【作者】 唐欣；

【导师】 罗文波；

【作者基本信息】 湘潭大学 ， 一般力学与力学基础， 2007， 硕士

【摘要】 高聚物被淬火到玻璃化转变温度(Tg)以下时,材料将在一个很长时间内处于非平衡态,其结构将随时间演化直至达到平衡状态,这种现象称为物理老化。随着老化时间的增加,材料的介电性能、热学性能和力学性能都相应地都会发生变化,如介电常数、热导率、比热容、蠕变柔量和抗拉强度等。高聚物和高聚物基复合材料正越来越广泛地作为结构材料应用在许多工程领域,且绝大部分的服役温度低于Tg,因此这些材料的物理老化行为已经越来越多地受到学术界和工程界的关注。本文着重研究高聚物物理老化对材料力学性能的影响。高聚物的力学性能是时间相关的,强烈地依赖于分子链的运动活性。分子链的运动活性则受温度、应力和老化时间的影响。蠕变是材料在恒定载荷作用下的时间相关的变形行为。除体积松弛和热焓松弛测量外,观测蠕变柔量的变化也是考察物理老化的一种非常普遍有效的方法。本文研究聚甲基丙烯酸甲酯(PMMA)在不同老化和不同应力条件下的蠕变行为,同时,通过常应变率拉伸测试得到的应力应变曲线考察材料初始瞬时弹性模量(E)和断裂强度(σf)随老化时间(te)的变化规律。在蠕变测试和常应变率拉伸测试前,试件首先被加热到115℃,约高于Tg 10℃,并保温30分钟以消除以前的热力历史,使所有试件均处于相同热力状态。然后迅速降温至不同的老化温度(Τa)下进行老化。老化温度分别为60℃, 40℃和27℃,老化时间最长超过1000小时,所有温度误差控制在±1℃以内。材料在固定老化时间不同应力作用下(≥14MPa)的蠕变测试结果表明,材料的蠕变柔量曲线互不重合,说明材料的蠕变行为是非线性的。我们应用时间-应力等效原理来分析这种非线性行为,通过把高应力下的曲线向低应力(14MPa)下的曲线进行移位构建了蠕变柔量主曲线。主曲线涵盖的时间标度与试验时长相比增大约两个数量级,表明时间-应力等效原理可用来对黏弹性材料的力学性能进行加速表征。简要介绍了几个重要的非线性黏弹性蠕变模型,并采用其中的Findley模型和Struik模型对PMMA(Τa=40℃, te=4h)在27℃不同应力下的非线性蠕变行为进行分析,通过遗传算法确定了模型参数。结果表明,Findley模型和Struik模型均能很好地拟合试验数据。此外,我们将Findley模型和Struik模型的模型预报结果与通过时间-应力等效原理得到的蠕变柔量主曲线进行比较,发现在105s内模型预报结果与主曲线基本吻合,当时间超过105s时,模型预报结果略高于主曲线。研究了不同应力(15MPa、20MPa、25MPa和30MPa)和不同老化时间下PMMA的非线性蠕变行为,讨论了老化时间的影响。结果表明,材料的等时蠕变柔量随log te而线性递减;同时蠕变柔量曲线可以沿对数时间轴平移而叠加到参考状态曲线上,说明时间-老化时间等效原理是适用的。本文取最长的老化时间为参考状态,通过老化移位因子(φa)构建了每个应力下的主曲线,移位因子与老化时间在双对数坐标图上呈现线性关系,其负斜率(μ)就是老化移位率。老化移位率随应力的增大而减小。此外,我们还从常应变率拉伸测试得到应力-应变曲线,并从中讨论老化时间对材料初始瞬时弹性模量(E)和断裂强度(σf)的影响,在本文讨论的老化时间范围内,两者都随log te而线性增大。本文工作受到国家自然科学基金(No.10572123)、湖南省自然科学基金(No.05JJ30014)和低维材料及其应用技术教育部重点实验室(湘潭大学)开放课题(No.KF0502)的资助。更多还原

