基于三点弯曲试验的聚丙烯塑料颗粒混凝土 声发射分析*

为探讨掺入聚丙烯塑料颗粒(PP)对混凝土疲劳损伤的影响,以不同塑料颗粒掺量Wf (0、10%和20%)混凝土为研究对象,采用声发射技术,开展一系列三点弯曲疲劳试验。试验结果表明：掺入塑料颗粒后,混凝土疲劳寿命增大,断裂能增大,可承受的最大荷载降低,但挠度增大,塑性变形能力增强;AE（Acoustic Emission）定位的损伤源主要集中在跨中,首次峰值荷载前,AE事件偏少,AE定位可识别的内部疲劳损伤点较少,当接近疲劳破坏阶段,AE信号源急剧增加,且随着Wf越大,AE事件数量明显减少,AE事件主要出现在循环荷载波谷处;掺入塑料颗粒后,声发射信号次数显著降低,持续时间更短,且随着Wf的增加,声发射信号次数进一步降低。掺入PP塑料颗粒有助于缓解混凝土内部疲劳损伤,延长其疲劳寿命。     

20~#钢不同晶粒度试件拉伸损伤试验的声发射特性研究

采用声发射技术,对四种不同晶粒度20~#钢试件的单轴拉伸过程进行监测,探究不同晶粒度对于金属材料损伤过程中声发射特性的影响。试验结果表明:声发射信号的幅值、能量和撞击计数等特征参数能够很好地描述材料不同晶粒度大小对材料拉伸过程中声发射特性的影响。细晶粒试件声发射信号数较少,强度较低,随着试件晶粒度的增大,声发射信号的强度和活性呈现明显增加的趋势。说明粗晶粒试件力学性能劣化严重,拉伸过程极易损伤,因此,试验结果也反映了金属材料微观损伤过程中声发射与材料内部的损伤演化密切相关。     

正交异性钢桥面板声发射波三维谱元法模拟及损伤定位*

针对大跨度桥梁正交异性钢桥面板的疲劳损伤评估与结构健康监测需求,开展基于声发射波场谱元法模拟的大型复杂板类结构损伤定位研究。采用Legendre高阶插值三维时域谱元法模拟声发射波在正交异性钢桥面板中的传播过程,验证了其内部显著的反射、衍射和频散现象,并代替人工预断铅实测试验获得大量声发射数据。然后,利用赤池信息准则判定声发射波到达各传感器的时间,通过高斯过程回归建立到达时差与声发射源位置的关系模型,用于未知损伤的定位监测。数值模型实验结果表明,赤池信息准则和高斯过程回归改进的时差图法在正交异性钢桥面板中的平均定位误差为37.3 mm(25 dB信噪比工况),平板的定位精度高于U肋。谱元法模拟有望代替繁琐的预断铅实测试验,提升声发射时差图系列损伤定位方法的实用性。     

三轴应力下花岗岩加载破坏的能量演化和损伤特征

为揭示不同围压下硬岩在破坏过程中的力学性质和能量演化规律,基于RMT-150B岩石力学试验系统对花岗岩试样进行不同围压条件下常规三轴压缩试验。研究结果表明:岩样的峰值应力和围压具有较强的线性关系,利用Mohr-Coulomb强度准则求出花岗岩的黏聚力为23.548 MPa,内摩擦角为57.629°。围压对花岗岩加载破坏过程中能量演化的影响显著,岩石的峰值能量、弹性应变能以及耗散能都随着围压的增大而增大,且两者呈线性增加关系。根据岩石的线性储能规律,提出了确定岩石应力阈值的方法。围压越大,起裂应力和扩容应力越大,且岩样起裂点处与扩容点处的能量也越大;当围压较低时,岩石破坏前储存的能量较少,破坏时能量释放速率低,岩样表现为典型低劈裂破坏;在高围压情况下,能量快速释放,岩样表现为剪切破坏。基于能量演化规律,提出了岩石损伤演化模型,得到了花岗岩的损伤变量D在不同围压下加载破坏过程中的演化规律。     

一种基于特征点提取的扬声器异常声检测方法

针对短时傅里叶变换在扬声器异常声检测中有效信息提取的随机性问题,提出了特征点法在扬声器异常声检测中的应用.此方法基于扬声器经扫频信号激励所得响应信号的短时傅里叶变换时频图,用改进的尺度不变特征转换算法对合格扬声器与异常声扬声器做特征提取,并将多组特征点经分割剔除后叠加组成特征矩阵模板.以合格扬声器样本提取特征曲线阈值构...     

