6H-SiC脆性切削声发射响应的分子动力学研究（英文）

采用分子动力学方法研究了6H-SiC脆性切削的声发射响应。研究了原子尺度下6H-SiC的微变形和裂纹形核,同时对加工过程中的声发射源进行了识别,分析了其相应的声发射特征。结果表明,6H-SiC在77 nm切削深度下的脆性变形过程简单但不寻常;在6HSiC切削过程中位错不会连续扩展,变形后的工件在刀具挤压作用下被分割成块,并由位错的快速扩展引发裂纹。对于影响声发射源特征的因素研究发现:初始压应力会导致声发射功率的下降;频率-能量分析中可见的3种声发射源分别是晶格振动、位错扩展和裂纹扩展。此外,在1 K温度下,2次明显的位错传播的声发射响应比晶格振动具有更高的频率特性,但总能量水平最低。相反地,裂纹扩展的声发射响应具有更为明显的频率分布特性和能量特性。     

单晶γ-TiAl合金纳米切削过程声发射响应的原子模拟

本文采用分子动力学方法研究了单晶γ-TiAl合金纳米切削过程的声发射响应。从原子尺度阐述了单晶γ-TiAl合金切削过程中裂纹形成机理。研究发现：切削初期随着切削力持续增大,剪切区域产生周期性的剪切带;与此同时,在高压应力和弹性应力波共同作用下,类晶粒晶界的非晶原子带产生并阻碍了剪切带的持续发射,使主剪切区的应力无法及时通过剪切带释放,产生局部应力集中现象,导致裂纹萌生并扩展;通过对采集的声发射信号分析,压应力会导致切削过程中声发射功率下降。在时域上,通过对微观缺陷演化和声发射功率-频率对比分析,阐述了纳米切削过程中晶格振动、剪切带以及裂纹萌生与扩展的声发射响应特征,并通过聚类分析得到损伤的功率和频率特性。     

孔洞对双晶TiAl合金断裂行为影响的声发射响应

本文基于分子动力学方法,对含孔洞的双晶TiAl合金试样进行了单轴拉伸模拟,在纳米尺度下研究了材料变形和断裂过程中的缺陷演化行为及其声发射响应。研究发现：孔洞大小和位置对材料的弹性模量影响较小,屈服强度随孔洞尺寸的增大而降低;进入塑性变形后,孪晶界对孔洞边缘连续发射的位错有阻碍作用,使晶体强度增加;达到屈服应力时,含晶界孔洞的试样更容易产生稳定的位错结构,阻碍其他位错运动,从而提高了晶体强度;通过对拉伸过程中的声发射信号进行分析,发现声发射信号主要来源于晶格振动,并且具有较大的功率值范围和较低的中值频率;位错滑移的声发射信号表现出宽频域的特点,位错增殖和位错塞积的声发射信号表现出低功率的特点;裂纹扩展的声发射信号属于突发型信号,表现为高频率、高功率的特征。     

起重机箱形梁结构表面裂纹扩展的声发射特性

采用声发射技术监测带有表面焊接裂纹的箱形梁结构的三点弯曲试验。分析了箱形梁试件受载弯曲过程中裂纹扩展的声发射信号特征,比较了加载过程中不同载荷水平下定位源信号的声发射参数特征。研究发现,表面裂纹扩展的声发射信号为突发型信号,其幅度主要为45～60dB,频率主要分布在100～450kHz。试验结果表明,线定位方法可以对箱形梁试件中的裂纹缺陷进行准确定位。     

Al薄膜纳米压痕过程的多尺度模拟

采用多尺度准连续介质法分别模拟无缺陷和具有初始缺陷两种状态下,单晶Al薄膜纳米压痕初始塑性变形过程,得到载荷-位移响应曲线和应变能-位移变化曲线.研究了初始缺陷对纳米压痕过程中位错形核与发射、Peierls应力以及位错发射.临界载荷的影响.结果表明,在整个纳米压痕过程中出现了多次位错形核与发射现象,初始缺陷对第1和第3对位错的形核与发射影响较小,而对第2对位错的形核与发射具有明显的推迟作用,并伴随有裂纹扩展现象;由于初始缺陷引起薄膜材料内部严重的晶格畸变,导致系统应变能和位错运动的Peierls应力增加;裂纹扩展前,发射第2对位错需要的临界载荷增加,裂纹失稳后,位错发射需要的临界载荷下降.模拟获得的纳米硬度和Peierls应力与实验结果吻合.     

