SHPB测试中的均匀性问题及恒应变率

利用一维应力波理论对霍普金森压杆(SHPB)测试中的均匀性问题作了较为详尽的讨论,对测试中各种加载波形的优缺点及各参数对均匀性的影响进行了分析与评估。给出了测试脆性材料时实现恒应变率加载的加载条件。对在满足应力均匀性要求下SHPB的可测应变率范围作了讨论并修正了前人不完善的结论。讨论了考虑均匀性时应采用的测试数据处理方法。利用图解的方法对弹塑性材料测试时的均匀性问题及相应加载要求作了定性分析,指出对弹塑性材料,测试中的应变不均匀也应予以考虑。     

高温下轻质泡沫铝动态力学性能实验

对传统的分离式Hopkinson压杆装置加以改进,设计了一种长杆直接撞击Hopkinson杆的实验方案,检测出低波阻抗材料在高温动态加载下的应力均匀性。对轻质泡沫铝材料的实验表明,在同一撞击速度下,温度越高,试件两端的应力均匀性越差,增加温度与提高撞击速度均会导致泡沫铝材料冲击端与支撑端的应力不均匀性。根据高温下应力均匀性的实验结果,确定高温下试件均匀变形对应的冲击速度,再通过传统的分离式Hopkinson压杆实验得出泡沫铝在高温动态下的力学性能。     

SHPB 实验数据处理的解耦方法

分析了ＳＨＰＢ试验中导致试件早期应力不均匀性的主要因素，指出试件中应力波的波速是与试件材料的应力应变曲线耦合在一起的，提出了考虑应力不均匀性的解耦方法，这种解耦方法具有普遍性，可广泛用于ＳＨＰＢ实验数据处理。     

关于支柱不均匀沉陷对扁壳的影响问题

本文討論了以双鉸拱支承的四边簡支扁壳在支柱发生不均匀沉陷时所产生的应力。我們首先給出了线性問题的精确解,然后又研究了不均匀沉陷較大的非线性問題。对于非縐性問題,我們把不均匀沉陷产生的应力分为两部分: (1)无載荷时不均匀沉陷产生的应力; (2)有載荷时不均匀沉陷产生的附加应力。 对于第一部分应力給出了理論上足够精确的公式;对于第二部分应力給出了估計量級的公式。 結果表明:选择适当的支承方式可使不均匀沉陷在壳体中产生的应力很小。本文結果亦适用于計算地面浇注的壳体在頂升时四角点高度不在同一平面內时所产生的应力。     

橡胶圆柱体的平面外剪切变形分析

利用Gao Y C给出的一类不可压缩的应变能函数,分析了不均匀橡胶圆柱体受平面外剪切变形问题.通过平面外剪切变形问题的变形模式,并结合应力和位移的边界条件以及界面连续条件得到了位移场和应力场在瞬时构型中的解析解,并讨论了材料的不均匀性对位移场及应力场分布的影响.结果表明:不均匀模型可以方便地退化成均匀模型;橡胶圆柱体的不均匀性对位移场和应力场的分布有着重要影响.     

浇铸类炸药应力应变曲线的SHPB测量

浇铸类炸药由于质地软、波阻抗及波速都很低，通过传统SHPB实验方法无法得到准确的应力应变数据。透射杆信号幅值过低、试样应力平衡均匀性不高以及大应变加载引起的入射波反射波重叠失效，是进行浇铸类炸药SHPB实验的难点所在。本文中对传统SHPB实验方法进行改进，在试样两端面加装石英晶体应力计，引入石英计所获得的应力数据与应变片测得数据共同对试样应力应变状态进行计算。该方法可以提高透射信号幅值，提供试样大应变加载，避免了入射波反射波重叠导致的信号失效问题，修正了SHPB实验过程中的应力时空不均匀性的影响，提高了实验结果的可靠性。利用改进后的实验方法对典型浇铸类炸药进行了实验研究，得到了较准确的应力应变曲线。     

肌肉类粘弹性超软材料SHPB实验的应力应变均匀性分析

试样的轴向应力和应变均匀问题是霍普金森杆实验行之有效的必要条件,而这对肌肉类粘弹性超软材料尤为重要.本文用量纲分析法,分析出了影响粘弹性材料SHPB实验应力和应变均匀性的无量纲量,结合LS-DYNA数值模拟方法,考察了各无量纲量对粘弹性材料均匀性的影响规律.结果表明,为使肌肉类材料SHPB实验更早达到均匀条件,理想的入...     