早龄期荷载及低温作用下的混凝土声发射特征试验研究*

井壁混凝土由于施工与环境特殊性导致其裂缝扩展情况异常,本文通过对不同早龄期荷载、低温条件下混凝土轴压破坏过程的声发射检测,分析井壁施工环境对混凝土内部裂缝产生、发展的影响。研究表明,早龄期荷载、低温单独作用时,与标养混凝土主要差异在轴压过程中的声发射初始阶段,对混凝土内部的原有收缩裂缝有所影响。当两者耦合作用时,早龄期荷载影响比低温作用显著,并且在早龄期荷载较大时会出现声发射信号"倒置"现象。     

不同卸围压速率下花岗岩的力学性质及声发射特征

为了研究不同卸围压速率下花岗岩的力学性质,利用RMT-150B岩石力学试验系统对花岗岩试样进行恒轴压卸围压应力路径试验。试验结果表明:相同的初始围压下,随着卸围压速率增大,岩样的延性减小,表现为脆性破坏。卸围压速率越大,卸围压阶段的应变率越高,但总变形量小;当卸围压的速率相同时,初始围压越高,卸围压阶段岩样的应变率和总变形量越大。采用Mogi-Coulomb强度准则对试验结果进行拟合,结果显示:卸围压速率对花岗岩的黏聚力有劣化作用,对岩石的内摩擦角有强化作用;卸围压速率越小,振铃计数的活跃期越长,表明在低卸围压速率下,花岗岩岩样内部损伤发展缓慢且完全。     

蒸养再生骨料混凝土断裂声发射特征*

为研究蒸汽养护对再生骨料混凝土断裂过程损伤特征的影响,开展了再生骨料混凝土三点弯断裂试验,并采用声发射技术对损伤过程进行了监测分析。结果表明,蒸养降低了再生骨料混凝土断裂峰值荷载,断裂过程中声发射信号强度低于标准养护试件。声发射振铃计数、撞击数、b值等参数能够准确反映再生骨料混凝土断裂过程损伤演变规律。加载上升阶段,声发射信号活动性很低,试件内以微裂纹的形成为主,加载至荷载峰值,声发射振铃计数累计值增长呈现平缓阶段,混凝土内微裂缝的聚合会持续一段时间。峰值后荷载迅速降低阶段声发射信号最活跃,为裂缝迅速发展的过程。峰后荷载缓慢降低的过程,声发射信号活跃性降低,裂缝扩展至试件顶部区域,主要是宏观裂缝的开展,不再产生新的裂缝。     

混凝土材料声发射信号的频率特征及其与强度参量的相关性试验研究

混凝土材料声发射信号频率特征与强度指标的关系是混凝土材料声发射检测技术的重要基础。通过实验,对声发射信号频率特征与混凝土强度指标的相关关系进行了探讨。研究结果表明,在较低的应力水平上,不同强度等级混凝土声发射信号大都是低频信号。而在较高应力水平时,随着强度等级的增加,声发射信号的优势频率则逐渐升高。     

基于声发射统计信息的2.25Cr-1Mo钢早期损伤判别方法

裂纹开裂是金属材料常见的损伤失效形式之一。而材料失效会经历从早期微小损伤到宏观变形的微观组织动态演化过程,如何在材料发生宏观变形之前,能够及早地发现损伤至关重要。该文基于对材料损伤失效过程不同声源的机理分析,提出了以不同声源信号数的统计占比值κ这一变量来判定表征材料早期损伤阶段的判别指标,制备6个含不同缺陷的不带堆焊层和带堆焊层的2.25Cr-1Mo钢试件,采用声发射技术对试件的弯曲损伤全过程进行在线测试,获得了6个试件早期损伤阶段的声学数据,通过对比分析以幅值和能量为主的单一参量以及基于支持向量机的多元参量κ值的统计结果,证明了利用κ值可以有效地对不带堆焊层和带堆焊层的2.25Cr-1Mo钢试件的早期损伤阶段进行判别,其中不带堆焊层试件的κ值范围为1:3.1～1:3.6之间,带堆焊层试件的κ值范围为1:2.2～1:2.6之间,具有明显的区分度。     

Dynamic three-point bending tester using inertial mass and optical interferometer

A method for generating and measuring varying force by means of levitating a rigid object is discussed in this paper. The inertial force is accurately measured using an optical interferometer and an electric frequency counter. The performance of the three-point bending tester based on the proposed idea is investigated. The 12 sets of three-point bending tests have been conducted against aluminum bars. The combined standard uncertainty in measuring the instantaneous value of the reaction force, with the sampling interval of approximately 1–2 ms, is approximately 0.11 N. This corresponds to approximately 3×10⁻³ (0.3%) of the maximum inertial force in the experiments.     

