常温下16MnR板材焊接缺陷声发射特性研究

对预制的含有未焊透缺陷的16MnR焊接矩形试样,进行了室内声发射试验.分析了试验中记录的试样拉伸曲线和声发射信号,阐述了试样声发射信号的分布情况和相应的应力状况.通过缺陷的断裂力学计算,预测了产生大量声发射信号的应力值,将声发射信号的产生和试样所受的应力状况有效地联系起来,对含缺陷容器的声发射检测有一定的指导作用.选择合适的小波函数,分析了试验中声发射信号的不同参量,对比了声发射信号不同参量在小波分析中的差异,得出了声发射信号的时域内分布特征和相应的应力状况.     

Q235钢细观损伤声发射源动态观测实验研究

金属材料细观损伤过程伴随有瞬态应力波的释放,利用声发射检测技术可采集到应力波的信息,但由于缺少直观可视的手段,利用声发射信息定量评价金属材料的细观损伤存在困难。本文将SEMtester1000原位拉伸试验机、MZ1000正置显微镜和PCI-2型声发射检测系统相结合,搭建了声发射源动态观测实验系统。以Q235钢可视化原位拉伸实验为例,获取了晶体的滑移和夹杂物的断裂两种典型声发射源图像及对应的声发射信号。通过对不同声发射源信号的分析可知,滑移是一个持续的过程,复杂的滑移活动会产生多类型的声发射信号。夹杂物断裂是瞬态过程,产生典型的突发型声发射信号,能量较滑移信号更高。依据声发射源动态观察结果及图像分析,建立了基于声发射机制的Q235钢细观损伤模型。     

Acoustic-emission analysis of fracture-toughness tests

Acoustic-emission behavior of 300M steel was investigated as a function of the fracture toughness and microstructure. A model is presented to correlate acoustic emission arising from plastic deformation as well as crack growth during fracture-toughness tests and experimental data demonstrate that a quantitative relationship exists between fracture toughness, fracture mode and acoustic emission. Finally, it is shown that, while very limited information can be obtained by monitoring the cumulative counts alone, amplitude-distribution analysis of acoustic emission can provide an improved quantitative characterization.     

Acoustic Emission Analysis of Nanoindentation-Induced Fracture Events

Monitoring of acoustic emission activity is employed to characterize the initiation and progression of local failure processes during nanoindentation-induced fracture. Specimens of various brittle materials are loaded with a cube-corner indenter and acoustic emission activity is monitored during the entire loading and unloading event using a transducer mounted inside the specimen holder. As observed from the nanoindentation and acoustic emission response, there are fundamental differences in the fracture behavior of the various materials. Post-failure observations are used to identify particular features in the acoustic emission signal that correspond to specific types of fracture events. Furthermore, analysis of the parametric and transient acoustic emission data is used to establish the crack-initiation threshold, crack-arrest threshold, and energy dissipation during failure. It is demonstrated that the monitoring of acoustic emission signals yields both qualitative and quantitative information regarding highly localized failure events in brittle materials.     

An Attempt to Evaluate Cohesion in WC/Co/Cr Coatings by Controlled Scratching

As well as phase composition, microstructural defects and residual stresses influence the wear resistance of WC-based coatings, so their integrity or cohesion may give a guide to their wear behaviour. Parallel scratching by a suitable probe induces increasing scratch interaction with ramp loading on each scratch. Controlled plastic strain has, thus, been accumulated, and ductile microfracture induced, on the surfaces of WC-Co–Cr thermal sprayed coatings. Cohesion criteria based on wear area, acoustic emission, grain fracture and tensile cracking in the scratch groove are described, and results are presented that correlate reasonably well with the erosive wear characteristics of a number of differently sprayed coatings.     

