Effect of stress biaxiality on the high-strain fatigue behavior of an aluminum-copper alloy

Although there is now a considerable volume of high-strain (<10⁵ cycles) fatigue data for uniaxial tension-compression and simple-bending conditions, relatively little information is available regarding the effects of stress and strain biaxiality. A method which has been used to study the effects of biaxiality on longlife fatigue strength is to subject thin-walled tubes to repeated internal pressure and an end load which is in phase with, and a linear function of, the pressure. The object of the present research was to use this method to study the influence of stress biaxiality on the high-strain fatigue behavior of a high strength, aluminum-4% copper alloy at room temperature. From a continuum-mechanics point of view, this material is completely elastic after the first few load cycles. Cylinder results for hoop to axial stress ratios of 2:1, 1:1, 1:2 and 2: ?1 suggest that fatigue failure of this material in the life range 10³ to 10⁵ cycles is primarily dependent on the maximum range of tensile stress. This conclusion and a study of fracture surfaces led to the use of linear-elastic fracture mechanics to interpret the fatigue and brittle fracture behavior of these cylinders.     

An analogue technique for determining torsion and flexure functions of uniform beams

This paper describes an extremely simple, and yet accurate technique, based on electric analogy, for determining torsion and flexure functions of beams of uniform section, acted upon by terminal loads. Equations governing the torsion and flexure of such beams have been expressed to state the problem as a Neumann-type boundary-value problem; thus, the problem reduces to finding a function (or functions), which is harmonic within the cross section of the beam, and whose normal derivatives at the boundary of the section are prescribed. Prescribed current densities are introduced at the boundary, and consequent voltages are obtained as the analogue of required functions. Instrumentation is very simple and, once what has been described here as the “influence matrix” of voltages is obtained, both torsion and flexure functions can be obtained simply by multiplying the “influence matrix” by vectors of appropriate normal derivatives at the boundary. None of the usual drawbacks of membrane analogy arise here. Use and accuracy of the technique have been demonstrated by a number of examples.     

钢筋混凝土柱抗弯承载力的概率模型与校准分析

为了克服传统确定性抗弯承载力模型和校准方法无法合理考虑不确定性所存在的缺陷,分别建立了钢筋混凝土(RC)柱的概率抗弯承载力模型与概率校准方法。首先,基于RC柱正截面受弯承载力的基本假定,结合偏心受压RC柱的截面内力平衡条件,分别建立了大(小)偏心受压RC柱的确定性抗弯承载力模型;然后,综合考虑固有不确定性和认知不确定性的影响,分别建立了大(小)偏心受压RC柱概率抗弯承载力模型的解析表达式,进而结合贝叶斯理论和MCMC法确定了概率模型参数的后验分布信息,从而建立了RC柱的概率抗弯承载力模型;最后,基于概率抗弯承载力模型所确定的概率密度函数、置信区间和置信水平,提出了传统确定性抗弯承载力模型的概率校准方法。研究结果表明,所建立的概率抗弯承载力模型不仅可以合理描述RC柱抗弯承载力的概率分布特性,而且可以校准传统确定性抗弯承载力模型的计算精度和置信水平。     

基于静强度分布参数的复合材料剩余强度及寿命预测

为探讨剩余强度及疲劳寿命与初始静强度分布参数之间的关系,构建基于初始静强度分布参数的剩余强度和疲劳寿命计算模型。在模型构建过程中未涉及复合材料结构的层数、铺层厚度和铺层方向,适应性强。模型参数可通过静力试验和剩余强度试验获得,试验成本相对疲劳试验较低。剩余强度和疲劳寿命与初始静强度变异系数及其分布参数有关,当初始静强度服从威布尔分布时,疲劳寿命亦服从威布尔分布,可为开展疲劳可靠性提供借鉴和参考。算例表明,基于建立的模型,剩余强度计算结果最大误差为-1.58%,疲劳寿命与试验数据吻合较好。     

Experimental and numerical investigation of fracture in a cast aluminium alloy

This paper describes an experimental and numerical investigation on the fracture behaviour of a cast AlSi9MgMn aluminium alloy. In the experiments, a modified Arcan test set-up was used to study mixed-mode fracture. During testing, the tension load and the displacement of the actuator of the test machine were recorded, simultaneously as a high-resolution digital camera was used to record a speckle-patterned surface of the specimen. The recorded images were post-processed using an in-house digital image correlation (DIC) software to obtain information of the displacement and strain fields in the specimen during the test. In addition, some newly implemented features in the DIC software allowed us to detect and follow the crack propagation in the material. The numerical calculations were carried out with a user-defined material model implemented in an explicit finite element code. In the model, the material behaviour is described by the classical J₂ flow theory, while fracture was modelled by the Cockcroft–Latham criterion, assuming the fracture parameter to follow a modified weakest-link Weibull distribution. With the proposed probabilistic fracture modelling approach, the fracture parameter can be introduced as a random variable in the finite element simulations. Crack propagation was modelled by element erosion, and a non-local damage formulation was used to reduce mesh-size sensitivity. To reveal the effect of mesh density and meshing technique on the force–displacement curves and the crack propagation, several different meshes were used in the numerical simulations of the modified Arcan tests. The numerical results were finally compared to the experimental data and the agreement between the measured and predicted response was evaluated.     

