50钢被动轴断裂分析

减速器被动轴在负荷试验过程中断裂，借助金相显微和ＳＥＭ等观察该轴的显微组织和断口形貌，结果表明，锻造加热时轴局部严重过热是造成轴心淬火裂纹，校直中裂纹扩展，开启，导致轴断裂的根本原因。提出了相应的热处理措施。     

汽车右后拖曳臂轴断裂原因分析

某汽车仅使用2d(天)、行驶56km,其右后拖曳臂轴即发生断裂。采用断口宏观和微观检验、硬度测试、金相检验、化学成分分析等方法,对该右后拖曳臂轴断裂的原因进行了分析。结果表明:该轴断裂性质为沿晶脆性断裂,断裂的主要原因是其锻造组织存在缺陷及未经调质处理,使轴的强度大大降低;次要原因是轴表面因磨削加工过热形成了表面磨削淬火层,增加了轴表面的脆性及拉应力,使微裂纹在轴表面产生。     

汽车变速器输入轴校直断裂失效分析

某汽车变速器输入轴在渗碳淬火热处理后校直时断裂,采用断口宏微观分析、低倍酸蚀检验、金相检验及硬度测试等方法对输入轴断裂原因进行了分析。结果表明:该输入轴断裂部位的横向剖面存在棒料未切除干净的残余缩孔缺陷,使轴直径变化的R部位有效承载面积减小,成为结构上的危险截面;加之输入轴表面渗碳层存在粗大的马氏体,脆性较大,从而导致输入轴在较大的校直外力作用下于应力集中的R部位表面萌生裂纹,并迅速扩展发生一次性断裂。     

摩托车主轴断裂失效分析

摩托车主轴在渗碳淬火热处理后校直时发生断裂,采用化学成分分析、金相检验、硬度测试及断口宏、微观分析等方法对断裂主轴进行了分析。结果表明：因为该主轴棒料冷挤压加工成形后轴表面附近区域产生的大量塑性变形已经达到了临界变形量,而主轴坯件冷锻后未按技术要求进行正火处理,使渗碳淬火热处理的原始组织不符合要求,从而导致主轴在渗碳加热过程中轴边缘区域发生再结晶转变,形成了极粗大的等轴晶粒区域,该区域经淬火和低温回火后形成了脆性大、硬度偏高的极粗大的回火板条马氏体组织,最终使得主轴在校直时的过高外力作用下发生脆性断裂。     

异型钢轨矫直断裂原因分析

某异型钢轨在经60kg／m热轧钢轨加工至50kg／m过渡轨后出现矫直断裂。经对钢轨断口的宏、微观检验和分析,认为,断裂裂纹起源于轨腰次表层;钢轨发生矫直断裂的原因是在加工异型钢轨的过程中,加热温度偏高,导致轨腰表层部分区域出现沿晶氧化,锻压时因轨腰表层受拉应力作用,沿晶氧化处形成微裂纹,随后矫直时轨腰三点弯曲受力,微裂纹缺陷处产生应力集中致使裂纹扩展,导致钢轨断裂。     

铁路货车车轴断裂分析

为查明铁路货车车轴卸荷槽部位断裂失效的原因，对断轴整体及断口部位做了全面的理化检验和宏、微观分析，发现轴表面疲劳源处存在许多腐蚀坑。进一步对腐蚀坑底部进行微观观察和电子能谱分析，发现坑底存在较多沿轴周向的微裂纹，坑内腐蚀产物含有高价硫元素。结果表明，较强腐蚀性物质使卸荷槽部位轴表面形成较深腐蚀坑，在该区域极易造成应力集中，这是引起裂纹萌生和扩展最终导致车轴断裂的主要原因。     

45钢厚板弯曲开裂原因分析

45钢厚板在弯曲时发生断裂，对断裂钢板进行了化学成分分析和宏、微观检验。认为板材在铆焊气割时，加热不均匀，局部过热且冷却速度过快，产生了粗大的魏氏组织，加之沿晶三次渗碳体的存在，使材质变脆；同时产生较大的内应力，使钢板生成微裂纹，造成弯卷过程中开裂。     

GCr15轴承钢连铸坯断裂原因分析

采用化学成分分析、宏观和微观检验等方法对GCr15轴承钢连铸坯在步进式加热炉内加热过程中发生断裂的原因进行了分析。结果表明：断坯中存在较多沿奥氏体晶界分布的网状裂纹,晶界呈熔凝特征,晶界上有许多含硅、铝、铬和锰等的液滴状复合氧化物颗粒,断口附近的沿晶裂纹周围氧化脱碳严重,局部有粗大魏氏组织。这些特征表明加热炉局部存在温度过高现象,进而导致坯料晶界弱化,并且在坯料和步进梁构成的悬臂拉力作用下最终造成沿晶断裂。     

