陶瓷刀具材料断口形貌及裂纹扩展的分形特征

通过热压烧结工艺,制备了一种高性能的Si3N4基陶瓷刀具材料。利用X射线衍射仪和扫描电子显微镜分别对材料物相组成、微观形貌及裂纹扩展路径进行了分析。借助图像处理技术和分形理论,计算了断口形貌及裂纹扩展路径的分形维数,并揭示材料断裂机制。研究表明,Si3N4基陶瓷刀具材料表现为穿晶/沿晶的混合断裂模式,其裂纹扩展方式主要是偏转和桥联,断口形貌及裂纹扩展均具有明显的分形特征。当材料断口形貌越粗糙,裂纹扩展路线越不规则,分形维数值增大,表明断口微观结构的粗糙程度、裂纹扩展路线的不规则程度可用分形维数来刻画。     

TiB2-TiN复合陶瓷刀具材料的显微结构和力学性能研究

热压烧结制备了不同TiN含量的复合陶瓷刀具材料TiB2-TiN-(Ni, Mo),对其性能测试表明,随着TiN含量的增加,材料的抗弯强度和断裂韧度逐渐提高,但是材料的硬度在TiN的含量达到40%(体积分数)时却大幅度降低.利用X衍射(XRD)、扫描电镜(SEM)和能谱(EDAX)分析了复合材料的物相和显微组织,结果表明,烧结过程中生成了MoNi相;随TiN含量增加,材料从以沿晶断裂为主转变为同时有沿晶断裂和穿晶断裂的断裂模式;裂纹扩展过程中有金属颗粒桥连现象.分析认为,材料的主要增韧机制是延性相颗粒桥连和裂纹偏转.     

超导磁体在NdFeB永磁材料制备技术中应用研究

概述了超导磁体技术的研究进展,分析了超导磁体在NdFeB永磁材料制备过程中的应用前景,重点讨论了超导磁体在烧结NdFeB磁体、NdFeB铸锭组织、HDDR(氢化-歧化)工艺制备NdFeB纳米晶粉末等领域的具体应用情况.     

合金元素对烧结Nd—Fe—B永磁材料断裂强度影响的研究

研究了含Ｃｏ和不含Ｃｏ的烧结Ｎｄ－Ｆｅ－Ｂ永磁材料的抗弯强度和断裂韧性，并采用微观分析方法对它们的断裂行为进行了研究。结果表明，烧结Ｎｄ－Ｆｅ－Ｂ永磁材料的静变断裂主要为沿晶断裂。     

烧结钕铁硼永磁材料腐蚀机理与表面防护技术

介绍了烧结NdFeB永磁材料在几种典型环境中的腐蚀机理,阐述了目前采用的NdFeB表面防护技术及研究进展情况,以及作者研究开发的烧结NdFeB永磁材料的表面防护涂层技术.     

温度对TC4-DT损伤容限型钛合金疲劳裂纹扩展行为的影响

对TC4-DT损伤容限型钛合金在150℃,25℃下的疲劳裂纹扩展速率da/dN进行了测试,给出了扩展速率和应力强度因子幅值△K之间的关系曲线.用SEM对2种温度下断口形貌进行了观测,实验结果表明,150℃的疲劳裂纹扩展速率试样具有较低的疲劳裂纹扩展速率,25℃的疲劳裂纹扩展速率试样具有较低的门槛值;稳态扩展区解理断裂和条带循环机制共存,150℃的da/dN试样中的疲劳辉间距比25℃试样细;快速扩展区的断口形貌呈韧窝型静载断裂特征,150℃的da/dN试样中的韧窝比25℃试样深.     

C形环小试样疲劳裂纹扩展试验方法与应用

航空航天、核反应堆工程等重要工程结构的安全控制对材料断裂性能有重要需求,小尺寸试样断裂韧性测试需求日益凸显。该文提出了一套用于小尺寸构件疲劳裂纹扩展速率测试的C形环小试样试验方法,设计了试样构形和试验夹具,并基于有限元分析和柔度法理论,建立了C形环小试样裂纹长度预测公式和应力强度因子K的表达式。对于低压转子材料26NiCrMoV11-5钢,采用C形环小试样和标准CT试样完成了疲劳裂纹扩展试验,获得了材料的疲劳裂纹扩展特性,并讨论了新方法的有效性。     

