (2D) finite element simulation for fracture and fatigue behaviours of pure alumina microstructures such as those found at hip prostheses. Finite element models are developed using actual Al2O3 microstructures and a bilinear cohesive zone law. Simulation conditions are similar to those found at a slip zone in a dry contact between a femoral head and an acetabular cup of hip prosthesis. Contact stresses are imposed to generate cracks in the models. Magnitudes of imposed stresses are higher than those found at the microscopic scale. Effects of microstructures and contact stresses are investigated in terms of crack formation. In addition, fatigue behaviour of the microstructure is determined by performing simulations under cyclic loading conditions. It is shown that crack density observed in a microstructure increases with increasing magnitude of applied contact stress. Moreover, crack density increases linearly with respect to the number of fatigue cycles within a given contact stress range. Meanwhile, as applied contact stress increases, number of cycles to failure decreases gradually. Finally, this proposed finite element simulation offers an effective method for identifying fracture and fatigue behaviours of a microstructure provided that microstructure images are available.     

Experimental and theoretical modal analysis of constructions with fatigue cracks

On the basis of experimental and theoretical modal analysis, the appearance of fatigue cracks with various depths within rotor systems is studied. For cracks with various depths, stress values in the place of occurrence of a defect are obtained. By means of obtained stresses, it is possible to reveal fatigue cracks with a depth of 15% of the cross-sectional area of a shaft.     

Experimental and analytical investigation of the heat transfer and thermal stresses in a cylinder head of a diesel engine

An experimental and theoretical study has been carried out to investigate the mechanism of crack development in cylinder heads of two-stroke diesel engines. The cracks initiate in the valve bridge region, between a pair of exhaust valves, where high temperatures exist during operation of the engine. The experiments included steady state and transient modes (cold start, loading and shut off), and temperature distributions were measured in the metal and the cooling water. The hot surface temperature as well as the maximum stresses are estimated using an analytical model. It is concluded that the dominant process of cracking is low cycle thermal fatigue and residual tensile stresses which appear after engine shut off.     

无缝钢管疲劳断裂分离新工艺及实验研究

针对传统的金属管材分离技术存在生产效率低、能耗高以及浪费原材料等问题,采用了一种新的管材分离方法,利用偏心加载工艺和应力集中效应促使管材表面环状V形槽底尖端处快速萌生疲劳裂纹并控制疲劳裂纹的稳定扩展。描述了管材的偏心循环加载疲劳断裂分离装置的工作原理,推导出了疲劳加载下管材V形槽根部裂纹是否起裂的判断公式。设计了线性递减、多层阶梯递减、1/4凸弧递减和1/4凹弧递减四种实验控制曲线,针对304不锈钢管材进行实验研究,实现了管材的精密分离。实验结果表明,利用V形槽的应力集中效应,在偏心加载力的持续作用下,加载频率采用多层阶梯递减控制曲线由高到低地逐渐降低加载频率,能够保证管材V形槽根部裂纹的快速萌生和稳定扩展,并可实现管材快速断裂,获得良好的断面质量。     

Progression of fretting fatigue damage in Ti–6Al–4V

An investigation was conducted to explore the nature of fretting fatigue damage in the stages prior to crack formation. In the unique experimental apparatus employed in this study, where total slip never occurs, several locations on each test specimen exist where cracks can develop due to local contact conditions. Under the test conditions used, not all of the sites had cracks upon test completion. This study evaluated the condition of non-cracked sites on several fretted specimens in an effort to identify differences between these and sites where small cracks were observed.A single test condition of 620 MPa average applied static clamping stress and 250 MPa applied axial fatigue stress for R=0.5 was selected, which corresponds to a fretting fatigue life of 10⁷ cycles based on prior work. For specimens tested to 10⁶ cycles, or 10% of life, several destructive and non-destructive characterization methods were chosen: scanning electron microscopy (SEM), residual stress measurement and transmission electron microscopy (TEM). Each site at which crack nucleation could be expected was inspected in the SEM and was then characterized using surface X-ray diffraction to quantify the residual stresses field near that location. Then TEM foils were cut from one area on a specimen with tiny cracks and dislocation densities were observed. A novel technique was used which permitted TEM samples to be obtained from regions in close proximity on the original specimen.Comparisons were made between as-received (AR) and stress-relief annealed (SRA) specimens, on which the stress-relief was applied prior to fretting fatigue testing. SEM inspection was useful for qualitative analysis of wear debris and identification of cracks as small as 20 μm, but was unable to provide quantitative data on the level of fretting fatigue damage beyond crack size. Although differences were noted in the residual stresses for the SRA versus the AR specimens, no residual stress peaks were noted in the edge of contact regions where cracks would eventually develop. TEM observations in the vicinity of the crack nucleation region showed that the dislocation structure decayed rapidly into the specimen thickness. The cause of the dislocations was attributed to plastic deformation caused by the clamping stresses.     

