A quantitative model for predicting the morphology of surface initiated rolling contact fatigue cracks in back-up roll steels

ABSTRACT A previously presented qualitative model for the rolling contact fatigue and spalling failure of back‐up rolls has been quantified in terms of crack lengths and growth directions. The morphologies of surface initiated fatigue cracks have been predicted using published data on the mode I and mode II thresholds in low carbon and roll steels, respectively, and the theoretical determination of the mode I and mode II stress intensity factors at the tips of the inclined surface cracks. The predictions have been validated by using the results of the metallographic examination of rolling contact fatigue cracks produced in test discs used in experimental simulations and the examination of spalled material from a back‐up roll.     

Failure analysis of an aircraft nose landing gear piston rod end

A failure investigation has been conducted on a piston rod end used in a hydraulic actuating cylinder of an aircraft landing gear. The failed piston rod end was found to be broken. An evaluation of the failed piston rod end was undertaken to assess its integrity that included a visual examination, photo documentation, chemical analysis, hardness measurement, tensile testing, and metallographic examination. The failure zones were examined with the help of a scanning electron microscope (SEM) equipped with EDX facility. A stress analysis is also carried out by the finite element technique for the determination of highly stressed regions on the piston rod end. The results indicated that the piston rod end failed by fatigue with cracks initiated at the surface close to the mechanically damaged region due to high stress concentrations.     

汽车发动机曲轴断裂失效分析

某厂生产的发动机曲轴在用户使用过程中,3个月内共发生了4起曲轴断裂失效事故,采用化学成分分析、金相检验、硬度测试以及断口的宏、微观形貌分析等方法对断裂曲轴进行了分析。结果表明：曲轴断裂为高周低应力弯曲疲劳断裂,导致其断裂的主要原因是在曲轴第一曲拐过渡圆角R附近的曲柄表面聚集分布着机加工刀痕,形成了应力集中;在用户行驶过程中因车况、路面等复杂因素形成的过载或冲击载荷等作用下,在第一曲拐轴颈尺附近曲柄表面应力集中的机加工刀痕处萌生疲劳裂纹,并逐步扩展直至断裂失效。     

Metallographic analysis of embedded crack in electron beam welded austenitic stainless steel chemical storage tank

An AISI 304L austenitic stainless steel tank used for chemical storage showed cracks during the post weld qualification programme. A crack of 75 mm length embedded within the weld pool was subjected to detailed metallographic analysis. The results revealed that the cracking was due to a shift in the solidification mode from primary ferrite to primary austenite. The residual stress introduced during rolling and forming of material as well as additional contractional strain during welding under fixtured condition, are additional factors which caused cracking.     

SHORT CRACK FATIGUE BEHAVIOUR IN A MEDIUM CARBON STEEL

The initiation stage and short crack behaviour in torsional fatigue of a 0.4% C steel was investigated by a replication technique. The fatigue cracks initiated and propagated in the ferrite phase which is located at the prior austenite grain boundaries in the form of long allotriomorphs. At this stage of crack development it is proposed that crack growth rate depends on the extent and intensity of plasticity at the tip of the crack. Crack growth per cycle is correspondingly proportional to the strength of the slip band. The ferrite-pearlite boundaries are strong barriers to crack propagation, which is manifested by a deceleration of growth and possible arrest. On raising the stress level the previously non-propagating cracks may continue to grow by branching or joining with other cracks in the ferrite phase. This process is repeated until the stress fields of one or more dominant cracks attain a critical value to sustain continued growth that leads to failure.     

吊钩断裂原因分析

某起重机吊钩使用1a(年)多突然断裂。采用宏、微观检验方法，对吊钩断裂失效原因进行了分析。结果表明，吊钩断裂的直接原因是由于低周疲劳所产生的裂纹，在外应力的作用下扩展所致；通过对吊钩材料化学成分分析和吊钩受力进行理论计算，认为材料选择不当，强度偏低是造成吊钩断裂的另一重要原因。     

Origin and mechanism of torsion fracture in cold-drawn pearlitic steel wires

The structural and mechanical factors that control the torsion fracture behavior of cold-drawn eutectoid steel wires are examined. Two types of the fracture are identified; namely, flat- and cleavage-type. Torsion cracks are found to initiate in ferrite and propagate along the ferrite/cementite interface. The shear stress distribution within the wires is affected not only by the applied torque, but also by the residual stress. The maximum shear stress occurs halfway from center to the surface, where the cracks initiated. The growth of torsion cracks is sensitive to the orientation of cementite lamellas in pearlite grains. The influence of thermal history on the occurrence of cleavage fracture is ascertained, with the assistance of atom probe. It shows that the cleavage fracture results from a decrease in dislocation mobility, caused by thermally activated diffusion of carbon atoms into ferrite.     

