玻璃在平面双向和单向应力状态下慢裂纹扩展特性

采用热弹性力学方法对含裂纹的玻璃薄片在双向和单向平面拉和下的亚临界裂纹扩展进行了观测和研究。该实验方法使得玻璃、陶瓷等危性材料的双向拉伸,直通裂纹预制和裂纹扩展过程记录这３个难度大的实验得以完成。对６０多个玻璃试件的断裂阻力随载荷速度的变化进行了观测和分析,从而较精确地了解了玻璃在平面和状态下的阻力曲线和断裂特性。     

直流电场对LiNbO3晶体亚临界裂纹扩展的影响

本工作采用双扭曲试验法(DT法),着重研究了极化条件、直流电场的强度和取向对晶体学取向不同的LiNbO_3晶体中裂纹亚临界扩展行为的影响,并对晶体由裂纹临界扩展导致断裂的形貌作了分析。实验结果表明:LiNbO_3晶体试样的裂纹亚临界扩展方程指数n随晶体学取向的不同而不同,这与裂纹扩展所沿晶面的断裂能有关。未极化的LiNbO_3晶体受直流电场作用时,其亚临界裂纹扩展的加快,系压电晶体电致伸缩效应所致;极化的LiNbO_3晶体试样在直流电场作用下,其亚临界裂纹扩展主要是受晶体的逆压电效应的影响。     

玻璃的脆性(四)

综述了裂纹的容忍度与玻璃脆性的关系。对不同品种玻璃的裂纹扩展力也作了阐述。此外,还讨论了应力腐蚀下各种玻璃的裂纹扩展速度和影响裂纹扩展的因素。     

加氢反应器裂纹扩展临界状态的研究

利用Hartman-Schijve公式计算加氢反应器材料2．25CrlMo钢裂纹疲劳扩展的应力强度因子理论门槛值,与实用门槛值比较结果表明后者偏于危险,二者相差约为42．55％。由此进一步引用推导公式,计算得到某加氢反应器材料断裂韧度的值。综合裂纹深度和长度方向两方面因素,对裂纹扩展状态进行分析,推导出裂纹扩展的临界尺寸,作为加氢反应器裂纹扩展评判依据。     

物理钢化对玻璃表面裂纹扩展和强度的影响

物理钢化对玻璃表面裂纹扩展和强度的影响宋力昕，胡行方，陈显求（中国科学院上海硅酸盐研究所２０００５０）ＴｈｅＥｆｆｅｃｔｓｏｆＴｈｅｒｍａｌＴｅｍｐｅｒｉｎｇｏｎｔｈｅＧｒｏｗｔｈｏｆＳｕｒｆａｃｅＣｒａｃｋｓａｎｄｔｈｅＳｔｒｅｎｇｔｈｏｆＧｌａｓ...     

工程陶瓷疲劳裂纹扩展的实验研究

本文提供了一种新的确定工程陶瓷疲劳裂纹扩展速率公式－－Ｐａｒｉｓ公式中两个常数参量的实验方法。避开了直接测量裂纹工度的麻烦，且成功率很高。给出了以Ａｌ２Ｏ３和ＨＰ０－Ｓｉ３Ｎ４陶瓷为试验材料的实验结果，并在这个结果的基础上对工程陶瓷的疲劳特性作了描述。     

电工陶瓷材料裂纹扩展规律的研究

本文应用双扭法研究了四种电瓷材料静态疲劳裂纹扩展规律。试验结果表明:在空气介质中,所有试验电瓷材料均存在明显的裂纹亚临界扩展现象,且随裂纹尖端应力强度因子(K_1)增大呈现三个阶段关系。在不同介质中,裂纹扩展的敏感性按下列顺序变小:水、空气和煤油。试验还表明电瓷材料静态疲劳裂纹扩展规律与材料的配方、环境和受力等因素有关。     

Nanoindentation Method for Measuring Residual Stress in Brittle Materials

The lowered threshold load for cracking with the cube-corner indenter has been used in developing a method that can be used to measure residual stresses in small volume brittle materials. By studying a series of orthogonal cracks generated at loads not exceeding 10 mN with the cube-corner indenter, a variation of crack length with position around a large Vickers impression in soda–lime glass was observed. Using an indentation fracture mechanics approach residual stresses were evaluated at the positions where the cube-corner indents had been made. The stress values thus evaluated were generally higher than those reported in the literature where micro-Vickers indents had been used to measure the stresses. Possible reasons for the disparity are discussed.     

