A viscoelastic analysis of the tensile weakening of deep femoral head articular cartilage

Articular cartilage from below the surface of the femoral head of the hip joint shows a profound age-dependent weakening in its tensile mechanical properties. This ageing is also associated with a reduced viscoelastic response in the older tissue. A constitutive model of the viscoelastic behaviour of deep articular cartilage (as discussed by Egan in 1988) is used to generate a graphical pattern which represents the mechanical behaviour. This constitutive approach suggests that the tensile weakening of the older cartilage is due to an age-related reduction in the recruitment of load-carrying structures as the tissue is deformed. The viscoelastic constitutive model also predicts a reduction in the tensile strength of deep articular cartilage with rate of deformation. This prediction is supported by experimental fracture stress data. A weakening of the tensile integrity of the microstructure of articular cartilage could make the tissue less able to sustain normal compressive physiological loading without damage and thus make the tissue more susceptible to osteoarthritic degeneration. The constitutive approach indicates that the weakening of the older tissue may be related to changes within the microstructure which determine how applied mechanical energy is stored and dissipated.     

Detachment of an elastic beam from a viscoelastic support: A variational approach

The paper is devoted to the analysis of failure in multi-layer composite materials. For the experimental study of the failure mechanism, the peel tests are used. The peel test is modeled as the detachment of an elastic beam attached to a rigid adherend by a viscoelastic adhesive layer.For solving this problem, we use the principle of minimum free energy and derive a new energy criterion of the detachment. We propose a model of an aging viscoelastic material describing the viscoelastic behavior in terms of elastic objects. Using this model we propose an expression for the specific potential energy of an aging viscoelastic medium and derive a new formulation of the principle of minimum free energy for a viscoelastic system.A numerical method for solving nonlinear integro-differential equations describing the detachment process is developed, and its convergence is established. We analyze numerically the dependence of the detachment on the construction and material parameters, and on the loading history. We demonstrate the existence of a limited training effect, which allows considerable reduction of the detachment zone by a special preliminary loading program (“training”).     

A new algorithm for computing the indentation of a rigid body of arbitrary shape on a viscoelastic half-space

In this paper the contact problem between a rigid indenter of arbitrary shape and a viscoelastic half-space is considered. Under the action of a normal force the penetration of the indenter and the distribution of contact pressure change. We wish to find the relations which link the pressure distribution, the resultant force on the indenter and the penetration on the assumption that the surfaces are frictionless. For indenters of arbitrary shape the problem may be solved numerically by using the Matrix Inversion Method (MIM), extended to viscoelastic case. In this method the boundary conditions are satisfied exactly at specified “matching points” (the mid-points of the boundary elements). It can be validated by comparing the numerical results to the analytic solutions in cases of a spherical asperity (loading and unloading) and a conical asperity (loading only). Finally, the method was implemented for a finite cylindrical shape with its curved face indenting the surface of the half-space. This last example shows the efficiency of the method in case of a prescribed penetration as well as a given normal load history.     

Static and dynamic behaviour of composite riveted joints in tension

The tensile response and failure of composite riveted joints are studied experimentally in the present paper. Seven joint configurations for aircraft application are tested at quasi-static, 4 and 8 m/s nominal loading rates. Joint specimens are made of CFRP in a number of lay-ups of unidirectional tapes and woven fabrics. A dynamic tensile test method is designed to give reliable test results. It is shown that the variation of tensile strength with loading rate is negligible for the tested composite riveted joints. However, for most of the tested specimens, the average total energy absorption of the composite joint increases with increasing loading rate. Various failure modes are identified for various joint designs and it was found that joint failure modes can change with varying loading rates.     

碳纤维/玻璃纤维/石墨协同改性PTFE复合材料力学性能

通过机械混合、冷压和烧结成型制备了碳纤维、玻璃纤维和石墨填充协同改性聚四氟乙烯(PTFE)复合材料。对比分析了不同样品的拉伸、冲击和压缩等力学性能。结果表明:玻纤和碳纤维使复合材料冲击强度下降;玻纤使复合材料拉伸强度下降,碳纤维则使复合材料拉伸强度稍有增强;玻纤和碳纤维均使复合材料压缩强度增加,但碳纤维的增强效果更为明显;石墨、玻纤和碳纤维协同增强PTFE复合材料的拉伸强度较高,弹性模量较大,断裂伸长率较高,抗压缩性能明显提高,且材料拉伸时呈塑性断裂,是综合力学性能较好的高性能润滑密封材料。     

