韧性颗粒增韧脆性材料的桥接变形计算模型

在脆性材料中弥散加入韧性颗粒可显著提高基体材料的断裂韧性。为了计算材料断裂韧性的增量，本文提出了新的基体与颗粒界面颈缩剥离变形假设，建立了应力与位移之间的关系式。文中还给出了精确的简化计算式。     

脆性材料的超精加工--脆性材料的钻石切削综述

在脆性材料的超精加工中，一个十分重要的现象是材料由脆性状态向可延展性状态的转变．为了成功地时脆性材料进行钻石切削加工以获得镜面表面，两种状态转变点的确定十分关键．本文阐述研究这一转变的实验技术，并讨论表面生成的机理．发现在脆性材料的钻石切削中，刀具磨损是一个严重后果．建议今后的研究应集中在这一领域．     

Dynamic Fracture of Nominally Brittle Materials

Current understanding of dynamic fracture mechanisms and the methods of modeling are reviewed critically. Experimental methods used in dynamic fracture investigations and key experimental observations are reviewed. This is followed by a critical review of the dynamic fracture models. Mechanistic and phenomenological models as well as discrete and continuum models and their ability to reproduce experimental results are discussed.     

The Brittle Versus the Ductile Nature of Fracture Modes I and II

Failure surface orientations are determined for states of uniaxail tensile stress and shiar stree appropriate to fracture Modes I and II. The method uses a newly developed failure criterion along with the associated flow rule. The failure angles show a sharp demarcation between ductile and brittle material types. Only the brittle materials class show consistency and likely represents a shear localization effect rether than explicit fracture.     

An analysis of Chipping in Brittle Materials

The chipping process in a brittle material subjected to a uniformly applied edge load has been investigated. The present analysis extends earlier work by recognizing that as the chip is formed it may bend and change the loading at the crack tip. This geometry change introduces a nonlinear effect and has significant influence on the phenomenon. The nonlinear effect was demonstrated by incorporating it into an analytical model for a crack propagating along an interface parallel to the free surface. A finite-element analysis was then conducted to examine the crack trajectory formed in a homogeneous material. This numerical analysis showed that the crack reaches a maximum depth, and then deviates back to the free surface to form a spall. The form of this trajectory results from the additional bending moment acting at the crack tip induced by the bending of the chip and the consequent displacement of the applied load. The length of the spall was found to be approximately proportional to the square root of êd^5/2/K_IC, where K_IC is the fracture toughness of the material, ê is the appropriate modulus of the material, and d is the depth over which the edge load is applied. This revised version was published online in August 2006 with corrections to the Cover Date.     

40Cr钢韧脆转变区域的一种细观随机模型

针对４０Ｃｒ钢提出孤细观模型认为；在统计意义上，材料的韧性断裂为空穴机制，临界空穴扩张比参数可以作为其判据；材料的脆性断裂可以用内嵌币状裂纹的脆性断裂模型来模拟；在韧脆转变区内，这两种机理并存并相互竞争。     

Ductile Shear Damage Model for Porous Materials

Based on a pre-existing yield function, an extended version of the well-known Gurson-Tvergaad-Needleman （GTN）isotropic hardening model is presented in this investigation. The yield function of the proposed constitutive model possesses the distinctiveness to be more accurate for arbitrary void volume fraction and especially to explicitly depend upon the third stress invariant. As a numerical example, the presented constitutive model and, for the purpose of comparison, the GTN model, are used to analyse the necking of a round tensile bar and the two-dimensional simple dynamic sheafing problem. The numerical results highlight similarities, good agreement as long as softening initiation of specimen is not reached, and discrepancy as soon as failure of specimen starts, between the proposed model and the GTN model.     

