吊车转盘连接螺栓断裂分析

吊车转盘后部的连接螺栓发生断裂,通过化学成分分析、宏观和微观检验等方法对断裂原因进行了分析。结果表明：螺栓为疲劳断裂,螺纹根部的细小裂纹是导致螺栓发生疲劳断裂的主要原因;螺栓松动后受到弯曲载荷是引起螺栓发生疲劳断裂的诱因。     

某铁路桥梁用20MnTiB钢高强螺栓断裂原因

某铁路主桥钢桁梁杆间20MnTiB钢高强螺栓发生断裂。采用复检试验、化学成分分析、金相检验、硬度测试、力学性能测试和断口分析等方法,分析了螺栓断裂的原因。结果表明：螺栓的断裂形式为氢致断裂,断裂原因是螺栓中锰元素含量偏高,导致耐腐蚀性下降,长期在潮湿环境下服役,致使螺栓发生氢致断裂。     

高强度钛合金螺栓断裂失效分析

规格为M4×30 mm高强度钛合金螺栓在正常预紧装配后自行断裂。采用金相检验、扫描电镜断口分析、二次离子质谱分析和螺栓的受力分析等方法对螺栓断裂的原因进行了分析。结果表明,螺栓断裂的性质为低应力脆性断裂,断裂机理为氢致延迟断裂,导致螺栓氢致延迟断裂的原因主要与该批棒材原始氢含量过高有关。     

柴油机用高强度螺栓断裂失效分析

某柴油机用30CrMoSi钢高强度螺栓在使用过程中发生断裂。通过化学成分分析、金相检验、断口分析、硬度测试等方法对螺栓断裂原因进行了分析。结果表明:螺栓断裂为早期疲劳断裂,螺栓螺纹根部存在加工缺陷以及螺栓材料中存在较多的非金属夹杂物是导致其疲劳断裂的主要原因。最后针对螺栓断裂原因提出了预防措施。     

镀锌螺栓的断裂分析

镀锌螺栓在组装后放置1～2d于根部或头部发生大量断裂，采用化学成分分析、金相检验、硬度检测及断口分析等方法对螺栓的断裂性质及断裂原因进行了分析。结果表明：螺栓断裂为典型的氢脆断裂；主要原因是螺栓在后期酸洗和电镀过程中除氢不彻底，吸入了大量的氢；次要原因是热处理工艺控制不当，使螺栓心度硬度偏高，增加了其对氢脆的敏感性；两者共同作用最终导致螺栓发生氢脆断裂。     

洗衣机螺栓断裂失效分析

某洗衣机螺栓在安装过程中发生断裂失效，为查明螺栓断裂的原因，对断裂螺栓进行了化学成分分析、宏观分析、金相检验和断口微观形貌分析。结果表明：螺栓断裂是由于其制造时产生了严重的带状组织，降低了螺栓的许可扭转力矩，导致螺栓在安装过程中发生扭转断裂。     

新能源客车电容支架安装螺栓的断裂原因分析

某新能源客车上的电容支架固定螺栓发生批量断裂。通过对断裂螺栓进行宏观观察、力学性能测试、金相检验、化学成分分析等,分析了其断裂原因。结果表明:螺栓的断裂为双向弯曲疲劳断裂;发生断裂的主要原因是电容支架与车架连接结构设计不当,导致结构松动,使螺栓受到弯曲交变应力作用;次要原因是螺纹表面脱碳,降低了螺栓的疲劳强度。     

电站稳压器支承裙座锚固螺栓断裂失效分析

某电站稳压器支承裙座锚固螺栓在安装一段时间后发生断裂,从力学和材料角度对该螺栓断裂原因进行了分析。对断裂螺栓进行了金相组织观察、化学成分分析、力学性能试验以及断口的宏观及微观形貌观察,发现螺栓断口附近组织异常,外部环状带晶粒粗大,硬度和抗拉强度偏低导致裂纹启裂,裂纹逐渐扩展引起螺栓整体失稳断裂;螺栓局部出现组织异常的原因为螺栓热处理过程中控温不当导致局部过热。     

掺稀泵压盖螺栓断裂原因

采用宏观和微观形貌分析、材料性能试验、金相检验、螺栓载荷校核等方法对某掺稀泵压盖螺栓断裂原因进行分析。结果表明：高周疲劳导致掺稀泵压盖螺栓断裂,螺栓螺纹表面脱碳层的厚度超标、材料性能下降等对螺栓的疲劳断裂具有促进作用。     

某挖掘机高强螺栓断裂原因分析

某挖掘机连接下车架与回转支承的36支高强螺栓在使用过程中全部发生断裂。采用宏观检验、化学成分分析、力学性能测试、金相检验等方法,结合螺栓装配、挖机工况等因素,对该批螺栓断裂的原因进行了调查分析。结果表明:螺栓断裂的性质为疲劳断裂;断裂的原因是螺栓装配不良,导致螺栓在使用中轻微松动,在挖掘机工作过程中的剪切力作用下,螺栓松动处产生疲劳裂纹,最终导致螺栓断裂。     

