【作者】 陈朝中；

【导师】 赵荣国；

【作者基本信息】 湘潭大学 ， 固体力学， 2009， 硕士

【摘要】 无定形聚合物的特点之一是具有粘弹性,即具有强烈的时间和温度依赖性,时间依赖性也表现为应变率的依赖性。在低应变率下,起主导作用的是代表主转变的链段运动,而在高应变率下,次级转变对材料的力学响应也发挥了重要作用,表现为主转变和次级转变的协同作用。无定形聚合物相对于金属、陶瓷等材料,其结构更加复杂,因此,研究其力学行为的应变率依赖性是一项复杂且困难的工作。无定形聚合物是热的不良导体,特别是在高应变率情况下,变形属于绝热变形,使得材料温度急剧上升,热软化十分突出,应变软化和热软化耦合作用。再者,从低应变率到高应变率,目前的本构模型预测结果尚不能与实验数据很好吻合,完全理解聚合物结构与力学性能的关系,还需要开展大量的研究工作。本文对聚甲基丙烯酸甲酯(PMMA)和聚碳酸酯(PC)两种无定形聚合物的应变率依赖性进行了分析和研究,主要研究内容及结论如下：1.介绍了无定形聚合物的特点和应用以及分子运动特性,分析评述了无定形聚合物应变率依赖性国内外研究现状,并论述了与应变率密切相关的热流变简单性和复杂性、固有变形行为和分子分析等内容。2.对PMMA进行动态力学分析,引入动态实验中应力应变的基本理论,建立起频率与等效应变率的关系,分析频率对PMMA玻璃化转变温度的影响以及升温速率、频率对PMMA动态力学性能的影响。结果表明：玻璃化温度Tg随频率和升温速率的增大而增大；应用损耗模量E″峰值温度定义的Tg比应用损耗因子tanδ峰值温度定义Tg偏小；储能模量E′随频率的增大呈现出缓慢增加的趋势；tanδ峰值对应的频率随温度的升高而增大,且随温度的升高tanδ曲线向高频方向漂移。3.进行了PC和PMMA的低应变率单轴压缩实验,分析和讨论了这两种无定形聚合物力学行为的应变率依赖性以及不同应变率下的分子运动。结果表明：PC和PMMA的屈服应力随对数应变率的增加基本呈线性增加趋势,而硬化模量则呈先增大后减小的趋势；PC和PMMA的非弹性行为指数I值并不完全符合Halary理论,而应变软化幅度SSA则与Halary理论基本相符。4.论述了聚合物材料SHPB实验技术中的关键问题。针对PMMA的力学响应分析表明,高应变率下弹性模量的应变率敏感性增加,屈服后材料存在明显的应变软化,由于高应变率下绝热变形导致的热软化,应力应变曲线不存在硬化阶段。更多还原

【Abstract】 Amorphous polymer, which possesses the properties of time-dependence and temperature-dependence, is a class of typical viscoelastic material. Time-dependence is also manifested in the strain rate dependence. Under conditions of low strain rate for various values, primary transition motions play a main role because secondary transition motions are relatively free, while in the cases of dynamic high strain rates, secondary transition motions also play an important role and the coupling effects between primary and secondary transition motions are responsible for the mechanical response for materials because the secondary transition motions are restricted. Compared to metals, ceramics and other materials, the structures of amorphous polymers are more complex. Therefore, it is a complex and difficult work to study the strain rate dependence of mechanical behavior of amorphous polymer. Firstly, due to the polymeric material’s poor thermal conductivity, especially in the case of high strain rate, its deformation belongs to adiabatic one, which results in the sharp rise in temperature, so the softening phenomenon is very prominent. In this case, strain softening and thermal softening coupling. Secondly, the theoretical results predicted by the current constitutive models are not well fitted with the experiment data as the strain rate varies from low values to high ones. there are a lot of work needed to be done to fully understand the relationship between amorphous polymers structure and mechanical properties.In this work, strain rate dependences of two kinds of amorphous polymers, poly methyl methacrylate (PMMA) and polycarbonate (PC), are analyzed and discussed. The main research contents and conclusions are as following:1. The properties, applications and molecular motions of amorphous polymers are introduced, and then the research situation and advances in the strain rate dependence of amorphous polymer are analyzed. Finally, the thermal rheological simplicity and complexity, the intrinsic deformation behavior and molecular analysis, which are closely related to strain rate dependence of polymer, are reviewed.2. The DMA tests of PMMA are carried out on a GABO EPLEXOR. The basic theory of stress and strain in the dynamic experiments is introduced, and the relation between frequency and equivalent strain rate is constructed. The influence of frequency on glass transition temperature, and the effects of heating rate and frequency on dynamic mechanical properties of PMMA are analyzed. It is showed that the glass transition temperature Tg increases with increasing frequency (equivalent strain rate) and heating rate. The value of glass transition temperature Tg definited by the peak temperature of the loss modulus E’ curve is smaller than that definited by the peak temperature of the loss factor tanδcurve. Storage modulus E increases slowly with increasing frequency. As the temperature increased, the value of frequency corresponding to the peak of tanδcurve increases, the tanδcurve is shifted from low frequency to high one.3. A series of uniaxial compressive experiments for PC and PMMA specimens are implemented. The strain rate dependence of mechanical behavior and molecular motions at various strain rates are analyzed and discussed. It is showed that the yield stress, which increases with increasing strain rate, is approximatively linear with logarithmic strain rate, but the hardening modulus of PC and PMMA firstly increases and then decreases with increasing strain rate. The calculated results of non-elastic behavior index I of PC and PMMA are not completely coincidence with Halary’s theory, while the strain-softening amplitude (SSA) is nearly consistent with Halary’s theory.4. The key problems of the split Hopkinson pressure bar (SHPB) test technique in the polymeric materials are summarized. Under dynamic high strain rates, it is found from the study on the mechanical response of PMMA that the strain rate sensitivity of elastic modulus of polymeric material increases at high strain rates, and an obvious strain-softening phenomenon exists at the post-yield stage. Due to the thermal softening caused by the adiabatic deformation at high strain rates, no strain harding stage is observed in the stress-strain curve.更多还原