NiTi形状记忆合金切削仿真变形行为研究

利用ABAQUS软件对NiTi形状记忆合金不同切削速度和进给量时的切屑和已加工表面层热力载荷分布进行仿真分析。结果表明：切削速度和进给量增加都会导致切屑温度升高,塑性应变和损伤加剧;切削速度对温度影响较大,而进给量对塑性应变和损伤影响明显;切削速度增加,已加工表面层温度变化速率变大,表层残余应力增大,深层残余应力减小;进给量增加,应变层深度增加,残余应力逐层变大。     

航空有机玻璃加工工艺对其残余应力影响的光弹性技术研究

航空有机玻璃在使用过程中的破坏主要是由于残余应力导致疲劳裂纹所致。残余应力会使航空有机玻璃材料链段或基团取向进而产生双折射效应。主要基于残余应力导致的双折射效应,运用数字相移光弹性法实现由钻孔工艺产生残余应力的无损检测,并对测试结果进行分析得出:钻头进给速度越小,加工时间越长,冻结的残余应力越大;而钻头转速越大,加工温度越高,冻结的残余应力越大。控制装卡使各个方向受到相同的约束,能够使孔周残余应力均匀,防止出现较大应力集中区域。加工直径稍大的孔时,应采用分级钻孔的方式,先钻小孔然后逐级扩大,能够保证孔周残余应力低。     

大块非晶合金Zr55Al10Ni5Cu30在过冷液相区中的性能

通过DSC实验确定了大块非晶合金Zr55A110Ni5Cu30的过冷液相区，并由等温DSC曲线得到此合金在过冷液相区中各个温度下可用于塑性加工的时间的估算公式。在前人研究的基础上，提出一种得到此合金在过冷液相区中的流动应力的新的测试方法：变应变速率压缩实验。这种方法可以由较少的试样得到较多的应力数据。这些试验为将来塑性加工中应变速率、加工温度、加工时间的选择提供了依据。     

拉伸曲线及形变硬化指数在塑性加工中的应用与探讨

本文从探讨影响材料加工性能的因素出发,论述了塑性材料在轴向拉伸条件下变形过程的力学行为,阐明了应变硬化、最大均匀真塑性应变、应变硬化指数的相互关系.通过真应力真应变曲线确定应变硬化指数,明确其数值与材料拉伸变形时的最大均匀应变相等,是衡量材料塑性均匀变形能力的力学指标.     

多元微合金化铜基形状记忆合金的超弹性及超塑性性能研究

用多组无微量合金元素Ti、B、Ce、Nb和Zr细化铜锌铝形状记忆合金的晶粒，改善了合金的加工性能、力学性能和形状记忆效应，联合添加Ti、B和Ce，其伪（超）弹性应变值在250℃温度下可稳定在4.5％左右;在500～750℃的温度下具有δt=112％～200％的良好超塑性性能，展示了良好的热加工性能。     

温度及应变速率对TWIP钢拉伸性能的影响

通过拉伸试验研究了温度和应变速率对孪生诱发塑性(TWIP)钢拉伸性能的影响。结果表明:随着温度升高,TWIP钢的强度逐渐下降,断后伸长率逐渐增加,应变硬化指数随真应变增加达到并维持在较高的水平,试验钢的均匀变形能力得到提高,宏观塑性增加;随应变速率增大,试验钢的流变应力升高,断后伸长率下降,抗拉强度基本保持不变,应变硬化率曲线上的平台区长度明显变短,这表明孪生受到抑制,较早达到硬化极值;应变硬化指数峰值随应变速率的增大而减小,TWIP钢的均匀变形能力及宏观塑性下降。     

