3044 t/h超超临界锅炉磨煤机拉杆断裂原因

某电厂3044t/h超超临界锅炉磨煤机拉杆发生断裂,通过化学成分分析、断口分析、力学性能测试和金相检验等方法对拉杆断裂原因进行了分析。结果表明:该拉杆供货态热处理质量较差,导致强度和冲击韧性指标不合格,且磨损补焊处因热处理不到位而产生冷裂纹,长期在往复拉应力及弯曲应力的作用下,裂纹疲劳扩展,最终导致拉杆疲劳断裂。     

Industrial experiences of bending fatigue strength in table liner for cement mill

A table liner for the vertical roller mill has been used to grind natural limestone. Unexpected fatigue failure accidents have occurred during portland cement manufacturing process. The design life of a table liner is 4 × 10⁷ cycles, but the actual fatigue life of a table liner is 2 × 10⁶ to 8 × 10⁶ cycles. The fatigue crack of a table liner initiates from the outside edge of the grinding path of the limestone. When such a crack occurs, the table liner has to be replaced, and this requires 30% of the total maintenance cost of the vertical roller mill. Therefore, this study examines the fatigue failure of a table liner by plane‐bending fatigue test, stress measurement test, finite element analysis and fatigue fracture analysis.     

Notch effect on creep–fatigue life for Sn–3.5Ag solder

This paper studies the creep–fatigue crack initiation and failure lives of Sn–3.5Ag solder notched specimens focused on the multiaxial strain at the notch root. Push–pull creep–fatigue tests were performed using three circumferential notched specimens using four kinds of creep–fatigue strain waveforms. Multiaxial strains at the notched section were calculated by finite element (FE) analysis under four kinds of creep–fatigue loading. Creep–fatigue damage laws were applied for evaluating the crack initiation and failure lives using the multiaxial strains obtained by the FE analysis. von Mises equivalent strain at the notch root estimated the crack initiation lives with a large scatter as well as the failure lives. Instead, the mean value of von Mises equivalent strain over the cross section of the notch root estimated the crack initiation and failure lives with a small scatter.     

Cyclic hysteresis of AZ31B extrusion under load‐control tests using embedded sensor technology

The fatigue behaviour of AZ31B extrusion magnesium alloy under load‐control cyclic test conditions is estimated using a combination of simulation and experimental results. The strain measurement of this asymmetric material is found experimentally using a Fibre Bragg Grating (FBG) sensor during rotating bending tests. Then, to analyse applied stresses in the sample – particularly in the plastic deformation range – the Variable Material Property (VMP) method is employed. Using this simulation method, the hysteresis loops of two critical top and bottom elements of the sample's cross section under different bending moments are obtained. Finally, the strain of the sample during rotating bending, as measured by the embedded FBG sensor, is related to the stresses obtained from the modeling using a mapping function. The hystereses obtained from this combination of the modeling and experimental results are compared with the results of a companion strain‐control pull–push test in which the input strain history was that of measured by the FBG sensor. Observations verify that the stresses of the combined VMP‐FBG hysteresis loops have good compatibility with the stress responses obtained through the experiment. The hybrid model introduced in this work can be employed to capture cyclic hysteresis, and hence estimate the fatigue life, under load‐controlled rotating bending tests.     

Shear mode fatigue crack growth in 1050 aluminium

The shear mode crack growth mechanism in 1050 aluminium was investigated using pre‐cracked specimens. A small blind hole was drilled in the centre section of the specimens in order to predetermine the crack initiation position, and a push–pull fatigue test was used to make a pre‐crack. Crack propagation tests were carried out using both push–pull and cyclic torsion with a static axial load. With push–pull testing, the main crack grew by a mixed mode. It is thus apparent that shear deformation affects the fatigue crack growth in pure aluminium. In tests using cyclic torsion, the fatigue crack grew by a shear mode. The micro‐cracks initiated perpendicular and parallel to the main crack's growth direction during the cyclic torsion tests. However, the growth direction of the main crack was not changed by the coalescence of the main crack and the micro‐cracks. Shear mode crack growth tends to occur in aluminium. The crack growth behaviour is related to a material's slip systems. The number of slip planes in aluminium is smaller than that of steel and the friction stress during edge dislocation motion of aluminium is lower than many other materials. Correlation between the crack propagation rate and the stress intensity factor range was almost the same in both push–pull and cyclic torsion with tension in this study.     

Contact Fatigue Failure of a Tapered Roller Bearing Used in a Lorry Wheel

Tapered roller bearings, which are also known as angular-contact bearing, are suitable for supporting radial and axial loads. The more frequent types of defects in such bearings are caused by contact fatigue in these machine components, and this examination focuses on a contact fatigue failure in a tapered rolling bearing. The examination included visual inspection, microscopic analysis (optical and scanning electron microscope), and microhardness measurements. These measurements were conducted to help understand the failure mechanisms. Based on the results of visual examination and microstructure and fracture surface analysis, it was determined that the tapered roller bearing failed by contact fatigue that was caused by overloading of the bearing.     

