二次应力存在时的J积分和断裂准则

本文对二次应力存在时的修正Ｊ积分参量进行了较全面的介绍和总结，用有限元法对各修正Ｊ积分的守恒性及其相互关系进行了比较和讨论，结果表明Ｊ′积分是适用于二次应力存在的具有明确物理意义的最普遍有效断裂参量，并建立了相应的Ｊ积分断裂准则     

泵车副梁断裂原因分析

通过外观检验、宏观断口检验、材料的化学成分与力学性能检验，并从理论和实践上用反证法对可能导致副梁断裂的因素进行逐一筛查，得出的结论是：冶金学缺口的存在相当于裂纹源，其与焊接拘束应力较大是导致副梁断裂的两个关键因素。提出了相应的改进建议。     

A MODEL FOR PREDICTING CRACK GROWTH RATE FOR MIXED MODE FRACTURE UNDER BIAXIAL LOADS

A model for predicting the crack growth rate of an initially angled crack under biaxial loads of arbitrary direction is suggested. The model is based on a combination of both the Manson–Coffin equation for low cycle fatigue and the Paris equation for fatigue crack propagation. The model takes into consideration the change in material plastic properties in the region around the crack tip due to the stress state, together with the initial orientation of the crack and also its trajectory of growth. Predictions of crack growth rate for any mixed mode fracture is based on the results of uniaxial tension experiments.     

A NEW APPROACH TO STRUCTURAL INTEGRITY ASSESSMENT FOR DUCTILE FRACTURE

Abstract— A new approach to structural integrity assessment based on ductile fracture is explored in the present paper. A simplified, yet more convenient methodology than the conventional EPRI elastic-plastic fracture analysis is outlined which assesses the ductile fracture instability of structures by an intersection rather than a tangent construction. The J -equivalence principle of crack growth is employed for tearing instability determination of single edge cracked plates. An admissible stress curve method is established with a peak value indicating instability. This approach gives reasonable predictions concerning the burst pressure of pre-flawed pressure vessels. A theoretical J resistance curve, which is justified by experimental measurements for steels with relatively high yield strength, can be incorporated for a complete analytical characterization of the defect assessment procedure.     

基于概率断裂力学的老龄钢桥使用安全评估

我国交通线上存在大量老龄钢桥,这些老龄钢桥承受着日益繁重的交通荷载,其疲劳剩余寿命已受到桥梁管理部门的高度重视.为确保老龄钢桥的使用安全,避免不必要的维护与更换,建立老龄钢桥疲劳剩余寿命与使用安全评估方法十分必要.建立了反映老龄铆接钢桥疲劳破坏机理的脆断和韧断概率失效模型,给出了用于疲劳可靠性分析的极限状态方程,合理确定了随机变量的参数取值.建立了铆接钢桥构件单角钢概率疲劳破坏模型,基于Monte Carlo算法实现了铆接钢桥构件单角钢疲劳断裂失效概率的计算,编制了相应的概率断裂分析程序SAPFF.进而将铆接构件概率断裂模型应用于上海市浙江路桥的时变疲劳可靠度分析,并给出了浙江路桥概率疲劳剩余寿命评估结果与维护对策.     

国内外“延性断裂评定”的各种实施方法及其可靠性研究

文章对压力容器及管道的延性断裂评定,评述了各国的不同实施方法,其中有EPRI的J～T稳定性评定图方法(美国)、PD6493/91失效评定图切线方法(英国)、相容应力的极值方法(中国)以及J～T曲线汇交方法(Pressure Vessel and Piping Codes)等。本文对个别评定方法的失当和不足进行了论证分析和对比计算,推荐了最佳的评定方法。推荐的方法便于微机操作,概念清晰,直观性强,可获得高可靠度的失稳预测。     

PROCESS ZONE SIZE FAILURE CRITERION AND PROBABILISTIC FRACTURE ASSESSMENT CURVES FOR CERAMICS

Data of fracture toughness and fracture stress of ceramics, generally show considerable size effects and scatter. This presents problems for the integrity of structural ceramics. Fracture mechanics test results on Si₃N₄ and A1₂O₃ reveal a sensitivity to crack size and the necessity to invoke probabilistic concepts to evaluate their integrity. A new failure criterion is proposed here, based on the existence of a crack-tip process zone. A probabilistic failure assessment curve is also proposed to quantitatively evaluate reliability.     

