齿轮轴表面发纹产生原因

某45钢齿轮轴在热处理后的精车加工过程中发现齿轮轴表面有纵向分布的发纹.采用宏观观察、化学成分分析、金相检验、扫描电镜及能谱分析等方法对发纹产生的原因进行了分析.结果表明:该齿轮轴表面的发纹是由硫化物夹杂聚集导致的,热处理过程中聚集的硫化物夹杂引起齿轮轴应力集中,产生沿夹杂物扩展的淬火裂纹,并最终形成发纹.     

机床主轴大端内锥孔裂纹分析

金相分析结果表明：４５＾＃钢机床主轴大端内锥孔裂纹属淬火裂纹。由正火魏氏体组织，加之淬火温度偏高所致。     

连杆淬火产生裂纹原因分析

采用化学成分分析、低倍组织检验、力学性能测试及其材料工艺性试验等方法对用42CrMo钢制成的发动机连杆经调质后发生的淬火裂纹问题进行了分析。结果表明,原材料中存在钨元素,导致连杆在淬火后发生淬裂。并提出相应的改进措施。     

压力机导柱裂纹分析

某压力机导柱在中频淬火后出现裂纹。采用化学成分分析，金相检验及力学性能试验等方法对压力机导柱的裂纹进行了分析。结果表明，导柱材料化学成分超标，同时存在严重的铸造缺陷组织，增加了材料应力集中的程度，降低了材料的力学性能，从而导致导柱零件在淬火时产生开裂。     

齿座裂纹分析

采用金相分析方法分析了齿座表面裂纹的性质和产生原因。根据齿座表面裂纹主要呈沿晶扩展,裂纹两侧无脱碳和氧化现象,认为该裂纹为淬火裂纹。渗碳温度过高、晶粒粗大、材质不良是产生淬火裂纹的原因。     

高强度螺栓断裂原因分析

表面磷化处理的高强度螺栓在装配时发生断裂。对断裂螺栓进行宏观和微观检验、化学成分分析、金相检验、能谱分析和力学性能分析。结果表明：该螺栓的裂纹表面有氧化物且存在与螺栓表面相似的磷化物,裂纹处没有脱碳现象,说明裂纹在磷化处理前已存在,判断该裂纹是淬火裂纹。螺栓的断裂是由淬火裂纹造成的。     

油压机柱塞裂纹分析

采用化学分析、金相检验和显微硬度梯度试验等方法对油压机柱塞表面淬火交界处裂纹进行了分析。结果表明，由于柱塞原材料含碳量偏高、晶粒粗大、存在严重的带状组织缺陷，加大了淬火开裂的倾向，同时降低了材料的力学性能，加之淬火时交界处产生大的组织应力，叠加的应力超过了材料的强度极限，导致裂纹的产生。     

拖拉机输出轴开裂原因分析

根据化学分析，金相检验和裂纹成因分析，认为拖拉机输出轴产生的裂纹和脱裂是由于碳，锰总量超出，降低了Ms点，使轴的淬火温度偏高，组织应力增大，当内应力超过材料强度极限时产生裂纹，调整热处理工艺由原来的840℃淬火为800℃淬火（水淬），540℃回火，弧形裂纹及脱裂现象不再出现。     

石油钻杆焊缝裂纹成因分析及解决方案

结合实际生产及热处理工艺,对钻杆焊缝生产检测过程中发现的裂纹成因进行了分析。结果表明:裂纹均出现在钻杆熔合线附近接头侧外壁,为淬火过程中内应力较大产生的淬火裂纹。为了解决该问题,把调质处理的淬火温度降低到Ac1~Ac3,即对钻杆进行亚温淬火,再高温回火;工艺调整后,在保证产品性能满足技术要求的基础上,完全避免了焊缝淬火裂纹的产生。     