Modelling fatigue damage accumulation in two-dimensional Voronoi honeycombs

Trabecular bone, a porous, cellular type of bone found at the ends of the long bones and within the vertebrae, is subject to cyclic compressive loading resulting from the activities of daily living. Such fatigue loading can result in fracture, especially in vertebrae of patients with osteoporosis. As an initial step in understanding compressive fatigue of trabecular bone we previously used finite-element analysis to model the progressive damage and failure of a simple, two-dimensional hexagonal honeycomb. In this study, the analysis is extended to a random, Voronoi honeycomb. Bending of the cell walls induces tensile stresses even when the overall loading is compressive. The cell walls are assumed to have a distribution of crack lengths in their tensile zones. The cracks are assumed to grow according to a Paris law and fail when the cracks reach 75% of the cell wall thickness. Failed cell walls are removed from the structure, the stress distribution recalculated and the next increment of fatigue loading are simulated. The Young's modulus of the honeycomb is calculated after each cell wall failure. Overall failure of the Voronoi structure is assumed to occur when the modulus is reduced by 5%; further loading reduces the modulus sharply. The slope of the S–N curve for the Voronoi honeycomb is the same as that for the hexagonal honeycomb. The model suggests that a random honeycomb is more sensitive to fatigue than a regular one.     

压弯组合应力下高强钢焊接板表面裂纹疲劳寿命计算

潜艇上浮下潜运动使得其耐压壳承受交变的外压载荷。潜艇耐压壳的疲劳热点为锥一柱过渡结构连接焊缝焊趾处，该处所受的应力特征为压弯组合应力。压弯组合应力下表面裂纹应力强度因子及其疲劳寿命的计算尚无报道。弄清楚压弯组合应力作用下带表面裂纹焊接板试件的疲劳特性对潜艇等结构的疲劳研究是必要的。文中提出用成一定角度的对接焊板试件和轴向加载获得压弯组合应力来模拟潜艇耐压壳锥一柱焊接结合区的应力特征的实验方法。用980高强钢作试件，研究焊趾处预制表面裂缝，并在压弯组合应力为特征的疲劳载荷作用下表面裂纹的疲劳行为。给出高强钢焊趾表面裂纹在压弯组合应力下应力强度因子及其疲劳寿命计算式。该结果可供海洋平台、压力容器及管道的某些受力特征为压弯组合应力的重要结构的疲劳设计时参考。     

螺栓连接单搭接件疲劳特性试验与全寿命估算方法研究

飞机搭接件结构在疲劳载荷下会产生裂纹并扩展,而目前对搭接件结构参数与其疲劳寿命的关系研究还不够系统,且寿命估算模型只考虑了全寿命的一部分。对不同结构参数的三螺栓单搭接件的疲劳特性进行了试验研究,并以现有工程方法为基础,构建了基于材料初始不连续状态的搭接件全寿命估算模型。结果表明,搭接件的疲劳破坏具有一定的隐蔽性,在一定条件下,采用直孔形式、增大孔间距和螺栓预紧力、减小孔直径等都可以改善搭接件的疲劳特性。数值估算结果与试验结果的对比表明,所建寿命模型具有一定的精度,能满足工程需要,计算结果和结论可作为该类结构损伤容限设计的参考依据。     

Fatigue strength depending on position of cracks for weldments

This is a study of fatigue strength of weld deposits with transverse cracks in plate up to 50 mm thick. It is concerned with the fatigue properties of welds already with transverse cracks. A previous study of transverse crack occurrence, location and microstructure in accordance with welding conditions was published in the Welding Journal (Lee et al., 1998). A fatigue crack develops as a result of stress concentration and extends with each load cycle until fatigue occurs, or until the cyclic loads are transferred to redundant members. The fatigue performance of a member is more dependent on the localized state of stress than the static strength of the base metal or the weld metal. Fatigue specimens were machined to have transverse cracks located on the surface and inside the specimen. Evaluation of fatigue strength depending on location of transverse cracks was then performed. When transverse cracks were propagated in a quarter-or half-circle shape, the specimen broke at low cycle in the presence of a surface crack. However, when the crack was inside the specimen, it propagated in a circular or elliptical shape and the specimen showed high fatigue strength, enough to reach the fatigue limit within tolerance of design stresses.     

