2019级

硕士毕业论文

论文题目：囊体材料蠕变力学行为及结构强度分析
硕士研究生：宋寅搏
指导老师：陈务军 教授
摘要
织物复合材料具有重量轻、比强度高、比模量高等一系列优良特性，广泛应用于航空航天、建筑、造船和汽车领域。织物复合材料一般分为刚性和柔性两种，前者通常采用碳纤维、玻璃纤维等浸渍于树脂基体中固化制备而成；而后者一般包括承力层和功能层两部分，由层压工艺制备而成。聚氟乙烯-聚芳酯（PVF-Vectran）囊体材料是一种典型的二维柔性织物复合材料，具有强度高、耐冲击、化学性质稳定、蠕变小等特点，主要于飞艇气囊等柔性结构。
飞艇通过气囊内大量氦气来获得升力，因驻空时间长、航行成本低等优势被视为一种优良的浮空器，能承受复杂空间环境的平流层飞艇更是当前世界各国临近空间资源开发装备研究的热点，具有通信中继、气象数据搜集、高分辨率对地观测等多方面的功能。飞艇的主要结构是囊体，囊体材料的破坏直接导致飞艇结构整体失效，所以囊体材料的力学性能研究和长期荷载作用下极限强度的确定对飞艇强度设计起着至关重要的作用。对囊体材料蠕变损伤和破坏机理的揭示需要依据微观、介观和宏观等多个尺度的试验现象观察和力学参数分析，故有必要借助显微观测、延时摄影等技术，从材料多尺度力学性能层面入手，研究囊体结构的蠕变强度性能。
首先，以飞艇用PVF-Vectran囊体材料为研究对象，制作单根纤维、纱线、囊体材料单轴拉伸等3个尺度的试件进行极限应力和极限应变的确定，分别进行单轴拉伸强度的85%、80%、75%、70%、65%五个应力水平下48h的单轴拉伸蠕变试验，并利用声发射仪器记录材料单轴拉伸过程中的声发射信号，从而获得各试件在不同应力水平下的损伤累积特征和蠕变破坏行为，可为相似结构的层压复合材料破坏行为预测提供参考。
其次，利用扫描电子显微镜观测记录试样蠕变破坏前后在不同尺度下的损伤与破坏形态，建立纤维、纱线、囊体材料在不均匀度、原始曲率、破坏准则等多方面的理论假设，引入损伤力学中均匀损伤场变量，利用基于声发射的间接损伤测试方法，通过声发射信号累积值得转换得到PVF-Vectran囊体材料在各应力水平下的单轴拉伸蠕变损伤演化方程，可以为后续囊体材料双轴拉伸蠕变损伤本构的建立及囊体结构层面的强度分析奠定基础。
然后，确定PVF-Vectran囊体裁切与拼接方案，依据方案制作球型和艇型囊体，利用三维激光扫描仪获取两种囊体在内压1kPa状态下的点云坐标并计算体积、直径、长细比等几何参数；随后进行两种囊体的充气承压破坏试验，基于囊体材料撕裂理论，对囊体残片进行裂缝定位，分析裂缝起始点位置；利用高速摄像机记录囊体结构在达到极限气压时的破坏起始点位置及裂缝发展形态，验证了残片分析结果的正确性，可以为囊体结构破坏后原因分析提供理论分析路线。
最后，基于囊体裁切效应和拼接位置焊缝加强等方面的考虑，在Abaqus中建立两种囊体的有限元精细化模型，依据材料双轴拉伸试验结果与理论模型编写VUMAT子程序定义PVF-Vectran囊体材料本构，验证了囊体结构充气承压破坏试验的结果与充气承压过程中各应力应变的发展历程，可以为囊体结构长期强度的深入研究提供参考。
关键词：飞艇结构，囊体材料，蠕变强度试验，损伤累积模型，失效分析