某核电机组柴油机紧固螺柱断裂原因分析

某核电机组柴油机紧固螺柱在回装过程中发生断裂。采用宏观观察、化学成分分析、金相检验、硬度测试、扫描电镜分析和能谱分析等方法对螺柱断裂原因进行了分析。结果表明:由于螺柱材料淬火温度控制不正确、淬火冷速不合适,导致螺柱中形成了淬火裂纹,在回装过程中的安装扭矩作用下,螺柱在淬火裂纹处发生一次性过载断裂。     

贮箱环焊缝开裂失效分析

一贮箱在水压试验时环焊缝开裂导致泄漏。对焊缝断口和材料组织进行了分析，认为焊缝泄漏是由于局部存在密集热裂纹所致。对热裂纹的产生机理进行了分析，提出了预防措施。     

Modeling of Coalescence of Dispersed Surface Cracks. Part 1. Probabilistic Model for Crack Coalescence

A probabilistic model was developed for coalescence of cracks randomly dispersed on the surface and having uniform orientation and statistically nonuniform length. The model makes it possible to calculate the probability of coalescence of any pair of closely located cracks with allowance for the interaction of the strain fields. The model has been modified for the case of coalescence of cracks having the biggest length in the sample. The initial parameters for determining the probability of coalescence are: mathematical expectation of the length of cracks, their surface density, the magnitude of the damaged area of the material (size-scale factor), and the level of the acting stress. The model obtained can be used to predict the life of a component from the criterion of formation of critical cracks by coalescence of dispersed defects.     

波音747SP飞机主起落架收放作动筒断裂分析

采用宏、微观检验方法对国外某航空公司波音747SP飞机主起落架收放作动筒断裂性质和原因进行了分析。查明作动筒断裂性质为腐蚀疲劳,腐蚀疲劳起源于作动筒外壁表面的网状裂纹,而这些网状裂纹产生于电镀之前,是一种加工缺陷。作动筒表面裂纹的网状形态以及初始深度符合磨削裂纹的特点。     

Fatigue crack initiation and growth in riveted specimens: an optical and acoustic microscopic study

While optical microscopy on riveted specimens reveals only surface cracks, the acoustic C-scan images reveal subsurface and buried cracks emerging to the surface. This is of particular interest for fatigue cracks that initiate below the outer surface such as with chamfered riveted panels. Fatigue crack initiation and growth in riveted panels of Alclad 2024-T3 were characterized using optical microscopy and scanning acoustic microscopy to obtain C-scan images. The C-scan images were obtained using a focused transducer with a center frequency of ca 50 MHz, and the peak value of the back surface echo of the plate with countersunk rivet holes was recorded in the C-scan images. Data on the initiation and development of fatigue cracks at rivets in riveted Alclad 2024-T3 are given.     

某厂房地面工程混凝土裂缝病害分析

正确检测、分析混凝土结构裂缝的性状、成因及发展趋势,是鉴定结构安全性、实用性和耐久性及制定处理决策的技术依据。地面工程中混凝土结构易出现裂缝问题,大多由配合比、施工方法、环境等多种因素共同作用导致。本文结合工程实例,对某厂生产车间地面工程裂缝原因进行鉴定和分析,为其他类似工程防治裂缝与施工提供借鉴和参考。     

Growth prediction of two interacting surface cracks of dissimilar sizes

When multiple cracks approach one another, the stress intensity factor changes due to the interaction of the stress field. This causes variation in the crack growth rate and shape of cracks. In particular, when cracks are parallel to the loading direction, their shape becomes non-planar due to the mixed mode stress intensity factor. In this study, the growth of interacting surface cracks was simulated by using the S-version finite element method, in which a local detailed finite element mesh (local model) is superposed on a coarse finite element model (global model) representing the global structure. First, simulations were performed for fatigue crack growth experiments and the method validity was shown. Second, simulations were conducted for various relative sizes and spacings of twin cracks. It was shown that the offset distance and the relative size were both important parameters to determine the interaction between two surface cracks; the smaller crack stopped growing when the difference in size was large. It was possible to judge whether the effect of interaction should be considered based on the correlation between the relative spacing and relative size.     

