Modelling effects of degree of crystallinity on mechanical behavior of semicrystalline polymers

Viscoplasticity theory based on overstress (VBO) which is one of the unified state variable theories is extended to account for crystallinity ratio () on mechanical behavior of semicrystalline polymers. The modifications on VBO are done considering the semicrystalline polymeric materials somewhat as a composite material since it consists of amorphous and crystalline phases. Amorphous and crystalline phase resistances are arranged in two different analog models: amorphous stiffness and flow are in parallel and series with crystalline phase. Apart from many existing work in the literature, not only uniaxial loading are modeled but also creep and relaxation behaviors are simulated for a hypothetical material. It is shown that when amorphous and crystalline phase resistances acting in parallel are considered in the model, creep, relaxation and uniaxial loading and unloading behaviors can be simulated well using the modified VBO. In addition, uniaxial compression loading and unloading behavior of highly crosslinked ultra-high molecular weight polyethylene (UHMWPE) and creep behavior of polytetrafluoroethylene (PTFE) with different crystallinity ratios are simulated using the proposed VBO model where amorphous and crystalline phases are parallel. Simulation results are compared to the experimental data by Kurtz et al. (2002) and Sun et al. (2005) [Kurtz, S.M., Villarragaa, M.L., Herra, M.P., Bergström, J.S., Rimnacc, C.M., Edidin, A.A., 2002. Thermomechanical behavior of virgin and highly crosslinked ultra-high molecular weight polyethylene used in total joint replacements. Biomaterials 23, 3681–3697; Sun, H., Cooke, R. S., Bates, W. D., Wynne, K.J., 2005. Supercritical CO₂ processing and annealing of polytetrafluoroethylene (PTFE) and modified PTFE for enhancement of crystallinity and creep resistance. Polymer 46, 8872–8882] respectively and good match with experimental data is obtained.     

A method for the evaluation of design limits for structural materials in a cyclic state of creep

The purpose of the present paper is to demonstrate how the minimum theorems proposed in an accompanying paper (Ponter and Boulbibane, 2002) can be utilised in the prediction of the deformation and life assessment of structures subjected to cyclic mechanical and thermal loadings. The developed method, which is based upon bounding theorems and an associate programming method, the Linear Matching method, takes into account the changes in residual stress field occurring within a cycle. Although the solution provided a bound on the inelastic work, it also appears that generally the displacements predicted by this solution are smaller than those that would be predicted by the rapid cycle solution. By way of illustration a simple non-linear viscous model is adopted and a number of solutions are presented involving a Bree plate problem subjected to cyclic histories of load and temperature. An elastic follow-up factor is identified as a key design parameter for high temperature dwell periods.     

Determination of creep compliance,recovery and Poisson's ratio of elastomers by means of digital image correlation (DIC)

This work aims to determine the creep compliance, creep recovery and Poisson's ratio of three common sealing elastomers by means of the digital image correlation (DIC). The tests were conducted by stressing specimens under three different constant stresses during short duration experiments (3 h) to see the prospective of DIC for this application. The strains were measured in x and y axes with time. Thus, the behavior of creep compliance, creep recovery, and the Poisson's ratio of each elastomer were obtained. The creep results exhibited repeatability, as well as, the mean Poisson's ratios estimated were close to reported values for elastomers. Finally, despite of some limitations from the DIC equipment, it was found that this procedure can be implemented as a suitable alternative for the characterization of creep compliance, creep recovery and Poisson's ratio of elastomers. Also, it may be enhanced by following some recommendations given.     

Creep assessment in hyperelastic material by a 3D Neural Network Reconstructor using bulge testing

A new methodology based on a Neural Network approach is developed for three-dimensional reconstruction of pseudo-hemispherical geometry of a hyperelastic specimen during a creep bulge test. On the basis of this procedure, the inflated membrane stress and strain states are established to determine the material mechanical properties. A new and economic experimental apparatus, used to conduct the bulge test, is provided with a sliding crossbar for the dome image acquisition in order to detect its strain state. A pressure regulator is used to ensure constant pressure, essential for creep. The artificial neural network was trained by using the position (x, y, z) of the membrane points at different values of the pressure measured in the bulge test chamber. The main characteristics of the system, which consist of the experimental apparatus and the neural post-processing, are accurate characterization of the materials and the ability to analyze materials with moderate anisotropy. This method has been verified on carbon black-reinforced SBR. The hyperelastic parameters that were obtained are in agreement with the values reported in scientific literature.     

