Effect of lubrication on the stress distribution in an extruded material

The visioplasticity method is used to find the complete strain, strain rate and stress distribution in the deformation zone, according to the deformation grid lines marked on the surface of the workpiece. From the experimental data (the values of the flow function for the extrusion), the velocity, strain rate and stress fields can be calculated by the finite-difference method from the stream function, equilibrium and plasticity equations. In this article, stress components distribution in forward extruded specimens of copper alloy are analysed using the visioplasticity method. Comparisons are made between stress distribution of the specimens extruded with three different coefficients of friction. The results are shown in the form of diagrams.     

Deformations of a simplified flip chip structure under thermal testing inspected using a real-time Moiré technique

Deformations of a Si-epoxy-FR4 (simplified flip chip) structure under thermal testing were inspected with a real-time Moiré technique. Specimens without cracks and specimens with a crack at the silicon-epoxy interface were prepared. The measurement results showed that the maximum deformation appeared at the edge. When the specimen was cooled to 20 °C, there was residual plastic deformation in the specimen. The creep effect was more dominant in the FR4-epoxy interface. Upon cooling to 20 °C, the specimen experienced partial strain recovery. To characterize the behavior of the interfacial crack, stress intensity factors K_I and K_II, and the strain energy release rate G in the vicinity of the crack tip were calculated using the measured deformations to conduct a quantitative study. It was observed that a sharp strain gradient occurred at the crack tip. K_I and K_II were dependent on temperature, and G was dominated by K_I for the interfacial crack in the specimen.     

Measurement of dynamic fracture toughness and failure behavior for explosive mock materials

In this work, a pre-cracked semi-circular shaped explosive simulant was loaded using a split Hopkinson pressure bar (SHPB). A high-speed camera was used to capture the deformation and fracture process of the specimen in situ. The digital images were processed using the digital image correlation (DIC) method. Next, full displacement and strain fields were obtained. The displacement vector field shows that the specimen fractured under tensile stress action. The strain field can be used to predict the crack propagation. Results show that the method of combined DIC and SHPB is effective to study the dynamic deformation and fracture behavior of explosive simulants. In addition, the specimen was loaded using a drop weight. The fracture toughness of the specimen was preliminary measured.     

Effect of constraint on fracture behavior of welded 17mn4 and aisi304 steels

In this study, 17Mn4 (P295GH) pressure vessels steel and AISI304 stainless steel were welded with ER309L austenitic consumable. In experimental part of the study, tensile tests were conducted on welded plates and variation of hardness values along specimen was measured. J-integral fracture toughness values were investigated for different crack locations. In order to determine the regions where plastic deformation did not take place due to constraint, uni-axial tensile test was performed on welded tensile specimen after attaching strain gauges. In numerical part of the study, finite element (FE) analyses were conducted by fixing 2-D models precracked on different locations by using ANSYS software. In these models, stress triaxiality and plastic deformation characteristics around crack tip were determined for each crack locations after stress — strain analyses. The limitation on the extension of plastic deformation at diffusion line causes extra increase in stress triaxiality at crack tip.     

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.     

橡胶纯剪试件变形与断裂的有限元分析

用非线性有限元法分析橡胶纯剪试件变形与理想纯剪变形之间的差异.结果表明试件中心接近纯剪,但是靠近自由边界的区域应力比较复杂,受自由边界影响的区域随应变的增加而减小,试件的宽高比越大变形越接近理想纯剪变形.对橡胶纯剪试件存在中心裂纹和边缘裂纹时的撕裂能进行计算,断裂形成大裂纹时撕裂能与裂纹尺寸无关,有限元解与以前的结果吻合较好;对于小裂纹,撕裂能与裂纹尺寸成线性关系.同时分析不同材料模型对橡胶纯剪试件变形和断裂的影响.分析表明,不同应变能函数的材料模型模拟的结果略有不同.     

6063铝合金在不同应力状态下的变形及损伤行为

用平板拉伸、缺口拉伸、剪切试验结合断口形貌观察及有限元模拟等方法,对6063铝合金在三种不同应力状态下的变形及损伤行为进行了研究.结果表明:在不同应力状态下该合金的变形及损伤行为存在明显的差异,剪切时材料的断裂应变远大于平板和缺口拉伸时的,而缺口拉伸时材料的屈服强度及峰值应力大于平板拉伸时的;平板拉伸时试样为拉剪混合型断裂,断口由一定比例的韧窝和剪切平面组成;缺口拉伸时试样有明显的颈缩,断口以韧窝为主;剪切断口以剪切平面为主;损伤行为的有限元模拟结果与试验结果吻合较好.     

Staging of a localized plastic deformation upon the tension of Д16AT alloy specimens on the basis of acoustic emission,surface deformation mapping,and strain gauging data. II. Specimens with notches of different depths

Studies of the development of deformations and damage in Д16AT alloy specimens with side notches, which modeled cracks in objects, were performed by combining the acoustic emission, surface deformation mapping, and strain gauging methods. It was shown that a significant portion of the loading time is associated with crack propagation, which is accompanied by the appearance of a segment with a negative value of the strain strengthening coefficient in loading diagrams. According to the data from the correlation of digital images at the plastic deformation stage (within stage 2), two substages were distinguished: the first is related to the deformation localization directly at the notch apex, while the second is associated with an increase in the area of this region to dimensions that are comparable with the specimen cross section. It was revealed that, in the case of a significantly localized deformation, the staging-based approach to the analysis of deformation and fracturing processes is characterized by the better correlation between data that are recorded by different in situ methods.     