【Abstract】 When a polymeric material is quenched to below its glass transition temperature, Tg, the material is in a thermodynamic non-equilibrium state for a very long time. During this long period of time, its structure will evolve with time towards the equilibrium state, leading to the aging-time dependent dielectric, thermal and mechanical properties such as dielectric permittivity, thermal conductivity, specific volume, creep compliance and tensile strength etc. This phenomenon is known as physical aging. Polymers and polymer composites are increasingly used in many structural applications, and most of them are used below their Tg, the physical aging behavior of such polymeric materials have been received more and more research attentions in the academic and industrial fields. This thesis focuses on the effects of physical aging via changes in the mechanical response.The mechanical properties of polymers are usually time dependent. Their mechanical response strongly dependents on the mobility of the chains which is affected by temperature, stress and aging time. Creep is the time dependent deformation of materials subjected to a continuous constant stress. Besides the volume recovery and enthalpy recovery measurements, the creep compliance measurement is a very popular and practical method to investigate the physical aging phenomenon. In this thesis, the creep behavior of polymethyl methacrylate (PMMA) at various stresses was measured for various aging times. Moreover, the ageing behavior was monitored by measuring the initial instantaneous elastic modulus (E) and the fracture strength (σf) from the stress-strain curves, which were obtained in uniaxial tensile loadings with a constant strain-rate of 0.00133s-1, as a function of ageing time (te).Prior to the creep tests and constant strain-rate tensile tests, the specimens were heated to 115℃, which was 10℃above the Tg, and hold at that temperature for 30 minutes to remove the previous thermal and mechanical history, and then were allowed to age at various aging temperatures, namely 60℃, 40℃and 27℃for different times of up to over 1000h. All the temperatures were controlled within the rang of±1℃.From the creep tests, it is observed that the creep compliance curves at different stresses (≥14MPa) for a fixed aging time depart from each other, indicating the nonlinear viscoelastic behavior. The time-stress superposition principle was used to analyze this nonlinear effect, and a creep master curve was constructed by shifting the compliance curves at higher stresses to the one at lowest stress (14MPa). It is shown that the master curve covers a wider time range, which is an approximate 2 decades wider than the test duration in the logarithmic time scale, and moreover the stress shift factors depends linearly on the stress in the stress range used in this study. This validates that the time-stress superposition principle provides an accelerated technique for characterization of viscoelastic properties of materials.Some important models for nonlinear viscoelastic creep were briefly reviewed, and two of them, namely Findley model and Struik model, were used to model the nonlinear creep behavior of PMMA, which was aged for 4h at 40℃and tested at 27℃at various stresses. The model parameters were determined via the genetic algorithm (GA). The two models are found to have strong potential to fit the test data. Moreover, the model predictions are compared with the creep master curve obtained by stress shifting technique, and an acceptable agreement is shown within the time range of 105s, though the model predictions are a bit higher than the creep master curve beyond 105s .The aging effect on creep behavior of PMMA was investigated by measuring creep compliance curves at four different stresses, 15-30MPa with an interval of 5MPa, for various aging times (te). The isochronous creep compliance is found to linearly decrease with log te. It is also shown that the creep compliance curves can be superimposed by horizontal shifting the creep response to the one at the reference aging time, indicating the applicability of the time-aging time superposition principle. In this study the reference aging time was chosen as the age of the longest aged test. The master curves for each stress level are created and the corresponding aging shift factor (φa) is found to be proportional to the aging time in a double logarithmic plot. The minus slop of the double logarithmic plot of logφa vs log te is the aging shift rate,μ. It is found thatμdecreases with increasing stress.Moreover, the aging effects on the initial instantaneous elastic modulus (E) and the fracture strength (σf), which obtained from the stress-strain curves, were also discussed. Both E andσf are found to linearly increase with log te in the aging time interval investigated in this study.This study was supported by the National Natural Science Foundation of China (No. 10572123), the Provincial Natural Science Foundation of Hunan (No.05JJ30014) and the Open Project Program of Key Laboratory of Low Dimensional Materials & Application Technology (Xiangtan University), Ministry of Education, China (No.KF0502).更多还原