高聚物粘弹性力学模型的等当性

以描述高聚物粘弹性的三元件模型之一的弹簧与沃-开氏并联单元串接的为例,通过运动方程式从而推导出应力松弛模量G(t)的表达式,用初级的数学替代就可把这个G(t)转换为另一个形式,正好对应于弹簧与麦氏串联单元并联的关系,说明弹簧与伏-开氏并联单元串接的和弹簧与麦氏串联单元并联的力学模型互为等当,它们描述的是同一个高聚物粘弹性现象。模型的这种等当性为人们提供了很大的方便,我们可以根据使用的方便而任意选择其中的任一个模型。     

形状记忆纤维热粘弹性基体复合材料的力学行为

应用热粘弹性理论和Voigt混合律,在变温场中针对马氏体逆相变过程建立了NiTi形状记忆纤维热粘弹性基体复合材料的应力-应变关系,在逆相变过程和基体呈现热粘弹态阶段,由于基体松弛其模量减小,在跃阶拉应力的作用下,复合材料的压缩应变迅速增大,纤维回复应力先增大后减小;在跃阶拉应变的作用下,复合材料的应力增加先变缓然后加快直至稳定,较高的温度和材料参数对NiTi纤维热粘弹性基体复合材料的力学行为和纤维的作动性能有明显的影响.     

高分子熔体和浓溶液流变性能的研究Ⅰ．具有高斯链限制作用的非线性粘弹性分子理论

根据多重缠结模型和多重蠕动机理，用统计力学和动力学相结合的方法，建立了具有缠结限制作用的非线性粘弹性分子理论，计算了处于多重缠结态高分子链的构象统计，得到了高聚物熔体的粘弹性形变自由能，推导出了４种简单形变方式的应力－应变关系。从理论和实验上证实了非线性粘弹理论的时间效应和形变影响是非相关地相互独立的，得到了高聚物熔体和非线性粘弹性模量和柔量的表达式，当其松弛模量和推迟模量中ｎ＇＝１时，相应模量和柔量表达式分别还原为线性粘弹性的表达式。该理论能较好地预测高聚物熔体的非线性和线性粘弹性力学行为，并成功地预测到η０∝Ｍ￣（３．３～３．７）的经验关系式。     

狗的多位点心肌拉伸性质的实验研究

介绍了自行研究和组装的测试心肌组织试件基本力学性质的实验设备和方法。完成了对6个狗心脏10个不同位点的肌肉组织的三种力学性质测试,包括:单轴拉伸、松弛和破坏实验,得到了这些位点心肌组织的应力-应变曲线、应力松弛曲线和强度极限。从实验数据中可知:左心室心外膜的刚度和强度最大,右心房侧壁、右心房心耳、房间膈的刚度和强度最小,其余位点介于两者之间。根据Pole-zero模型对所得拉伸数据进行了参数拟合,试验数据和模型之间的相关性较好。根据松弛实验的结果对Pole-zero模型进行了修正,增加了一个粘弹性项。     

介电弹性体力学松弛的试验与理论研究

针对介电弹性体的粘弹性应力松弛现象,通过单轴拉伸的应力松弛试验研究了材料应力在不同预拉伸率下的时变特征,建立了材料基于Prony级数的准线性粘弹性模型。研究结果表明:通过理论计算和实验结果的对比验证了模型的准确性,预拉伸率是影响介电弹性体薄膜应力松弛曲线的重要因素,基于二阶Prony级数的准线性粘弹性模型能够有效模拟介电弹性体薄膜预拉伸率在[2,6]区间内的非线性力学松弛特性,而且与实验值拟合度较高;在预拉伸率为4,400s条件下,介电弹性体薄膜的应力达到0.126MPa。     

高聚物材料动态本构关系对PP/PA共混物的应用研究

用霍布金森实验技术(SHPB)研究了两种PP/PA共混高聚物材料在高应变率(102～103s-1)下的粘弹性力学行为.实验表明,PP/PA共混高聚物材料的动态应力-应变曲线能用一个简单实用的非线性粘弹性本构方程(ZWT方程)较好地表达;通过最小二乘法拟合了材料参数;在加载过程中,理论计算值与实验结果较吻合.     