AZ31镁合金的热变形行为及加工图

用Gleeble-1500D型热模拟试验机对AZ31镁合金在变形温度为473～723 K,应变速率为0.001～1 s~(-1)下的热变形行为进行了试验研究;用双曲正弦关系式描述了该合金在热变形过程中的稳态流变应力;根据合金动态模型,计算并分析了该合金的加工图。结果表明:利用加工图可确定出该合金热变形的流变失稳区,并且得到试验条件下热变形的最佳工艺参数,即变形温度为473～523 K,应变速率为0.01 s~(-1)左右;当变形温度为623～673 K、应变速率为0.001～0.006 s~(-1)时可进行超塑性加工;当变形温度高于673 K后可在较宽的应变速率范围进行热加工。     

Anand模型预测63Sn37Pb焊锡钎料的应力应变行为

通过在温度313K～398K、应变率10^-3％／s～10％／s内的一系列恒应变率拉伸实验,研究63Sn37Pb焊锡钎料的力学行为,发现该材料的应力应变关系与温度和应变率有很大的相关性。采用统一型Anand粘塑性本构方程对该材料在较大温度和应变率范围内的应力应变行为进行数值模拟。结果表明Anand粘塑性方程可以有效地描述63Sn37Pb焊锡钎料在10％应变下的温度和应变率相关粘塑性本构行为。     

多元微合金化铜基形状记忆合金的超弹性及超塑性性能研究

用多组元微量合金元素Ｔｉ、Ｂ、Ｃｅ、Ｎｂ和Ｚｒ细化铜锌铝形状记忆合金的晶粒，改善了合金的加工性能、力学性能和形状记忆效应，联合添加Ｔｉ、Ｂ和Ｃｅ，其伪（超）弹性应变值在２５０℃温度下可稳定在４．５％左右；在５００￣７５０℃的下具有δｔ＝１１２％￣２００％的良好超塑性性能，展示了良好的热加工性能。     

S30408碟形封头应变强化特性及极限载荷分析

利用磁性测量仪FMP30测量不同塑性应变量下S30408板状拉伸试件形变马氏体体积分数,确定形变马氏体体积分数与塑性应变的关系.测量某低温液体半挂车罐体内容器碟形封头上各区域的形变马氏体体积分数,并对马氏体体积分数沿经向的分布规律进行简化,据此推算封头各部位加工历史造成的塑性应变.根据封头各部位的塑性应变确定考虑加工历史时各部位材料的应力应变曲线,并通过非线性有限元计算确定结构的极限承载能力.与不考虑加工历史仅考虑真应力应变关系以及不考虑材料强化仅按弹性理想塑性材料计算的结果相比,得到以下结论:对于S30408碟形封头,考虑材料应变强化特征计算得到的垮塌载荷与极限载荷均明显高于按弹性理想塑性模型计算的结果.加工历史造成的应变强化对结构的垮塌载荷影响不大,但对按双切线法确定的极限载荷影响较大,可较明显地提高结构的极限载荷.     

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.     

Evaluation of mechanical characteristics of the plate-type polymer hyperfine pit pattern by the nanoindentation process

It’s important to measure quantitative properties about the thermal-nano behavior of polymers in order to produce high quality components using the nanoimprint lithography process. Nanoscale indentation can be used to make the cells for molecular electronics, drug delivery, slots for integration into nanodevices and defects for tailoring both the structure and properties. In this study, the formability of polymethylmetacrylate (PMMA) and polycarbonate (PC) were characterized. Thermo-mechanical properties during formation at a high temperature. Polymers become softer at elevated temperature due to heating. In this case it is particularly important to study the high temperature-induced mechanical properties of the polymer. Nanoindentation was used to measure the thermo-mechanical properties of both PMMA and PC. The polymer was heated with the heating stage on a NanoXP. For a CSM (Continuous Stiffness Method) mode test, the heating temperature was 110°C, 120°C, 130°C, 140°C and 150°C for the PMMA, and 140°C, 150°C, 160°C, 170°C and 180°C for the PC. The maximum indentation depth for this test was 2000 nm. For the basic mode test, the heating temperature was 90°C and 110°C for the PMMA, and 140°C and 160°C for the PC. The maximum loads for this test were 10 mN, 20 mN and 40 mN. An indented pattern was also observed by using AFM. The pile-up phenomenon was mitigated due to the indentation at elevated temperature but the sink-in phenomenon occurred in this instance. When patterning at a high temperature, one should consider the variation in the indentation profile and depth after unloading when designing a structure. It was thought that the mechanical properties decrease when the working temperature increases because PMMA and PC are thermoplastics which soften or melt by heating. Further research in this area is required about the molecular weight and molecular movement at elevated temperature when the free volume of molecules increases.     