Modeling of geometry effects in fretting fatigue

The stress field that results from two bodies in contact is an important aspect that governs the fretting fatigue behavior of materials. Applied loads as well as contact geometries influence the contact stresses. The profile of an indenter and the boundary conditions provide sufficient information from which the surface tractions and the corresponding subsurface stresses have been calculated in a semi-infinite halfspace using singular integral equations. In this investigation, a numerical subroutine was developed to calculate the surface tractions and the corresponding surface and subsurface stresses of an arbitrary finite thickness infinite plate subjected to loading through a random indenter. The results from the detailed stress analysis of the contact region are required by both an initiation and fracture mechanics approach. While initiation criteria involving stress gradient fields, such as sharp notches and edges of contact in fretting fatigue, are not well established or agreed upon, stress intensity factor calculations using tools such as weight functions are more reliable. The stress intensity analysis, which is used to determine whether an initiated crack will continue to grow if it is above the threshold, depends on many variables in the stress analysis such as pad and specimen geometry, loading configuration and friction coefficient. The contact stress analysis has been used to determine equivalent stress parameters that are related to the initiation of a crack. Similarly the numerical subroutine for the contact stresses is used in conjunction with the stress intensity analysis to determine the influence of the geometry, loading configuration and friction coefficient on the stress intensity factor. Results from high-cycle fretting fatigue experiments are used to determine the threshold stress intensity factor for a given configuration. The combination of the numerical and experimental analysis is then used to develop a tool for high-cycle fretting fatigue based on a threshold approach involving a go–no go criterion.     

Fatigue crack propagation behavior in STS304 under mixed-mode loading

The use of fracture mechanics has traditionally concentrated on crack growth under an opening mechanism. However, many service failures occur from cracks subjected to mixed-mode loading. Hence, it is necessary to evaluate the fatigue behavior under mixed-mode loading. Under mixed-mode loading, not only the fatigue crack propagation rate is of importance, but also the crack propagation direction. In modified range 0.3≤a/W≤0.5, the stress intensity factors (SIFs) of mode I and mode II for the compact tension shear (CTS) specimen were calculated by using elastic finite element analysis. The propagation behavior of the fatigue cracks of cold rolled stainless steels (STS304) under mixed-mode conditions was evaluated by using K_I and K_II(SIFs of mode I and mode II). The maximum tangential stress (MTS) criterion and stress intensity factor were applied to predict the crack propagation direction and the propagation behavior of fatigue cracks.     

多轴随机应变加载下铝合金缺口件有限元分析及寿命预测

在应变控制下对7075-T7451铝合金实心棒带U型环状缺口试件进行拉—扭比例、非比例恒幅和随机加载疲劳试验。查明名义剪切应力最大值与轴向应力最大值的下降规律,并用两者最先开始下降时的循环数比值来评估裂纹萌生寿命。利用弹塑性有限元法分析不同加载条件下试件缺口根部的循环应力、应变响应。基于有限元计算得到的应力、应变结果,采用拉伸型多轴疲劳损伤模型预测随机加载条件下缺口试件的疲劳裂纹萌生寿命,计算结果与试验得到的寿命比较吻合。     

火焰筒热疲劳分析与寿命估计

发动机火焰筒的热疲劳损伤是导致火焰筒失效的主要原因.在火焰筒温度急剧变化过程中,结构上存在温升滞后现象,热冲击载荷造成局部高温区压缩塑性变形;在持续升温与停车降温过程中,结构自身约束导致此处形成拉伸应力,引发循环塑性行为,从而产生疲劳裂纹.基于该机理,采用有限元方法模拟不均匀温升和循环过程及应力应变的变化,并结合Coffin-Manson规律对火焰筒寿命进行预测,与实际情况对比,说明该机理的合理性.     