Fracture Mechanics of Corrosion Cracking in Concrete by Acoustic Emission

Nucleation of microcracks can be detected and analyzed by acoustic emission (AE), by which crack kinematics of locations, types and rientations are quantitatively estimated. The procedure was applied to clarify mechanisms of corrosion cracking. Based on fracture mechanics, numerical analysis was conducted by the boundary element method (BEM). Relations between the stress intensity factors and crack types were investigated by BEM solutions. In experiments, four types of crack patterns were nucleated by employing expansive agent. Following the surface crack, the diagonal crack and/or the horizontal crack propagated. The internal crack extended after the surface crack was terminated. Depending on the crack types, contributions of mode-I and mode-II were varied. According to AE results, four crack patterns observed differently consisted of tensile, mixed-mode and shear cracks. It is demonstrated that mechanisms of corrosion cracking in concrete are dominantly of mode-I failure along with a minor contribution of mixed-mode and mode-II.     

混凝土材料三点弯曲破坏的声发射特性

为了研究碎石混凝土材料在不同应力状态下的声发射信号特征,在MTS材料试验机上对混凝土梁进行了三点弯曲实验,并利用先进的多通道声发射系统监测了混凝土梁在三点弯曲载荷下的破坏过程。通过对声发射信号的持续时间、振铃计数、撞击数进行滤波,将混凝土材料破坏过程的声发射信号分成七种类型,同时结合前人关于混凝土材料Kaiser效应的试验研究,将七种类型的声发射信号对应于不同的相对应力水平。研究结果表明,声发射信号的持续时间、振铃计数以及撞击数不仅可以表征混凝土破坏过程的类型,还可以表征混凝土材料所处的应力状态,从而为声发射技术在线监测混凝土材料提供了一种新的途径。     

单轴压缩条件下破裂岩样声发射及能耗特性试验研究

为揭示岩石变形破裂的内在机理及正确评价工程破裂岩体的特性,本文对破裂岩石再破裂过程声发射、能耗特性进行了综合研究。借助MTS伺服试验机及AE21C声发射检测仪,对破裂岩样进行了单轴压缩条件下再破裂过程中的声发射及能耗特性实验研究。研究结果表明:破裂岩样声发射事件-时间曲线主要属于峰前阶段加速增长型,其声发射事件-时间曲线大致可分为声发射初始发展、剧烈、降低三个阶段;在整个压缩破裂过程中,声发射活跃程度总体上大于相对完整岩样;峰值点是声发射特征、能耗特征的转折点;岩样变形破裂过程中,声发射与能耗指标之间存在内在的、必然联系;随着破裂程度的提高,破裂岩样峰值点吸收能量与峰前段耗散能量均逐步降低;破裂岩样表面破裂面分布分形维数同峰值点吸收能量与峰前段耗散能量之间总体上均遵循非线性的二次多项式函数关系。     

The measurement and significance of energy in acoustic-emission testing

A technique for measuring the energy sensed at an acoustic-emission transducer is presented that utilizes a squaring circuit and digital integrator. Theoretical relationships between energy and other more conventional acoustic-emission parameters, such as counts and RMS voltage, are derived for certain idealized cases. Experimental results from the following types of tests are presented: (1) unflawed tensile (‘continuous’ emission); (2) precracked stress-corrosion cracking; (3) precracked fracture toughness; and (4) fatigue-crack growth. Energy, counts, RMS-voltage, energy/event and counts/event measurements are included. In the case of unflawed tensile specimens, energy techniques appeared somewhat superior to counts. In all other cases, a direct relationship between counts and energy was obtained. Energy measurements tended to give a larger weight to higher amplitude events. Other than this, energy measurements appeared to have no advantage over counts. The theoretical relationship predicted between energy/event and count/event agreed quite well with experimental observations. Overall, the test results presented indicate that energy techniques provide no significant advantage over counting threshold crossings in cases in which crack extension in metals is the primary source of acoustic emission.     