Flexural vibrations and stability of beams with variable parameters

Bialystok Polytechnic Institute, Poland. Translated from Prikladnaya Mekhanika, Vol. 30, No. 9, pp. 75–81, September, 1994.     

Optimizing diffusion bonding parameters to maximize the strength of AA6061 aluminum and AZ61A magnesium alloy joints

Experimental Techniques - The main difficulty when joining magnesium (Mg) and aluminum (Al) alloys by fusion welding process lies in the existence of oxide films and formation of brittle...     

无腹筋RC悬臂梁抗剪强度及尺寸效应理论研究

不同于混凝土材料,钢筋混凝土(RC)构件的破坏模式与机制更为复杂,采用混凝土材料尺寸效应理论难以描述构件的尺寸效应行为。我国相关规范中并没有系统考虑构件尺寸及纵筋率对RC梁抗剪承载力的影响。为研究剪跨比及纵筋率对无腹筋RC梁剪切破坏及抗剪强度尺寸效应的影响,采用三维细观数值模拟方法,建立了RC梁剪切破坏力学分析模型,研究了剪跨比及纵筋率对RC梁剪切破坏及抗剪强度尺寸效应的影响机制与规律。研究结果表明,RC梁抗剪强度表现出极为显著的尺寸效应现象;RC梁抗剪强度随剪跨比的增大而减小,随纵筋率的增大而增大;剪跨比较小时,纵筋率对抗剪强度的影响尤为显著。此外,基于Ba■ant材料层次尺寸效应律,提出了考虑剪跨比及纵筋率影响的RC梁抗剪强度尺寸效应理论公式。对比试验结果,验证了所提公式的准确性与合理性。     

Fracture resistance and strength of two-phase WC–Ni alloy

Microcracking near a crack tip in a material with microinhomogeneity is regarded as a stochastic process. Both the critical deformation energy density and minimum ligament size of a structural element at global failure are related to a fracture resistance parameter which depends also on the strength. This parameter is also known as the critical energy density factor. For composites it is shown that transition of material behaviour from one structural state to another could alter the fracture resistance depending on character of the critical defect size and strength. The theoretical result is shown to be in agreement with the experimental data.     

汽车板弹簧减振与等强度梁教学实验

汽车板弹簧减振是等强度梁优良动力特性在工程应用中的典型例子. 以此为背景,设计了一个等强度梁动载实验,并与相应的等直梁动载实验对比,介绍了实验装置、原理、步骤和实验结果. 实验揭示了汽车板弹簧减振的机理,综合了《材料力学》中“等强度梁”、“动载荷”和《理论力学》中“质点振动”等内容.     

颗粒冰与钛合金冻结强度的剪切试验分析

飞机在湿度较大的高空云层飞行时,空气中液滴碰撞飞机发动机叶片后易产生具有颗粒结构的冰体。为研究叶片上冻结冰对飞机发动机运行性能的影响,本文在低温实验室内采用喷雾法在钛合金表面制备颗粒结构冰,模拟飞机发动机叶片上冻结冰的形成过程。通过自主研制的剪切试验仪对不同环境温度下冻结的冰体与钛金属表面的剪切强度进行试验测量,得到了温度(T)与剪切强度(τs)的对应关系。试验结果显示,冻结冰与钛金属粘剪切强度随着环境温度降低而增加,并呈现出明显的线性关系。应力时程曲线显示,颗粒冰与钛合金表面粘结处发生的剪切破坏为典型的脆性破坏过程。本文为研究飞机发动机上冰体的力学性质、飞机防冰设计以及相应数值模拟提供了重要的参考数据。     

Dissimilar ultrasonic spot welding of Mg-Al and Mg-high strength low alloy steel

Sound dissimilar lap joints were achieved via ultrasonic spot welding (USW), which is a solid-state joining technique. The addition of Sn interlayer during USW effectively blocked the formation of brittle al₁₂Mg₁₇ intermetallic compound in the Mg-Al dissimilar joints without interlayer, and led to the presence of a distinctive composite-like Sn and Mg₂Sn eutectic structure in both Mg-Al and Mg-high strength low alloy (HSLA) steel joints. The lap shear strength of both types of dissimilar joints with a Sn interlayer was significantly higher than that of the corresponding dissimilar joints without interlayer. Failure during the tensile lap shear tests occurred mainly in the mode of cohesive failure in the Mg-Al dissimilar joints and in the mode of partial cohesive failure and partial nugget pull-out in the Mg-HSLA steel dissimilar joints.     