Effects of Temperature on the Microscale Adhesion Behavior of Thermoplastic Polymer Film

Adhesion tests were carried out under various temperature conditions to investigate the effects of temperature on adhesion behavior between a fused silica lens and a poly(methylmethacrylate) (PMMA) film, materials used for molds and as thermoplastic polymer films in nanoimprint lithography (NIL). The pull-off force and the contact area were measured as the temperature of the PMMA film was increased from 300 to 443 K, and as it was decreased back to 300 K. As the temperature increased, the PMMA film changed from a glassy state to a viscous flow state, and the adhesion behavior showed significant variation corresponding to this change in state. In the glassy state (below 363 K), the pull-off force did not change noticeably as the temperature increased. In the rubbery state (383–413 K), the pull-off force increased significantly as the temperature increased. In addition, the contact area was enlarged. In the viscous state (above 423 K), fingering instability was observed in the contact area and the pull-off force decreased, while the contact area increased, as the temperature increased. The adhesion behavior was also found to vary markedly with thermal history of the PMMA film. The causes of these changes are discussed.     

用有机玻璃的记忆性研究塑性变形的应变分布

有机玻璃是一种具有形记忆性的热塑性材料。本文利用有机玻璃对圆柱体镦粗和直齿圆柱齿轮精锻的变形状态进行模拟实验,将变形后的试样没子午面剖分,在一个剖分面上划上网格,然后使其恢复原状,利用恢复后的网格尺寸变化反向计算应变分布。     

Prediction of cement volume for vertebroplasty based on imaging and biomechanical results

Control of bone cement volume (PMMA) may be critical for preventing complications in vertebroplasty, the percutaneous injection of PMMA into vertebra. The purpose of this study was to predict the optimal volume of PMMA injection based on CT images. For this, correlation between PMMA volume and textural features of CT images was examined before and after surgery to evaluate the appropriate PMMA amount. The gray level run length analysis was used to determine the textural features of the trabecular bone. Estimation of PMMA volume was done using 3D visualization with semi-automatic segmentation on postoperative CT images. Then, finite element (FE) models were constructed based on the CT image data of patients and PMMA volume. Appropriate material properties for the trabecular bone were assigned by converting BMD to elastic modulus. Structural reinforcement due to the changes in PMMA volume and BMD was assessed in terms of axial displacement of the superior endplate. A strong correlation was found between the injected PMMA volume and the area of the intertrabecular space and that of trabecular bone calculated from the CT images (r-0.90 and −0.90, respectively). FE results suggested that vertebroplasty could effectively reinforce the osteoporotic vertebra regardless of BMD or PMMA volume. Effectiveness of additional PMMA injection tended to decrease. For patients with BMD well lower than 50mg/ml, injection of up to 30% volume of the vertebral body is recommended. However, less than 30% is recommended otherwise to avoid any complications from excessive PMMA because the strength has already reached the normal level.     

An attempt to determine the true stress-strain curves of amorphous polymers by nanoindentation

We have made an attempt to identify the true material stress-strain curves of some amorphous polymers (PC, PMMA, CR39 (diethylene glycol bis(allyl carbonate)) from nanoindentation experiments by using non-self similar tip shapes, i.e. tips promoting an increasing strain with an increasing indentation depth. We firstly found that, for a defined strain rate, the relationship linking the contact pressure to the ratio between the contact depth and the contact radius is intrinsic to the material, i.e. it doesn’t depend on the tip shape used. From this relationship, for each material, we calculated the evolution of the average stress with the average strain beneath the tip, by using the theoretical background developed for elasto-perfectly plastic materials. By comparing to compression test results, we concluded that this approach works well for PC and PMMA only for an average strain below 12%. This approach is inapplicable for the CR39 material even at low strains. We assumed that this is mainly due to the occurrence of a significant strain hardening, that doesn’t allow us to neglect the strain gradient existing beneath the tip.     