激光选区烧结超高分子量聚乙烯工艺及性能

研究了激光选区烧结(SLS)成型工艺中不同工艺参数以及后续热处理工艺对超高分子量聚乙烯(UHMWPE)材料成型性能的影响。通过调整扫描间距、激光功率、扫描速度等不同工艺参数,描述了SLS成型UHMWPE零件的致密度、拉伸强度以及断裂伸长率,并对热处理前后的SLS成型UHMWPE零件的力学性能进行了比较。结果显示,致密度、拉伸强度、断裂伸长率总体上与激光功率呈正相关关系,与扫描间距、扫描速度呈负相关关系。经热处理后,SLS成型UHMWPE零件的力学性能有明显提高,致密度达到95.12%,抗拉强度达到24.08 MPa,断裂伸长率达到334.82 MPa。实验结果表明:SLS成型UHMWPE零件与模塑成型UHMWPE零件性能尚有差距,仅优化成型工艺不足以得到理想性能,但经热处理后,零件性能基本满足使用要求。     

Study on vital static properties of fine blanking of GFRP composites with that of conventional drilling

Among the high performance engineering materials, fiber-reinforced plastics play an important role. The present work is concerned with the comparison of vital static strength properties of fine blanking with conventional drilling on hand lay-up made glass fiber-reinforced plastic (GFRP) laminates of four different reinforcement lay-up sequences such as unidirectional [0/0]_n, angle ply [0?±?45]_ns, quasi-isotropic [0/45/90]_ns, and cross-ply [0/90]_n. Observation includes tensile and flexural bending strengths of the specimens without hole and with hole by conventional drilling and fine blanking. In this work, an endeavor has been made to simulate the service conditions to determine their effect on the response of composite laminates. Detailed studies on GFRP composites when subjected to different loading environments such as static loading, particularly tensile loading, and low frequency high amplitude (fatigue) loading were carried out. The response of the composite laminates to these service environments has been evaluated in terms of flexural strength and modulus. From the tensile study, it was observed that by inserting a hole at center by drilling, the strength was reduced to one third, and by inserting a hole at center by fine blanking, the strength was increased nearly 20% than that of drilling. Apart from this, the flexural test conducted on polymeric composite specimens showed that an exposure to low frequency and high amplitude loading enhances the flexural strength up to certain duration of exposure, beyond which, due to accumulation of damage within the composites, the flexural strength reduces with number of cycles. This can be attributed to possible strain-induced stiffening of fibers and interface.     

Finite element analysis of coating adhesion failure in pre-existing crack field

Abstract

The influence of a pre-existing crack field on coating adhesion failure in a steel surface coated with a 2 μm thick titanium nitride (TiN) coating was investigated by finite element method modelling and simulation. The stress and strain fields were determined in contact conditions with a spherical diamond tip sliding over the coated surface at a loading of 8 N. One crack in or at the coating increased the maximum tensile stresses with six times from 82 to 540 MPa when the crack was vertical through the coating or L shaped and with nine times when the crack was horizontal at the coating/substrate interface. A simulated multicrack pattern relaxed the tensile stresses compared to single cracks. The results indicate that a cracked coated surface needs to have about five to nine times higher adhesive and cohesive bonds to resist the same loading without crack growth compared to a crack free surface. For optimal coated surface design, the strength of the adhesive bonds between the coating and the substrate in the vertical direction needs to be 50% higher than the cohesive bonds within the coating and the substrate in the horizontal direction. The first crack is prone to start at the top of the coating and grows vertically down to coating/substrate interface, and there it stops due to the bigger cohesion within the steel material. After this, there are two effects influencing that the crack will grow in the lateral direction. One is that steel cohesion is normally bigger than the coating/interface adhesion, and the second is that there are higher tensile stresses in the horizontal than in the vertical cracks. Several vertical cracks can stop the horizontal crack growth due to stress relaxation.     

Mechanotransduction-The relationship between gravity,cells and tensile loading in extracellular matrix

Gravity plays a central role in vertebrate development and evolution. Mechanotransduction involves the tensile tethering of veins and arteries, connections between the epidermis and dermis in skin, tensile stress concentrations that occur at tissue interfaces, cell-cell interactions, cell-collagen fiber stress transfer in extracellular matrix and fluid shear flow. While attention in the past has been directed at understanding the myriad of biochemical players associated with mechanotransduction pathways, less attention has been focused on determining the tensile mechanical behavior of tissues in vivo. Fibroblasts sit on the surface of collagen fibers in living skin and exert a retractile force on the fibers. This retractile force pulls against the tension in collagen fibers in skin. After fibroblast-collagen fiber interactions are altered either by changes in fibroblast adhesion or after formation of cancer associated fibroblasts, and changes in cell junctions, alterations in the retractive force leads to changes in mechanotransduction. The purpose of this paper is to present a model of tensile forces that occur at the fibroblast-collagen fiber interface and how these forces are important in extracellular matrix physiology in health and disease.     