挖掘装载机长销轴断裂分析

挖掘装载机服役仅三个月铲斗长销轴即断裂。用化学分析、金相检验、扫描电镜等方法对断轴进行了分析。结果表明，长销轴表面加工时留下的切削缺陷，使销轴形成缺口敏感；加之，因热处理淬火加热温度偏低，基体组织中铁素体未完全熔解，降低了材料强度，导致长销轴断裂。     

驱动铲片棘轮断裂分析

针对某型号筑路平地机驱动铲片棘轮发生的断裂事故进行了失效分析,通过对棘轮的材料成分分析、断口分析、金相检验及渗碳层硬度分布检测,判断出棘轮渗碳过程中表面产生了脱碳,在淬火过程中产生淬火裂纹,运行过程中在断裂前由于承受较大的冲击载荷作用,在表面脱碳层形成裂纹,再加上棘轮中心晶粒及晶界较粗大,有过热倾向,最终导致棘轮发生断裂。     

Internal fatigue cracks are growing in vacuum

Cylindrical specimens of 2024 and 7075 Al alloy material were heat treated with a cold water quench to obtain high residual tensile stresses at the interior. Fatigue tests showed internal cracks growing in the shear mode. By drilling a hole along the centre line internal cracks were given access to air, which then produced tensile mode cracks. Prestraining of specimens eliminated residual stresses thus producing crack initiation at the outer surface with crack growth in the tensile mode. Cracking in the tensile mode was sensitive to mean stress, whereas cracking in the shear mode was not. The shear mode crack on a micro level appeared to be slip band cracking.     

给水管座开裂原因分析

锅炉锅筒的给水管座与锅筒在焊接和消除应力热处理后经探伤检查发现沿焊缝有环向裂纹,并扩展至给水管座的内壁形成纵向裂纹。采用化学成分分析、宏观和微观检验以及力学性能分析等方法对给水管座的裂纹产生原因进行了分析。结果表明：由于焊接工艺规范控制不当,未及时进行消氢处理,焊件的碳当量提高具有明显的淬硬倾向,焊接结构和较大的焊件厚度引起拘束应力等综合作用产生了延迟裂纹。     

Failure Analysis of Cracked Gear Shafts

Two gear shafts cracked during the fabrication. The cracks are through the grooves associated with the gear teeth and propagated completely across the shaft. Fractography on the cracked teeth indicates that the crack origins are about 3.5 to 4.0 mm from the groove and exhibit pointlike features. Fracture surfaces near the crack origin regions were intergranular, and the fracture appeared to have occurred instantaneously. Metallurgical analysis indicates that decarburization had occurred on the surfaces of the gear shafts, but occurrence of decarburization has no relation to the cracking. Cracking occurred during the carburization-quenching process and originated at a cluster of Al₂O₂ complex inclusions. The thermal stress produced by quenching drove the cracking process.     

Cracking due to Cu and Ni segregation in a 17-4 PH stainless steel piston rod

The 17-4 PH stainless steel is employed to produce piston rods in industry due to the high strength and toughness, good workability and nice corrosion resistance. In the present failure analysis, obvious long cracks were observed along the longitudinal direction on the surface of the commercial 17-4 PH stainless steel piston rod after heat treatment. The cracks were carefully looked into by observing the crack tip and characterizing the microstructure along the cracks. The results showed that the cracks were mostly initiated from the surface of the rod and propagated along the phase boundary between martensite and δ ferrite. The EDXA showed that the segregation of Cu and Ni should be responsible for the cracking after heat treatment. In order to define when the crack was coming into being, oxidation film along the crack was considered as a clue. The scrutiny of the oxidation film on the crack edge illustrated that the crack should be formed right in the heat treatment of aging.     

The true toughness of human cortical bone measured with realistically short cracks

Bone is more difficult to break than to split. Although this is well known, and many studies exist on the behaviour of long cracks in bone, there is a need for data on the orientation-dependent crack-growth resistance behaviour of human cortical bone that accurately assesses its toughness at appropriate size scales. Here, we use in situ mechanical testing to examine how physiologically pertinent short (<600 microm) cracks propagate in both the transverse and longitudinal orientations in cortical bone, using both crack-deflection/twist mechanics and nonlinear-elastic fracture mechanics to determine crack-resistance curves. We find that after only 500 microm of cracking, the driving force for crack propagation was more than five times higher in the transverse (breaking) direction than in the longitudinal (splitting) direction owing to major crack deflections/twists, principally at cement sheaths. Indeed, our results show that the true transverse toughness of cortical bone is far higher than previously reported. However, the toughness in the longitudinal orientation, where cracks tend to follow the cement lines, is quite low at these small crack sizes; it is only when cracks become several millimetres in length that bridging mechanisms can fully develop leading to the (larger-crack) toughnesses generally quoted for bone.     