等离子喷涂铁基涂层的疲劳磨损裂纹行为

采用等离子喷涂制备了铁基涂层,使用球盘式疲劳磨损试验机进行涂层的疲劳磨损试验,利用声发射技术在线判断涂层开裂,并通用透射电子显微镜（TEM）分析了试验前后涂层内部微观结构,研究了喷涂层疲劳磨损裂纹的扩展行为。结果表明：制备的涂层致密,结合状态良好,主要相为铁素体,同时存在大量的非晶-纳米晶相,利用声发射技术能够表征涂层的临界失效状态,其内部疲劳磨损裂纹主要以韧性穿晶断裂的形式扩展。     

Acoustic emission assessment of interface cracking in thermal barrier coatings

In this paper, acoustic emission (AE) and digital image correlation methods were applied to monitor interface cracking in thermal barrier coatings under compression. The interface failure process can be identified via its AE features, including buckling, delamination incubation and spallation. According to the Fourier transformation of AE signals, there are four different failure modes: surface vertical cracks, opening and sliding interface cracks, and substrate deformation. The characteristic frequency of AE signals from surface vertical cracks is 0.21 MHz, whilst that of the two types of interface cracks are 0.43 and 0.29 MHz, respectively. The energy released of the two types of interface cracks are 0.43 and 0.29 MHz, respectively. Based on the energy released from cracking and the AE signals, a relationship is established between the interface crack length and AE parameters, which is in good agreement with experimental results.     

2D-C/SiC拉伸损伤的声发射信号聚类分析

摘要：对2D-C/SiC复合材料进行常温拉伸试验,利用声发射（AE）仪在线监测其损伤过程。应用K-均值聚类算法对AE信号进行模式识别,结合扫描电镜观察,发现2D-C/SiC复合材料拉伸过程的损伤模式包括基体开裂、界面层脱粘、层间剥离、纤维断裂以及纤维束断裂等,并得到了各种损伤模式AE信号参数的中心值。通过对不同损伤模式AE累积事件数和累积能量随时间变化的统计分析,描述了2D-C/SiC复合材料拉伸损伤演化过程。     

Inverse Estimation of Cohesive Fracture Properties of Asphalt Mixtures Using an Optimization Approach

Tensile cracking in asphalt pavements due to vehicular and thermal loads has become an experimental and numerical research focus in the asphalt materials community. Previous studies have used the discrete element method (DEM) to study asphalt concrete fracture. These studies used trial-and-error to obtain local fracture properties such that the DEM models approximate the experimental load-crack mouth opening displacement response. In the current study, we identify the cohesive fracture properties of asphalt mixtures via a nonlinear optimization method. The method encompasses a comparative investigation of displacement fields obtained using both digital image correlation (DIC) and heterogeneous DEM fracture simulations. The proposed method is applied to two standard fracture test geometries: the single-edge notched beam test, SE(B), under three-point bending, and the disk-shaped compact tension test, DC(T). For each test, the Subset Splitting DIC algorithm is used to determine the displacement field in a predefined region near the notch tip. Then, a given number of DEM simulations are performed on the same specimen. The DEM is used to simulate the fracture of asphalt concrete with a linear softening cohesive contact model, where fracture-related properties (e.g., maximum tensile force and maximum crack opening) are varied within a predefined range. The difference between DIC and DEM displacement fields for each set of fracture parameters is then computed and converted to a continuous function via multivariate Lagrange interpolation. Finally, we use a Newton-like optimization technique to minimize Lagrange multinomials, yielding a set of fracture parameters that minimizes the difference between the DEM and DIC displacement fields. The optimized set of fracture parameters from this nonlinear optimization procedure led to DEM results which are consistent with the experimental results for both SE(B) and DC(T) geometries.     

ACOUSTIC EMISSION AND FRACTAL CHARACTERISTICS OF COAL ROCK SAMPLES UNDER TRIAXIAL COMPRESSION

为了研究煤样在不同围压下的声发射及分形特征,利用RMT-150C 型岩石力学伺服试验系统和CDAE-1 型声发射检测仪对煤样试样在不同围压（5MPa,10MPa,15MPa） 进行了实验研究. 对煤样试样的声发射计数和累计计数以及声发射序列的分形特征进行了对比研究. 研究结果表明：不同围压下煤样的声发射特性具有一定的差异性,围压越高前兆信息越明显. 不同围压下煤样都具有分形特征,随着围压的增大,分形特征有所增强. 煤样在失稳破坏过程中声发射序列的分形维值都会出现一个波动-上升-突降的过程,提出可以作为煤样体失稳破坏的前兆信息.     