CREEP BEHAVIOR OF ENVELOPE MATERIAL FOR AIRSHIP AND STRUCTURAL STRENGTH ANALYSIS
ABSTRACT
Braided composites reinforced with fiber, yarn or fabric have a series of excellent characteristics such as light weight, high strength and high modulus. They are widely used in aerospace, architecture, shipbuilding and automobile. Braided composites are generally divided into two types: rigid composite and flexible one. The former is usually prepared by impregnating and curing carbon fiber, glass fiber and other high modulus fibers in resin matrix. The latter includes at least two parts: reinforcement base cloth and coating layer. Braided composites are prepared by lamination process without fixed shape. PVF-Vectran membrane is a typical two-dimensional flexible braided composite, which has the characteristics of high strength, impact resistance, stable chemical properties and low creep. It is mainly used in flexible structures such as airbags of airships.
Airships obtain lift through a large amount of hydrogen or helium in the airbag. Due to the advantages of long dwell time and low navigation cost, airships in the stratosphere, which can withstand complex space environment, are the focus of research on near space resources development equipment all over the world. They have the functions of communication relay station, meteorological data collection, high-resolution earth observation and so on. The main structure of airship is envelope, and the failure of envelope materials may directly lead to the overall failure of airship structure. Therefore, the research on the mechanical properties of envelope materials and the determination of ultimate strength under long-term load plays an important role in the strength design of airship. To reveal the creep damage and failure mechanism of the envelope material, it is necessary to observe the experimental phenomena and analyze the mechanical parameters at the micro, meso and macro scales. Therefore, it is necessary to study the creep strength of the airship structures from the multi-scale mechanical properties of the envelope material with the help of microscopic observation and time-delay photography.
Firstly, based on the PVF-Vectran envelope material used for airship structures, three scales of specimens such as single fiber, yarn and uniaxial tensile specimen are made to determine the ultimate stress and ultimate strain. Uniaxial tensile creep tests are carried out at five stress levels of 85%, 80%, 75%, 70% and 65% respectively for 48 hours, and acoustic emission signals during uniaxial tension tests are collected by acoustic emission instrument, The damage accumulation characteristics and creep failure behavior of each specimen under different stress levels are obtained, which can provide a reference for the prediction of failure behavior of laminated composites with similar structures.
Secondly, the damage and failure modes of the sample at different scales before and after creep failure are observed and recorded by scanning electronic microscope, the theoretical assumptions of fiber, yarn and envelope material in non-uniformity, original curvature and failure criterion are established, and the acoustic emission signal during uniaxial tension is recorded by acoustic emission instrument, so as to obtain the damage accumulation characteristics and creep failure behavior of each specimen under different stress levels, which can provide a reference for the establishment of biaxial tensile creep damage constitutive model of membrane and the strength analysis of airship structure.
After then, through the plan of PVF-Vectran envelope plane cutting and splicing, two types of scale model which are spherical and parabolic airship structures are obtained. The point cloud coordinates of the two models under the internal pressure of 1kPa are measures by using a three-dimensional laser scanner, which provide adequate data for the calculation of geometric parameters such as volume, diameter and slenderness ratio. Then, the inflatable bearing failure tests of two kinds of capsules were carried out. Based on the tearing theory of membrane material, the cracks of the capsule fragments were located and the starting point of the cracks was analyzed. The high-speed camera is used to record the failure starting point and crack development shape of the capsule structure when reaching the ultimate air pressure, which verifies the correctness of the fragment analysis results, and the results can provide a theoretical basis for the cause analysis of the capsule structure after failure.
Finally, based on the consideration of the cutting effect of the envelope material and the reinforcement of the weld at the splicing position, the finite element refinement models of the two kinds of capsules are established in ABAQUS. According to the previous biaxial tensile test results and the existing theoretical models, the VUMAT subroutine is compiled to define the constitutive relationship of PVF-Vectran membrane, and the results of the inflatable pressure bearing test of the capsule structure are verified, and the development history of stress and strain in the process of inflation and pressure bearing is obtained, which can provide a reference for the in-depth study of the long-term strength of the capsule structure.
KEY WORDS: airships, composite films, creep strength tests, damage accumulation models, failure analysis