The fracture of bulk metallic glasses

The fracture of metallic glasses has received relatively little attention until recently. The development of bulk metallic glasses (BMGs) with more compositions, large sample sizes and diverse fracture behaviors provides a series of ideal model systems for the study of fracture in glassy materials. The fracture toughness of different BMGs varies significantly from approaching ideally brittle to the highest known damage tolerance. Diverse fracture patterns on the fracture surface, fracture modes and dynamic propagation of cracks have been observed in different BMGs. In this review paper, we present a comprehensive view of the state-of-the-art research on various aspects of the fracture of BMGs, including fracture behavior and characteristics, fracture mode, fracture criterion, fracture toughness, and fracture morphology. Accumulated experimental data on BMG fracture are presented and their possible theoretical connections with continuum fracture mechanics and the atomic-scale process are introduced and discussed. Modeling studies of the fracture of BMGs by various computational methods are also reviewed. The review also presents a number of perspectives, including the relation of BMG fracture study to other topics, and unsolved issues for future investigation.     

木材断裂解离分形特征与分形断裂力学模型研究

目的 以椴木为研究对象,研究冲击载荷作用下椴木试件的断裂解离形貌特征和断裂力学特性,建立适用于木材原料断裂解离的分形断裂力学模型,并对其断裂解离力学行为进行描述。方法 对椴木试件进行冲击加载试验,分析试件断口的形貌特征和断裂力学特性,构建适用于木材原料断裂解离的分形断裂力学模型。结果 椴木试件横向冲击断裂断口裂纹形状和断口形貌特征比纵向冲击复杂,横、纵向冲击断裂断口均具有分形特征;椴木试件纵向冲击断裂韧性均值是横向冲击断裂韧性均值的1.112倍,椴木试件横、纵向冲击断口的分形维数均值分别为2.063 5和2.075 1,椴木试件横、纵向冲击韧性与其断口分形维数之间存在线性正相关关系,拟合优度分别为0.778 7和0.812 2;构建的木材原料断裂解离临界解离应力和断裂韧性的分形断裂力学模型也适用于脆性材料。结论 在木材原料冲击断裂解离时,木材原料初始裂纹长度越短,断裂解离断口越粗糙复杂,木材原料断裂解离所需要的能量越大;当裂纹沿着与冲击加载力方向垂直成大约1.055rad方向扩展时所需的能量最小,木材原料最易沿该方向进行断裂解离。     

材料断口分形研究现状及发展前景

讨论了分形几何应用于断裂研究的几个基本问题,主要包括断面的分形特征与分形测量,分维与断裂韧性的关系以及金属断裂的多种分形模型.进一步阐述了分形理论在金属断裂方面的应用前景.     

Investigation on fracture mechanisms of metals under various stress states

Fracture mechanisms for widely used metal materials are investigated under various loading conditions. Several specimens and different loading methods are deliberately designed to produce various stress states. The stress triaxiality is used to rate the level of tension and compression under various stress states. The stress triaxiality increases with adding a notch in the specimen under tension loading and decreases by changing the loading from tension to compression. Scanning electron microscopes are used to observe the microscopic features on the fracture surfaces. The fracture surfaces observed in the tests indicate that with the decreasing stress triaxiality the fracture mechanism for a given metal material includes intergranular cleavage, nucleation, growth, void coalescence, and local shear band expansion. With the fracture mechanisms changing from intergranular cleavage to nucleation, growth, and coalescence of voids, and expansion of a local shear band, four possible fracture modes can be observed, which are quasi-cleavage brittle fracture, normal fracture with void, shear fracture with void, and shear fracture without void. Quasi-cleavage brittle fracture and normal fracture with void are both normal stress-dominated fracture modes; however, their mechanisms are different. Shear fracture with and without void are both shear stress-dominated fracture, and shear fracture with void is also influenced by the normal stress. To a certain metal material, under high stress triaxiality, quasi-cleavage brittle fracture and normal fracture with void tend to occur, and under low stress triaxiality, shear fracture with and without void tend to occur. In addition, the critical positions and fracture criteria adapted to each fracture mode will also be different.     

