2016级

博士毕业论文

论文题目：基于囊体材料拉剪本构和精细建模的飞艇囊体力学行为
博士研究生：高成军
指导老师：陈务军 教授
摘要
飞艇囊体是飞艇的核心部件，囊体结构性能决定飞艇的安全和工作性能。飞艇囊体由囊体材料平面裁切后热合拼接而成，囊体材料是由织物承力层和多层高分子薄膜复合而成的高性能薄膜材料。目前针对新型高性能囊体材料的拉剪力学行为研究尚不完备，同时受囊体结构制作工艺影响，大尺度飞艇的结构安全性分析和强度预测仍面临挑战。因此，本文针对囊体材料拉剪性能和囊体结构数值分析问题，开展试验、模拟和分析研究，主要包括：
（1）针对新型囊体材料拉伸力学性能，进行了单轴拉伸强度和低周循环拉伸试验，分析了7个偏轴角度的拉伸弹性刚度和泊松比，揭示了囊体材料单轴拉伸非线性力学行为机理；通过11个应力比的双轴循环拉伸试验建立双轴拉伸刚度响应面和泊松比响应面，通过7个应力比的双轴拉伸强度试验建立应力-应变响应面和双轴拉伸五参数强度准则，揭示了囊体材料双轴拉伸非线性力学行为机理。
（2）针对新型囊体材料剪切力学性能，进行了45°偏轴拉伸试验，提出了基于平面应力关系的偏轴拉伸剪切力学性能表征方法，建立了囊体材料非线性剪应力-剪应变关系；进行了双轴剪切试验，得到了剪应力-剪应变滞回曲线，提出了低剪应力下剪切刚度计算方法，揭示了囊体材料剪切力学行为机理。
（3）建立基于双轴循环拉伸弹性参数响应面和双轴剪切刚度的囊体材料拉剪本构模型，编写可用于囊体结构隐式分析的UMAT子程序；建立基于双轴拉伸强度试验的应力-应变响应面和偏轴拉伸剪应力-剪应变曲线的囊体材料本构模型，编写可用于囊体结构显式分析的VUMAT子程序；通过材料试验的数值分析，验证本构模型和分析方法的准确性。
（4）提出了囊体结构精细建模方法，根据囊体母线方程计算展平后平面裁片，然后对平面裁片施加位移逆向提升形成三维裁片，最后将裁片组装成囊体结构精细模型；分别进行了精细模型与母线回转模型的数值模拟与分析，结果表明精细模型裁片宽度方向变形呈现中间大边缘小的特点，而母线回转体裁片宽度方向变形均匀无差异。
（5）针对环索增强囊体结构，考虑囊体材料双轴拉伸和剪切本构，引入热合缝和加劲环索本构模型，考虑加劲环索与囊体接触关系，建立了环索增强囊体结构精细模型，进行了强度数值分析和充气临界压力预测；数值分析结果表明加劲环索能够显著提高囊体承压能力（本文分析案例中提高17.81%），进行了环索增强囊体结构充气极限承压试验，试验得到充气临界压力与精细模型数值分析强度结果相差仅1.19%，验证了数值分析方法的有效性。