Hypochlorite Cleaning Causes Degradation of Polymer Membranes

Polymer membranes are often used in water treatment industry, as a reliable economic solution for purification of surface water streams. Among a variety of membrane processes the ultrafiltration (UF) membranes combine high pathogen removal ability with minimal energy consumption. Since the membranes are considered to be chemically and mechanically stable, in-place cleaning is routinely performed with strong oxidizing agents, such as hypochlorite. The results of the current study, however, clearly indicate that the mechanical strength of UF membranes deteriorates upon hypochlorite cleaning. Values of ultimate tensile strength, ultimate elongation and Young’s modulus decreased as the dose (concentration × contact time) of hypochlorite was increased, with the decreases being more marked for cellulose acetate membranes than for polyethersulfone membranes. Scanning probe microscopy showed membrane aging in response to hypochlorite treatment. The aging was related to gradual chain breaking in membrane skin layer, as reflected by the disappearance of O–C–O and C–S absorbance peaks in ATR-FTIR spectra of treated cellulose acetate and polyethersulfone membranes, respectively. Since the degree of scission was linearly related to the deterioration in ultimate tensile strength, we here propose that ultimate tensile strength measurements can be used as a simple macroscopic test for periodic inspection of the degree of degradation of UF membranes exposed to in-place chemical cleaning.     

构件等效动态的强度试验

基于构件的动态冲击试验和相应的静态试验,以动静态试验的响应近似等效为原则,提出了以静态试验代替动态试验的等效动态强度试验方法。以两端固支圆管构件为例,首先对构件进行动态冲击试验和相应的静态试验,并建立相应的有限元分析模型;基于试验数据,修正相关的有限元仿真模型;通过正交试验设计,基于动静态试验的响应近似等效原则,提出了动静态试验输入之间的关系模型;并通过追加试验,分析了用静态试验强度预测动态冲击试验强度的误差。提出的方法可对承受动态冲击载荷的构件在初步设计阶段,利用简单的静态试验强度快速判断设计方案是否满足动态冲击设计要求。     

Estimation of rubber sliding friction from asperity interaction modeling

Interaction between a soft rubber asperity and its hard counterpart is traced with the help of a finite element computation. The analysis is aimed to estimate the influence of adhesion between rubber and rigid surfaces and the energy losses arising from the deformation of rubber bulk to the sliding resistance. At the contact zone, interfacial bonds are formed due to adhesion and their resistance to sliding is represented by the shear strength of the contact interface. In the rubber bulk, the hysteresis loss is calculated using an appropriate model of the viscoelastic mechanical behavior of rubber for large strains. Dependence of friction on sliding speeds and temperature is hence detected. Influence of surface roughness and contact pressure on friction is also examined.     

Enriched finite element method for three-dimensional viscoelastic interface crack problems

The enriched finite element method is developed for three-dimensional problems of an interface crack between elastic and viscoelastic (including dissimilar viscoelastic) materials. According to the displacement fields of elastic interface crack, the displacement fields of viscoelastic interface crack are derived through the correspondence principle. By incorporating the displacement expressions into the displacement model of regular element, the incremental formulations of enriched element are derived. The stress intensity factors and strain energy release rates can be solved based on the enriched degree of freedoms. A 3-D through interface crack at the center of jointed dissimilar viscoelastic plate subjected to remote tension and a quarter-circular viscoelastic interface corner crack subjected to uniform thermal loading was investigated using the enriched finite element method. It is shown that the present solutions are consistent with the analytical solutions, which indicates the present method is correct and efficient.

     

提升ZL111合金抗拉强度的工艺研究

铝合金材料的理化性能主要受抗拉强度、延伸率及硬度等影响,其中,抗拉强度的高低直接决定了材料所能承受的力的大小。从材料化学成分及热处理数两个环节来研究提升ZL111合金抗拉强度的工艺方法,该方法的研究,对于提升ZL111合金的抗拉强度具有现实的指导意义。     

