基于动力响应的不确定性结构概率损伤识别

基于被测加速度响应,研究了考虑测量数据不确定性的结构概率损伤识别问题。系统中的不确定性被处理为具有给定方差的零均值正态随机变量。基于理论模型和测量的加速度响应,经过两步模型修正方法分别得到了无损和损伤结构概率模型,进而利用损伤判据确定了各单元的损伤概率。通过对十杆平面桁架结构的损伤识别,讨论了不同采样点个数对识别结果的影响,结果表明了本文方法的有效性和可行性。     

运用改进残余力向量法的结构损伤识别研究

针对结构损伤识别时测试结构模态振型不完备的情况,采用了自由度凝聚方法进行模型凝聚,并给出了缩聚模型的残余力向量计算公式,证明了残余力向量对结构损伤单元的敏感性.为减小自由度凝聚及测量等因素带来的误差影响,提出了一种改进的残余力向量法.该方法首先通过测试损伤结构多阶模态得到残余力向量,并计算相对于损伤前结构的残余力向量得到残余力向量差矩阵,选取向量差矩阵各行元素中最大绝对值组成改进的残余力向量.损伤识别时,先运用改进的残余力向量确定可能损伤单元,再运用筛选法计算单元损伤程度.数值仿真算例说明,采用该方法只需结构的前几阶低阶频率和振型,就可以快速准确地识别出结构的损伤,显示了该方法具有较好的抗噪性.     

基于模态频率的简支梁桥损伤识别模糊推理方法

为了有效降低不确定性因素对结构损伤识别结果的影响,提出了一种基于模糊推理系统的损伤识别方法。该方法以结构的前四阶频率变化率为损伤识别参数,以单元的损伤状态（轻微、中度、严重损伤等）为输出参数。通过在原始数据中添加高斯白噪声来模拟不确定因素的影响,简支梁桥的数值模拟分析验证了该方法用于桥梁结构损伤识别的有效性。结果表明,该方法的抗噪水平达到10％以上。     

基于柔度矩阵和神经网络的结构损伤识别法

提出一种分步识别结构损伤的方法。首先利用测量模态参数建立结构柔度矩阵来确定结构损伤的大体位置，然后应用神经网络技术和结构的加速度响应对确定的损伤范围进行参数识别，根据识别的刚度值判别结构的损伤程度。通过一个8自由度结构的仿真计算表明，该方法稳定性好，计算精度高，对噪声具有很高的鲁棒性，在10％噪声情况下，应用神经网络技术能较精确地得到结构的损伤程度，显示了该方法对大型复杂结构进行损伤诊断的潜力。     

核密度估计法在板件概率损伤识别中的应用

基于谱元法将核密度估计方法用于解决结构的损伤识别,从而得到了损伤位置的概率密度函数。通过建立压电-结构耦合的三维谱元法模型,模拟Lamb波在铝板完好及损伤结构中的传播过程。利用连续小波变换计算响应信号在传感器之间的飞行时间,得到Lamb波在结构中的传播速度。通过对Rayleigh-Lamb方程的数值分析,得到Lamb波的理论传播速度,并将其与谱元法得到的结果进行对比,证实了谱元法模型的准确性。在椭圆定位技术的基础上,考虑环境不确定性对测量信号的影响,引入核密度估计方法将损伤位置识别转化为一种概率性问题。讨论了3种噪声水平情况下的损伤位置的概率密度函数,并给出了最终识别的结果。结果表明,核密度估计方法能够有效地识别出损伤位置,最大误差在5%左右。     

基于贝叶斯方法的统计模型修正和结构概率损伤识别

提出了一个用马尔科夫链抽样的贝叶斯模型修正和损伤识别方法,用于对梁结构的损伤识别.首先建立了基于结构振动频率测量,确定其振型的目标函数,然后采用延缓拒绝自适应的马尔科夫链蒙特卡洛方法进行随机抽样,得到在完好状态和损伤状态下结构参数的后验概率,通过比较这两种状态下识别参数的概率密度函数,最终得到结构损伤的概率和损伤程度....     

