PMMA动态拉伸破坏实验研究

本文利用Hopkinson压杆系统对PMMA变截面柱锥试样进行冲击加载,研究材料动态拉伸断裂特性和破坏机理.由于PMMA的拉压强度差别不大,采用圆柱状试件难以产生反射拉伸破坏,因此,将试样设计为圆柱锥状,实验时,将试样的圆柱端粘贴于输入杆上.实验结果表明,在一定的撞击速度下,由于应力波的汇聚作用,在试样锥段产生了反射拉伸破坏;当撞击速度较高时,在产生反射拉伸破坏的同时,试件中还产生了银纹区,随着打击速度的提高,银纹区域增大.同时根据断口的SEM观察,研究了材料的破坏机制.     

爆炸驱动下飞板运动速度的实验研究

在理论分析的基础上 ,建立了双反射镜法实验测量系统 ,设计了一套实验装置和闪光灯充电电路 ,解决了双反射镜实验中被测试件的安装精度和外照明光源问题。实验测量了滑移爆轰作用下 ,不同爆炸焊接复板材料的运动速度 ,结果表明该方法有效实用。     

充气压力容器高速撞击实验研究

为了确定高速撞击条件下压力容器发生具有撕裂的简单穿孔和裂纹失稳扩展的界限 ,实验采用长径比为 0 56的铝柱状弹丸在约 1 7km/s的速度下正撞击铝罐。大部分实验采用的是未防护的铝罐 ,铝罐的压力从 0～ 0 4MPa变化来探索获得不同损伤形式的压力条件。给出了铝罐前、后壁从穿孔到裂纹失稳扩展的实验结果。防护铝罐的主要损伤是其前壁的裂纹失稳扩展。确定了发生裂纹失稳扩展的临界压力 ,并对发生裂纹失稳扩展的临界尺寸进行了分析。     

弹丸高速撞击压力容器损伤实验研究

微流星体及空间碎片的高速撞击威胁着航天器的安全运行。而压力容器是航天器上受微流星体及空间碎片撞击威胁最大的关键部件之一。对压力容器的高速撞击能导致其发生破裂而过早终止航天器的使命。本文的目的是通过实验研究,确定高速撞击条件下压力容器发生具有撕裂的简单穿孔和裂纹失稳破裂的界限。实验样件选择6063铝合金管焊接制成。高速撞击实验采用铝柱状弹丸和球形弹丸在3km/s左右的速度下正撞击铝压力容器,铝压力容器的压力从0～30Bar变化来探索获得不同损伤形式的压力条件。给出了铝压力容器前、后壁从穿孔到裂纹失稳破裂的实验结果。未防护充水铝压力容器的主要损伤是其前壁的裂纹失稳破裂。而防护充水铝压力容器在前璧未穿孔时未发生破裂。未防护充气压力容器在给定的实验条件下前璧未发生破裂而后璧发生破裂。防护充气压力容器在小防护间距时前璧发生破裂。     

准静态一维应变实验装置的设计与验证研究

准一维应变状态测试技术对混凝土等脆性材料的力学性能研究具有重要意义,而目前准一维应变实验装置设计单一、互换性较差,所以无法实现普及。针对上述情况,本文在套管围压装置的基础上进行叠加式设计与组合式分离设计,旨在建立一套结构简单、适用性广的准静态被动围压装置。基于数值模拟方法,本文对混凝土试样在一维应变状态与套管围压作用下的力学特性进行研究,从而分析混凝土试样在套管装置下能达到一维应变状态的程度;同时,对叠加式套管与整体式套管的性能进行比较,发现叠加式套管理论上能在准一维应变实验中很好地代替整体式套管。组合式分离设计的有效性最终通过相关实验得到验证。     

完整岩体卸荷破坏的模型试验研究

本文在真三轴应力状态下，采用三种与工程开挖对应的卸荷方式，对完整岩体模型试样进行了卸荷破坏试验．通过对试样在整个试验过程中的应力、应变及声发射等进行的量测，以期揭示岩体在工程卸荷条件下的变形破坏特征     

准静态压缩应力-应变曲线测量方法的探索

本文介绍了用应变片直接测量材料的准静态应力-应变曲线的试验研究。在MTS810材料试验机上分别对93W、G50、砂浆等几种材料进行了准静态压缩试验。由于仪器的系统误差不能由MTSCod规准确的得到材料的真实的弹性变形。为此在试件的中部贴应变片得到材料的弹性变形,塑性变形仍旧由MTSCod规记录,从而得到试样的真实的应变,准确获得准静态压缩应力-应变曲线。试验结果表明:在试件中部贴应变片的方法能够准确得到该材料的杨氏模量;在试件两端垫块的刀口上安置Cod规可得到材料的应力应变曲线。两者的合理组合即可得到准确而完整的准静态压缩应力-应变曲线。试验中还发现准静态实验中试件的断面加工不平,偏心压缩等都会影响E的准确测量。     