考虑孔洞影响的铸造镁合金ZM6疲劳寿命预估方法

铸造镁合金ZM6是一种应用于直升机减速器机匣制造的典型材料。然而,在铸造过程中产生的内部缺陷对材料的疲劳性能有显著影响。本文研究了含内部孔洞缺陷ZM6材料的疲劳损伤模型和寿命预估方法。首先,采用X射线断层扫描技术,对三个批次毛坯料制成的试验件进行扫描观察,获得了试验件内部孔洞的分布特征。进而,对48件试验件进行了两种应力比下多级应力水平的疲劳试验,获得了各批次试验件寿命结果,并通过观察与分析,得出了孔隙率和近表面较大孔洞为影响试验件疲劳寿命的两个关键因素。然后,基于损伤力学理论提出了通过材料初始弹性模量和等效孔洞局部应力应变场来分别反映孔隙率和近表面较大孔洞影响的疲劳损伤模型和寿命计算方法,并结合ABAQUS软件平台实现了含孔洞试验件的疲劳损伤计算和寿命计算。最后,采用所提理论模型和计算方法给出了试验件的疲劳寿命预测结果,并与试验结果进行了不同维度的对比,验证了所提模型与方法的有效性和适用性。     

刨削参数对刨刀载荷特性影响研究及其强度分析

为研究刨刀刨削煤岩过程中刨削参数对刨刀载荷的影响,建立了基于碰撞接触模型的刨刀刨削煤岩动力学模型。采用软件模拟、现场试验的手段,对不同工况条件下的刨刀载荷进行了分析,得到刨削速度、刨削深度及煤岩硬度等刨削参数对刀头载荷的影响规律;并基于仿真结果,对刨刀强度进行了分析和预测。研究结果表明:随着刨削深度的增加,刨刀载荷特征值均有较大幅度的增加,刨削速度、煤岩硬度等刨削参数与刨刀载荷特征值也呈正相关关系,但变化幅度较小。根据仿真结果选取一组最恶劣工况条件进行刨刀强度分析,发现除焊缝局部在接触载荷作用下产生强度失效外,其余大部分区域仍可以满足材料强度使用需求,并不会产生失效;在极限测试载荷条件下刨刀焊缝部位也可能发生强度失效,且有较小概率引发刀头、刀体部分发生断裂失效。研究结果可为刨煤机关键零部件设计优化提供一定的理论基础。     

近源堆沉积土石体细观结构与抗剪强度指标关联分析

细观结构是认知土石体力学行为本质的关键科学问题。本文通过引入物理学和数学等方法,借助颗粒物质力学理论,从几何排列与接触力的空间分布来定量刻画水平固结与山前坡地堆积两种典型环境下土石体的细观结构特征,并建立其与抗剪强度指标的关联。研究表明,（1）两种环境的土石体在细观结构上存在较大差异。在几何排列上,水平固结环境下的土石体具有长程无序和短程有序的特点,坡地堆沉积环境下的土石体表现出了无序的无定形结构;在接触力与单位接触向量的空间分布上,两者较为相似,绝大多数接触力以小于均值接触力的形式存在,其概率密度曲线P（f）呈幂函数衰减;90%以上接触方位角集中在40°~160°和220°~340°范围内。（2）基于径向分布函数、接触力概率密度和单位接触向量分别定义细观结构的特征量K_a,K_s和K_o,发现三个特征量的增大对内摩擦角呈线性促进作用,对黏聚力呈非线性削弱作用。     

Investigation of the effect of pressure on some physical parameters and thermoelastic phase transformation of NiAl alloy

The Ni–Al alloys which exhibit the thermoelastic phase transformations in the composition range from 60 to 65 atomic percentage of Ni are widely used in the high technology applications. In this study, thermoelastic phase transformations of Ni–37.5 at.% Al alloys at 0, 1 and 2 GPa pressures were investigated by using MD simulation. Physical interactions among atoms in the alloy system were modelled using Sutton–Chen version of the embedded atom method based on many-body interactions. The potential parameters for cross interactions between Ni and Al atoms were estimated by optimising the results obtained from the molecular dynamics simulations. Moreover, the effect of applied pressure on transformation temperatures, enthalpy, entropy and elastic energy of model alloy system were investigated. The obtained result showed that the transformation temperature increased with applied pressure while enthalpy, entropy and elastic energy decreased. The values of the thermodynamical parameters that obtained in this study are in very good agreement with results of experimental studies.