结构损伤诊断的改进静力方法

利用柔度分解技术,提出一种改进的静力方法用于工程结构损伤诊断中。通过在结构静力反应方程中引入柔度分解式,得到求解柔度扰动参数的线性方程组。由该线性方程组首先计算出各单元柔度扰动参数,再根据柔度扰动参数和刚度扰动参数(损伤参数)之间的关系式,即可计算出各单元损伤参数,据此即可评估结构的损伤状况。所提方法避免了已有方法中需要求解非线性方程组的缺点,无论损伤程度大小如何,均不需要进行迭代运算即可获得精度良好的计算结果。以一个桁架结构为例对所提方法进行验证。结果表明:无论损伤程度大小如何,所提方法只需经过一次计算即可获得精度较高的识别结果,避免多次迭代,显示该方法在计算上的优越性。     

藏式古建筑木结构损伤识别的数值模拟

为保护藏式古建筑木结构,针对实际中众多结构存在的梁、柱构件损伤问题进行了损伤识别理论的研究。基于实地勘查结果,对梁、柱构件的残损类型和残损机理进行了汇总和分析。以某典型藏式古建筑多层梁柱排架结构为损伤识别对象,在建立其有限元模型的基础上,采用振动响应灵敏度损伤识别方法对该结构梁、柱构件的3种不同损伤状态进行了数值模拟识别。为了模拟古建筑结构构件之间物理参数离散性大的特点,给每个构件的弹性模量添加了随机误差。识别时,为了减小离散的物理参数对结果收敛性和准确性的影响,采用修正的正则化方法求解识别方程。结果表明,该方法可以克服测量噪声的影响,准确识别出藏式古建筑木结构梁、柱构件的损伤,为该类型结构实际的损伤识别提供了理论依据。     

测量噪声对海洋平台损伤检测的影响研究

振动信号模态识别是检测海洋平台损伤的有效手段,但在实际检测中存在多种干扰因素,其中测量噪声对损伤检测结果有显著影响。基于此,在局部测量的基础上进行振动试验研究,明确测量噪声、模态参数与海洋平台损伤检测结果的关系。     

基于模型修正的大跨斜拉桥损伤识别方法

首先,根据斜拉桥的结构特点,将主梁等效为弹性地基梁,由此可推知结构损伤而导致的内力重分布只影响损伤部位附近单元,以此作为结构损伤位置的初步判断的依据;然后,由于内力重分布程度与结构损伤前的初始位移成线性关系,可知汽车荷载作用下的索力变化对损伤更为敏感.根据基准有限元模型,采用优化算法,以损伤单元的损伤程度为设计变量,以实测索力变化和理论索力变化之差为目标函数,对有限元模型进行修正,进而实现结构损伤程度的识别;最后,以苏通长江大桥为例,对4种损伤工况进行了分析识别.计算结果表明,根据同一汽车荷载作用下的索力变化,可以有效地识别主梁单元的损伤位置和程度.     

High laser damage threshold coatings and damage testing technology

1Introduction Thelaser induceddamagethreshold(LIDT)ofopticalcoatingsisalimitedfactorfordevelop mentofahighpeakpowerlaser.Theoutput fluencesthatcanbeattainedontheNationalIg nitionFacility(NIF)whichcontains192beam linesbuiltbytheUniteStatesareultimatlylim itedbyopticalcoatingswithinthechain.These opticalcoatingsneedtosatisfythefluencere quest,22J/cm2forreflectivemirrorsand11J/cm2forpolarizers(1064nm,3nspulselength).Becausethedamagemechanismofthecoatings understronglaserirradiationisverycom…     

Cavitation damage prediction

New results from cavitating venturi water tests were used to reinforce the concept of cavitation erosion efficiency previously developed from tests in a vibratory facility with both water and sodium. The concept emerges from a technique which allows a priori prediction of eventual cavitation erosion rates in flow machines. Bubble collapse pulse height spectra obtained from submerged microprobes are correlated with measured erosion rates in given laboratory and/or field devices to allow this prediction. Preliminary results from such correlations are presented together with other measurements of the effects of gas content, velocity and cavitation condition upon the mechanical cavitation intensity as measured by the pulse height spectra.New results from vibratory facility tests in tap water and synthetic seawater upon three materials of variable corrodability (304 stainless steel, 1018 carbon steel and 1100-0 aluminum) are presented. The ratio between maximum erosion rates for the saltwater and freshwater tests were found to increase toward unity as the mechanical cavitation intensity is increased, i.e. increased mean depth to penetration (MDPR), as expected on theoretical grounds.The relation between the incubation period and MDPR_max was examined from the vibratory test results, and was found to depend upon the material properties as well as the fluid flow conditions.     

Incremental damage mechanics of particle or short-fiber reinforced composites including cracking damage

In particle or short-fiber reinforced composites, cracking of the reinforcements is a significant damage mode because the cracked reinforcements lose load carrying capacity. This paper deals with an incremental damage theory of particle or short-fiber reinforced composites. The composite undergoing damage process contains intact and broken reinforcements in a matrix. To describe the load carrying capacity of cracked reinforcement, the average stress of cracked ellipsoidal inhomogeneity in an infinite body as proposed in the previous paper is introduced. An incremental constitutive relation on particle or short-fiber reinforced composites including progressive cracking of the reinforcements is developed based on Eshelby’s (1957) equivalent inclusion method and Mon and Tanaka’s (1973) mean field concept. Influence of the cracking damage on the stress-strain response of composites is demonstrated.     