AN EXPERIMENTAL INVESTIGATION ON THE IMPACT PERFORATION FAILURE OF WATER-FILLED-PRESSURIZED PIPELINES

Some experimental data recorded from impact tests on empty and water-filled pressurized mild steel pipes are presented. The pipes were supported as a three-span continuous beam and impacted laterally by a rigid indenter at the mid-span of middle span. Three kinds of indenter nose shapes were used: blunt-nose, hemisphere-nose and 90?conical-nose. The internal pressure ranged up to 20 MPa. The perforation failure modes and corresponding critical impact energies were obtained under different test conditions. The time-history curves of the internal pressure and impact force were given. The experiments show that the media filled in the tube greatly decreased the ballistic limit energy.     

气固射流加砂方式中颗粒速度场及浓度场的实验研究

将激光数字式粒子成像测速技术(DPIV)方法应用于砂尘环境实验中颗粒速度场及浓度场的实验研究,成功地对环境风洞中颗粒速度场及浓度分布进行了无干扰瞬态测量,实验结果较真实地反映了气固射流加砂方式中粒子速度场及浓度场的分布规律。文中所获得的规律性认识为我国自行研制大型砂尘实验设备,确定加砂/尘方法及实验段参数等提供了依据。     

Computational Investigation of Wing Flutter

Numerical aeroelastic simulation of a high-aspect-ratio transport type wing model in transonic region is presented. The aeroelastic responses of the wing are extracted by integrating compressible thin-layer Navier-Stokes equations coupled with the equations of motion of the wing structure, in a time dependent manner. The Yee-Harten implicit TVD scheme and the Wilson's θ method are employed to integrate these equations, respectively. Flutter boundaries were found for Mach number range, 0.7 to 0.85 and the results were compared with experimental flutter boundaries. Futhermore, Limit Cycle Oscillations were found and the characteristics of the flutter and limit cycle oscillations are investigated and discussed.     

Experimental Investigation of the Velocity Field near a Cylinder in a Contonuously Stratified Fluid

The velocity distributions in the upstream disturbance and lagging wake are measured using markers in the shadow flow pattern near a horizontal cylinder uniformly towed in a stratified fluid. The dimensions of the upstream total blocking zone and the velocity damping laws in the upstream disturbance are determined. On the basis of the experimental results the actual limits of applicability of existing methods of calculating the structure of stratified flow past obstacles are found for small Froude numbers.     

Failure analysis of the total knee prosthesis anchored with PMMA

Stress and failure analysis of the bone/cement/metal prosthesis of the femorotibial joint are made. The system is modelled by the finite element method in two dimensions while the strain energy density (SED) criterion is applied to locate the potential medial and lateral failure sites in the PMMA for five different prosthesis positions. Examined also is the influence of the PMMA cement that may or may not completely fill the ligament between the stems of the femoral prosthetic component.According to the SED criterion, the cement next to the base of the femoral component systems is the weakest, particularly when the implant is tilted locally resulting in an uneven spread of the cement lining. A 10° local misalignment of the metal implant with reference to the mechanical axis of the femur is more detrimental than the 20° valgus or 20° varus position. The precise locations of possible failure sites on the cement are predicted even though the overall structural integrity of the total knee prosthesis system is found to be adequate for the ideal case investigated. Loadings of a dynamic nature combined with imperfections such as voids and defects in the cement are additional factors that can enhance failure initiation.     

第一门槛值区间单防护屏防护正撞击实验研究

微流星及空间碎片的高速撞击威胁着航天器的安全运行,导致其严重的损伤和灾难性的失效。本文级出和分析了柱状弹丸在第一门槛值速度区间正撞击铝合金单防护屏防护结构实验研究的结果。结果表明：单防护屏防护结构在此速度区间受柱状弹丸正撞击其损伤是比较严重的,并得到了柱状弹丸撞击防护结构损伤的特点和规律。     

载荷作用下泡沫铝的波速与变形关系的实验研究

对单轴压缩下两种密度的泡沫铝进行了三种频率(50kHz、200kHz和300kHz)的声波测试,同时运用CCD对实验进行跟踪拍摄。分析结果表明:P波波速随载荷增大而增加,当载荷达到屈服力时,波速有所下降;S波有相同趋势,但变化比P波小。应用数字图像相关方法对CCD拍摄图片进行处理,得到不同载荷阶段下泡沫铝的全场应变;采用Weibull函数对全场应变分布进行拟合,研究了Weibull分布参数随载荷变化的规律。由此,初步建立了载荷作用下材料结构变化与波速的关系。此研究对于声波探测领域有很好的指导意义。     