Cryogenic Friction Behavior of PTFE based Solid Lubricant Composites

Solid lubricants used in aerospace applications must provide low friction and a predictable operation life over an extreme range of temperatures, environments and contact conditions. PTFE and PTFE composites have shown favorable tribological performance as solid lubricants. This study evaluates the effect of temperature on the friction coefficient of neat PTFE, a PTFE/PEEK composite and an expanded PTFE (ePTFE)/epoxy coating. These experiments evaluate friction coefficient over a temperature span which, to the investigators’ knowledge, has not been previously examined. Results show a monotonic increase in friction coefficient as sample surface temperature was decreased from 317 to 173 K for all three samples. The frictional performance of these and other published solid lubricant polymers was modeled using an adjusted Arrhenius equation, which correlates the coefficient of friction of the polymer materials to their viscoelastic behavior. A model fit of all the polymer data from 173 to 450 K gives an activation energy of 3.7 kJ/mol. This value suggests that breaking of van der Waals bonds is the likely mechanism responsible for the frictional behavior over this temperature range.     

我国射线化学合成SiC纤维及相关材料的现状与展望

传统CVD法合成的SiC纤维其最高使用温度仅有900℃,氧化先驱丝法制备的SiC纤维也只能用于1 000℃以下。主要介绍了我国射线化学合成法制备耐高温抗氧化的SiC纤维(使用温度高于1 600℃,单丝抗拉强度达2.2～2.7GPa)的系列关键技术,及其基于这些技术制备圆形截面管状SiC纤维(外径为10～15μm,壁厚2～3μm)的技术;同时也简介了射线化学应用于纳米微孔SiC功能陶瓷以及SiCf/SiC陶瓷基复合材料等方面的研究进展。     

双相不锈钢各组相循环变形行为的纳米压痕试验和有限元表征方法研究

对双相不锈钢的奥氏体相和铁素体相,分别开展了不同加载模式（接触载荷和压入位移）和不同加载波形下的单向、循环纳米压痕试验,对比分析了两相的基本力学性能和压痕循环变形行为的演化规律。基于压痕试验结果和修正ABDEL-KARIM-OHNO非线性随动硬化准则的弹塑性本构模型,提出一套双相不锈钢奥氏体相和铁素体相的塑性和循环塑性行为的本构模型参数表征方法。通过对微结构代表性体积单元整体拉伸和循环变形行为进行模拟,并与宏观试验结果对比,验证了参数表征方法的合理性。研究结果表明,铁素体相的强度、硬度和抗棘轮变形的能力均高于奥氏体相,两相之间通过晶界产生一定的交互作用;在接触载荷控制的循环加载条件下,奥氏体相与铁素体相均产生明显的压痕棘轮现象,且载荷水平越高压痕棘轮变形程度越大;所发展的本构模型参数表征方法可为研究多相材料各组相、小体积材料的循环变形行为提供借鉴和参考。     

RESEARCH ON BEHAVIORS OF ELECTRORHEOLOGICAL FLUID-BASED SEMIACTIVE ROBOT FINGERTIPS

ＲＥＳＥＡＲＣＨＯＮＢＥＨＡＶＩＯＲＳＯＦＥＬＥＣＴＲＯＲＨＥＯＬＯＧＩＣＡＬＦＬＵＩＤＢＡＳＥＤＳＥＭＩＡＣＴＩＶＥＲＯＢＯＴＦＩＮＧＥＲＴＩＰＳ①ＹｕｅＨｏｎｇＬｉＴｉｅｊｕｎＤｅｐａｒｔｍｅｎｔｏｆＭｅｃｈａｎｉｃａｌＥｎｇｉｎｅｒｉｎｇ,Ｈ...     

Friction stir welding of AZ61A magnesium alloy

This paper deals with the development of an empirical relationship to predict tensile strength of friction stir welded AZ61A magnesium alloy. The process parameters such as tool rotational speed, welding speed, axial force and tool pin profile play a major role in deciding the tensile strength. The response surface method (RSM) was used to develop the empirical relationship. The four-factor, five-level central composite design was used to minimize the number of experimental conditions. The developed empirical relationship can be effectively used to predict tensile strength of friction stir welded AZ61A magnesium alloy joints at 95 % confidence level.     

Drilling-induced damage in uni-directional glass fiber reinforced plastic (UD-GFRP) composite laminates

Uni-directional glass fiber reinforced plastic (UD-FRP) composite materials are a feasible alternative to structural members that bear loads in only one direction. FRP composite materials have excellent properties in the direction of the fibers. Drilling- induced damage acts as an inhibitor to their application, as the holes act as stress concentration sites for failure under loading. The present study is an attempt to study the influence of drilling-induced damage on the residual tensile strength of uni-directional composite laminates and to propose a mathematical model correlating the residual strength with the drilling parameters. A finite element model (FEM) is also developed to study the drilling-induced damage in composite laminates.