关键词：囊体材料；拉剪本构；囊体结构；精细建模；结构强度
 
MECHANICAL BEHAVIOR OF AIRSHIP ENVELOPE BASED ON TENSILE-SHEAR CONSTITUTIVE AND REFINED MODELLING

Abstract

The envelope is the most important part of airship, which determines the safety and structural performance of the airship. The envelope is welded by cutting pieces of envelope fabrics, which is composed of load-bearing fabric and multi-functional foil. The structural safety analysis and strength prediction of large-scale airships are facing challenges, due to the incomplete research on the tensile and shear mechanical behavior of the novel high-performance envelope fabrics, and the influence of envelope manufacturing process. Therefore, experiments, simulation and analysis were carried out to solve these problems in this paper.
Firstly, for the tensile properties of a novel envelope fabrics, uniaxial tensile strength test and low cycle tensile test were carried out, the tensile elastic modulus and Poisson's ratio in 7 off-axis angles were measured, and the nonlinear mechanical behavior uniaxial tension was revealed. Biaxial cyclic tensile test with 11 stress ratios and biaxial monotonic tensile tests with 7 stress ratios were carried out, the nonlinear properties of envelope fabrics were expressed by stress-strain response surface and elastic parameters (elastic stiffness and Poisson's ratio) response surface, the strength criterion with five parameters was obtained according to biaxial monotonic tensile tests, and the nonlinear behavior of biaxial tension was revealed.
Secondly, for the shear properties of a novel envelope fabrics, bias-extension test with 45° specimen were carried out, the characterization method of shear properties based on the angle variation between warp and weft was proposed, and the nonlinear shear constitutive model was established. The hysteretic curve of shear stress-strain was obtained by biaxial cyclic shear test, and the calculation method of shear stiffness under low shear stress level was proposed, and the shear behavior of envelope fabrics was revealed.
Thirdly, the constitutive model of envelope fabrics was established based on the elastic parameters response surface and shear stiffness, and the UMAT subroutine was developed for implicit analysis; the constitutive model of envelope fabrics also can be expressed based on the stress-strain response surface of biaxial strength test and the shear stress-strain curve of bias-extension test, and the VUMAT subroutine was developed for explicit analysis. Furthermore, the accuracy of the constitutive model were verified by the numerical analysis of material test.
Fourly, refined modelling method of envelope structure was proposed. The flatten-cutting were calculated according to the generatrix equation, and then the reverse displacement was applied to the flatten-cutting which was assembled into a refined model of envelope structure in final step. The numerical results shown that the deformation of cutting pieces in refined model was larger in the middle and smaller in the edge, while the deformation of cutting pieces in ideal rotator was uniform
Finally, the constitutive models of envelope fabric, weld and rope were introduced to refined model of rope reinforced envelope structure, the strength numerical analysis and inflation critical pressure prediction was carried out. The numerical results shown that the hoop rope can significantly improve the pressure bearing capacity of envelope strucrure (the critical internal pressure of refined model with rope was 18% higher than that without rope in this paper). The inflation failure test of rope reinforced envelope structure was carried out, and the difference of critical pressure between experiment and refined model numerical analysis was 1.19%, which verified the effectiveness and accuracy of numerical analysis method.

KEY WORDS: envelope fabrics;tensile-shear constitutive model;envelope structure; refined modelling,;structure strength

硕士毕业论文

论文题目： ETFE气枕平面裁切成形方法试验与数值模拟
研究生：李一坡
指导老师：陈务军 教授
学科专业：结构工程

摘要
具有高透光、自洁性和可循环利用等特性的乙烯-四氟乙烯（ethylene- tetra-fluoro-ethylene，ETFE）薄膜是一种绿色可持续的轻质高强的透明建筑材料，常与钢结构、铝合金结构等一起形成大跨膜结构，被广泛应用于建筑领域。本文以ETFE薄膜和ETFE气枕为研究对象，对ETFE薄膜进行了不同温度下的单轴拉伸和徐变试验，并对ETFE气枕不同温度下的平面裁切成形过程进行了试验和模拟。
首先，本文对15组250μm厚的ETFE薄膜长条形试件在40℃、60℃、80℃的温度下进行了不同应变速率（1%/min、10%/min、50%/min、100%/min、200%/min）的单轴拉伸试验，得到了材料的屈服应力、屈服应变在不同温度下随应变速率的变化规律。试验结果对不同温度下的工程分析具有参考价值。
其次，本文对12组ETFE薄膜在40℃到80℃下进行了不同温度和不同应力下的徐变试验。对试验数据采用对数拟合，分析了拟合参数随试验条件的变化规律，并得出考虑加载应力和温度的函数关系式，以用于描述ETFE薄膜在不同温度和应力下的徐变性能。该函数关系式对后续分析ETFE薄膜力学性能和长期荷载作用下结构性能有重要作用。试验和分析结果表明，ETFE薄膜的徐变特性随加载应力的增大和温度的升高而明显增大，且呈非线性关系；40℃以下且应力在6MPa以下时材料的徐变较小且增加缓慢；40℃以上或较高高应力状态下，徐变显著。
再次，基于修正的时间强化理论模型建立了ETFE在不同温度下的徐变模型，该徐变模型考虑不同温度下徐变应变与应力水平、徐变时间的关系。通过对徐变试验结果的拟合确定不同温度下的材料模型参数。利用该模型，通过有限元软件ANSYS对徐变试验结果进行数值模拟，结果表明修正的时间强化模型对ETFE徐变的描述具有适用性。
最后，基于各向同性强化弹塑性模型和修正的时间硬化模型对不同温度、不同内压下的ETFE气枕的充成形过程进行了数值模拟，研究了基于平面裁切的ETFE气枕在不同温度下充气成形过程的力学特性，对高温条件下ETFE气枕的徐变特性研究具有一定的参考意义。