Diagnosing and forecasting the residual service life of welded metal structures of bridge cranes using metal foils

Problems related to safe operation of metal structures of bridge cranes are considered and ways to resolve them are proposed. A method of metal foils is proposed for the qualitative assessment of the accumulation of fatigue cracks in the base metals and welded joints. Techniques for determining the equivalent stresses and the number of loading cycles, as well as a method for calculating the residual service life of welded metal structures, are proposed. Results of forecasting the residual service life of metal structures of bridge cranes are considered.     

Characterisation of fretting fatigue damage using synchrotron X-ray micro-tomography

The fretting fatigue damage of mechanical joints is studied experimentally by considering the following scenario: first, the crack nucleation in the joint is investigated via fretting tests. Second, the propagation of the fretting cracks under cyclic uniaxial tensile loading is carried out and analysed. Synchrotron radiation X-ray micro-tomography is used to image for the very first time the three-dimensional shape of the initial fretting cracks as well as that of the propagating cracks at different stages of the fatigue life, thanks to a dedicated set-up. This technique brings new experimental data on the influence of the local microstructure on fretting fatigue damage, initiation and growth.     

汽车车轮结构强度分析

针对车轮动态弯曲疲劳试验建立了两种轿车车轮的静态线性有限元模型，它们可以有效地用来确定车轮结构的危险点，即结构中计算应力（von Mises应力）比较大的点。结构危险点的计算应力反映该处的应力集中程度。对车轮结构上计算应力较大的测点进行实验应力分析，验证有限元分析结果。对结构危险点的应力状态进行分析，有助于预测车轮疲劳裂纹的发生方向和引起疲劳损伤的主要应力循环，在所研究的车轮结构中也就是沿着车轮径向的正应力变程。分析还表明，在动态弯曲疲劳试验中，车轮结构各点所承受的可能是非对称应力循环。     

Dynamic deformation of hot temperature degraded POM and PP using a modified SHPB with pulse shaper technique

The conventional split Hopkinson pressure bar (C-SHPB) technique with a special experimental apparatus is used to obtain a dynamic deformation material behavior under a high strain rate loading condition. An experimental modification is introduced to reduce the non-equilibrium on the dynamic material response during a short test period for two polymeric materials. The proposed method uses aluminum pressure bars to achieve a closer impedance match between the pressure bars and the specimen materials such as hot temperature degraded POM (Poly Oxy Methylene) and PP (Poly Propylene) to obtain more distinguishable experimental signals. In addition, a pulse shaper technique is used for increasing the rise time of the incident pulse to ensure the dynamic stress equilibrium and the homogeneous deformation in the specimen under dynamic compression loading condition. The details on the dynamic stress equilibrium and the duration of uniform strain rate during the dynamic deformation of the specimen are experimentally investigated. The effects of degradation at a few different hot temperatures on the maximum compressive stresses are also experimentally studied under varying impulsive loading conditions.     

An investigation into the fixation of hip replacements.

This paper describes an experimental study of hip prosthesis stems fixed with bone cement in model femora and subjected to cyclic loading. Loosening of the stems did not occur due to fatigue of the bone cement, even when reduction of medial support from the calcar had been simulated. Measurement of stresses in the cement layer and comparison with published properties of bone cement indicated that fatigue failure was not likely under the experimental conditions. Fatigue failure of cement could not be solely regarded as initiating loosening. It is concluded that the results uphold theories that biological factors must also contribute to the initiation of loosening, rather than being a response to mechanical failure.     

含夹杂物轴承钢中裂纹的萌生与扩展

针对轴承钢中夹杂物周围应力集中导致的疲劳剥落,建立了一种结合连续损伤力学的内聚力模型,用于模拟滚动接触循环加载下的裂纹萌生与扩展。基于内聚力模型的损伤起始准则和损伤演化规律,利用VUMAT子程序结合连续损伤力学构造了新的损伤演化方式,实现循环加载下的损伤累积,建立了基于内聚力模型的疲劳损伤累积失效模型,对含夹杂物模型的疲劳裂纹萌生与扩展进行了模拟,并研究了载荷条件和接触区摩擦因数对裂纹萌生与扩展以及疲劳寿命的影响。研究结果揭示了微观裂纹的萌生与扩展过程,为认识滚动接触疲劳提供了基础。