铝锂合金材料损伤演化方程和层裂准则的研究

 通过平板撞击实验对铝锂合金层裂现象进行实验研究。以细观统计和唯象分析相结合的思想,提出了一种新的微孔洞型损伤演化方程,方程同时计及了孔洞长大、成核以及材料可压缩性的耦合效应对损伤演化的影响。在一维应变波模拟程序中嵌入考虑损伤演化影响的铝锂合金本构方程和状态方程,模拟平板撞击过程靶板自由面质点速度时程曲线,并与实验曲线相对比,得到了铝锂合金材料损伤演化方程及其层裂准则的相关参数。     

Tuler-Butcher模型中考虑已有损伤影响的一种修正

 基于Tuler-Butcher模型提出了一种修正的损伤演化模型。在此基础上,将新的损伤演化方程编入一维Lagrange有限差分动力学程序,对45钢和铝锂合金两种延性金属材料的平板正碰撞层裂实验进行了数值模拟,计算结果与实验结果吻合得较好,验证了本模型的正确性。     

一种新的概念性层裂模型

 在重建Cochran-Banner模型的基础上提出了一种新的概念性层裂模型。这种新模型仅保留Cochran-Banner模型中的强度函数,重新定义损伤,并抛弃了基本假设：一旦微损伤形成,使微损伤演化远远易于使固体进一步体积应变,进而修正了差分微元中固体比容的计算。在新的模型中,一旦拉伸应力达到层裂强度,重新定义的损伤将由强度函数确定的应力松弛方程、计及损伤的能量守恒方程、状态方程以及本构方程等一系列封闭方程组确定。新模型中也仅包含两个参数：层裂强度及临界损伤度,它们的确定能使在一定初、边值条件下的层裂试验的数值计算结果与实验测得的靶自由面速度历史或靶-低阻抗界面应力历史以及回收观测的层裂面上的损伤一致。强调指出,选定强度函数或应力松弛方程提供了确定损伤的可能,同时排除了任何外加的损伤演化方程。     

高温高应变率下混凝土材料的损伤演化方程

采用?74 mm大口径分离式霍普金森压杆(SHPB)对不同温度(20、200、400℃)下的C45混凝土材料进行动态力学性能实验,得到了不同温度、不同应变率下混凝土材料的应力-应变曲线。实验结果表明:在20~400℃温度范围内,混凝土材料具有温度硬化和应变率硬化现象。基于上述实验数据给出了损伤变量关于塑性应变的关系式,并通过相关实验数据确定了不同温度、不同应变率下损伤演化方程的材料参数。将该损伤演化方程应用于混凝土材料的本构关系中,预测结果与实验数据具有较好的一致性,证明了所提出的高温、高应变率下混凝土材料损伤演化方程的合理性。     

一种新型微孔洞损伤模型

 在前人的基础上引入单位体积微损伤个数概念，从而将宏观损伤参量与微观孔洞体积结合起来，并且在孔洞长大扩展时计入惯性效应的影响，得到了一个新的二次型微孔洞损伤演化方程，指出表面扩张效应和惯性效应分别在孔洞长大的不同阶段起主导作用，最后以921钢板撞击实验为参考，通过比较自由面速度时程曲线，得出的结果与实验相符，验证了本模型的正确性。     

简单拉伸下高聚物粘结炸药的损伤测量及损伤演化研究

 在简单拉伸载荷下高聚物粘结炸药呈现出非线性力学行为,这种非线性特性是由于材料内部损伤的演化和发展所致。采用弹性模量下降法测量了高聚物粘结炸药的损伤变量,据此建立了这类材料的损伤演化方程和本构关系,理论结果与实验结果吻合,并进一步讨论了高聚物粘结炸药的损伤度量方法和损伤特性。     

模拟细观非均质材料破坏演化的物理元胞自动机理论

根据非均质材料的细观特征,从基本的能量传递定律出发,建立了一种新的描述细观非均质材料破坏演化的物理元胞自动机(PCA)理论.该理论能够对岩石、混凝土等非均质材料破坏演化进行有效模拟,突破了传统元胞自动机仅限于数学规则运算的框架,使之成为一种有效的物理力学方法.与岩石力学实验结果对比分析表明,PCA模拟结果与实验结果基本符合 关键词： 元胞自动机 细观非均质材料     