Axisymmetric crack terminating at the interface of transversely isotropic dissimilar media

In this paper, the axisymmetric elasticity problem of an infinitely long transversely isotropic solid cylinder imbedded in a transversely isotropic medium is considered. The cylinder contains an annular or a penny shaped crack subjected to uniform pressure on its surfaces. It is assumed that the cylinder is perfectly bonded to the medium. A singular integral equation of the first kind (whose unknown is the derivative of crack surface displacement) is derived by using Fourier and Hankel transforms. By performing an asymptotic analysis of the Fredholm kernel, the generalized Cauchy kernel associated with the case of `crack terminating at the interface' is derived. The stress singularity associated with this case is obtained. The singular integral equation is solved numerically for sample cases. Stress intensity factors are given for various crack geometries (internal annular and penny-shaped cracks, annular cracks and penny-shaped cracks terminating at the interface) for sample material pairs.     

Fracture Behavior of Particle Reinforced Metal Matrix Composites

The contributions of the reinforcement volume fraction and annealing temperatures to crack opening force and propagation energy are systematically studied by three point bending tests and by SEM investigations. The bending test data show that for the same reinforcement volume fraction, 2618 and 7075 Al composites require much higher force to open the cracks than 6061 matrix. This relates to the much higher levels of solute elements which causes matrix hardening. Studies reveal that the energy absorption level of the materials during crack propagation depends on both matrix strength and ductility which relates to the reinforcement volume fraction, composition and heat treatment conditions. Large deformation zones are found in front of the crack tip before crack propagation which indicate a ductile failure mode for the composites. Studies also reveal that cracks initiate generally at the particle/matrix interfaces for the low volume fraction reinforced composites. However, for the high volume fraction reinforced composites, crack initiation has been found from both reinforcement/matrix interfaces and broken particles. This indicates that increasing reinforcement volume fraction and matrix strengthening tend to change the fracture mode from interface debonding to particle cleavage cracking.     

利用无网格方法分析钢筋混凝土梁开裂问题

裂缝处理一直是混凝土数值分析中的一个难点,而无网格方法由于不需要单元网格,非常适用于分析断裂问题,因此可以考虑将其引入混凝土断裂分析领域。首先将混凝土中的裂缝分为微观裂缝和宏观裂缝,对于微观裂缝仍然使用传统的弥散裂缝模型;而宏观裂缝则利用无网格方法可以非常方便地调整节点分布的特点,通过增加裂缝节点和裂面边界的方法加以模拟。并给出了宏观裂缝产生,扩展的具体模拟方法。通过算例表明,利用提出的方法,可以较准确地分析混凝土宏观裂缝的产生、扩展以及裂缝宽度变化,得到一些传统有限元方法难以得到的计算结果。     

纵肋与横隔板交叉构造细节穿透型疲劳裂纹扩展特性及其加固方法研究

纵肋与横隔板交叉构造细节是正交异性钢桥面板最易发生疲劳开裂的构造细节,通过建立有限元数值模型,采用断裂力学方法,研究栓接角钢加固方式对该处疲劳易损细节穿透型裂纹的加固效果。基于疲劳试验足尺节段模型相对应有限元模型,建立了纵肋与横隔板焊接处穿透型疲劳裂纹模型,针对栓接角钢和纵肋外侧栓接钢板两种加固技术的加固效果进行评估。研究结果表明:钢桥面板纵肋与横隔板交叉构造细节的疲劳裂纹扩展至一定长度后将发展成穿透型裂纹,裂纹面受力复杂,纵肋腹板内外侧疲劳裂纹扩展特性表现的不一样,但是随着裂纹扩展的逐步进行,裂纹尖端的开裂模式均以复合型开裂为主;栓接角钢加固方式主要抑制纵肋与横隔板交叉构造细节易损部位疲劳裂纹的I型开裂,因此能很好地抑制短裂纹的扩展,但对于该细节处以复合形式扩展的穿透型疲劳裂纹的加固效果并不显著;在纵肋外侧栓接半U形钢板的加固方法能有效改善穿透型疲劳裂纹的等效应力强度因子,并且加固之后均保持在裂纹扩展阈值以下,表明该加固方式对穿透型疲劳裂纹有良好加固效果。