关键词：ETFE薄膜；单轴拉伸力学性能；徐变特性；温度应力效应；修正的时间强化模型；ETFE气枕；充气成形过程；平面裁切
 
EXPERIMENTAL STUDY AND SIMULATION OF ETFE CUSHION BASED ON FLAT-PATTERNING FORMING METHOD

Abstract
ETFE (ethylene-tetra-fluoro-ethylene) is a new type of transparent membrane material with the superior properties of lightweight, high light transmission, self-cleaning and recyclability, and is widely used in the large-scale membrane structures, often combined with steel or aluminum alloy structures. In this paper, ETFE foil and ETFE cushion were studied, experiment method and FEM method were performed to investigate the mechanical properties under different temperature. Main content of this paper includes uniaxial tensile test and creep test under different temperature, and the experimental study and simulation of ETFE cushion based on flat-patterning forming method were also conducted.
Firstly, uniaxial tensile tests on fifteen groups of ETFE foil samples of 250μm thick were conducted under different temperature (40℃, 60℃ and 80℃) and loading strain rate (1%/min, 10%/min, 50%/min, 100%/min and 200%/min). The yeild strength of ETFE under different temperature and strain rate were determined by area method, and the variation law of the yeild strength was obtained by curve fitting.The results of the experiments are helpful to practical engineering application.
Secondly, creep tests on twelf groups of ETFE foil samples were conducted under different temperature and stress. According to the test results, this paper adopted the logarithmic curve for the fitting of time-deformation relations, and analysed the changing regularity of fit parameters under different test conditions. The functional relationship between creep strain, stress and temperature was got and used to describe the creep properties of ETFE foils under different temperature and stress. The experiment and analysis results show that large creep strain happens under high temperature or large stress, and the relationship between strain, stress and temperature is nonlinear. It is also found that the creep strain keeps small if the temperature is under 40℃ and the stress is under 6MPa. Large creep strain is observed in case of a high temperature or a large stress.
Subsequently, the creep model of ETFE at different temperatures was established based on the Modified Time Hardening Theory model. The relationship between true creep strain, stress and creep time at different temperatures is considered in this model. The parameters of the model were determined by fitting the results of the 24h creep tests. In addition, the numerical simulation of the creep test results by the finite element software ANSYS shows that the Modified Time Hardening model is applicable to describe the creep properties of ETFE foils.
Finally, the flat-patterning forming process of ETFE cushions under different temperatures and loading conditions were simulated based on the Multilinear Isotropic Hardening model and the Modified Time Hardening model. The mechanical properties of ETFE cushions under different conditions were studied, which are helpful to understanding the flat-patterning forming process of ETFE cushions under different temperatures in practical engineering application.

KEY WORDS: ethylene-tetrafluoroethylene (ETFE) foil; uniaxial mechanical properties; creep properties; temperature-stress effects;Modified Time Hardening model; ETFE cushion; forming method;flat patterning
 