Multi-crack analysis of hydraulically pumped cone fracture in brittle solids under cyclic spherical contact

The evolution of surface damage in bilayers due to cyclic spherical indentation in the presence of incompressible lubricant is studied using an all-transparent glass/polycarbonate system as a model for more practical applications such as dental crowns and rolling contact fatigue. In situ observations and post-mortem material sectioning reveal that inner cone cracks evolve sequentially from the contact edge inward by slow growth in a process controlled by stress shielding from preceding cracks. The embryonic cracks are then accelerated by the action of fluid pressure into the flexural tensile stress at the lower part of the coating, where crossover fracture leading to delamination between the coating and substrate may ensue. A consistent FEM brittle fracture analysis incorporating multiple cracks, rate-dependent toughness and liquid pressure is used to follow the damage evolution in the coating. Crack trajectories are determined incrementally under the dual constraint K _I = K _II = 0, which maximize the tension at the crack tip upon the application of fluid pressure. The latter, evaluated at each increment with the aid of a fluid entrapment model, helps drive the leading crack past the compression zone beneath the contact via a hydraulic pump like action. In the early stages of fracture, the liquid pressure is reasonably well approximated by the Hertzian radial surface stress at the crack mouth. Fluid trapped in secondary cracks accentuate the compression beneath the contact. This helps squeeze more liquid into the tip of the leading crack in a zipping like action, which further enhance the crack driving force in the far field. The analytic predictions generally collaborate well with the tests.     

An extended Layerwise method for composite laminated beams with multiple delaminations and matrix cracks

For the delamination and matrix crack prediction of composite laminated structures, the methods based on the damage mechanics and fracture mechanics are most commonly used. However, there are very few methods that can accurately simulate the delaminations together with matrix cracks, although the in‐plane matrix cracks always exist alongside the delaminations under impact loading. In this work, an extended layerwise method is developed to model the composite laminated beam with multiple delaminations and matrix cracks. In the displacement field, the nodes in the thickness direction are located at the middle surface of each single layer, the top surface and the bottom surface of the composite beams. The displacement field contains the linear Lagrange interpolation functions, the one‐dimensional weak discontinuous function and strong discontinuous function. The strong and weak discontinuous function are applied to model the displacement discontinuity induced by delaminations and the strain discontinuity induced by the interface between the layers, respectively. Because the nodes in the thickness direction are located at the middle surface of each single layer, the extended layerwise method can be conveniently employed to deal with the in‐plane matrix cracks combined with the extend FEM. Copyright © 2014 John Wiley & Sons, Ltd.     

Interaction of two unequal cracks in a prestressed fiber reinforced composite

We consider an elastic orthotropic material representing a fiber reinforced composite. The composite is prestressed and contains two collinear cracks having different lengths. The faces of the cracks are acted by symmetrically distributed constant normal incremental stresses. We determine the critical values of the applied incremental stresses for which the cracks tips start to propagate and we analyse the interaction of the cracks as function of their lengths and of the distance between the cracks. This revised version was published online in July 2006 with corrections to the Cover Date.     

Growth evaluation of multiple interacting surface cracks. Part II: Growth evaluation of parallel cracks

In Part I of the current work, experiments on fatigue crack growth from notches and crack growth simulation for a coalesced crack with re-entrant portion were conducted. It was revealed that the growth rate in area is the same for the same applied stress and area of crack face. The main conclusion was that a crack with a re-entrant portion can be replaced with a semi-elliptical crack of the same area for the growth prediction. In this study, the influence of the interaction on the growth of semi-elliptical/semi-circular surface cracks in the parallel position was investigated. The stress intensity factor for various relative positions and shapes was evaluated by finite element analyses, and the magnitude of the interaction was quantified. Then a crack growth simulation for parallel surface cracks was developed. It was revealed that the magnitude of the interaction increases continuously during the crack growth and that, for a growth prediction, the parallel surface cracks can be replaced with a single crack of the same size on the projected plane when the relative spacing is close enough. It was concluded that the simulation can estimate the crack growth of interacting cracks and the replacement can be carried out when the offset distance is less than the crack depth.