FIB-induced damage in silicon

The damage created in silicon transmission electron microscope specimens prepared using a focused ion beam miller is assessed using cross‐sections of trenches milled under different beam conditions. Side‐wall damage consists of an amorphous layer formed by direct interaction with the energetic gallium ion beam; a small amount of implanted gallium is also detected. By contrast, bottom‐wall damage layers are more complex and contain both amorphous films and crystalline regions that are richer in implanted gallium. More complex milling sequences show that redeposition of milled material, enriched in gallium, can occur depending on the geometry of the mill employed. The thickness of the damage layers depends strongly on beam energy, but is independent of beam current. Monte Carlo modelling of the damage formed indicates that recoil silicon atoms contribute significantly to the damaged formed in the specimen.     

RB-SiC亚表面损伤检测及其旋转超声磨削亚表面损伤特征

分别采用截面抛光法(包括以硅片作陪衬与以聚酯作陪衬两种形式)和界面黏接法检测了反应烧结碳化硅(Reaction Bonded SiC,RB-SiC)旋转超声磨削加工的亚表面损伤。为确定其中的最佳检测形式,采用表面破碎层深度、最大破碎层深度、平均裂纹深度、最大裂纹深度4个亚表面损伤评价指标对两种方法分别检测到的RB-SiC旋转超声磨削亚表面损伤进行对比分析。结果显示:截面抛光法(硅片作陪衬)检测到的4个指标值依次为3.30μm、6.59μm、8.64μm、17.44μm;截面抛光法(聚酯作陪衬)检测到的4个指标值依次为5.71μm、14.33μm、15.36μm、54.82μm;而界面黏接法检测到的4个指标值依次为9.19μm、19.45μm、13.04μm、32.20μm。试验结果表明,截面抛光法(硅片作陪衬)检测的精度更高,检测的亚表面损伤更符合实际情况。最后,基于此方法,对旋转超声磨削RB-SiC材料的亚表面损伤特征进行了总结。     

A fretting-fatigue damage threshold concept

Fretting fatigue studies were conducted on Ti-6Al-4V and 7075-T6 aluminum specimens cycled in axial fatigue loading at a fatigue ratio (R) of +0.1. Axial fatigue loading was applied at a frequency of 30 Hz in a laboratory environment with the fretting applied to the specimen central section through a fretting pad made of the same material as the fatigue specimen. Tests were conducted at various maximum axial fatigue loads and normal pressures.The fretting damage that occurred resulted in a significant reduction in fatigue life. The reduction in fatigue strength was greater for both materials studied in the long life region. A fretting fatigue damage threshold that results from the fretting was found to exist for both materials. At all load levels a given amount of fretting damage is required before any fatigue life reduction occurs. Presumably the damage leads to the development of cracks in the fretted areas. The concept of the fretting damage threshold is related to the development of an initial crack that causes the local stress intensity to exceed the threshold value at a much smaller number of applied cycles. Thus, the concepts of fracture mechanics are related to the “initiation” of fretting damage.     

Erosion damage in engine bearings

Erosion damage in engine bearings has until recently been of little more than academic interest with insignificant effect on bearing performance. However, over the last four or five years, erosion damage in plain bearings has become more prevalent. The types of erosion damage are categorised and common features examined.     

Soft-X-ray damage to biological samples

X-ray damage to biological samples was investigated in the wavelength region of 2.7–5 nm, which overlaps the so-called 'water window', the wavelength range of 2.4–4.3 nm usually used in X-ray microscopy. Yeast cells and myofibrils were chosen as representatives of whole cell samples and motile protein systems, respectively. The samples were exposed to X-rays using an apparatus composed mainly of a laser-plasma X-ray source, a Wolter mirror condenser, and a sample cell. The yeast cells lost their dye exclusion ability when the X-ray flux was higher than 1 × 10⁶ photons μm⁻², while the myofibrils lost contractility when the X-ray flux was higher than 4 × 10⁵ photons μm⁻². These X-ray fluxes are lower than the flux required for the X-ray microscope observation of biological samples at a resolution higher than that of light microscopes.     

Wear damage to spur gears

Electron microscopic studies of damage to spur gears in an experimental gearbox have shown that rolling contact fatigue plays a major role supplemented by abrasive wear. It is noteworthy that the most voluminous part of the wear debris consisted of extremely fine particles of thicknesses below 0.2 μm.