论文题目：织物膜材拉剪试验方法与拉剪力学行为研究
硕士研究生：刘魏鹏宇
指导老师：陈务军 教授
摘要
复合织物膜材具有轻质高强的力学性能，优良的抗紫外线、抗老化性能和阻燃等建筑物理性能，较好的焊接、耐揉搓等加工工艺性能，因此，复合织物膜材是目前建筑膜结构主要的材料，也是新型建筑膜材料研发和产业重要方向。同时，复合织物膜材也广泛应用于航空航天等工业领域。虽然关于织物膜材的力学研究较多，但主要集中在拉伸力学特性，关于剪切、拉剪耦合特性研究较少。
本文针对复合织物膜材拉剪力学问题，结合新型交联涂层织物膜材力学机理，开展试验与分析研究，主要包括：织物膜材的拉伸力学性能、双轴拉伸力学性能、双轴剪切力学性能、图框拉剪试验与图框拉剪有限元模拟，可为膜结构工程设计参考应用。
首先，对复合织物膜材法拉利TX30-II分别进行单轴循环与强度试验、双轴拉伸循环与强度试验、双轴剪切试验，以测量材料的单轴拉伸强度、单轴循环弹性模量、双轴循环弹性模量、双轴循环泊松比与双轴剪切模量等力学行为参数，以为后续拉剪试验与模拟提供力学参数。
其次，提出了图框剪切试验设计方法和测试方案，试验首先对十字形试件施加不同初始双轴预应力，并通过数字图像法对卸载回弹后试件的应变损失进行测量分析，最后将图框安装至单轴试验机上进行图框剪切试验，测得不同初始预应力下的材料剪切性能。本文图框背面采用支杆相连，使图框夹具保持较大的初始双轴预拉力，可对较大初始预应力下材料的剪切力学行为进行研究。
再次，对囊体材料UN5120与法拉利TX30-II建筑织物膜材在双轴预紧力作用下的剪切力学行为进行试验，研究复合织物类膜材在双轴拉应力与剪应力的耦合作用下的力学行为，得到复合织物类膜材在不同双轴预应力作用下的单轴拉力-位移曲线与剪应力-剪应变曲线，对不同试验结果进行分析与讨论。
最后，对囊体材料UN5120材料图框试验进行有限元模拟，针对十字形试件的双轴图框试验，探究了膜材拉剪耦合应力分布规律，建立考虑切缝及过渡区影响的数值模型，实现了双轴图框试验的全过程模拟。
关键词：织物膜材;双轴剪切;图框试验;拉剪耦合

TENSILE SHEAR TEST METHOD AND TENSILE SHEAR BEHAVIOR OF FABRIC MEMBRANE
ABSTRACT
Composite fabric membrane material has light and high strength mechanical properties, excellent uv resistance, aging resistance and flame retardant and other building physical properties, good welding, rubbing and other processing properties, therefore, composite fabric membrane material is the main building structure material, and it also is an important direction of new building materials. At the same time, composite fabric membrane material is also widely used in aerospace and other industrial fields. Although there are many researches on the mechanics of fabric membrane materials, they mainly focus on the tensile mechanical properties, but there are few researches on the tension-shear coupling properties.
Based on compound fabric membrane material tensile shear learning problems, combined with new crosslinking coated fabric membrane materials mechanics mechanism, this paper carries out the test and analysis, mainly includes: fabric tensile mechanical properties of membrane material, biaxial tensile mechanical properties, biaxial shear mechanics performance, tensile frame finite element simulation of tensile shear test and tensile frame simulation, which can be applied for the membrane structure engineering design reference.
Firstly, uniaxial cycling and strength test, biaxial tensile cycling and strength test and biaxial shear test are respectively carried out on ferrari TX30-II composite fabric membrane material to measure the mechanical behavior parameters such as uniaxial tensile strength, uniaxial cyclic elastic modulus, biaxial cyclic elastic modulus, biaxial cyclic Poisson's ratio and biaxial shear modulus of the material. It can provide mechanical parameters for subsequent tensile and shear test.
Secondly, presents the design tensile frame shear test method and test scheme, test first to exert different initial biaxial prestressed cruciform specimen, and through the method of unloading rebound after the specimen strain loss measurement by DIC, finally install drawing frame to uniaxial test on tensile frame shear test, shear with measured under different initial prestressing material performance. In this paper, the back of the tensile frame is connected with a support rod, so that the tensile frame clamp maintains a large initial biaxial pre-tension, which can be used to study the shear mechanical behavior of the material under a large initial prestress.
Thirdly, the shear mechanical behavior of the capsule material UN5120 and Ferrari TX30-II building fabric membrane material under the action of biaxial preload was tested to study the mechanical behavior of composite fabric membrane material under the coupling action of biaxial tensile stress and shear stress. The uniaxial tension displacement curve and shear stress shear strain curve of composite fabric membrane under different biaxial prestress were obtained. The different test results were analyzed and discussed.
Finally, finite element simulation was carried out for the material frame test of capsule material UN5120. Aiming at the biaxial frame test of cruciform specimen, the distribution law of the tensile and shear coupling stress of membrane material was explored, and a numerical model was established considering the influence of slit and transition zone, so as to realize the whole process simulation of the tensile frame test.
KEY WORDS: fabric membrane materials;biaxial shearing; frame test; tension-shear coupling