5083铝合金宽应变率实验与基于损伤的本构模型研究

5083铝合金材料在工程领域应用广泛,会受到包括冲击和碰撞等多种不同情况的强动加载,亟需对其宽应变率加载下的力学性能及其本构模型开展研究。首先,对5083铝合金进行了系统的准静态实验及中、高应变率加载下的拉伸和压缩实验,得到了宽应变率加载下的应力-应变曲线。实验结果表明,该材料在同一实验条件下所得到的应力-应变曲线,其强化阶段的拉伸曲线总是低于压缩曲线,并从微观机制上对这一现象进行了合理解释。然后,通过引入损伤,考虑了损伤对该材料拉伸加载情况下的力学性能影响。基于连续介质力学及其实验结果,获得了损伤演化方程。最后,借助改进的Johnson-Cook(JC)本构模型,并基于已确定的损伤演化方程,得到了考虑损伤的5083铝合金本构模型。通过实验曲线与所得模型曲线的对比,吻合良好,表明该模拟具有很好的适用性,能够对该材料的工程应用提供有效的科学依据、分析模型和必要的参考。     

玄武岩纤维布超高速撞击损伤分析

 在航天器空间碎片超高速撞击防护领域中,采用高技术纤维作为防护材料是当今防护结构发展的趋势之一,玄武岩纤维（Basalt Fiber）是近年来受到关注的一种高强度纤维。对玄武岩纤维织物受铝合金弹丸超高速撞击时的宏观穿孔损伤特性和细观纤维丝断裂损伤特性进行了分析研究,观察到了冲击高压造成的材料熔化现象,根据实验结果拟合得到了玄武岩纤维布撞击孔的孔径方程,根据纤维丝断口形貌分析了纤维丝的断裂原因。研究结果可为玄武岩纤维材料在空间碎片防护结构中的应用提供有益参考。     

基于孔洞体胞模型铸造镁合金ML308的本构方程

铸造镁合金不可避免地包含许多微孔洞,这些微孔洞在材料的后续加工及服役过程中将发生演化,并对材料的力学行为产生重要影响.基于球形孔洞体胞模型,提出微孔洞长大及形核方程,它们构成微孔洞的演化方程.根据孔洞演化将造成材料性质弱化的物理机制,将微孔洞演化以弱化函数的形式引入到非经典弹塑性本构方程,得到考虑孔洞演化的铸造镁合金弹塑性本构方程.发展与本构方程相应的有限元数值分析程序,用其模拟了铸造镁合金ML308的微孔洞演化及力学行为,计算结果与实验结果符合较好. 关键词： 铸造镁合金 孔洞体胞模型 孔洞演化方程 本构方程     

Laser-Induced Damage Growth on Larger-Aperture Fused Silica Optical Components at 351nm

Laser-induced damage is a key lifetime limiter for optics in high-power laser facility. Damage initiation and growth under 351 nm high-fluence laser irradiation are observed on larger-aperture fused silica optics. The input surface of one fused silica component is damaged most severely and an explanation is presented. Obscurations and the area of a scratch on it are found to grow exponentially with the shot number. The area of damage site grows linearly. Micrographs of damage sites support the micro-explosion damage model which could be used to qualitatively explain the phenomena.     

Analysis of thresholds of optical damage to the potassium and sodium chloride surface by femtosecond laser pulses

Comparative analysis of optical damage thresholds of potassium and sodium chloride surface by pulses with durations of 80 and 40 fs has been carried out. The critical radiation power in a pulse at the optical damage threshold for the ion crystals studied has been determined. An estimate of the optical damage thresholds of the sodium chloride surface by laser pulses with a duration of 20 fs is presented.     

The use of pseudo-faults for novelty detection in SHM

The main problem associated with pattern recognition based approaches to Structural Health Monitoring (SHM) is that damage localisation and quantification almost always require supervised learning. In the case of high-value engineering structures like aircraft, it is simply not possible to generate the training data associated with damage by experiment. It is also unlikely that data can always be generated by simulation as the models required would often need to be of such high fidelity that the costs of development and the run-times would again be prohibitive. The object of this paper is to explore the potential of a simple experimental strategy, which involves adding masses to the structure, in the attempt to extract features for novelty detection. The idea itself is not presented as revolutionary based on the fact that adding masses has been considered as a case of damage before, however, an in-depth investigation of its suitability for guiding feature selection is presented here. The approach is illustrated first on a simple structure by using data generated from a finite-element (FE) simulation and then validated experimentally on a more complicated laboratory structure. Simulated damage, in the form of a loss in the stiffness in the case of the numerical model and of a saw-cut in the case of the structure is used for comparison. The results show similar patterns in both cases which suggests a potential use of the method for higher level damage detection.     