Failure Analysis on Fractured Diesel Engine Camshafts

The diesel engine injection pump camshafts used in a truck failed after only 13,000 km of service. The failed camshafts were made from a 20Cr steel. The failure occurred at the keyway region of the camshafts. Fractographic studies indicate that fatigue fracture was the main failure mechanism, and the circumferential tangent stress resulting from insufficient static friction force between the cone of the camshaft and the timer acting on the edges of the keyway caused the initiation of the fatigue cracks at the root of the keyway. An abnormal Widmannstätten structure ferrite microstructure was found at the cone position of the camshafts, and initiation and growth of the cracks was facilitated by the microstructure.     

Experimental and Numerical Failure Analysis of Vertical Rods Used in Paper Molding Machine

A failure investigation has been conducted on the vertical rods which used in paper molding machines. The vertical rods are important elements for paper molding machines to support press forces. Each of the vertical rods that are analyzed was broken into two parts in service. The two failed vertical rod specimens were exposed to various tests such as visual inspection, chemical analysis, hardness measurement, metallographic examination with optical and scanning electron microscopes. A stress analysis was also carried out by the finite element technique for the determination of highly stressed regions on the vertical rods. The results indicated that due to high stress concentrations, the vertical rods failed by fatigue with cracks initiated at the surface close to the shoulder region which has sharp corner.     

Investigation of crack initiation and propagation behavior of AISI 310 stainless steel up to very high cycle fatigue

Fatigue behavior up to very high cycles for AISI 310 stainless steel has been investigated. The fatigue crack initiated from the surface of the material. It was found that up to 10⁶ cycles, cracks initiated from the carbide precipitates at grain boundaries. However, above 10⁶ cycles, the cracks initiated from persistent slip bands found at the surface of the specimen. At lower stress levels, slip bands were developed without initiating the cracks. The horizontal asymptote S–N curve from 10⁶ to 10⁹ cycles was attributed to the development of slip bands all over the surface of the specimen, before crack initiation.     

Failure analysis of a T-joint of AISI type 316 L stainless steel

The flow of sodium through the various circuits of a PFBR is controlled by centrifugal pumps. T-joints, made of type 316 LN stainless steel, will be used as pipe fittings to connect sodium pumps in the secondary side of the PFBR. However, two such T-joints received from the manufacturer were found to have developed fine cracks on the surface, which were seen after the pickling and passivation operation was carried out at the user's site. Visual, metallographic and fractographic examinations suggested that the failure occurred due to fatigue, which initiated because of surface roughening caused by the fabrication process. Initial large grain size of the material caused surface roughening to occur during fabrication. The rough surface led to initiation of fatigue cracks during fabrication, which was not carried out in one stretch as recommended by the user. Absence of corrosion product on the surface suggested no role of corrosion in the failure. The high hardness of the material indicated non-compliance with the user's specifications by the manufacturer vis-à-vis post-fabrication annealing. To avoid future failures, it was recommended that proper selection of material with respect to starting grain size should be made; qualified fabrication procedures should be adopted and followed so as to avoid cyclic loading on the component during fabrication; and appropriate solution annealing at 1323 K should be carried out to relieve cold work.     

TEM研究氢对双相组织的影响

用电镜观察了充氢后双相钢的亚结构,探讨了氢致双相钢机械性能下降的原因。观察表明:充氢后,F相中位错增殖,最后形成胞状亚结构;M相中位错组态变化不明显,但随充氢量增加,裂纹将在M板条界萌生和扩展,有时也横穿M板条;F/M相界充氢后出现宽化现象,并向F相一侧增殖位错,进而界面起裂。M量越多,则此现象越明显。M相与F/M相界起裂对充氢电流密度比充氢时间更为敏感。M相及F/M相界充氢后损伤严重,导致双相钢塑性下降,成为宏观拉伸时易于裂纹萌生和扩展的薄弱部位。     

Failure analysis of stress corrosion cracking in heat exchanger tubes during start-up operation

The cracking failure of a new heat exchanger during first start-up operation has been analyzed. Through the investigation of the operating history of the equipment, analysis of the chemical compositions of tube material and corrosion products, metallographic test of specimens with cracks, the cracking mode can be described as the Stress Corrosion Cracking (SCC) of austenitic stainless steel. This kind of cracking was induced by the chloride in high temperature steam and tensile stress. The residual tensile stress due to seal expansion has been proved by numerical calculation. The pre-heating steam which was polluted by the catalyst with chloride is the main reason for the tube cracking in this case.     

Fatigue Fracture of a Locomotive Diesel Engine Cardan-Shaft

A locomotive diesel engine cardan-shaft fractured when trial test for 360 h. Fractographic investigation reveals that the failure mechanism of cardan-shaft is multiple origins fatigue fracture. The crack origins are situated on the external circle surface of the transition fillet root of the splined portion, which acts as a stress concentrator. The macro-structure segregation and corresponding inhomogeneous microstructure are presented in cardan-shaft material. And the presence of more ferrite and abnormal Widmannstätten structure ferrite in core microstructure led to the low core hardness (below the minimum value required). Therefore, the estimated fatigue endurance limit of the cardan-shaft material is lower than the expected value by approximately 20%. Along with the stress concentrator in transition fillet, the fatigue crack initiated and propagated under the torsion load and the axial tensile load, ultimately leading to premature fatigue fracture of cardan-shaft. The defective forging and the heat treatment processes should be responsible for the occurrence of defective microstructure and the consequence of low core hardness.     