热致非晶态形状记忆聚合物的热粘弹性参数及回复行为

分别采用非线性拟合应力松弛主曲线及单个常频率条件下变温动态力学分析(DMA)实验中储能模量与损耗角正切曲线的方式,获取热致非晶态形状记忆聚合物(SMP)多重松弛模型中的热粘弹性参数。结合广义有限变形粘弹性理论和KAHR33参数热变形模型在有限元软件平台研究了该SMP的自由回复与约束回复行为。仿真结果与实验数据相符较好,说明该建模方法的合理性及获取的热粘弹性参数的准确性。相比构建应力松弛和储能模量主曲线而言,单个DMA实验数据更易得到,因此对其拟合能大幅减少实验成本。     

基于细观力学的纤维沥青混凝土有效松弛模量

为了研究纤维沥青混凝土的本构模型,将其视为以沥青混合料为粘弹性基体,纤维为弹性夹杂的两相复合材料。对基于复合材料细观力学理论建立的有效模量表达式进行了修正,提出了纤维沥青混凝土的割线有效松弛模量。以聚酯纤维沥青混凝土为例进行了有效松弛模量的解析分析和模拟蠕变实验的有限元分析,分析结果与试验数据的比较表明,该文提出的割线有效松弛模量模型对于纤维沥青混凝土粘弹性力学行为具有很好的预测能力。应用该模型对路面弯沉变形进行了有限元分析,结果表明:纤维的加入有效的改善了沥青混凝土路面的粘弹性性能。     

沥青混合料热粘弹性本构关系试验测定及其力学应用

基于热粘弹性力学理论,就不同的温度条件下沥青混合料的应力松弛特征开展了试验研究,应用热流变简单材料的时温等效原理对试验结果进行了分析和参数拟合,根据试验结果建立了描述沥青混合料粘弹特性的广义Maxwell模型;通过理论推导提出了沥青混合料非定常和非均匀变温条件下增量型热粘弹性本构关系,在此基础上,给出应用本构关系进行沥青路面热粘弹性力学分析的数值实现方法;通过对TSRST试验的模拟,对得到的沥青混合料热粘弹性本构关系及其数值实现方法的合理性进行了验证,并给出一个工程计算实例。     

基于LM算法的粘弹性材料剪切松弛模量的拟合

在对粘弹性材料进行有限元计算时,需要材料的剪切松弛模量,必须对经验公式或实验数据进行转换。根据KWW经验公式与广义Maxwell剪切松弛函数的对应关系,列出了求解松弛时间与剪切松弛模量的超定非线性方程组,用LM优化算法得到了方程组的最优解。初步讨论了初值的选取方法。根据此方法得到的松弛时间与剪切松弛模量的拟合曲线的拟合度达到了0.99以上。所提出的方法也适用于用实验数据(随时间变化的应力应变)求解材料的剪切松弛模量。     

Biaxial Yield for Nonlinearly Viscoelastic Materials with a Strain Clock

A constitutive equation with a dilatation dependent reduced time is used to model the mechanical response of solid amorphous polymers such as polycarbonate. Such constitutive equations have the property that stress relaxation occurs faster with increasing dilatation. In previous work, it has been shown that this constitutive equation can account for yield in materials undergoing uniaxial strain or stress control histories. In the present work, yield is discussed when materials described by this constitutive equation undergo homogeneous biaxial and triaxial strain histories. Four sets of conditions are considered: in-plane biaxial constant strain rate histories and in-plane biaxial constant stress rate histories, for both plane stress and plane strain states. Yield is defined in a manner analogous to that in the corresponding strain and stress control conditions in the uniaxial case.     

A stress relaxation model for the viscoelastic solids based on the steady-state creep equation

Creep and stress relaxation are inherent mechanical behaviors of viscoelastic materials. It is considered that both are different performances of one identical physical phenomenon. The relationship between the decay stress and time during stress relaxation has been derived from the power law equation of the steady-state creep. The model was used to analyse the stress relaxation curves of various different viscoelastic materials (such as pure polycrystalline ice, polymers, foods, bones, metal, animal tissues, etc.). The calculated results using the theoretical model agree with the experimental data very well. Here we show that the new mathematical formula is not only simple but its parameters have the clear physical meanings. It is suitable to materials with a very broad scope and has a strong predictive ability.     

On the determination of the relaxation modulus of PP compounds from arbitrary strain histories

Due to the viscoelastic behavior of polymers mechanical properties are strongly affected by the loading history. To obtain the time-dependent Poisson’s ratio without further data manipulation, stress relaxation tests have to be carried out. Only few results for viscoelastic materials have been published to date, but the theory of Poisson’s ratio in the framework of linear viscoelasticity has received some attention with respect to loading histories other than relaxation, i.e. creep and constant rate of strain tests.     