SHPB试验中粘弹性材料的应力均匀性分析

采用特征线解法，对满足ZWT方程的粘弹性材料在高应变率SHPB试验中的应力均匀性进行了数值研究。着重分析了不同的材料本构粘性(松弛时间)、瞬态波阻抗比和入射波升时对于试样中应力均匀性、应变均匀性和平均应变率等的影响。为今后动态试验的试样设计提供了一定的理论依据。     

La30Ce30Al15Co25金属玻璃应力松弛行为

作为潜在的工程材料, 金属玻璃在材料科学和凝聚态物理等领域引起广泛的研究兴趣. 金属玻璃结构与性能的关系表明, 金属玻璃的动态非均匀性与其黏弹性和塑性紧密相关. 然而, 宏观应力松弛行为与动态弛豫之间的物理图像并不清晰. 与传统金属材料不同, 金属玻璃的变形机理非常复杂. 应力松弛是一种表征玻璃体系黏弹性和塑性变形机制的有效手段, 从而探索结构和动态非均匀性. 本研究以La₃₀Ce₃₀Al₁₅Co₂₅金属玻璃为模型体系, 在较宽的温度窗口研究了其应力松弛行为. 研究结果表明, 与传统金属玻璃不同, La₃₀Ce₃₀Al₁₅Co₂₅金属玻璃具有明显的β弛豫行为. 基于Kohlarausch-Willams-Watts (KWW)方程的分析表明, 金属玻璃应力松弛为动态不均匀过程; 热动力学分析发现La₃₀Ce₃₀Al₁₅Co₂₅金属玻璃应力松弛存在显著的双阶段行为, 即从高应力条件下应力驱动为主导的松弛行为, 向低应力下热激活为主导的松弛行为发生转变. 通过激活能谱模型分析表明, 应力松弛单元的激活并非均匀, 而是存在能量上的起伏, 金属玻璃对于外力响应是一个渐进过程, 具有动力学不均匀性. 本研究进一步构建了金属玻璃的结构和动态非均匀性之间的关联, 为研究金属玻璃的α弛豫和β弛豫提供了强有力的支撑.      

La30Ce30Al15Co25金属玻璃应力松弛行为

作为潜在的工程材料,金属玻璃在材料科学和凝聚态物理等领域引起广泛的研究兴趣.金属玻璃结构与性能的关系表明,金属玻璃的动态非均匀性与其黏弹性和塑性紧密相关.然而,宏观应力松弛行为与动态弛豫之间的物理图像并不清晰.与传统金属材料不同,金属玻璃的变形机理非常复杂.应力松弛是一种表征玻璃体系黏弹性和塑性变形机制的有效手段,从而探索结构和动态非均匀性.本研究以La_(30)Ce_(30)Al_(15)Co_(25)金属玻璃为模型体系,在较宽的温度窗口研究了其应力松弛行为.研究结果表明,与传统金属玻璃不同,La_(30)Ce_(30)Al_(15)Co_(25)金属玻璃具有明显的β弛豫行为.基于Kohlarausch-Willams-Watts(KWW)方程的分析表明,金属玻璃应力松弛为动态不均匀过程;热动力学分析发现La_(30)Ce_(30)Al_(15)Co_(25)金属玻璃应力松弛存在显著的双阶段行为,即从高应力条件下应力驱动为主导的松弛行为,向低应力下热激活为主导的松弛行为发生转变.通过激活能谱模型分析表明,应力松弛单元的激活并非均匀,而是存在能量上的起伏,金属玻璃对于外力响应是一个渐进过程,具有动力学不均匀性.本研究进一步构建了金属玻璃的结构和动态非均匀性之间的关联,为研究金属玻璃的α弛豫和β弛豫提供了强有力的支撑.     