Controlled Crack Growth Specimen for Brittle Systems

A pure Mode I fracture specimen and test procedure has been developed which provides extended, stable, through-thickness crack growth in ceramics and other brittle, nonmetallic materials. Fixed displacement loading, applied at the crack mouth, promotes stable crack extension by reducing the stored elastic strain energy. Extremely fine control of applied displacements is achieved by utilizing the Poisson expansion of a compressively loaded cylindrical pin. Stable cracks have been successfully grown in soda-lime glass and monolithic Al₂O₃ for lengths in excess of 20 mm without uncontrollable catastrophic failure.     

Application of Load-Relaxation Techniques to Study Subcritical Crack Growth in Brittle Materials

The load relaxation technique was applied to double- cantilever-beam specimens for the determination of diagrams of the stress intensity factor (K₁ ) vs crack velocity (V) for soda-lime glass. The effect of machine compliance was explicitly incorporated into the velocity equation. The data were in good agreement with published data from different techniques. It was shown that a hard machine (small compliance) is best suited for the load-relaxation technique. The use of a soft machine (high compliance) will lead to catastrophic failure during load relaxation. A single-edge notched-beam specimen in bending is unsuitable for the relaxation technique since catastrophic failure will occur during relaxation regardless of machine compliance.     

Statistics of Flaw Interaction in Brittle Materials

The problem of elastic interaction of an array of microcracks is considered. The solution is based on an efficient surface integral method using distribution of edge dislocations to represent the microcracks in the mathematical model. From the analysis of the interaction between two identical cracks, it is seen that interaction is most likely to produce a slight enhancement in the stress intensity factor. By performing computer experiments on a random array of microcracks, the effect of crack interaction is studied statistically as a function of the density of the microcracks. The significance of the interaction effects for fracture in ceramics is discussed.     

脆性材料强度评价

本文介绍以先进结构陶瓷为主要对象的脆性材料抗张强度的各种评价方法。包括直接单轴拉伸法和各种间接测定法。     

Factors Controlling the Fracture Resistance of Brittle Cellular Materials

Previous work on brittle cellular solids has indicated that the fracture toughness may be independent of cell size. The results showed agreement with two theoretical approaches proposed by Maiti et al. to predict the toughness of porous cellular solids. One approach indicates a strut strength and cell size dependence of the bulk material toughness, whereas the other depends only on the toughness of the solid portion. The present work attempted to experimentally discern between the two approaches and identify the controlling parameters governing the mechanical behavior. Reticulated vitreous carbon samples were oxidized to introduce surface flaws into the struts. The toughness and compressive strength of the bulk material were measured before and after oxidation. Fractographic analysis of the fracture surface of individual struts was essential in being able to estimate the strut strength. It was concluded that it is the strut strength which controls the strength and toughness of the bulk cellular solid.     

Fiber Debonding in Residually Stressed Brittle Matrix Composites

The competition between initial fiber debonding versus fiber failure marks a crucial event of the microstructural failure process in fiber-reinforced brittle matrix composites. In this study, the role of a thermal residual stress field on the debonding conditions is examined theoretically and analytically. The analysis is based on two critical observations, the first being that the mechanics at the tip of a kink crack are driven only by the singularity at the main crack tip. Following from the first is the second observation that any thermal stress effects on the debonding criteria should enter only through the phase angle ψ ^T of the total stress intensity factor at the main crack tip. In general, this stress intensity factor has a thermal as well as a mechanical load contribution. It is shown that when the thermal and mechanical stress intensities, K ^R and K ^t , respectively, are in phase , i.e., ψ ^R =ψ ^t , the existing debonding conditions are universal and can be used even in the presence of thermal loads. On the contrary, when K ^R and K ^t are out of phase , i.e., ψ ^R ≠ψ ^t , events such as the delamination of thick films or debonding of inclined aligned fibers in brittle matrix composites become sensitive to the presence of the thermal stresses.     

Multibarrier Kinetics of Brittle Fracture: I, Stress Dependence of the Subcritical Crack Velocity

A general equation for the subcritical crack velocity in brittle materials is derived using steady-state, multibarrier kinetics. It provides a basis from which specialized expressions can be developed for the crack velocity in specific cases. One social equation for v=v(K) at constant temperature, derived from the general expression, has a mathematic form that fits data representative of a wide variety of specific cases. Examples are presented and the equation is discussed in terms of its physical foundations.     

Subcritical Crack Growth in Sintered Silicon Nitride Exhibiting a Rising R-Curve

Subscritical crack growth of sintered silicon nitride was analyzed in terms of the R -curve. Provided that the stress intensity at the crack tip governs the subcritical crack-growth velocity, the K _I– V relationship of sintered silicon nitride exhibiting a rising R -curve is shown to shift to the high- K _I region as the crack advances.