用示波落锤试验法评价14Mn VTiRe钢的韧脆断裂行为

采用示波落锤试验法对 5mm热连轧 1 4MnVTiRe钢的韧脆断裂行为进行了研究并对落锤撕裂试样的动态断裂特征进行了评价。结果表明 ,当裂纹扩展功远大于裂纹萌生功时 ,试样处于韧性断裂状态 ;当裂纹扩展功小于裂纹萌生功时 ,试样处于脆性断裂状态 ;当裂纹扩展功与裂纹萌生功相当时 ,试样处于混合断裂状态。该结果与按标准测定的剪切面积百分数有良好的对应关系 ,说明示波落锤试验法可作为一种评定材料韧脆断裂特征的实用方法     

含氢的Ti-2Al-2.5Zr合金的静态和动态断裂行为研究

采用紧凑拉伸(CT)试样,研究了含氢110μg/g的Ti-2Al-2.5Zr钛合金在室温的静载延迟裂纹扩展(da/dt)和疲劳加载裂纹扩展(da/dN)行为,用扫描电镜观察了断口形貌.结果表明,Ti-2Al-2.5Zr合金含氢110μg/g即足以引起材料的静载氢致延迟断裂,氢在裂纹尖端扩散聚集并析出氢化物,导致材料变脆,是氢致延迟断裂的微观机制;而相同含氢量时对疲劳断裂过程的影响微弱,疲劳断裂受通常的裂纹萌生、稳态扩展和瞬断机制控制.     

预制裂纹角度对橡胶拉伸断裂影响的有限元分析

初始裂纹的形态影响着裂纹尖端的应力场和扩展方向,进而决定着橡胶材料的使用寿命。目前人们关于预制裂纹试样拉伸断裂的研究主要集中在直裂纹,很少涉及预制裂纹角度的改变对橡胶拉伸断裂的影响。文中应用ANSYS有限元分析软件计算拉伸状态下含不同裂纹角度橡胶试样裂纹尖端的等效应力值和撕裂能的大小,判断裂纹是否扩展及扩展方向,并对橡胶试样进行拉伸验证试验测试。结果表明,在拉伸断裂过程中,裂纹尖端的应力值和撕裂能随着初始预制裂纹角度的增大而增大,裂纹尖端形状均由初始的尖点变成圆弧状;含不同裂纹角度橡胶试样的拉伸断裂形貌与裂纹预测扩展方向基本一致,验证了有限元分析的正确性。     

摩托车气门外弹簧的断裂分析

某摩托车在行驶l500km后气门外弹簧发生断裂。采用扫描电镜、金相显微镜、等离子光谱发射仪等分析手段，对其断口进行了分析。结果表明，断裂呈现典型的疲劳断裂特征，材料表面缺陷是引起该弹簧疲劳断裂的主要原因。     

Microstructure and fracture mechanical response of wood

Investigations on the fracture properties of wood in relation to its microstructure are reported. The inhomogeneous and hierarchical structure of wood is addressed. Wood species, the influence of orientation, the role of structural features, like rays are considered and discussed. Likewise the mode of loading, which determines the mode of fracturing, and the influence of humidity have been studied by using new fracture mechanical techniques and ways of evaluation. The specific fracture energy has been determined under crack opening conditions. In-situ loading in an environmental scanning electron microscope (ESEM), which allows observation in moistured condition, has been performed in order to investigate the mechanisms of fracturing of wood on a sub-microscopic scale. In the nanometer range, especially the influence of the microfibril angle on deformation and fracture behaviour has been studied.     

Effect of aging treatment on fracture toughness in ZM61 magnesium alloy

The effect of aging treatment on fracture toughness in Mg–6Zn–1Mn (wt-%) was investigated by optical microscopy, scanning electron microscopy, transmission electron microscopy, scanning transmission electron microscopy, uniaxial tensile and fracture toughness tests, respectively. The results showed that the fracture toughness of Mg–6Zn–1Mn alloy can be enhanced by aging treatment. The fracture toughness and strength showed a reverse trend in single aged and double aged alloy. Synergetic effect of fine grains and precipitates improved the fracture toughness more sharply than aging treatment. The precipitate free zones and grain boundary precipitates made the largest contribution to the reduction of toughness. Under as extruded and aged conditions, the main origins of cracks were elastic incompatibility and plastic deformation.     

Computational evaluation of flexural toughness of FRC and fracture properties of plain concrete

In this paper, a computational tool concerning the computation of flexural and fracture toughness of cement based composites is presented. Firstly, RILEM’s (Réunion Internationale des Laboratoires d’Essais de Matériaux) recommendations related to the analysis of FRC in three-point bend tests are discussed in their relevant aspects regarding the computational implementations. The determination of other mechanical properties such as the Young modulus has been added to the program. Taking this into account, a new formulation based on displacements is used. In the second part of the paper, the determination of fracture properties of concrete, such as the fracture energy, G _F , and the fracture toughness, K _IC ^S , is discussed regarding the computational strategies used in the implementations. Several features whereby anterior data can be reanalyzed, obtained from other standards and recommendations, have been incorporated into the program, therefore allowing comparative studies and back analysis activities.     