侧向载荷作用下的套管塑性破坏力学模型与计算分析

首次针对套管柱承受屈服页岩横向载荷展开研究,运用虚功原理建立了一个横向载荷均匀分布页岩剖面上的力学模型,得出在套管柱径向变形阶段,其横向承载强度迅速下降;对于轴向变形,其横向承载强度变化不大.本文通过计算实例,为优选套管方案提供了可靠的分析工具,增强屈服粘土层段套管柱承受横向载荷的能力,减小套管失效率,降低油田操作费用.     

Computational and Experimental Investigation of the Development of Turbulent Mixing in a Gas Layering in Passage of a Shock Wave

The experimental data and the results of direct numerical simulation of the flow developed in a constant-cross-section tube in passage of a shock wave through a three-layer gas system are presented. The three-layer systemis formed as a result ofmounting two thin films in the tube and filling the space between them with gases of different densities. The first interface (thin film) makes an angle of 45? with the shock front and the second interface is located in parallel to the front. The shock wave is formed at the left tube end and moves towards the first interface at the Mach number M = 2.4. The results of simulation of the problem are compared with the experimental data.     

含冲击损伤复合材料层压板压缩破坏机制的声发射特性研究

复合材料层压板在压缩破坏过程中包含了丰富的声发射信息.为了研究含冲击损伤的复合材料层压板的压缩破坏机制,采用声发射观察层压板的压缩破坏过程,通过分析声发射信号的特征规律,表征了在压缩载荷下材料损伤的形式及其演化过程.结果表明:通过对声发射参数(撞击计数、能量、幅值、事件位置)和载荷曲线进行综合分析,发现损伤的发展过程经过了初始阶段、平稳扩展期和断裂阶段,冲击造成的分层区域最先出现屈曲并最早破坏;在损伤初始阶段和平稳扩展期间,损伤是一种渐进式的增长,层压板具有一定的承载能力;在断裂阶段损伤快速扩展,层压板的承载能力迅速下降,在出现纤维密集断裂的现象后整体破坏.     

压缩-剪切复合应力下PMMA材料的静态力学特性研究

通过压缩具有一定倾斜角(0°,10°,15°,20°和25°)试件和双剪切模型试件,实现了单轴压缩、压缩-剪切复合应力以及纯剪切三种应力状态,得到PMMA(聚甲基丙烯酸甲酯)在相应应力状态下的应力-应变曲线,同时对不同应力状态下试件的破坏模式进行了分析。结果表明:在不同受力环境中材料的强度和破坏的机理不同;同单轴压缩状态下相比,材料在压缩-剪切复合应力状态下屈服极限、强度极限以及破坏应变均不同程度的增大,呈现明显的"剪切增强"现象。单轴压缩与压缩-剪切应力状态下试件的破坏模式均为在试件短对角面上出现明显的剪切屈服带,由应力分析得出试件剪应力在短对角面上达到最大,引起在此平面上分子链间滑动从而产生应变软化形成剪切屈服带;双剪切试件的破坏模式为与剪切面呈45°的斜面。     

缺陷对玄武岩中声波波速影响的实验研究

岩体是具有多种微结构的复杂体,具有明显的频散效应.过去的研究基本上将岩体中的孔隙率作为研究岩体微观结构的参数,这种方法无法考虑到岩体中细观缺陷的局部影响.本文选用水泥砂浆为参考材料,通过在试件中预制裂缝来研究含缺陷玄武岩的弹性波传播特性.对含不同裂缝的试样进行了5种频率的声波测试.结果表明:缺陷对玄武岩的弹性波频散效应影响很大,随缺陷所占比例的增大而增大.同时单轴加载下的弹性波测试则说明,在0～20MPa范围内,玄武岩弹性波波速随压力增大开始时候有一定幅度的升高,到8MPa以后由于玄武岩硬度相对较高,波速基本保持在一定水平.     

Experimental Determination of Lagrangian Velocity Statistics in Turbulent Pipe Flow

The calculation of Lagrangian statistics out of experimentally determined data from homogeneously seeded inhomogeneous turbulent flows is far from straightforward since statistical properties are position-dependent, necessitating local sampling. Two solutions for the preferential sampling of faster particles at a certain position in the flow are proposed. The performance of both methods was tested using DNS calculations for turbulent pipe flow. Both methods show a good performance for various statistical properties, thus providing two reliable ways to analyze experimental data from inhomogeneous turbulent flows.