FRACTURE AND DEFECT ASSESSMENT OF CERAMIC COMPOSITES

Abstract— Industrial ceramic crucibles manufactured by hot-pressing a mixture of ceramic powders sometimes fail by delamination when operated under extreme conditions. Results of laboratory tests under similar conditions suggest that failure may be caused by the constrained evaporation of a water rich liquid absorbed by relative large aggregate particles. Fracture mechanics test results, from specimens cut parallel and transverse to the direction of hot pressing, revealed a clear anisotropic behaviour and a large sensitivity to notch size for notches smaller than a millimetre in depth. A model based on the existence of a notch-tip damage zone is proposed and used to present fracture curves that can be employed for failure assessment of industrial crucibles.     

压杆法研究厚度对小圆片试样断裂韧性的影响

采用SP压杆实验方法,在常温下对六种不同厚度的小圆薄片的断裂韧性进行测试。实验结果表明,随着厚度的增加,断裂变形能和断裂韧性增加,断裂部分的外表面表现出微突起,微突起四周存在微小裂纹明显的特征。考虑弹塑性变形能,通过SP冲压断裂变形能计算得到断裂韧性,进而得到临界断裂变形能密度。采用塑性断裂应变作为裂纹起裂判据,临界断裂变形能密度作为裂纹扩展判据的断裂模型,对SP压杆实验进行模拟,得到与实验结果比较相符的模拟结果。采用GTN模型研究厚度对空洞增长的影响,得到厚度增加导致微突起明显的空洞率增加。     

A FRACTURE MECHANICS APPROACH TO HIGH-CYCLE FATIGUE CRACK GROWTH UNDER IN-PLANE BIAXIAL LOADS

Abstract Fatigue crack growth under biaxial tensile load conditions is reported for a structural sheet steel. The new test facility can operate at high frequency (0–40 Hz) thereby permitting real-time testing required for threshold investigations; specimens are of the cruciform type.
It is found that crack growth rate is affected by a cyclic tensile load applied in the direction of growth which decreases as the said load increases. The rate however increases if the biaxial loads are increasingly out of phase.
Within the test conditions reported LEFM can be applied to crack growth under biaxial load conditions. The threshold stress intensity range is shown to be a function of load biaxiality, phase difference and stress ratio.     

建筑结构相关荷载组合的平稳二项随机过程方法

目前建筑结构设计和分析的荷载组合采用的是经验性的荷载组合规则,荷载组合结果具有较大的随意性,不能计算荷载组合值的保证率。在将随时间变化的荷载随机过程假定为平稳二项随机过程的基础上,研究多个相关或不相关荷载(或荷载效应)组合问题。考虑参与组合的荷载随机变量间的相关性,确定了组合荷载随机过程的参数;根据最大熵原理确定了荷载组合随机过程的任意时段幅值随机变量的概率分布,从而建立了相关随机荷载的组合方法;结合现行规范设计方法,进一步讨论了荷载组合系数的确定;最后采用MonteCarlo试验验证了本文的荷载组合方法,并对常见的几种经验荷载组合规则进行了概率校核。     

爆炸荷载作用下钢筋混凝土柱损伤FEM分析

采用有限元分析方法,对钢筋混凝土柱在爆炸荷载作用下的损伤程度进行了数值分析和评估,为建筑物抗爆设计和选择防爆安全距离提供一定的参考。采用有限元软件对钢筋混凝土框架柱在外部爆炸荷载作用下的非线性反应特征和损伤程度进行分析,采用一种塑性损伤模型,数值模拟分析时考虑材料的应变率效应。将混凝土框架柱的损伤划分为四个等级,并给出柱的防爆等级与距离的评估图。     

建筑屋面滑移雪荷载的模拟方法研究

滑移雪荷载是屋面雪荷载的重要组成部分, 该文发展了计算屋面滑移雪荷载的模拟方法. 首先, 基于前人提出的能量及质量平衡模型, 并针对建筑屋面积雪的特点, 构造了建筑屋面融雪模型. 在此基础上, 提出将屋面积雪中液态水超过积雪最大持水量时作为屋面雪层开始滑移的判定条件. 利用该文方法对我国几个代表性地区的屋面滑移雪荷载进行了模拟, 分析了不同地区的气象特征、邻近建筑对屋面积雪的遮挡等因素对滑移雪荷载的影响;同时定量分析了各个能量对滑移雪的贡献大小.     

基于延性撕裂的“PD6493高级评定方法及EPRI方洁”的优化评定和应用对比研究

本文从工程应用角度重点研究了考虑延性撕裂的英国ＢＳＩＰＤ６４９３高级评定方法及美国ＥＰＲＩ工程方法。作者所建立的优化评定方法，分析简单，概念清晰，克服了传统图解方法的繁琐和不确切。对比表明：修订的ＰＤ６４９３高级评定法（１９９１）与ＥＰＲＩ工程方法具有相似的精度水平。因而，利用两种延性撕裂评定方法所获得的结果，都是可以信赖的。     

汽车螺栓断裂失效分析

对汽车上某一连接发动机和车身的高强度级别螺栓在使用过程中发生的断裂进行了分析.结果表明,螺栓的化学成分和金相组织均正常,其断裂为多源疲劳断裂.经对螺栓使用过程中的受力情况分析,认为螺栓断裂主要是由于该螺栓在安装时预紧力不足或汽车在运行过程中,发动机自身振动和汽车行进中颠簸而使螺栓产生了松动,从而受到交变载荷而在应力集中最为严重的螺纹根部发生疲劳断裂.     

