复合材料尖劈和接头端部奇性场的反平面问题研究

提出了一个基于位移的分析尖劈端部奇性位移场和应力场反平面问题的非协调元特征法．该方法与过去原有求解裂纹尖端近似场的有限元特征法有几点不同：(1)导出虚功原理的出发点为二维扇区的散度原理;(2)有限元的单元形式为非协调元;(3)尖劈端部邻域内的位移场假定没有采用奇异变换技术．运用该方法给出了求解正交各向异性复合材料尖劈端部附近奇性应力指数、奇性位移和应力角分布函数的算例．计算结果表明,该方法较原来的有限元特征法所用的单元少而且精度高．     

随机晶界分布和连通性对晶间破坏行为的影响

采用随机有限元方法研究了两个具有相同不均匀材料属性和相同随机晶界含量但不同随机晶界分布的特殊多晶体样本在准静态拉伸下的晶间破坏行为,发现随机晶界连通性强的样本的断裂能比较低．在不同随机晶界含量下分别构造的50个晶界随机分布的样本的计算结果表明,断裂能与平均晶界连通数的关联并不明显,甚至还出现了相反的结果,即在相同随机晶界含量下,平均晶界连通数大的计算样本的断裂能反而大．     

A variational approach to analyze a natural fault with hydraulic fracture based on the strain energy density criterion

A simplified analytical model of the interaction between a hydraulic fracture and an existing natural fault is developed. The mechanical activation of the natural fault as a result of contact with a pressurized fracture is described for plane strain conditions and quasi-static fracture propagation approximation. Using a variational approach, the normal and shear stresses, as well as the boundaries of the open and sliding zones along the fault, are predicted for three stages of the fracturing process (fracture approaching, coalescence, and fluid penetration). An accumulated concentration of shear stress at the tip of the fault’s sliding zone is shown to create sufficient tensile stress to initiate a new tensile crack on the opposite side of the fault, provided either the differential in situ stress is low or the friction coefficient is sufficiently large. The results of direct numerical simulation of the fracture interaction fit the model predictions made from the strain energy density fracture criterion.     

纳米材料力学行为的原子尺度模拟研究

对几何尺寸极小的纳米材料而言,数值模拟是与实验测试同样有效的研究手段, 而且,当材料特征尺寸更小、缺乏可用的测试系统时,数值模拟可能是唯一的方法.介绍了近年来纳米材料力学行为的原子尺度数值模拟研究方面的若干新进展,重点综述了采用分子动力学模拟与第一原理计算对纳米材料的晶格不稳定性、理想强度、界面断裂、碳纳米管的力电特性和铁电纳米材料的力电特性等问题的研究结果.总结介绍了纳米材料原子尺度模拟中一些实用的计算策略和方法,并提出了若干需要进一步研究的问题.      

A statistical, physical-based, micro-mechanical model of hydrogen-induced intergranular fracture in steel

Intergranular cracking associated with hydrogen embrittlement represents a particularly severe degradation mechanism in metallic structures which can lead to sudden and unexpected catastrophic fractures. As a basis for a strategy for the prognosis of such failures, here we present a comprehensive physical-based statistical micro-mechanical model of such embrittlement which we use to quantitatively predict the degradation in fracture strength of a high-strength steel with increasing hydrogen concentration, with the predictions verified by experiment. The mechanistic role of dissolved hydrogen is identified by the transition to a locally stress-controlled fracture, which is modeled as being initiated by a dislocation pile-up against a grain-boundary carbide which in turn leads to interface decohesion and intergranular fracture. Akin to cleavage fracture in steel, the “strength” of these carbides is modeled using weakest-link statistics. We associate the dominant role of hydrogen with trapping at dislocations; this trapped hydrogen reduces the stress that impedes dislocation motion and also lowers the reversible work of decohesion at the tip of dislocation pile-up at the carbide/matrix interface. Mechanistically, the model advocates the synergistic action of both the hydrogen-enhanced local plasticity and decohesion mechanisms in dictating failure.     

Dynamic crack growth under Rayleigh wave

A nonuniform crack growth problem is considered for a homogeneous isotropic elastic medium subjected to the action of remote oscillatory and static loads. In the case of a plane problem, the former results in Rayleigh waves propagating toward the crack tip. For the antiplane problem the shear waves play a similar role. Under the considered conditions the crack cannot move uniformly, and if the static prestress is not sufficiently high, the crack moves interruptedly. For fracture modes I and II the established, crack speed periodic regimes are examined. For mode III a complete transient solution is derived with the periodic regime as an asymptote. Examples of the crack motion are presented. The crack speed time-period and the time-averaged crack speeds are found. The ratio of the fracture energy to the energy carried by the Rayleigh wave is derived. An issue concerning two equivalent forms of the general solution is discussed.     

碳纳米管增韧Sm_2Zr_2O_7基热障涂层复合材料的制备

本文提纯和分散了碳纳米管,采用共沉淀法制备出Sm2Zr2O7-CNT复合材料粉末,并用热压法制备出复合材料块体试样。研究了碳纳米管对Sm2Zr2O7微观形貌、相结构以及断裂韧性的影响。结果表明:在所得的复合材料中,碳纳米管均匀分散在Sm2Zr2O7基体中;复合材料的断裂韧性高于纯Sm2Zr2O7试样,并且随着碳纳米管含量的增多,复合材料强度和韧性有所提高,可以用界面结合强度及纤维拔出、纤维与基体的脱粘、纤维搭桥等理论来分析碳纳米管增韧复合材料的机制。     

3Y-TZP粉体烧结性能研究

分析了烧结温度对四种Y2O3氧化锆粉体烧结体力学性能的影响,并对影响机理进行了初步分析。研究发现:在1450~1550℃烧结温度范围内,这四种商业氧化锆制得样品的硬度随着烧结温度的提高均呈下降趋势;原始粉体与烧结体的相含量对材料的断裂韧性有一定影响。实际应用中,可根据断裂韧性和硬度的实际需求来选用原料粉体,并确定烧结温度。     

Crack nucleation sensitivity analysis

A simple analytical expression for crack nucleation sensitivity analysis is proposed relying on the concept of topological derivative and applied within a two‐dimensional linear elastic fracture mechanics theory (LEFM). In particular, the topological asymptotic expansion of the total potential energy together with a Griffith‐type energy of an elastic cracked body is calculated. As a main result, we derive a crack nucleation criterion based on the topological derivative and a criterion for determining the direction of crack growth based on the topological gradient. The proposed methodology leads to an axiomatic approach of crack nucleation sensitivity analysis. Copyright © 2010 John Wiley & Sons, Ltd.