Numerical-graphical method for determining characteristics of damaged viscoelastic materials

A method for determining the material functions of nonlinear endochronic theory of aging viscoelastic materials (NETAVEM) with preliminary mechanical damage was developed. The proposed method is based on an analysis of the differences between two graphs of the stress dependence on time obtained in tension with the same constant speed of two specimens made of the same filled polymer material. One of the specimens was not preloaded, and the other was preloaded. The reduced time [1] contained in the NETAVEM constitutive relations and its dependence on the actual time are determined by the distances from the stress axis to two points corresponding to the same stress value and lying on the graphs for the damaged and undamaged specimens. The relaxation kernel is determined in the experiment with the undamaged specimen. These two material functions and the curve obtained for the damaged specimen are used to obtain the NETAVEM aging function, and then the function of viscosity can be calculated. As a result, all characteristics of the damaged material become known, and the strength of structures made of this material can be calculated.     

应力平衡和接触状态在Hopkinson杆测试聚合物动态弹性模量中的影响研究

针对用Hopkinson杆试验能否准确测量聚合物动态弹性模量以及其中主要影响因素的问题,本文基于重构试样初始加载阶段的应力波反射透射过程,分别计算了6个特征时间内的前三次反射波和透射波,得到试样的应力平衡度和试样的应力应变曲线。对于所研究的聚合物材料,通过比较重构的应力应变曲线的弹性模量与输入的材料弹性模量,发现在4个特征时间后,误差仅在3%左右。因此试样变形过程中的应力平衡与否不是材料在Hopkinson杆试验中弹性模量测不准的原因。通过环氧树脂试样试验发现,根据Hopkinson杆理论计算的应变结果要大于试样上应变片实测的结果,误差在11%左右。相应的数值模拟研究发现:试样和杆子端面接触状态直接决定着试样弹性模量测量的精度。关于惯性效应和压痕效应的研究也证实它们的影响是可以忽略的。     

SHPB帽形试样尺寸效应研究

利用ANSYS/LS-DYNA软件开展不同尺寸帽形试样的SHPB数值模拟，研究帽形试样的尺寸效应。结果表明，当帽形试样剪切变形区域宽度大于0.2 mm时，根据经典公式处理得到的应力值将随t的增大而显著增大, 严重偏离理论值，当t=1.0 mm时，计算得到的应力值甚至接近理论值的2倍。采用分体模型进行的进一步计算表明，数据处理结果中应力偏差主要来源于帽形试样中环状部位的膨胀变形。提出改进的数据处理方法，并采用圆柱样与几种不同尺寸帽形试样开展了验证实验，结果与计算结果基本一致。     

Anisotropic thermal conductivity in sheared polypropylene

We discuss the anisotropy of the thermal conductivity tensor in polymer flow in this paper. Isotactic polypropylene (iPP) specimens were deformed by injection moulding at high shear rates and by steady shear at low shear rates, and were then quenched. The thermal conductivities parallel and perpendicular to the shear direction were measured using modulated differential scanning calorimetry (MDSC) in accordance with the ASTM E1952-01. The measured results showed that the thermal conductivity of the sheared polymer was anisotropic with an increase in the shear direction. The thermal conductivity can be regarded as varying either with the strain or the stress, as suggested by Van den Brule (1989). In addition to the Van den Brule mechanism, crystallization during flow also changes the thermal conductivity and this effect may often be dominant. Suggestions for procedures in processing computations, based on both effects, are given.     

老化对PP/SSFs导电复合材料结构及 应力松弛性能的影响

导电高聚物复合材料具有柔性好、导电性可调、容易成型及生产成本低等优点,并具有抗静电、电磁屏蔽/吸波、压力/温度敏感性 等特点,可以作为功能材料在诸多领域应用. 但是在加工、储运和使用过程中,由于多种因素综合影响,不可避免发生老化并导致其性能劣化. 通过熔融共混、注塑成型的方法制备不锈钢短纤维(SSFs)填充聚丙烯(PP)导电复合材料,将其进行加速湿热老化和紫外老化,测定应 力松弛曲线以及松弛条件下电阻率变化规律,通过X射线粉末衍射仪分析老化前后结晶度的变化,借助扫描电子显微镜(SEM)观察研究老化前后材 料的微观形貌,并进行能谱(EDS)分析. 研究结果表明,应力松弛曲线有明显的三阶段特征,且湿热老化材料松弛后的应力水平降低. 材料的初始电阻率随填料含量的增加而降低,老化使材料的初始电阻率增大. 由于导电高聚物的压阻效应,使得达到应力松弛起始应力前,即材料在获得特定应力的加载阶段,电阻率随载荷增加显著降低,而在随后 的松弛阶段,材料电阻率则趋于一个稳定值并在较小的范围内波动. SEM/EDS分析结果表明,随着老化时间的延长,试样表面氧元素含量增加,且氧元素含量随距表面深度的增加而减小. XRD分析结果表明,材料的结晶度随填料含量的增加而减小,随老化时间的延长也呈减小趋势. 研究结果可为研究导电高聚物复合材料的老化性能提供实验依据.      