论文题目：飞艇结构力学特性试验与数值模拟
研究生：李意
指导老师：陈务军 教授
学科专业：结构工程

摘要
随着技术革新，高分子材料的应用以及太阳能电池的发展，平流层飞艇技术的研究再次回到人们的视野，平流层飞艇作为近地空间作业的主要平台之一，具有覆盖范围广，工作寿命长，能源消耗低等优势。目前关于平流层飞艇的研究主要集中在总体布局设计，囊体材料，飞行控制以及能源系统等方面。本文基于充气膜结构的基本理论，对柔性飞艇结构，半柔性飞艇结构的静力学特性进行研究，主要内容包括柔性飞艇充气与静载试验、数值模拟，半柔性飞艇结构静载试验与分析。
首先，以飞艇囊体缩比模型为研究对象，开展了充气试验和静载试验，对飞艇主气囊在充气过程中和静载过程中的结构响应进行测量，得到囊体静刚度特性的一般规律;
其次，基于充气式结构裁切效应对囊体裁片建模，得到柔性飞艇主气囊的初始构形，并对囊体充气过程和静载过程进行数值模拟，通过与试验结果对比，验证建模方法和试验测量的正确性，分析囊体静力学结构响应特性；
再次，介绍半柔性结构飞艇的组成和设计原理，为了提高龙骨的整体刚度，内部采用预应力钢丝绳进行张拉，通过对预应力的优化分析，得到龙骨最终的几何形式；将龙骨与主气囊组装成整体，并开展静载试验，对半柔性飞艇的结构响应进行测量，得到半柔性飞艇结构的静刚度特性的一般规律；
最后，为了验证预应力优化的结果，对龙骨施加预应力过程进行数值模拟，得到预应力施加结束后，龙骨中应力分布的初始状态；对半柔性结构飞艇静载过程进行模拟，通过对比试验结果，对半柔性飞艇的力学响应规律进行研究，为半柔性飞艇的结构设计提供参考。

关键词：平流层飞艇；充气膜结构；静力学特性；裁切效应；半柔性飞艇
 
 
EXPERIMENTS AND NUMERICAL SIMULATION OF STRUCTURAL MECHANICAL CHARACTERISTICS OF AIRSHIP

Abstract
With the technological innovation, the application of polymer materials and the development of solar cells, the research on stratospheric airship technology has once again returned to people's vision. As a main platform for near-Earth space operations, stratospheric airships have a wide coverage and working life. Long, low energy consumption and other advantages. Current research on stratospheric airships focuses on overall layout design, capsule materials, flight control, and energy systems. Based on the basic theory of inflatable membrane structure, this paper studies the static characteristics of flexible airship structure and semi-flexible airship structure. The main contents include flexible airship inflation test and static load test, flexible airship capsule inflation process and static load process numerical simulation. Composition and design of semi-flexible structure airship, static load test and analysis of semi-flexible airship structure.
Firstly, based on self-designed airship capsule shrinkage model, the inflation test and the static load test were carried out to measure the structural response of the airship during the inflation process and the static load process, and the static stiffness characteristics of the capsule were obtained.
Secondly, the initial configuration of the flexible airship is obtained based on the structural cutting effect of inflation, and the inflation process and the static load process are numerically simulated. The model is verified by comparison with the test results. And the correctness of the test measurements, analysis of the static structural response characteristics of the capsule.
Thirdly, the composition and design principle of the semi-flexible structure airship are introduced. In order to improve the overall rigidity of the keel, the stress in pre-stressed steel wire rope is optimized and the final geometric form of the keel is obtained; then the keel is assembled with the main capsule, through the implementation of static load test, the structural response of the semi-flexible airship is measured, and the general law of the static stiffness characteristics of the semi-flexible airship structure is obtained.
Finally, in order to verify the results of prestress optimization, the prestressing process of the keel is simulated. Based on the comparison of experimental results and Simulated results, the static load process of the semi-flexible structure airship is studied, which provides reference for the structural design of semi-flexible airships.

KEY WORDS: stratospheric airship;inflatable membrane structure; static characteristics; semi-flexible airships
 