位错和位错偶沿单─滑移系从裂纹尖端的发射

采用计算机模拟了位错和位错偶沿单一滑移系从裂纹尖端的发射,考察了滑移面取向、外加载荷、晶格摩擦力以及位错发射的临界应力强度因子对所发射的位错数量、塑性区与无位错区大小以及裂关残余应力强度因子的影响研究表明,位错从裂纹尖端发射的临界应力强度因子对无位错区的存在和其大小起决定作用,而外加载荷与晶格摩擦力主要影响位错发射的数量以及塑性区大小．在Ｉ型载荷作用下,滑移面与裂纹面的夹角越大,从裂尖发射出的位错数量越多,位错对裂纹的屏蔽效应也越大当裂纹发射位错后的残余应力强度因子仍然较大时,位错偶就有可能在裂纹尖端附近产生井沿着几个滑移面发射,但发射出的位错偶对裂纹没有明显的屏蔽作用在滑移面不垂直于裂纹面时,发射出的位错或位错偶关于裂纹面呈不对称分布     

IN SITU TEM STUDY OF DISLOCATION EMISSION AND MICROCRACK NUCLEATION FOR α-Ti AFTER ADSORBING Hg

用自制的恒位移加载台，在ＴＥＭ中原位观察了α－Ｔｉ在Ｈｇ蒸汽中放置前后加载裂纹前端位错组态的变化以及脆性微裂纹的形核和扩展。并和α－Ｔｉ在ＴＥＭ中原位伸的结果进行了比较。结果表明：加载裂纹吸附Ｈｇ原子后能促进位错的发射，增殖和运动；当吸附促进位错发射和运动达到临界状态时，脆性微裂纹就在原裂纹顶端或在无位错区中形核并解理扩展。     

齿轮表层疲劳裂纹扩展的声发射模型

齿轮表面的疲劳磨损是齿轮故障的主要形式之一。从弹性力学的能量理论角度,推导出了齿轮齿表疲劳裂纹在扩展时的能量关系,并根据声发射信号的特征,构造出齿表疲劳裂纹在扩展时产生的声发射信号的理论模型,说明了影响声发射信号能量强度的主要因素为裂纹扩展时间、形变体积及载荷和弹性模量的变化率,从理论上证明了声发射技术用于检测疲劳磨损的可行性。     

结构裂纹对弹性梁振动特性的影响

研究了裂纹扩展引起的结构柔度变化以及裂纹界面接触对弹性梁振动特性的影响。基于断裂力学和能量原理推导了含横向裂纹弹性梁的局部柔度模型,分析了常见形式裂纹均匀梁的局部柔度,给出了相应的无量纲柔度系数计算公式;结合梁的弯曲振动方程,探讨了含裂纹悬臂梁的振动特性。算例表明：裂纹形式对弹性梁的柔度影响明显,裂纹界面接触会引发参数振动,界面滑动摩擦阻尼对稳定梁的振动响应具有重要作用。     

铝合金液体金属脆断过程的TEM原位观察

采用自制的恒位称加载台，在透射电镜（ＴＥＭ）中原位观察了７０７５铝合金吸附液体金属（Ｈｇ＋３％Ｇａ）后加载裂纹前方位错组态的变化以及脆性微裂纹的形核和扩展，结果表明：液体金属吸附后能促进位错的发射、增殖和运动；当吸附促进位错发射和运动达到临界状态时，脆性微裂纹就在原裂纹顶端或在无位错区中形核并解理扩展。     

Interface fracture toughness and fracture mechanisms of thermal barrier coatings investigated by indentation test and acoustic emission technique

Interface fracture toughness and fracture mechanisms of plasma-/sprayed thermal barrier coatings (TBCs) were investigated by interfacial indentation test (IIT) in combination with acoustic emission (AE) measurement. Critical load and AE energy were employed to calculate interface fracture toughness. The critical point at which crack appears at the interface was determined by the IIT. AE signals produced during total indentation test not only are used to investigate the interface cracking behavior by Fast Fourier Transform (FFT) and wavelet transforms but also supply the mechanical information. The result shows that the AE signals associated with coating plastic deformation during indentation are of a more continuous type with a lower characteristic frequency content (30-60 kHz), whereas the instantaneous relaxation associated with interface crack initiation produces burst type AE signals with a characteristic frequency in the range 70-200 kHz. The AE signals energy is concentrated on different scales for the coating plastic deformation, interface crack initiation and interface crack propagation. Interface fracture toughness calculated by AE energy was 1.19 MPam_1/2 close to 1.58 MPam_1/2 calculated by critical load. It indicates that the acoustic emission energy is suitable to reflect the interface fracture toughness.     

Ductile and brittle fracture of titanium

Conclusions Crack propagation in ductile samples leads to a sharp increase of dislocation density in the vicinity of the crack. Since no increase in noted in the density of pits on cleavage steps in brittle fractures, it can be assumed that all the energy of fracture is spent on the joining of cracks and the plastic deformation associated with it. Consequently, deformation in brittle fractures is localized at the boundaries of the cleavage steps.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 48–49, May, 1974.     