热塑性聚合物复合材料蠕变性能的综述

热塑性聚合物复合材料(TPCs)由于其力学性能优异、产量巨大,并具有价格优势,已被作为重要的结构材料得到了广泛的应用.然而,由于聚合物的粘弹本性,其不可避免地会在外力的作用下产生随着时间延长而增加的形变,即蠕变.这种作用将对材料造成永久形变以及不可恢复的破坏,从而限制材料的应用.本文通过对TPCs蠕变的研究综述,详细探讨了TPCs蠕变的机理特征,分析了各类因素(诸如分子结构、填料、应力、温度等)对TPCs蠕变作用的影响规律.在此基础上,简明扼要的分析并提出了有助于获得抗蠕变性能优异的TPCs的方法.     

Nanomechanical characterizations of InGaN thin films

In_xGa_1−xN thin films with In concentration ranging from 25 to 34 at.% were deposited on sapphire substrate by metal-organic chemical vapor deposition (MOCVD). Crystalline structure and surface morphology of the deposited films were studied by using X-ray diffraction (XRD) and atomic force microscopy (AFM). Hardness, Young's modulus and creep resistance were measured using a nanoindenter. Among the deposited films, In_0.25Ga_0.75N film exhibits a larger grain size and a higher surface roughness. Results indicate that hardness decreases slightly with increasing In concentration in the In_xGa_1−xN films ranged from 16.6 ± 1.1 to 16.1 ± 0.7 GPa and, Young's modulus for the In_0.25Ga_0.75N, In_0.3Ga_0.7N and In_0.34Ga_0.66N films are 375.8 ± 23.1, 322.4 ± 13.5 and 373.9 ± 28.6 GPa, respectively. In addition, the time-dependent nanoindentation creep experiments are presented in this article.     

Correlation of Cooperatively Localized Rearrangement on the “Fluidized Domain”of Polymers to Their Nonexponentially Viscoelastic Behavior and Lifetime at Double Aging Processes II: Estimation of Long-term Mechanical Behavior and Lifetime of Polymeric Mate

以四种不同类型高聚物(半晶聚丙烯、聚甲基烯酸甲酯、交联环氧树酯和炭黑填充丁苯橡胶)的短时标应力松弛和蠕变测试实验数据作为实例,按照两套应力松弛模量和蠕变柔量普适对比态方程,从理论组合主直线上直线外推了四种高聚物的长时标力学行为和寿命,并将其外推预测值同理论计算值和实测值进行了对比. 结果证实四种高聚物的外推预测值均同理论计算值和实测值很好地符合. 又讨论了它们的尺寸稳定性、负荷承载性,失效寿命和失效形变量等对老化时间的依赖性. 最终证实该从理论组合主直线上来直线外推长时标力学行为和寿命的方法是有效可行的.     

顺层蠕动边坡变形破坏机理及稳定性动态分析

本文以阜新海州露天矿边坡为例, 分析了边坡变形规律和破坏机制。在弱层流变试验的基础上, 建立了弱层流变力学模型, 通过确定加速蠕变阶段来临的极限应变量和应变速率, 建立了弱层长期强度的时间效应方程, 对于不同边坡工况进行了动态稳定性评价, 为采取抗滑措施控制蠕动边坡的变形破坏发展提供了科学依据。最后提出了一个边坡加速蠕变阶段来临的预测预报方法, 分析结果和实测资料较为一致。     

An anisotropic flow law for incompressible polycrystalline materials

New and explicit anisotropic constitutive equations between the stretching and deviatoric stress tensors for the two- and three-dimensional cases of incompressible polycrystalline materials are presented. The anisotropy is assumed to be driven by an Orientation Distribution Function (ODF). The polycrystal is composed of transversally isotropic crystallites, the lattice orientation of which can be characterized by a single unit vector. The proposed constitutive equations are valid for any frame of reference and for every state of deformation. The basic assumption of this method is that the principle directions of the stretching and of the stress deviator are the same in the isotropic as well as in the anisotropic case. This means that the proposed constitutive laws are able to model the effects of anisotropy only via a change of the fluidity due to a change of the ODF. Such an assumption is justified to guarantee that, besides knowledge of the parameters involved in the isotropic constitutive equation, the anisotropic material response is completely characterized by only one additional parameter, a type of enhancement factor. Explicit comparisons with experimental data are conducted for Ih–ice. Dedicated to Prof. L.W. Morland on the occasion of his 70^th birthday Received: July 6, 2004; revised: November 8, 2004