2017级

 硕士毕业论文

论文题目：ECTFE薄膜材料力学性能与气枕结构行为试验研究
硕士研究生：刘昶江
指导老师：陈务军 教授
学科专业：结构工程

摘要
乙烯-三氟氯乙烯共聚物（ethylene- chloro- tri- fluoro- ethylene，简称ECTFE）为有两相结构的半晶聚合物，具有粘弹塑性。ECTFE薄膜的自重小、抗冲击性强、使用温度范围较广、雾度超低、自洁性优异、抗破损性好。ECTFE薄膜的工程力学性能研究较少，因此对ECTFE材料、薄膜和气枕的性能探究亟需开展。本文对ECTFE薄膜展开了不同应变速率、温度、循环拉伸条件下的单轴拉伸试验和分析，对ECTFE气枕的平面裁切成形过程展开了模拟分析和试验论证。
首先，本文对不同试样形状的ECTFE薄膜分别进行单轴拉伸试验，发现条形试样和哑铃形试样的屈服强度相近。对MD（Machine Direction）和TD（Transverse direction）方向的试样进行了8种应变速率（=0.02/min~10.00/min）的单轴拉伸试验，得到了多种力学参数。分析表明：ECTFE薄膜为各向同性材料；应力应变曲线共经历弹性、应变软化、冷拉和应变硬化四个主要阶段；力学参数与应变速率的拟合公式可预测膜材在不同应变速率下的力学参数。
其次，基于实际环境中高聚物薄膜的最高使用温度以及材料的使用温度范围，对ECTFE薄膜进行了不同的低-高温度（=-50 ℃~ 80 ℃）下的单轴拉伸试验。试验结果表明：随着温度的降低，应力应变曲线整体抬升，拉伸强度、弹性模量、屈服强度增加；在不同温度下（-50 ℃~ 80 ℃），拟合公式保证误差系数达93 %，屈服强度的差值可达到89 %；给出了主要力学参数和温度的拟合公式，可较好地描述温度相关性。
再次，针对ECTFE薄膜在实际工程中的受力情况，对条形ECTFE薄膜试样在MD方向进行单轴循环拉伸试验，通过应力和应变控制循环的开始。ECTFE薄膜在屈服点之前开始循环拉伸，弹性模量所受影响较弱；随着循环次数的增加，薄膜的弹性模量增加。分别在非线性的三个阶段：软化阶段、冷拉阶段、应变硬化阶段中，给出了ECTFE薄膜的弹性模量随循环次数变化的拟合方程。循环拉伸结束后继续拉伸至膜材屈服或断裂，材料保持单轴拉伸的应力应变特性。
在最后，基于几何非线性材料线性模型，对不同内压下的ECTFE气枕的充气成形过程进行了数值模拟，进行了1.0 m边长正六边形ECTFE双层薄膜气枕的平面裁切充气成形试验，通过将试验测得的矢高与模拟结果进行对比，确认了数值模拟的可靠度，通过数值模拟得到了不同内压下的膜面应力、应变、变形的基本特征，对不同气压下的最大应力、最大应变和矢高进行了曲线的拟合，对常温条件下ECTFE气枕的形变特性研究具有一定的参考意义。
 