CRACK-FOLLOWING FOR ACCURATELY DETERMINING FRACTURE TOUGHNESS

Current standard fracture-mechanics methods encourage sudden, explosive and total testpiece fracturing in which one catastrophic event produces one set of measurements. By demanding conformance to the overall elasticity requirement while relaxing thickness and crack-length specifications and following a slow crack-growth sequence, series of simultaneous measurements were made on single testpieces. From these measurements independent values of the Mode-I critical stress-intensity factor and strain-energy release rate were determined. Polyethersulphone injection-moulded discs were prepared as round-compact testpieces. The crack grown from a chevron-notch was propagated under control, slowly. A large number of measurements were made as the crack extended. The fractures were smooth, square to the body of each disc and had a stable crack path across the diameter. The independent fracture-toughness values were small but their derived modulus agreed well with directly measured values of Young's modulus, thus confirming the elastic integrity of the test.     

FRACTURE MECHANICS ASSESSMENT OF PLANAR BRANCHED CRACKS IN AN EQUIBIAXIAL STRESS FIELD

Abstract A simplified fracture mechanics assessment is presented of branched planar cracks in an equibiaxial stress state. In linear-elastic fracture mechanics the stress intensity factors which characterize the load at the crack tips depend, for a given external load, only on the crack geometry. The stress intensity factors of a large number of branched cracks were evaluated using the Boundary Element method, and correlations between the stress intensity factors and the crack geometry were investigated. Formulae are presented which assign an individual effective crack length to each crack tip of a branched crack and hence allow approximate stress intensity factors to be determined for very complicated crack geometries. An algorithm is used for the stochastic simulation of an irregular crack pattern formation in thermal fatigue.     

ELASTIC-PLASTIC FRACTURE MECHANICS FOR PRESSURE VESSEL DESIGN

The project has been concerned with the verification of the J-philosophy for initiation and growth of cracks on laboratory type specimens as well as on larger surface cracked plates. The analysis of the experiments involved extensive fully three-dimensional finite element calculations. It was found that the initiation events for the six surface cracked plates occurred at approximately the same J-value. A corresponding relation to the laboratory type specimen was less successful, mainly because of the short-comings of current methods for J_IC-evaluations. A somewhat puzzling feature of the surface crack experiments was that unstable crack growth occurred immediately after crack growth initiation in four out of six tests, while some amount of stable crack growth was evident in the remaining two and in all of the laboratory type specimens. An evaluation of the surface experiments by a type of R6-analysis was also performed. Using as K_cr the mean value from the surface crack experiments, it was found that the R6-method yielded conservative predictions.     

含初始缺陷钢结构损伤累积至断裂及后期的调查分析

为分析钢结构损伤累积至断裂的演变过程,从含初始缺陷钢结构的敏感性出发,提出了基于钢结构动态损伤响应分析的断裂数值模拟理论框架,指出该问题的实质就是求解钢结构在工作应力远低于屈服应力的情况下发生断裂对应于初始缺陷对疲劳寿命增加的百分数.设定初始缺陷为敏感激励参数,并给出疲劳寿命、强度退化以及二者间的数学计算模式.应用确定安全期限的缺陷敏感性理论,运用强度退化方法计算并比较不同初始缺陷情况下钢结构抗断裂韧性和疲劳寿命数值之间的偏差,表明了本法的可行性和有效性.这些结论可为钢结构安全设防提供指导和参考.     

复杂载荷下多层刚架的快速解析求解

对于有复杂载荷的多层刚架,常用的方法无法求得解析解。提出了一种快速求解多层刚架解析解的连续分段独立一体化积分法。该方法首先将刚架进行连续分段,建立一种复杂载荷下刚架求解的通用力学模型,从该模型推导出任一截面的具有四阶导数的挠度微分方程。然后独立积分四次,得到挠度的通解。再根据边界条件、力边界条件和连续性条件确定积分常数。用MAPLE语言开发出相应的求解解析解程序,实现了对复杂载荷多层刚架解析解的计算机求解。在多层刚架模型建立和求解推导过程中,不需进行载荷和刚架结构的简化,工程实例表明,连续分段独立一体化积分法编程程式化,可以得到一体化解析解,不仅适用于静定刚架,而且适用于超静定刚架。