应变硬化对冷轧301L不锈钢点焊接头疲劳强度的影响

将应变硬化程度不同、厚度均为1.5 mm的国产和进口301L不锈钢冷轧板采用相同的焊接工艺分别制成标准电阻点焊试样,进行了疲劳试验和微观结构分析.结果表明:应变硬化程度较高的国产板点焊试样疲劳极限低于应变硬化程度较低的进口板点焊试样;国产板点焊试样焊点边缘产生了折叠变形,该部位同时存在再结晶组织和应变硬化组织,所以不但有结构应力集中而且出现了组织性能不均匀所造成的应力集中,折叠变形和应力集中是造成疲劳极限降低的主要原因.     

Measurements of plastic strain in copper due to sliding wear

Wear experiments have been conducted to determine the plastic strains that are introduced in surface material near sliding wear tracks. Both oillubricated and dry-sliding experiments have been carried out at different sliding distances on surfaces of copper. The strain values were determined from selected-area electron channelling patterns obtained from regions as small as 10 μm in size and 0.05 μm deep around the wear track using a scanning electron microscope. A deformed calibration specimen was used to relate the electron channelling band contrast to the deformation strain. Strain maps were obtained on the wear surface lateral to the wear track and also below the surface using electropolishing metal removal techniques. Particular attention was given to the near-surface strain values. In all cases the maximum strain was found at the wear surface located at the track center and the strains decreased uniformly with depth. Significant large strains were also found outside the wear tracks. The results are compared with those previously reported for iron and with other relevant studies.     

直角切削过程中应变的有限元分析

基于材料模型,断裂准则,刀具与工件以及切屑的接触关系等建立了分析直角切削过程中应变的有限元模型。根据有限元分析结果分析了正常切削过程中3个变形区和两侧方向应变以及应变造成的主要影响,两侧方向应变形成了两侧方向毛刺;第一变形区和第三变形区的应变使工件表面产生了变质层;第一变形区和第二变形区形成切屑并影响切屑的应变分布。     

A methodology for measuring in situ lattice strain of bulk polycrystalline material under cyclic load

The material response of polycrystalline materials under cyclic loading is not fully understood. Even during uniaxial loading, individual grains embedded within the polycrystalline material can experience complicated strain histories. By quantifying the deformation state at the crystal level, we can begin to understand the conditions that lead to fatigue failure. An in situ powder diffraction method was developed and employed at the Cornell High Energy Synchrotron Source to measure the aggregate crystal response at various points in a material's life using synchrotron x ray. A set of experiments was conducted using a load frame capable of exerting cyclic uniaxial loads on a specimen. A high speed x-ray shutter was developed to synchronize the x-ray beam and the loading cycle. Using the high speed shutter, the evolution of the lattice strains for the families of crystallographic planes was measured while the aggregate was under cyclic uniaxial loading, thus monitoring a live evolution of lattice strain in a cyclically loaded specimen. The methodology is demonstrated using uniaxial cyclic specimens machined from oxygen free conductivity (OFHC) copper sheet.     

Comparison of convergent beam electron diffraction and geometric phase analysis for strain measurement in a strained silicon device

Convergent beam electron diffraction and geometric phase analysis were used to measure strain in the gate channel of a p-type strained silicon metal-oxide-semiconductor field-effect transistor. These measurements were made on exactly the same transmission electron microscopy specimen allowing for direct comparison of the relative advantages of each technique. The trends in the strain values show good agreement in both the [110] and [001] directions, but the absolute strain values are offset from each other. This difference in the absolute strain measured using the two techniques is attributed to the way the reference strain is defined for each.     

铝基复合材料─不锈钢摩擦焊焊接接头显微组织的电镜研究

利用电子显微镜技术系统地观察了铝基复合材料（ＭＭＣ）与奥氏体不锈钢之间摩擦焊焊接接头中的显微组织变化规律。发现强度较高的奥氏体不锈钢在焊接过程中发生了明显的塑性变形，焊接界面附近不同部位变形机制和特征不同，变形方式主要是形成形变孪晶、滑移带和位错亚结构。首次在透射电镜（ＴＥＭ）下观察到了一个由微晶氧化物组成的过渡层组织。     

Experimental evaluation of palm oil as lubricant in cold forward extrusion process

Today, vegetable oil is much desired for its application as a lubricant in metal forming processes, because it is a renewable resource and has high biodegradability compared to mineral oil. According to the Organization for Economic Cooperation and Development for the European Union 301C (OECD) testing method, the biodegradability levels of vegetable oils are better compared to petroleum-based lubricants. Palm oil is used more often than other vegetable oils. Therefore, palm oil has the potential to fulfill the demand for vegetable-based lubricants. The purpose of this paper is to evaluate the viability of palm oil when used as a lubricant in cold work such as the forward plane strain extrusion process. The performances of palm oil were compared with additive-free paraffinic mineral oil. Experimental work with a plane strain extrusion apparatus with a symmetrical workpiece was carried out at room temperature. The material of the workpiece is annealed pure aluminum A1100. The visioplasticity method was used to calculate the velocities and effective strain in the deformation zone of the workpiece. The results obtained from the experimental work showed that palm oil has satisfactory lubrication performances, as compared to paraffinic mineral oil, and has advantages in reducing the extrusion load.     