关键词：ECTFE薄膜，单轴拉伸试验，率相关性，温度相关性，单轴循环拉伸，ECTFE气枕，充气成形，平面裁切

EXPERIMENT ON UNI TENSILE MECHANICAL PROPERTIES OF ECTFE FOILS AND STRUCTURAL BEHAVIOR OF ECTFE CUSHION

Ethylene-chlorotrifluoroethylene (ECTFE) is a kind of fluorinated copolymers with two-phase structure and visco-elastoplastic property. There are a variety of advantages of ECTFE foil, including light weight, good toughness, high hardness, heat resistance, ultra-high light transmittance, excellent self-cleaning, etc. For the point of engineering application of ECTFE in architectural field, the research on the mechanical properties of ECTFE foil are still insufficient. Thus, mechanical experiments on ECTFE film and ECTFE cushion are indispensable. Unitensile experiments under various rates, temperature and cyclic load are carried out in detail. And inflating forming experiments are performed for ECTFE cushion through planar patterning method.
Firstly, uniaxial tensile experiments were performed on ECTFE foil samples of different specimen. It was found that the broken elongation and strength of dumbbell samples are much larger than those of strip samples, with a little effect on the yield strength. Then, dumbbell ECTFE foil specimen were cut in the machine direction (MD) and transverse direction (TD) respectively, and were stretched under 8 various tensile rates (0.02 /min~ 10.00 /min). With the analysis of stress-strain curve, correlation between mechanical parameters and tensile rates were investigated comprehensively. The results show that mechanical properties in the MD and TD are basically the same, that is, the ECTFE foil could be assumed as isotropic material. The stress-strain curves show obvious non-linear characteristics, where four phases of linear elasticity, strain softening, cold drawn and strain hardening are distinguished and defined. The nonlinear exponential formulas characterizing the effects of tensile rate are fitted, which could be used to predict the mechanical parameters of ECTFE foil under different tensile rates.
Secondly, based on the maximum temperature of the polymer film in actual environments and the service temperature range of ECTFE material, the uniaxial tensile stress-strain curves of ECTFE foil were obtained under fourteen temperatures (-50 ℃~ 80 ℃). The experiments show that the stress-strain curve rises as temperature decreases, accordingly yield strength, tensile strength, cold drawn stress, and elastic modulus increase, and contrarily, broken elongation and toughness decreases. The difference of yield strength can be 89 %. The temperature-dependent parameters are analyzed thoroughly. The exponential and logarithmic fitting formulas between mechanical parameters and temperature are given, of which the guarantee factor error is greater than 93%, which can well describe the temperature dependence of ECTFE foil.
Thirdly, according to the practical loading condition of ECTFE membrane structure, uniaxial cyclic tensile tests are conducted on the MD strip specimen. The cycling load amplitude are controlled by stress or strain, alternatively. Under each working condition, the stress-strain characteristic remains similar. The elastic modulus is less affected as the cyclic stretch starts before the yield point. Furthermore, the elastic modulus increases with the augmentation of cyclic periods, especially in the late stage of strain hardening phase. Fitting equations for the relationship between elastic modulus and cyclic period are given during the three non-linear phases: strain softening, cold drawn and strain hardening. After the end of the cycle loading, ECTFE film is stretched consecutively to the next yield point or broken strength point. The material maintains stress-strain characteristics under uniaxial tensile test.
Finally, based on nonlinear geometrically and elastic material method, numerical simulations were performed for the inflating forming of ECTFE cushion under various pressures. Meanwhile, inflating forming experiments are carried out for a hexagonal ECTFE cushion with planar patterning method. The numerical method is validated through good agreement of rise between experiment and simulation. The parametric analysis and structural behavior of ECTFE cushion are performed and investigated thoroughly, including distribution of strain and deformation field, maximum stress and rise, etc. And the variation of the maximum stress, strain and rise are best-fitted with respect to pressure variables. The present work is valuable to research and practical design of ECTFE cushion in near future.
 
KEY WORDS: Ethylene-chlorotrifluoroethylene (ECTFE) copolymer foil, uniaxial tensile test, rate-dependent performance, temperature-dependent performance, uniaxial cyclic tensile test, ECTFE cushion, inflating forming, planar patterning