嵌段共聚物F68增韧改性PMMA的性能

采用嵌段共聚物F68对聚甲基丙烯酸甲酯(PMMA)进行原位增韧改性,研究了改性PMMA的力学性能、光学透明性和断面形貌,并探讨了其改性机理。结果表明:F68对PMMA有较好的增韧作用,随着F68含量的增加,改性PMMA的冲击强度不断增大,当F68的质量分数为20%时,其冲击强度提高了117.61%;改性PMMA的储能模量在玻璃化转度温度以上时随着F68含量的增加不断增大,而损耗因子的峰值温度逐渐降低;改性PMMA具有较好的光学透明性能,当F68的质量分数在20%以内时,改性PMMA透光率的降低小于2%。     

应变片温度特性检测

涡桨发动机功率输出轴载荷测量过程中,环境温度变化范围大,如果应变片灵敏度系数变化,将给试验带来误差,影响试验结果,因此,必须检测应变片的温度特性是否稳定。基于某型螺旋桨飞行试验的应变片温度特性试验检测方法和试验过程,对试验结果进行比对分析,得出应变片温度试验中需要注意的事项,应变片特性试验检测结果有效可行。     

基于桥梁结构的FBG传感器温度与应变交叉敏感问题的研究

对光纤布拉格光栅(FBG)传感器在桥梁结构健康监测中产生的温度与应变交叉敏感问题进行了研究。采用参考光纤光栅法在应变传感光纤光栅附近额外加入一个温度测量光纤光栅,对应变光栅实现温度补偿功能。设计了基于参考光纤光栅法的FBG传感器及FBG传感器封装的机械结构,并通过实验来验证FBG传感器的性能。实验数据表明,温度传感光纤光栅几乎不受应变的影响,应变传感光栅的中心波长变化与温度变化呈一阶线性关系,修正后的测量结果更加精确,达到了双参数同时测量的目的,应变与布拉格波长的线性关系非常好,相关系数达到0.99以上。参考光纤光栅法能够很好地解决FBG传感器温度与应变交叉敏感的问题。     

Effect of curing characteristics on residual stress generation in polymethyl methacrylate bone cements

Residual stresses resulting from the shrinkage of polymethyl methacrylate (PMMA) bone cement have been implicated in the formation of cracks in cement mantles following total hip arthroplasty. This study investigates whether two such cements, with differentiated solidification characteristics (i.e. working and setting times), display significant differences in their residual stress characteristics in an experiment designed to replicate the physical conditions of total hip arthroplasty. Experiments were performed using a representative femoral construct to measure and compare the temperatures and residual strains developed for standard PMMA cement mantles (CMW 1 Gentamicin) and slow curing cement mantles (SmartSet HV Gentamicin) during and following polymerization. These experimental results revealed no statistically significant difference (t-test, p > 0.05) for peak exotherm temperature and residual strain levels between the cements (measured after 3 h). The tailored polymerization characteristics of the slow-curing cement do not significantly affect residual stress generation, compared with the standard cement. It is often considered that residual stresses significantly relax following polymerization and before biomechanical loads are first applied during rehabilitation (up to 3 days later). This was examined for durations of 18 h to 3 days. Axial strains in the model femur and stem reduced by averages of 5.5 and 7.9 per cent respectively, while hoop strains in the stem exhibited larger reductions. An axisymmetric transient thermoelastic finite element model of the experiment was developed, allowing residual stresses to be predicted based on differential scanning calorimetry (DSC) measurements of the heat released throughout the exothermic curing reaction. The model predictions closely replicated the experimental measurements of both temperature and residual strain at 3 h, suggesting that residual strains can be fully accounted for by the thermal contraction mechanism associated with cooling after solidification.