Ti基非晶合金及其SiC陶瓷骨架复合材料动态压缩力学行为研究

为了验证Ti基非晶合金和陶瓷两种材料三维连通网状结构的复合优势,制备出具有优良抗冲击性能的复合材料,本文采用铜模吸铸法制备了Ti基非晶合金,并用渗流铸造法制备出孔隙率分别为30.86%、18.14%和15.28%的Ti基非晶合金/SiC陶瓷骨架复合材料。采用X射线衍射仪对纯Ti基非晶合金以及SiC陶瓷骨架复合材料进行相分析,确认了试件材料的非晶状态;在不同应变速率下,用分离式霍普金森压杆(SHPB)实验装置对试样进行室温轴向动态压缩力学性能测试,并利用能谱型场发射扫描电镜(SEM)等设备观察了试件的微观组织和断面特征,对比分析了Ti基非晶合金和SiC陶瓷骨架复合材料的动态压缩力学性能和失效机理。研究表明,Ti基非晶合金/SiC陶瓷骨架复合材料内部的微裂纹最初萌生于应力集中的两相界面处,并在SiC相内部或两相界面处扩展,继续加载,SiC相失效后,Ti基非晶合金相在远超过其动态压缩强度的应力下迅速失效,复合材料整体失效。SiC相内的断裂形貌主要有微裂纹与解理台阶,Ti基非晶合金相内的断裂形貌有脉状花样、多重脊状条带、蜂窝状花样与光滑无特征区,其中以光滑无特征区为主。复合材料的抗压强度随Si...     

Numerical life estimation after fatigue failure of a complex component

Failure of a large ethylene‐reciprocating compressor was found to be due to fatigue growth of cracks in the crosshead of one of the cylinders, initiated at material defects near stress raisers. Total fatigue crack growth time was required in order to identify the cause of the failure. The applied stress field near the initiation sites and along fatigue path was estimated using FEM. The stresses were found to vary steeply and become partly compressive along a large part of the fatigue crack path. A weight function based on numerical method was developed, which was able to predict exactly the shape of the crack front during propagation. Fatigue crack initiation was traced to a disassembly 6 months before final failure. This failure was found to be jointly the result of non‐conformities in manufacture and maintenance.     

调整螺母裂纹成因分析

调整螺母在调质处理后探伤时发现有裂纹,采用宏观检验、化学成分分析、金相检验等方法对其开裂原因进行了分析。结果表明：该批调整螺母开裂是由于明显的带状偏析和较多的夹杂物引起的淬火裂纹,同时由于回火温度过高,使得裂纹两侧脱碳出现网状铁素体。     

Failure analysis of fretting fatigue initiation and growth on railway axle press-fits

Fretting fatigue failure of press fitted railway axle-wheel assembly was presented. Size, distribution and propagation profiles of the circumferential fretting cracks on the full-scale axles were determined by magnetic particle and metallographic slicing methods. The distribution of multiaxial stress cycle along the press-fit seat was obtained by finite element analysis (FEA). The obtained stress path was used for interpretation of fractographic evidence collected at crack initiation sites and the crack propagation plane. Metallurgical and mechanical characterization of the axle material (34CrMo4) was made on the specimens sampled from the broken axles. The threshold conditions for propagation of small fretting cracks were determined by Kitagawa analysis and El-Haddad correction method. The results of the experimental study were compared with EA1N grade steel which is given as the reference axle material in the EN standards. The metallurgical factors affecting the fretting fatigue crack initiation and propagation were investigated. The causes of the examined axle failures were associated with the deteriorated mechanical properties of the axle material.     

某型弹用尾翼片裂纹分析

目的分析弹用尾翼片淬火、回火后表面裂纹产生的原因。方法通过对存在裂纹的尾翼片进行金相检测、化学成分分析、硬度检测、断口检测、力学性能检测,对尾翼片产生的原因进行了分析和讨论。结果确定了尾翼片裂纹的性质和产生原因。结论尾翼片裂纹为淬火裂纹;尾翼片裂纹产生的主要原因是由于原材料钢板剪尾翼片条料过程中,在条料的表面产生了冷变形开裂缺陷,出现了较深的线状缺口,导致淬火热处理时产生了严重的应力集中,诱发了裂纹的萌生,同时,原材料钢板中存在的魏氏组织和硫化物夹杂等材质缺陷,也对促进了淬火裂纹的萌生。     

Mixed Mode Fracture of Cracks and Wedge Shaped Notches in Expanded PVC Foam

Fracture initiated from a sharp crack or wedge shaped notch in a homogeneous material, subjected to different loading is considered. Singularities in the stress fields at edges and vertices are discussed. A point-stress criterion is used to predict fracture for sharp cracks as well as 90° wedge notches in expanded PVC foam. The point-stress criterion is formulated in a manner allowing failure predictions in general 3D stress situations. The influence of nonsingular T-stress at cracks is discussed and substantiated by experimental results.