Failure Behavior of Plasma-Sprayed Yttria-Stabilized Zirconia Thermal Barrier Coatings Under Three-Point Bending Test via Acoustic Emission Technique

In this paper, the failure behavior of plasma-sprayed yttria-stabilized zirconia thermal barrier coatings fabricated by atmospheric plasma spraying (APS-TBCs) under three-point bending (3PB) test has been characterized via acoustic emission (AE) technique. Linear positioning method has been adopted to monitor dynamic failure process of the APS-TBCs under 3PB test. The investigation results indicate that the variation of AE parameters (AE event counts, amplitudes and AE energy) corresponds well with the change of stress–strain curve of the loading processes. The failure mechanism was analyzed based on the characteristics of AE parameters. The distribution of frequency of crack propagation has been obtained. The AE signals came from two aspects: i.e., plastic deformation of substrates, initiation and propagation of the cracks in the coatings. The AE analysis combined with cross-sectional observation has indicated that many critical cracks initiate at the surface of the top-coat. And some main cracks tend to propagate toward the substrate/bond-coat interface. The actual failure mechanism of the APS-TBCs under 3PB test is attributed to the debonding of metallic coating from the substrates and the propagation of the horizontal crack along the substrate/bond-coat interface under the action of flexural moment.     

Study of the mechanism of nanoscale ductile mode cutting of silicon using molecular dynamics simulation

In cutting of brittle materials, it was observed that there is a brittle-ductile transition when two conditions are satisfied. One is that the undeformed chip thickness is smaller than the tool edge radius; the other is that the tool cutting edge radius should be small enough—on a nanoscale. However, the mechanism has not been clearly understood. In this study, the Molecular Dynamics method is employed to model and simulate the nanoscale ductile mode cutting of monocrystalline silicon wafer. From the simulated results, it is found that when the ductile cutting mode is achieved in the cutting process, the thrust force acting on the cutting tool is larger than the cutting force. As the undeformed chip thickness increases, the compressive stress in the cutting zone decreases, giving way to crack propagation in the chip formation zone. As the tool cutting edge radius increases, the shear stress in the workpiece material around the cutting edge decreases down to a lower level, at which the shear stress is insufficient to sustain dislocation emission in the chip formation zone, and crack propagation becomes dominating. Consequently, the chip formation mode changes from ductile to brittle.     

A new cutting method for bone based on its crack propagation characteristics

The goal of the paper is to develop a machining process for biomaterials. Efficiency, mechanical stress, and precision are crucial parameters in the machining of living tissues, such as bone. The authors observed and analyzed the fracture of and crack propagation in bone, since crack generation cannot be avoided in bone machining, due to the brittle nature of the workpiece. A new cutting method is proposed based on the determined crack propagation characteristics. Through the use of this method, the required machining energy was decreased by utilizing the new insights concerning crack propagation and the surface roughness was improved, owing to the small finishing depth of cut.     

声发射源特征识别的最新方法

介绍多传感器数据融合、时频能量模式分析及分析理论识别声发射源特征的方法，说明基于波形分析的现代信号处理技术是揭示声发射源及信号传播规律的重要手段。     

声发射源特性的神经网络模式识别研究

提出一种对声发射源活动情况进行模式识别的方法。该方法以裂纹的形成、扩展和断裂三个阶段为声发射源,通过对声发射的参数进行采样和分析,利用不同类型的人工神经网络进行模式识别,以判断理解纹的危害程度。该方法也可用于区别噪声、泄漏、裂纹等不同声发射源的声发射信号,以判断缺陷的类别。     

Effects of Nb Addition on Charpy Impact Properties of TiVTa Refractory High-Entropy Alloy

In the present study, the effects of Nb addition on Charpy impact properties of TiVTa refractory high-entropy alloy with high strength-ductility trade-off were systematically studied by using the instrumented Charpy impact testing machine. The experimental results showed that the impact toughness was remarkably improved by Nb addition in TiVTa to form TiVTaNb alloy. The crack initiation energy and propagation energy of TiVTaNb were 67.3% and 24.9% higher than that of TiVTa, indicating that Nb addition simultaneously reinforced the resistance to crack initiation and propagation. The impact fracture of TiVTaNb exhibited larger bending degree of shear lips, deeper dimples and more secondary cracks which effectively dissipated more impact energy. The deformation mechanism of TiVTa alloy was dominated by dislocation activities. While in TiVTaNb, the deformation mechanism was synergized by dislocation activities and deformation twinning, which were the main contributors for the improved impact properties and the stronger crack resistance of TiVTaNb alloy under impact loading.