Time-dependent poisson’s ratio of polypropylene compounds for various strain histories

Due to the viscoelastic behavior of polymers mechanical properties are strongly affected by the loading history. To obtain the time-dependent Poisson’s ratio without further data manipulation, stress relaxation tests have to be carried out. Only few results for viscoelastic materials have been published to date, but the theory of Poisson’s ratio in the framework of linear viscoelasticity has received some attention with respect to loading histories other than relaxation, i.e. creep and constant rate of strain tests.The main objective of this work is to compare the potential of different testing methods to determine Poisson’s ratio. Transverse and axial strain have been measured in relaxation tests, creep experiments and displacement rate controlled tensile tests. Relaxation tests are evaluated accounting for the finite loading time and the results are compared with those of tensile creep and displacement rate controlled tensile tests. An optimization based method to determine linear viscoelastic material functions developed previously is applied to calculate Poisson’s ratio.     

Aggregatzustände und Molekülbewegungen bei Kunststoffen

States of Aggregation and Molecular Movements in Plastics The molecular structure of amorphous polymers is discussed. Possibilities for molecular movements in the different states of aggregation are described and the relaxation phenomena are in vestigated which are responsible for the transition between the states of aggregation. The essentials of the mechanical behaviour of amorphous polymers in the various states of aggregationare described in relation to molecular structure.     

钛合金高温短时蠕变与应力松弛的关系研究*

蠕变或应力松弛被认为是钛合金板材热成形降低回弹的主要机理。目前对热校形阶段中的蠕变与应力松弛的区别及联系尚缺乏深入研究。本文主要进行了钛合金高温短时蠕变及应力松弛实验, 利用TEM对实验后的显微组织进行了观察。分别研究了温度、应力及时间对蠕变和应力松弛行为的影响规律, 从蠕变率-时间和蠕变-时间角度建立了蠕变与应力松弛之间的联系。研究表明: 钛合金在低温低应力下蠕变以原子扩散为主, 高温高应力下以位错滑移和攀移为主, 而应力松弛在不同温度时均以位错攀移为主要变形机制, 基于蠕变数据预测的应力松弛行为与实验结果符合较好。     

Dynamic Phosphorescent Probe for Facile and Reversible Stress Sensing

Dynamic phosphorescent copper complex incorporated into the main chain of polyurethanes produces a facile and reversible response to tensile stress. In contrast to common deformation sensors, the applied stress does not lead to bond scission, or alters the phosphor structure. The suppression of dynamics responsible for the nonradiative relaxation is found to be the major pathway governing stress response. As a result, the response of dynamic phosphor described in this work is stress specific. Compared to initial unloaded state, a nearly twofold increase of photoluminescence intensity occurs in response to a 5–35 MPa stress applied to pristine metalated polymers or their blends with various polyurethanes. Finally, the dynamic sensor proves useful for mapping stress distribution patterns and tracking dynamic phenomena in polyurethanes using simple optical imaging techniques.     

Stress relaxation of starch/synthetic polymer blends

Stress relaxation behaviour of injection-moulded starch/synthetic polymer blends were studied. In one experiment, the starch content was kept constant at 70% while the amylose to amylopectin ratio was varied. The synthetic polymers in the blend included high-density (HPDE) and low-density polyethylene (LPDE), and ethylene vinyl acetate (EVA). In the second experiment, the starch content in the blend was varied. A small amount of maleic anhydride-functionalized synthetic polymer (5% by weight) was added to compatibilize reactively the starch and the synthetic polymer. Starch/HDPE and starch/LPDE blends had a ductile behaviour, while starch/EVA blends displayed rubbery characteristics. Blends containing EVA relaxed the fastest while those containing LPDE took the longest time. A double logarithmic plot of stress versus time at constant strain was linear and the slopes of the plots were insensitive to starch type but were affected by the starch content. Residual internal stress which developed during moulding was also estimated from the stress relaxation measurement. The stress–relaxation data fitted several empirical stress–time non-linear models well. © 1998 Kluwer Academic Publishers     

A unified model of stress relaxation and creep applied to oriented polyethylene

The stress relaxation behaviour of high-modulus oriented polyethylene fibre has been studied with regard to the response to successive small strain increments imposed on an initial relatively large strain deformation. For isotropic polymers, the results of such experiments have previously been interpreted in terms of a single thermally activated process modified by strain hardening. It has been found that, although this approach can describe satisfactorily some of the stress relaxation experiments on the oriented polyethylene fibres, it is unsatisfactory once the strain increments have exceeded a certain size, and that it is at variance with stress recovery experiments. It is shown that both the present stress relaxation and stress recovery experiments can be interpreted in terms of a model comprising two thermally activated processes acting in parallel. Furthermore, the parameters obtained for the stress relaxation data are consistent with those required to fit creep data obtained in a comparable stress range. The essential feature of the mechanical behaviour which was previously attributed to strain hardening can now be seen to arise from the transfer of stress between the two thermally activated processes in the two-process model.