凸模直径以及试件形状对成形极限图的影响

成形极限图(FLD)反映了板料在各种应变路径下的应变极限,是综合衡量板料成形性能的一个重要指标。在用半球形凸模进行试验时发现,所用凸模直径和试件形状的不同对FLD有影响。采用两种不同直径的凸模和两组不同形状的试件进行了试验,获得了4种情况下的FLD,对比分析了凸模大小和试件形状对试验结果的影响。结果表明:采用直径较大的凸模得出的FLD低于采用小直径的凸模得到的FLD,试件形状对于FLD的左半部影响较大,中间带圆弧形的试件得出的FLD左半部低于中间部分平行的试件得到的FLD,试件形状对右半部FLD没有影响。     

The influence of strain hardening of polymers on the piling-up phenomenon in scratch tests: Experiments and numerical modelling

The aim of this study was to relate the scratching behaviour of polymers to their mechanical properties. A thermosetting resin (CR39) and a thermoplastic polymer (PMMA) were studied using a microscratch tester allowing in situ observation of the contact area. These two polymers exhibit different elastic and viscoplastic properties, the main difference being the large ability of CR39 to strain harden, whereas PMMA softens. A spherical indenter was used to vary the level of deformation imposed on the samples. The response was initially elastic, then viscoelastic and finally mainly viscoplastic with increasing penetration of the indenter into the material. The two polymers displayed the same response for small levels of deformation, while at larger strains PMMA showed more pronounced plastic behaviour. The origin of this difference in behaviour was investigated by means of a three dimensional finite element analysis. The rheology of PMMA and CR39 was simplified and modelled by assuming linear elastic behaviour and a viscoplastic law taking into account their strain hardening capacity at high strains. Strain hardening was found to be a key factor to correctly model the material flow around the indenter. The response of the polymers was governed by the ratio between the plastic and elastic strains involved in the deformation in the contact region. In first approximation, the representative strain was imposed mainly by the geometry of the indenter, while the elastic deformation was controlled by the mechanical properties of the material, a larger strain hardening leading to a greater elastic deformation and a lower plastic strain thus a better scratch resistance of the specimen.     

面向驱动桥壳再制造的磁记忆无损检测

为研究汽车驱动桥壳再制造的金属磁记忆检测,选用510L钢拉伸试样,分析了塑性应变不同阶段磁记忆信号的变化规律;以材料拉伸试验为基础,针对废旧驱动桥壳进行磁记忆损伤检测及变形量检测,得到了桥壳变形量与磁记忆信号特征值的关系。结果表明,随着应变量的增大,拉伸试样表面磁记忆信号法向分量Hp(y)由斜线、无异变峰变化为反转且出现过零点,磁记忆信号法向分量梯度Kmax早期缓慢增大,颈缩时出现快速增大;桥壳的变形检测显示,桥壳变形量与Kmax存在量化关系,当桥壳发生较大变形时,梯度Kmax发生突变,磁记忆检测可为再制造损伤评估提供依据。     

铀表面铝镀层热应力的有限元分析

对铀表面磁控溅射沉积铝镀层的热应力进行了热弹塑性有限元分析。结果表明：镀层内的热应力较大,达到铝的屈服强度,镀层界面两侧存在明显应力梯度,试样侧边存在由于边缘效应引起的应力分布不均匀性,至侧边约4倍镀层厚度处,应力分布不均匀性逐渐消失;沉积温度升高,界面塑性应变明显增大,镀层弹性模量和泊松比对镀层界面热应力和塑性应变的影响较小,而屈服强度的影响较大,减薄镀层厚度有利于改善镀层界面的热应力和塑性应变。     

Dynamic shear stress of tough-pitch copper at high strain and high strain-rate

Dynamic shear tests for the tough-pitch copper at high strain and high strain rate was performed. The Split Hopkinson Pressure Bar (SHPB) compression test system was modified to yield a shear deformation in the specimen. Hat-shaped specimens for the tough-pitch copper were adopted to generate high strain of γ=3–4 and high strain-rate of γ=10⁴/s. The dynamic analysis by ABAQUS 5.5/EXPLICIT code verified that shear zone can be localized in hat-shaped specimens. A proper impact velocity and the axial length of the shear localization region were determined through the elastic wave analysis. The displacement in a hat-shaped specimen is limited by a spacer ring which was installed between the specimen and the incident bar. The shear bands were obtained by measuring the direction of shear deformation and the width of deformed grain in the shear zone. The decrease of specimen length has been measured on the optical displacement transducer. Dynamic shear stress-strain relations in the tough-pitch copper were obtained at two strain-rates.