Non-isothermal developing flow of a power-law fluid in a converging slit

A numerical solution of the developing non-isothermal flow of a generalised power-law fluid in a slightly converging slit is presented, a problem which is relevance to some polymer processing operations.The results are presented in graphical form. They indicate that inertia forces and kinetic energy increases along the slit and these factors affect the development of the velocity and stress distributions. The effect of other parameters on the process is also brought out.     

Cyclic viscoelastoplasticity and low-cycle fatigue of polymer composites

Observations are reported on a polymer composite (polyamide-6 reinforced with short glass fibers) in tensile relaxation tests with various strains, tensile creep tests with various stresses, and cyclic tests with a stress-controlled program (ratcheting with a fixed maximum stress and various minimum stresses). Constitutive equations are developed in cyclic viscoelastoplasticity of polymer composites. Adjustable parameters in the stress–strain relations are found by fitting observations in relaxation tests and cyclic tests (16 cycles of loading–unloading). It is demonstrated that the model correctly predicts experimental data in creep tests and dependencies of maximum and minimum strains per cycle on number of cycles up to fatigue fracture of specimens. The influence of strain rate and minimum stress on number of cycles to failure is studied numerically.     

A method for measuring the creep behavior of pressurized polymer tubing

Polymer components have been proposed for use in domestic solar hot-water heating systems. A polymer heat exchanger is under development for such systems. For heat transfer considerations, the heat exchanger will comprise many thin-walled tubes. The heat exchanger must survive 10 years of service at high pressure (1.55 MPa) and high temperature (82°C). A novel method has been developed for evaluating the long-term performance (creep) of the polymer tubing. Traditional creep testing, performed with dog bone test specimens, cannot be applied because the thin-walled tubing has anisotropic material properties. Consequently, performance must be evaluated directly on the extruded tubing. The method entails wrapping a Constantan wire around the tube specimen to continuously record the hoop strain. For pressure loading of tubing, this method offers significant improvements over strain gage instrumentation. In this paper, the test method is described, an analysis of the strain transfer between the tubing and wire wrap is presented, and strain data for polypropylene tubing measured with a strain gage and wire wrap are compared. The data show that the wire measurement method can be successfully used for the characterization of long-term mechanical behavior of polymer tubes.     

Prediction of nonlinear viscoelastic behavior of polymeric composites using an artificial neural network

Creep tests at constant stress are performed for the carbon-fiber reinforced epoxy composite at various temperatures and initial stresses. A nonlinear viscoelastic constitutive model is developed, and its material parameters are determined by fitting it to creep test data. Model results are found to agree very well with the experimental data at low temperature and low stress conditions. However, the agreement deteriorates at high temperatures, particularly in the vicinity of the glass transition temperature.An alternative model based on an artificial neural network (ANN) is developed to predict the stress relaxation of the polymer matrix composite. The ANN model is trained and validated with 9000 experimental data sets obtained from stress relaxation tests performed at various constant strain (initial stress) and constant temperature conditions. Training of the ANN employs a scaled conjugate gradient method. The optimal brain surgeon algorithm is employed to optimize the topology. The optimal ANN configuration has 88 processing elements (3 in the input layer, 45 in the first hidden layer, 39 in the second hidden layer, and 1 in the output layer) and 410 links. The predictions of the ANN model are found to be more accurate over a wider range of stress and temperature conditions than those of the explicit nonlinear viscoelastic model, in particular near the glass transition temperature.