论文题目：内置氦气囊的非稳定形态机理分析
研究生：段元洪
指导老师：张大旭 副教授，陈务军 教授
学科专业：结构工程

摘要
内置氦气囊处于高空重型硬式飞艇气室内，外部主气囊为空气囊，内部充满氦气用于产生静升力来平衡艇体绝大部分自重。作为提供飞艇浮力的核心载体，气囊在不同充气比例状态下形态的改变会引起浮心和重心的改变，进而影响飞艇的飞行姿态控制。本文基于控制体积法和双向流固耦合的数值模拟分析方法分别建立了数值仿真模型，对内置氦气囊进行泄气模拟分析，得到氦气囊在不同充盈度下的形态位移。
首先，通过非线性显式动力学分析软件LS-DYNA，基于控制体积法建立了氦气囊的充气、稳定、泄气模型，结合ANSYS/LS-DYNA求解器计算得到了氦气囊在泄气过程中的形态位移。结果发现：基于控制体积法建立的氦气囊模型在泄气过程中不能考虑由囊体内外空气、氦气的密度差引起的浮力载荷作用，无法反映浮力对泄气过程的影响。
然后，基于ANSYS的瞬态动力学分析和CFX的多物理场耦合分析，考虑空气与氦气密度差导致的浮力效应，建立了单个氦气囊的泄气仿真模型，进行空气和氦气流体及柔性囊体的多场耦合分析。首先，基于囊体无应力状态进行充气模拟得到初始充气平衡形态，校验数值模拟方法的正确性；然后通过氦气排气量控制模拟氦气囊在0、10、20、30度仰角以及充盈度分别为100%、90%、80%、70%、60%时的形态，从而模拟氦气囊的泄气过程。
最后，通过分析不同仰角姿态、不同充盈度下的氦气囊泄气模型，得到了结论：在仰角为0度时，随充盈度减小，囊体底部约1/3区域发生形态失稳，呈环向均匀小凹陷模态，且向顶部扩展，环向贯通为大凹陷；当有仰角时，凹陷先发生于下腹部，环向非对称，且随仰角增大，下腹部凹陷向顶部扩大。
关键词：内置氦气囊；浮力效应；双向流固耦合；控制体积法；氦气充盈度；非稳定形态
 
UNSTABLE MORPHOLOGY MECHANISM ANALYSIS OF HELIUM BAG BEDDED-INSIDE ENVELOP

Abstract
The helium bag bedded-inside envelop is located in the air chamber of the heavy airship at high altitude, and the outer main bag is an air bag. The helium gas is filled inside to generate static lift to balance most of the self-weight of the hull. As the core carrier to provide the buoyancy of airship, the change of the shape of airbag under different aeration ratio will cause the change of the buoyancy center and center of gravity, and then affect the flight attitude control of airship. Based on the control volume method and the two-way fluid-solid coupling numerical simulation analysis method, the numerical simulation models are established respectively. The helium bag bedded-inside envelop is simulated and analyzed, and the shape displacement of the helium bag at different filling degrees is obtained.
Firstly, by using LS-DYNA, a non-linear explicit dynamic analysis software, the inflatable, stable and degassing model of helium bag was established based on the control volume method. The shape displacement of helium bag during degassing was calculated by ANSYS/LS-DYNA solver. The results show that the helium bag model based on the control volume method could not consider the density difference between air and helium which caused the buoyancy load. Therefore, it could not reflect the influence of buoyancy on venting process.
Then, based on ANSYS transient dynamics analysis and CFX multi-physical field coupling analysis, considering the buoyancy effect caused by the density difference between air and helium, a multi-phase coupling analysis of air, helium and flexible membrane bag has been carried out. Firstly, the initial inflated equilibrium shape was commutated from the unstressed geometry of the helium bag by inflating simulation, the buoyancy and gravity equilibrium and deformation validate the numerical simulation method. Then, the helium gas flow control method was employed to simulate the helium bag at elevation angles of 0, 10, 20, 30 degrees and at filling percentage of 100, 90, 80, 70 and 60 respectively. Thus, the degassing process of helium bag was simulated.
Finally, through the analysis of helium bag deflation models with different elevation angles postures and different filling degrees, it is concluded that when the elevation angle is zero, approximately 1/3 of the bottom of the helium bag becomes unstable as the filling percentage decreases. The circular uniform sag modes were observed to extend toward the top and a large sag in the circumferential direction; when there is an elevation angle, the sag first occurs in the lower part and exhibit circumferential asymmetry. With the increase of the elevation angle, the sag in the lower part expands toward the top.
KEY WORDS: helium bag bedded-inside envelop; buoyancy effect; two-way fluid-structure coupling; control volume method; helium filling percentage; unstable configuration