硬脆材料的旋转超声辅助加工

光学玻璃、功能晶体、工程陶瓷等硬脆材料具有高强度、耐高温、耐磨损等优良的物理和机械性能,目前已被广泛应用于航空航天、汽车、军工等领域.但由于这些材料具有较高的硬度和脆性,难以用传统的加工方法进行加工,从而在很大程度上制约了这些材料的进一步推广和应用.旋转超声辅助加工技术由于其特殊的加工性能,具有切削力小、低切削热等优点,适应于各种硬脆材料的加工,因此得到了广泛的应用和迅速的发展,在硬脆难加工材料的制造领域已经占据了重要的地位.本文回顾了旋转超声辅助加工技术的发展历程和特点,介绍了旋转超声辅助加工的机理及工艺设备的研究进展,并对旋转超声辅助加工技术和设备的发展趋势进行了展望.     

脆性材料截面透射电镜样品的制备

针对脆性材料截面透射电镜样品制样难的问题,详细介绍一种改进的离子减薄法制备脆性材料截面TEM样品的方法,即先制备层叠矩形薄片,然后使用低速锯切取出层叠薄片,再用切割工具切割出φ3 mm的圆片,最后采用凹坑研磨和抛光及离子减薄法完成样品的制备.大量操作试验证明该方法具有成本低、操作方便等优点,提高了截面TEM样品的制样成...     

Temperature Dependence of Brittle Fracture Toughness of Reactor Pressure-Vessel Steels upon Ductile Crack Growth

The main mechanisms of brittle fracture upon ductile crack growth are studied on the basis of the probabilistic model of brittle fracture and the deterministic model of ductile fracture, which were put forward by the authors earlier. The investigations are carried out on the reactor pressure-vessel steel 15Kh2NMFAA in the initial and embrittled states. The dependences of brittle-fracture probability on the stress intensity factor and the value of ductile crack growth are calculated for various temperatures. The temperature dependence of brittle fracture toughness in the initial and embrittled states is predicted with and without regard for ductile crack growth. The authors analyze the main factors that govern the above-mentioned relationships. The calculated results are compared to test data for CT-type compact specimens.     

Tensile Behavior of NiAl-9Mo Eutectic Alloy around Brittle to Ductile Transition Temperature

The influence of strain rate and temperature on the tensile behavior of as-cast and HIPed NiAl-9Mo eutectic alloywas investigated in the temperature range of 700～950℃ and over a strain rate range from 2.08×10~(-4) s~(-1) to2.08×10~(-2)s~(-l). The results indicate that HIP process causes an enhancement in ductility and a decrease in ultimatetensile strength (UTS), yield strength (YS), average strain hardening rate as well as a drop in brittle to ductiletransition temperature(BDTT) under the same condition. It is noticed that the BDTT of as-cast NiAl-9Mo is moredependent on strain rate than that of HIPed one. The brittle to ductile transition process of the alloy is related toa sharp drop in strain hardening rate. Regardless of strain rate, the fracture morphology changes from cleavage inNiAI phase and debonding along NiAI/Mo interface below the BDTT to microvoid coalescence above BDTT. Theapparent activation energy of the BDT of HIPed and as-cast material are calculated to be 327 and 263 kJ/mol,respe     

A One-Dimensional Nonlocal Damage-Plasticity Model for Ductile Materials

This paper presents a new 1-D non-local damage-plasticity deformation model for ductile materials. It uses the thermodynamic framework described in Houlsby and Puzrin (2000) and holds, nevertheless, some similarities with Lemaitre’s (1971) approach. A 1D finite element (FE) model of a bar fixed at one end and loaded in tension at the other end is introduced. This simple model demonstrates how the approach can be implemented within the finite element framework, and that it is capable of capturing both the pre-peak hardening and post-peak softening (generally responsible for models instability) due to damage-induced stiffness and strength reduction characteristic of ductile materials. It is also shown that the approach has further advantages of achieving some degree of mesh independence, and of being able to capture deformation size effects. Finally, it is illustrated how the model permits the calculation of essential work of rupture (EWR), i.e. the specific energy per unit cross-sectional area that is needed to cause tensile failure of a specimen.     