论文题目：退役PVDF涂层建筑膜材力学试验研究
硕士研究生：李坤泽
指导老师：陈务军 教授
摘要
随着膜结构建筑进入永久性建筑的行列，相关结构设计与建筑织物材料的使用逐渐成为人们关注的重点。膜结构建筑具有结构多样化、施工方便的优势，被广泛应用于大型体育场馆、仓储结构等公共建筑。由于建筑膜材在长期服役过程中力学性能下降，为确保建筑美观及结构设计指标，以及考虑台风等不可抗力对结构的影响，服役建筑膜材达到特定使用年限需退役，此时退役膜材的力学性能变化研究，对膜结构建筑的设计具有参考意义。目前建筑膜材老化的研究大多为单一变量的人工老化，与实际工程的膜材老化情况有所区别。本文基于实际工程退役膜材的使用，针对相关问题，进行试验、模拟和研究分析，主要内容包括：
（1）选用服役年限分别为18.5年和14.5年，位于江苏省南京市和上海市的膜结构建筑，南京国际展览中心和闵行体育馆（Ferrari 1202T2）的退役膜材，并进行相关力学性能试验，通过与全新膜材的力学参数对比，并用老化率表示各力学性能的变化情况；
（2）基于南京国际展览中心退役膜材的双轴循环拉伸试验数据，对第三组循环的平均值取特征点用于应变-应力、弹性模量-应力以及泊松比-应力关系的拟合，拟合效果较好。其中在弹性模量-应力响应面的拟合过程中采用了对数坐标变换，因此增量形式的函数拟合为指数函数形式；
（3）采用全微分增量形式并利用偏导数原理拟合弹性模量-应力响应面，结果表明仅有对数坐标下求得的二元一次方程基本满足要求，这是由于增量形式运用到偏导数时，对应变-应力函数响应面的精度要求很高，因此建议使用该种方法时准备9组应力组数据；模量分离法的使用通过参考经、纬向试验数据可合理简化模量的计算，从而达到5组应力比试验数据所能达到的较好拟合精度；
（4）基于Abaqus软件，对试验试件进行建模，同时进行材料子程序UMAT二次开发，从而引入第三章的弹性模量响应面，建立唯象本构模型。拟合结果与二次、三次应变-应力响应面进行对比，可知Abaqus拟合和响应面结果具有一致的变化趋势，建立的本构模型准确性较好。

关键词： 退役膜材；膜结构；响应面；涂层织物；老化材料
 
Experimental investigations on mechanical properties of dismantled PVDF-coated polyester fabrics
ABSTRACT

As membrane structures are recognized as permanent buildings, relevant structural designs and the use of membrane materials are gradually paid more attention. Membrane structures have the advantages of diversity and convenient construction. Due to the decline of mechanical properties of building membrane materials in the long-term service, in order to ensure the architectural beauty and structural design performance, and to consider the impact of typhoon and other force majeure on the structure, the building membrane materials on service need to be retired after reaching a specific period. At this time, the researches on mechanical properties between dismantled and the new membrane materials are meaningful to structural designs. At present, most researches on aging membrane materials are based on artificial aging with single variable, which may be different from the fact of actual engineering. Therefore, it is of great significance to study the mechanical properties of dismantled membrane materials. In this dissertation, based on the use of dismantled membrane materials in practical engineering, tests, simulation and research analysis are carried out. The main contents are as follows:
(1) The dismantled membrane materials of Nanjing International Exhibition Center and Minhang Stadium, with service life of 18.5 years and 14.5 years respectively, are selected and tested for their mechanical properties. By compared with the mechanical data of the new membrane materials, the changing characteristics are expressed by their aging rate;
(2) Based on the data of biaxial cyclic tensile test of membranes of Nanjing International Exhibition Center, the characteristic points of the average values of the third group of cycles are used to fit the curves of strain-stress, elastic modulus-stress and Poisson's ratio-stress, and the fitting results is good. The logarithmic coordinate transformation is used in fitting process of Elastic modulus-stress response surface, therefore, incremental function fitting is exponential form;
(3) The total differential increment form and partial derivative are used to calculate the modulus. The results show that only the first-order polynomial obtained by logarithmic coordinates basically meets the requirements. The reason is that when the incremental form is applied to the partial derivative, the accuracy of the response surface of the strain-stress function is very high. Therefore, it is suggested to prepare 9 groups of data when using this method; The use of modulus separation method can reasonably simplify the calculation of  by referring to the longitude and latitude test data, so as to achieve better fitting accuracy;
(4) Based on the commercial software ABAQUS, the modeling of the test specimen is carried out, and the secondary development of material subroutine UMAT is carried out, so as to introduce the Elastic modulus response surface in Chapter 3 and establish the phenomenological constitutive model. The fitting results are compared with the quadratic and cubic strain-stress response surfaces, which can be seen that the results of ABAQUS fitting and response surface have the same variation tendency, and the established constitutive model is accurate.

KEY WORDS: dismantled membrane material; response surface; membrane structure; coated fabric; aging materials