Amplitude-distribution analysis of acoustic emission

Amplitude distribution of acoustic-emission events generated during fracture of materials contains the potential information about the fracture mechanisms. A methodology is developed for the analysis of acousticemission burst signals. Based on this, the amplitude distributions can be characterized from the cumulative counts and events data alone. This simplifies the equipment as well as the data analysis. A model for amplitude distribution is proposed here, which can potentially identify two mechanisms operating simultaneously. It is also shown analytically that cumulative counts (N) are linearly related to summation of amplitudes of events and thatN can be considered a measure of the total energy of acoustic emission only if the amplitude distribution does not change significantly.     

三维编织CMC断裂韧性表征形式的试验研究

在试验的基础上 ,发现三维编织陶瓷基复合材料断裂试件的裂纹扩展沿着编织角方向进行 ,表现出一种非自相似的裂纹扩展模式 ,表明三维编织CMC的断裂是复合型断裂。利用材料的载荷 -位移曲线和声发射技术 ,分析了三维编织CMC在外载荷作用下的损伤行为和断裂机理 ,并且根据不同的外载荷类型 ,将三维编织CMC的断裂韧性表征分别界定在线弹性和弹塑性的两个领域里 ,初步确定三维编织CMC的断裂韧性表征形式     

Effects of surface diffusion and resolved shear stress intensity factor on environmentally assisted cracking

Hydrogen induced crack-tip plastic deformation has been known as the primary mechanism of hydrogen assisted cracking and stress corrosion cracking. It has been systematically shown that the same mechanism of environmentally assisted crack-tip dislocation emission causes hydrogen assisted cracking, stress corrosion cracking, and liquid metal embrittlement cracking.An embrittling chemical species has to reach a crack tip in order to accelerate crack growth. Very close to a sharp crack tip, surface diffusion is shown to be the dominant transport process of embrittling species for stage-II crack growth. The role of surface diffusion in stage II crack growth is analyzed. The constant cracking velocity is proportional to the surface diffusion coefficient of an embrittling species and inversely proportional to a length parameter, , which is related to the transport process upstream.Dislocation emission at a crack tip is driven by crack-tip resolved shear stress. Crack-tip resolved shear stress field is characterized by resolved shear stress intensity factor, K_RSS·K_RSS is defined, the procedure for its calculation outlined, and its applications to crack-tip dislocation emission and environmentally assisted cracking discussed.     

Principles of the kinetic approach of fracture prediction

It is shown that during crack formation in loaded solids considered as a thermofluctuational process of generation of stable cracks two stages exist, i.e. a delocalized accumulation which spontaneously results in an enlargement of cracks and a relatively fast autocatalytic growth of enlarged cracks in the field of overstresses produced by them. The change of stages takes place at the critical concentration of cracks when an average relative distance between them is equal to e. The effect of the enlargement is the increase of the energy emission during further crack formation. This effect allows to state the going of the object into the prerupturing state and to estimate its residual longevity according to acoustic emission data corresponding to the crack formation. This permits to formulate the physically valid principle of fracture prediction of elements of engineering constructions and geophysics objects.     

Acoustic-emission crack monitoring in fracture-toughness tests for AISI 4340 and SA533B steels

The feasibility of using acoustic-emission techniques for the characterization of fracture resistance in AISI 4340 and SA533B steels is examined. The critical value of J-integral (J _IC ) is measured with a single small specimen (compact tension) loaded in the elastic-plastic range at room temperature. Initiation is detected during loading by acoustic emission. A new AE procedure for crack-growth monitoring in fracture-toughness specimens has been proposed. The possibility of discriminating AE signals from noncritical sources, such as void nucleation during crack-tip plastic deformation and signals from a growing crack, is discussed.