脆性材料在双向应力下的断裂和失效研究

采用热力学方法对脆性薄片试样成功地进行了双向和单向平面拉伸试验,通过观察和记录试片中心的直通裂纹的扩展和断裂过程,测试出玻璃和陶瓷薄片的断裂韧性在双向和单向拉伸载荷时的差别．结果表明双向拉伸使裂纹阻力增强,平行于裂纹的应力对裂纹扩展有影响．该研究表明,对线弹性材料在双向载荷作用下,传统的应力强度因子准则不适用．裂纹张开的应变依赖性被证实在双向应力的断裂评价中．     

航空结构用增韧复合材料的研究与开发

先进复合材料国防科技重点实验室在拥有自主知识产权的"离位"增韧新概念基础上,成功开发了兼具优异工艺性和高抗冲击损伤性的航空级"离位"增韧的热压罐固化复合材料体系和液态成型(RTM及RFI等)复合材料体系.这些增韧的材料体系包括高性能的环氧树脂体系和双马来酰亚胺体系的复合材料.经试验验证,"离位"增韧可在显著提高复合材料层问韧性、冲击损伤阻抗和容限的同时,保持材料原有优异的成型工艺性和湿热性能.     

Mixed Mode Brittle and Ductile Fracture of a High Strength Rotor Steel at Room Temperature

The mixed mode fracture of a high strength rotor steel has been investigated at room temperature using single edge notched specimens. In mode I, and for limited amounts of shear loading, the steel exhibited cleavage fracture. For conditions near mode~II ductile fracture occurred. A transition from brittle to ductile fracture occurred for mixed mode loading. Finite element analysis provided estimates of the extent of near crack tip yielding and elastic-plastic stress intensity factors. Test results agreed with the maximum tensile stress (MTS) criterion for small scale yielding for limited amounts of shear loading. The load for mode II fracture was lower than predicted from the MTS criterion, but higher than predicted from plastic collapse predictions. Observed fracture angles where in broad agreement with the predicted fracture mechanisms. The load for the transition from brittle to ductile fracture was found to agree approximately with the predicted load. This revised version was published online in July 2006 with corrections to the Cover Date.     

硬铝约束对纯铝韧脆转变的影响

本文通过四点弯曲试验以及数值模拟方法对爆炸焊接的LY12/Al/LY12层合材料中纯铝中间层在不同界面强度错配比条件下裂端应力场及韧脆转变行为进行了研究。结果表明:在双侧硬铝约束下,界面强度错配比增加对裂端应力三轴度及其分布具有显著影响,裂端前沿应力三轴度的提高对中间纯铝层韧脆转变起主导作用,在一定应力三轴度及最大主拉应力条件下,裂端前沿将发生脆性解理启裂。     

Nondestructive Evaluation of Ductile to Brittle Transition Temperature for Martensitic Steels Based on Magnetic Measurements

Interest in magnetic nondestructive evaluation (NDE) methods for nuclear reactors materials to quantify material embrittlement and ensure reactor safety is increased. In this paper, ductile–brittle transition temperature (DBTT) and temperature dependence of the coercive field (\(H_{C})\) and magnetization (M) of martensitic steels (F82H, Eurofer97, SCRAM, T91 and cold rolled T91 steels) were investigated from 100 to 350 K to non-destructively evaluate their DBTT. Results show that a characteristic temperature in both ln(\(H_{C})\) and d(lnM)/d(1/T) vs temperature curves coincides well with the corresponding DBTT measured by Charpy impact test. The curves decrease linearly above and below DBTT respectively, the absolute slope above DBTT is larger than that below. The phenomenon is closely related to the different motion status and the pinning strength of dislocations and magnetic domain wall in the ductile and brittle regions. Results indicate the magnetic evaluation method could be a promising technique with good reproducibility and high sensitivity to evaluate DBTT of martensitic steels used in nuclear reactors.