FRF-based damage localization method with noise suppression approach

In this paper a noise-robust damage identification method is presented for localization of structural damage in presence of heavy noise influences. The method works based on Frequency Response Functions (FRFs) of the damaged structure without any prior knowledge of the healthy state. The main innovation of this study starts with convolving FRFs with Gaussian kernel to suppress the noise. Denoised signals are then used to develop shape signals according to the second derivative of the operational mode shapes at frequencies in the half-power bandwidth of the center resonant frequencies. The scheme is followed by normalization of shape signals to create a two-dimensional map indicating the damage pattern. The validation of the method was carried out based on simulated data and experimental measurements. The simulated data polluted with 10 percent random noise considering four different conditions: (i) un-correlated noise with Gaussian distribution (ii) noise with non-Gaussian exponential distribution (iii) noise with non-Gaussian Log-normal distribution and (iv) correlated colored noise. The robustness of the method was examined with respect to the damage severity with various damage conditions. Finally, damage detection experiments of a fixed–fixed steel beam are presented to illustrate the feasibility and effectiveness of the proposed method. According to the numerical and experimental investigations, it was demonstrated that the proposed approach presents satisfactory damage indices both in single and multiple damage states in presence of high level noise. Hence, the method can overcome the problems of output measurement noise and deliver encouraging results on damage localization.     

光学材料的激光损伤形态研究

 提出了破坏形态因子的概念并以高功率连续激光作用下的光学材料的热力学响应为例，通过积分变换的方法给出激光作用期间温度场和应力场的解析形式，继而得到破坏形态因子的表达式及其简化形式，研究破坏形态因子与材料性质、激光参数的关系，从而预言特定的材料在特定的激光作用下的破坏形态。     

高纯铜初始层裂的微损伤特性研究

本文对平面冲击加载下高纯铜初始层裂的微损伤特性进行了研究. 利用准三维的表面轮廓测试技术, 对冲击加载“软回收”的样品截面进行测试. 通过对测试数据的重构、量化和统计分析, 结果表明: 拉伸应力持续时间和加载应力幅值的增加, 都会加剧样品内部损伤局域化程度. 样品内损伤区域宽度是亚微米尺度的损伤演化的结果, 并且亚微米尺度的演化速率随着拉伸应变率的增加而单调递增. 通过统计获得了样品内微损伤的尺寸分布特征, 并分析了其与损伤演化进程的关联.     

A generalized flexibility matrix based approach for structural damage detection

In this paper, a new structural damage detection approach based on changes in the generalized flexibility matrix is presented. The generalized flexibility matrix is first introduced; its sensitivity and change are then used to detect structural damage location and damage extent. Compared with the original flexibility matrix based approach, the effect of truncating higher-order modes can be considerably reduced in this new approach. Finally, a numerical example for a simply supported beam is used to illustrate the effectiveness of this proposed method.     

The importance of "temporal pattern" in traumatic impulse noise exposures

The equal energy hypothesis (EEH) was evaluated for impulse noise. Specifically, the experiments evaluated the importance of the temporal distribution of impulses; the trading relation between the number of impulses and peak level and the difference between continuous and impulse noise. Monaural chinchillas were exposed to one of seven conditions. Their hearing was evaluated before, immediately after, and 30 days after the exposure. Hair cell damage was reported in the form of a cochleogram. The experiments show that the EEH is more appropriate for low-level impulse (135-dB peak); for equal amounts of energy, 150-dB impulses produce more hearing loss and hair cell damage than 135-dB impulses; for equal amounts of energy, impulses presented in rapid bursts cause less damage than impulses presented at "1/s" and 50 microseconds. Pairs of impulses presented at "1/s" produce the largest amount of damage. The results are discussed in terms of implications for the EEH.     

Damage Identification of Truss Structures Based on Force Method

An computationally efficient damage identification technique for the planar and space truss structures is presented based on the force method and the micro genetic algorithm. For this purpose, the general equilibrium equations and the kinematic relations in which the reaction forces and the displacements at nodes are take into account, respectively, are formulated. The compatibility equations in terms of forces are explicitly presented using the singular value decomposition (SVD) technique. Then governing equations with unknown reaction forces and initial elongations are derived. Next, the micro genetic algorithm (MGA) is used to properly identify the site and extent of multiple damage cases in truss structures. In order to verify the accuracy and the superiority of the proposed damage detection technique, the numerical solutions are presented for the planar and space truss models. The numerical results indicate that the combination of the force method and the MGA can provide a reliable tool to accurately and efficiently identify the multiple damages of the truss structures.