换热器紧固螺栓的应力腐蚀断裂分析

紧固螺栓服役2年后发生成批断裂.采用宏观观察、化学成分分析、金相检验和扫描电镜与能谱分析等方法对失效螺栓进行了分析.结果表明,由于紧固螺栓长期浸在含有H2S介质的油料中,并在所承受的各种应力下又与腐蚀环境相互作用便产生了应力腐蚀,加上螺栓材质选用不当和硬度、显微组织不合理等其他因素,最终导致螺栓断裂.     

我国混凝土断裂力学研究的部分新进展

混凝土、钢筋混凝土及钢纤维混凝土是非均匀的多相复合材料,其断裂性能十分复杂。本文论述了近年来我国在这方面的部分进展情况,内容包括:一、简述了发展历史;二、素混凝土断裂问题的研究;三、钢筋混凝土断裂问题的研究;四、钢纤维混凝土断裂问题的研究;五、指出了值得进一步探讨的问题。     

几种实用的断裂试验方法

为克服标准断裂试验方法测定中，低强度材料KIC值的困难，若干实用性断裂试验方法被提出，介绍了几种比较实用的试验方法，这种方法具有简便，经济的显著特点，在理化检验中有很大的应用价值。     

Cleavage Fracture of Brittle Semiconductors from the Nanometre to the Centimetre Scale

The objective of this paper is to present the fundamental phenomena occurring during the scribing and subsequent fracturing process usually performed when preparing surfaces of brittle semiconductors. In the first part, an overview of nano‐scratching experiments of different semiconductor surfaces (InP, Si and GaAs) is given. It is shown how phase transformation can occur in Si under a diamond tip, how single dislocations can be induced in InP wafers and how higher scratching load of GaAs wafer leads to the apparition of a crack network below the surface. A nano‐scratching device, inside a scanning electron microscope (SEM), has been used to observe how spalling (crack and detachment of chips) and/or ductile formation of chips may happen at the semiconductor surface. In the second part cleavage experiments are described. The breaking load of thin GaAs (100) wafers is directly related to the presence of initial sharp cracks induced by scratching. By performing finite element modelling (FEM) of samples under specific loading conditions, it is found that the depth of the median crack below the scratch determines quantitatively the onset of crack propagation. By carefully controlling the position and measuring the force during the cleavage, it is demonstrated that crack propagation through a wafer can be controlled. Besides, the influence of the loading configuration on crack propagation and on the cleaved surface quality is explained.     

吊装过程中吊具断裂失效分析

在吊装重364.8 t的船用机座过程中由于吊具断裂造成人员伤亡和机座严重摔坏的重大事故.采用化学分析、金相检验和断口分析等方法对断裂件材质、断裂机理和显微组织等方面进行了分析和研究,并从力学及断裂力学两方面对断裂件的安全性进行了分析和评价.金相分析证实,卸扣锻造质量不佳、锻后没有进行适当的热处理、卸扣拐点处的分型面存在补焊现象和焊接微裂纹等缺陷.上述诸多因素均导致卸扣的承载能力和安全性下降,静载荷检验达不到JB 10603-2000技术要求.断裂力学计算也证实在上述条件下卸扣的实际承载能力将大大下降,实际断裂卸扣裂纹的深度已经非常接近所允许的临界裂纹长度.基于上述结果进一步对断裂事故的发生过程、断裂次序等进行了分析和讨论.事故中卸扣G最先断裂,其次是卸扣D;之后,螺栓B,E,F由于卸扣G和D断裂后受较大的弯曲载荷而断裂.     

Multiscale modeling of dynamic crack propagation

A multiscale approach is employed to investigate a center-cracked specimen with the purpose to redefine fracture toughness from the atomistic perspective and to simulate different modes of crack propagation. The specimen is divided into three regions: (1) far field, modeled by classical fracture mechanics, (2) near field, modeled by a multiscale field theory and analyzed by a generalized finite element method, and (3) crack tip atomic region, modeled by molecular dynamics (MD). The exact and analytical solution of the far field is utilized to specify boundary conditions at the interface between the far field and the near field. The interaction between the near field and the crack tip region is described by full-blown interatomic forces. In this work, crystals of perovskite (Barium Titanate) and rocksalt (Magnesia) have been studied. Fracture toughness is defined as a material property associated with instability of the MD simulation. Mode I, Mode II, and mixed mode fracture have been investigated and numerical results will be presented and discussed.