Using the local approach to evaluate scaling effects in ductile fracture

This paper uses a local model to predict ductile fracture in geometrically similar structures of different sizes containing either sharp cracks or blunt stress concentrators. Simple theoretical considerations suggest that when fracture occurs by quasi-isotropic void growth, fracture initiation at blunt notches follows replica scaling, whereas fracture initiation at sharp cracks does not. Simulations with a local fracture model of fracture events in (1) fatigue precracked compact specimens and (2) three-point-bend bars containing blunt notches confirm these conclusions. However, a comparison of simulations with actual experimental results with HY-130 steel specimens leads to mixed conclusions. Predicted and observed behaviors for fracture at sharp cracks agree well, but the discrepancy is considerable for fracture initiating at blunt notches loaded in bending. Significant scaling effects are observed in the experiments for the conditions of fracture initiation at blunt notches. Fractographic analysis reveals that the reason for this discrepancy is a difference in the micromechanisms controlling fracture at sharp cracks as opposed to blunt notches. At sharp cracks, quasi-isotropic void growth dominates, whereas fracture initiates at blunt notches by a shear localization process and the nucleation, growth, and coalescence of voids in a mixed shear and tensile deformation field. The transition from one mode to the other may be governed by the hardening rate and, if so, is material dependent. Therefore, when using local fracture models for predicting fracture under generalized geometric and loading conditions, care must be taken, that the micromechanisms of ductile fracture invoked in the actual material match those assumed by the local fracture model. If this correspondence is verified, local fracture models can be used to predict fracture conditions and associated scaling effects for situations not amenable to treatment by classical elasto-plastic fracture mechanics. However, new or expanded models that can treat ductile fracture in localized shear zones should be developed to realize the full potential of these local fracture methodologies. This revised version was published online in August 2006 with corrections to the Cover Date.     

混凝土断裂过程区长度计算方法研究

该文基于粘聚裂缝概念,以起裂韧度作为裂缝起裂和扩展的准则,提出了混凝土断裂过程区长度的计算方法。以Ⅰ型裂缝为例,计算了不同初始缝长和起裂韧度情况下的断裂过程区长度值,结合以往大体积混凝土的试验数据对其进行了验证。进而分析了断裂过程区长度的影响因素,结果表明:断裂过程区长度随初始缝长的增大而逐渐增大,随起裂韧度的增大而逐渐减小。     

基于微观机制的复杂应力状态下钢材韧性断裂行为研究

韧性断裂是钢材最常见的破坏形式,研究钢材韧性断裂机理并准确预测钢材韧性断裂行为具有重要的理论意义和工程实用价值。基于微观机制的断裂预测方法对研究钢材韧性断裂行为有较好的适用性。该文基于体胞模型空穴演化机理改进了现有的韧性断裂模型,校核了Q345钢材断裂模型参数。此外,在韧性断裂模型中引入损伤因子,以考虑应力状态在加载过程中的变化,使断裂模型能准确描述每一加载时刻的累积损伤值。文末采用Fortran语言将断裂模型编写USDFLD子程序,并将其植入有限元程序ABAQUS,对一组十字型刚节点试件单轴拉伸试验进行数值模拟。结果表明,该断裂模型在拉-剪复合应力状态下具有良好的预测精度,且能够准确捕捉钢材断裂起始位置及裂缝扩展路径。该文改进的韧性断裂模型也可用于其它韧性金属材料断裂预测分析。     

新老混凝土粘结面断裂损伤过程区研究

在新老混凝土粘结面断裂性能试验的基础上,建立了新老混凝土粘结面断裂破坏模式,分析讨论了界面层特性及界面微裂缝扩展对新老混凝土粘结面断裂性能的影响。建立了新老混凝土粘结面带状微裂缝断裂过程区模型,根据该模型可求出断裂过程区尺寸。结合裂缝过程区的荷载~应变试验结果,用临界断裂韧度对新老混凝土粘结断裂韧度进行了断裂过程区影响修正。     

Equivalence between stress intensity factor and energy approach based fracture parameters of concrete

The paper presents numerical study and relationship between the double-K fracture parameters and the double-G fracture parameters using two standard tests. The data required for calculation is obtained using cohesive crack model. It is observed that both the corresponding parameters of the double-K fracture model and the double-G fracture model at the onset of crack initiation and unstable fracture are equivalent. This observation agrees well with experimental results available in literature. It is also found that the fracture parameters of double-K fracture criterion and double-G fracture criterion are influenced by initial notch-length/depth ratio, specimen shape, size and softening function.     

Evaluation of six fracture models in high velocity perforation

A systematic evaluation of six ductile fracture models is performed to identify the most suitable fracture criterion for high velocity perforation problems. Included in the paper are the Wilkins, the Johnson-Cook, the maximum shear stress, the modified Cockcroft-Latham, the constant fracture strain, and the Bao-Wierzbicki fracture models. These six fracture models are implemented into ABAQUS/Explicit by means of a user material subroutine (VUMAT), and applied to model the failure processes of a steel and an aluminum target plate impacted by a projectile. The numerical simulations are examined by comparing with the experimental results published in the open literature. The Wilkins fracture model predicts spallation of the impacted zone of the target plate beneath the projectile. This unrealistic result is due to its power law form of the pressure term. The maximum shear stress fracture criterion fails to capture the shear plugging failure pattern in a wide range of the impact velocity. Material fracture properties cannot be fully characterized with the constant fracture strain and the modified Cockcroft-Latham fracture models. Various tensile tests on round bars do not give a consistent critical damage. The calculated residual velocities of the projectile are sensitive to the magnitude of the fracture parameters. The Johnson-Cook and the Bao-Wierzbicki fracture models formulated in the space of the stress triaxiality and the equivalent plastic strain to fracture are capable of predicting the realistic fracture patterns and at the same time the correct residual velocities. Finally, the limitations of the Johnson-Cook fracture model are discussed.