Mechanical behavior and health monitoring by Acoustic Emission of unidirectional and cross-ply laminates integrated by piezoelectric implant

Recent progress in sensor technologies, signal processing and electronics has made it possible to fulfill the need for the development of in-service structural health monitoring (SHM) systems. This study presents a health monitoring of composite materials integrated by piezoelectric sensor using Acoustic Emission (AE) technique. A series of specimens of composite laminates with and without piezoelectric implant were subject to three-point bending in static and creep tests while continuously monitoring the response by the AE technique. The analysis and observation of AE signals lead to the identification of the acoustic signatures of damage mechanisms in composite laminates. The mechanical behavior of composites with and without integrated sensor shows no difference in the form. The incorporation of piezoelectric sensor influences specially the fracture load and causes low degradation of mechanical properties of materials. One of the major differences between the two types of materials (with and without embedded sensor) is the intense acoustic activity in the integrated material.     

Mechanical behavior and acoustic emission technique for detecting damage in sandwich structures

The present study investigates the mechanical behavior under static and dynamic loadings and assesses damage by the acoustic emission method of two types of sandwich composite materials. The sandwich structures under study are both made of cross-ply laminates as skins and PVC closed-cell as foam with different densities: 60 kg m⁻³ and 100 kg m⁻³. The mechanical behavior tests were conducted in static and cyclic fatigue loadings under 4-point bending. The sandwich structures considered in fatigue tests were damaged by a various number of shear damages in the foam. Static tests were performed to determine the failure parameters and characteristics used in fatigue tests. The damage density effect on the stiffness, hysteresis loops, dissipated energy and damping of sandwich structures, were studied for various numbers of cycles during cyclic fatigue tests. The acoustic emission method was used to identify and characterize the local damage in both types of sandwich materials under static 4-point bending tests.     

Experimental and theoretical analyses of package-on-package structure under three-point bending loading

<正>High density packaging is developing toward miniaturization and integration,which causes many difficulties in designing,manufacturing,and reliability testing.Package-on-Package(PoP) is a promising three-dimensional highdensity packaging method that integrates a chip scale package(CSP) in the top package and a fine-pitch ball grid array (FBGA) in the bottom package.In this paper,in-situ scanning electron microscopy(SEM) observation is carried out to detect the deformation and damage of the PoP structure under three-point bending loading.The results indicate that the cracks occur in the die of the top package,then cause the crack deflection and bridging in the die attaching layer.Furthermore,the mechanical principles are used to analyse the cracking process of the PoP structure based on the multi-layer laminating hypothesis and the theoretical analysis results are found to be in good agreement with the experimental results.     

Fatigue behaviour and structural health monitoring by acoustic emission of E-glass/epoxy laminates with piezoelectric implant

This paper represents the continuation of our research on built-in piezoelectric sensor for structural health monitoring of composite materials. Experimental research is focused on examining the effects of the embedded sensors on the structural integrity of composite laminates subjected to mechanical tests. A series of composite specimens with and without embedded sensor are tested in fatigue loading while constantly monitoring the response by acoustic emission technique. The acoustic signals are analysed using the classification k-means method in order to identify the different damage mechanisms and to follow the evolution of these mechanisms for both types of composite materials (with and without sensor). The mechanical behaviour of composites with and without embedded sensor shows no difference in the form. The incorporation of piezoelectric sensor causes low degradation of mechanical properties of composites. Comparing embedded sensor to sensor mounted on the surface, the embedded sensor showed a much higher sensitivity. It is thus verified that the embedded acoustic emission sensor had great potential for acoustic emission monitoring in fibre reinforced composite structures.     

木塑复合材料缺陷及损伤的声发射信号特征分析及神经网络模式识别

针对木塑复合材料五种典型的缺陷及损伤机制，选择合适的木塑试样，应用三点弯曲的加载方法采集声发射信号。对主损伤区附近的声发射事件，应用小波变换提取特征参数，确定五类主要损伤机制所对应的声发射信号特征。采用B—P型反向传播神经网络构成的智能化模式分类器，对此五类声发射信号进行识别，获得了满意的效果。     

Feasibility study of a digital speckle pattern interferometry and bending test combined technique to investigate coating adhesion

This paper presents a feasibility study to assess whether digital speckle pattern interferometry could be used as a possible technique to investigate the adhesive performance of coatings. The approach is based on the measurement of the deflections produced by a pre-notched coated specimen subjected to a four-point bending test. When the bending load is increased, a delamination between the coating and the substrate is propagated with its length depending on the adhesion strength. Experiments carried out with specimens having simulated delaminations confirm that digital speckle pattern interferometry can be used to estimate the delamination length.