Numerical simulation of fatigue crack propagation in compressive residual stress fields of notched round bars

In this paper, the influence of the residual compressive stresses induced by roller burnishing on fatigue crack propagation in the fillet of notched round bar is investigated. A 3D finite element simulation model of rolling has allowed to introduce a residual stress profile as an initial condition. After the rolling process, fatigue loading has been applied to three‐point bending specimens in which an initial crack has been introduced. A numerical predictive method of crack propagation in roller burnished specimens has also been implemented. It is based on a step‐by‐step process of stress intensity factor calculations by elastic finite element analyses. These stress intensity factor results are combined with the Paris law to estimate the fatigue crack growth rate. In the case of roller burnished specimens, a numerical modification concerning experimental crack closure has to be considered. This method is applied to three specimens: without roller burnishing, and with two levels of roller burnishing (type A and type B). In all these cases, the computational finite element predictions of fatigue crack growth rate agree well with the experimental measurements. The developed model can be easily extended to crankshafts in real operating conditions.     

Three-dimensional stress intensity factor analysis of a surface crack in a high-speed bearing

The boundary element method is applied to calculate the stress intensity factors of a surface crack in the rotating inner raceway of a high-speed roller bearing. The three-dimensional model consists of an axially stressed surface cracked plate subjected to a moving Hertzian contact loading. A multi-domain formulation and singular crack-tip elements were employed to calculate the stress intensity factors accurately and efficiently for a wide range of configuration parameters. The results can provide the basis for crack growth calculations and fatigue life predictions of high performance rolling element bearings that are used in aircraft engines.     

Repeated plastic deformation as a cause of mechanical surface damage in fatigue, wear, fretting-fatigue, and rolling fatigue: A review

After studying statements bringing together essential features of the fatigue behaviour of metal structures in service, it is shown that the elementary phenomena of plastic deformation and fatigue crack propagation, which explain the fatigue behaviour of notched parts, also play an important role in other modes of damage of machinery parts. This is the case for mechanical surface damage under repeated bearing pressures in the absence of any apparent lateral sliding, for fretting-fatigue in which there is alternating lateral displacement of very low amplitude, and for false-brinelling marks in ball or roller bearings. It is also the case for unidirectional friction wear and for rolling damage, either with friction in gears or with very low friction in ball or roller bearings.     

EFFECT OF BIAXIAL MEAN STRESS ON CYCLIC STRESS-STRAIN RESPONSE AND BEHAVIOUR OF SHORT FATIGUE CRACKS IN A HIGH STRENGTH SPRING STEEL

Abstract— Biaxial fatigue tests were conducted on a high strength spring steel using hour-glass shaped smooth specimens. Four types of loading system were employed, i.e. (a) fully reversed cyclic torsion, (b) uniaxial push—pull, (c) fully reversed torsion with a superimposed axial static tension or compression stress, and (d) uniaxial push—pull with a superimposed static torque, to evaluate the effects of mean stress on the cyclic stress—strain response and short fatigue crack growth behaviour. Experimental results indicate that a biaxial mean stress has no apparent influence on the stress—strain response in torsion, however a superimposed tensile mean stress was detrimental to torsional fatigue strength. Similarly a superimposed static shear stress reduced the push—pull fatigue lifetime. A compressive mean stress was seen to be beneficial to torsion fatigue life. The role of mean stress on fatigue lifetime, under mixed mode loading, was investigated through experimental observations and theoretical analyses of short crack initiation and propagation. Using a plastic replication technique the effects of biaxial mean stress on both Stage I (mode II) and Stage II (mode I) short cracks were evaluated and analysed in detail. A two stage biaxial short fatigue crack growth model incorporating the influence of mean stress was subsequently developed and applied to correlate data of crack growth rate and fatigue life.     

Crack propagation of CCT foam specimen under low strain rate fatigue

The objective of this study is to investigate the effect of holes on the low strain rate fatigue properties of the nickel foam material and to understand the lifetime of this material which is subjected to the repeated loads. Failures of foam materials under single and repeated loads analogous to fatigue are essential to designers and users in military and aerospace structures. The material failure induced by repeated low strain rate loading becomes a critical issue because of significant loss of stiffness and compressive strength in the foam material. Testing methods to study low strain rate (that is, strain rate) fatigue are quite numerous; no single standard testing procedure is defined for studying the low strain rate fatigue property of a material. The increasing application of foam material in aerospace structures, owing to high specific stiffness and strength has attracted a great concern about the high sensitivity to low strain rate damage introduced during manufacture or in service, and the effects of such damage on structural degradation. To investigate this issue, this study sets up an experimental procedure to determine the low strain rate fatigue properties of nickel foam material. This study performs both experimental and numerical investigations to catch the low strain rate fatigue behavior of nickel foam with open-cell type. The experiments are conducted by rod up and down at the strain rate fatigue of loading. The crack length at the specific cycles are measured experimentally by taking pictures with a paper ruler attached on the surface of specimen and these values are apply to the computer simulations as crack seam model. The simulation result of stress intensity factors are compared with a well known theoretical calculation. Design life and probability of failure or survival at specified life can be calculated so that the fatigue life of nickel core material subjected to repeated low strain rate loading is predicted.     

Failure analysis and fatigue performance evaluation of a failed connecting rod of reciprocating air compressor

A connecting rod of a reciprocating air compressor is subjected to complex dynamic loads therefore it is of a critical machine element. Failure of this type of connecting rod was reported to occur at the rounded fillet of the big connecting rod end. The present investigation is aimed to identify the cause of failure and to evaluate fatigue performance of the failed connecting rod. Factors affecting failure including structural design, type of material and dynamic loads were assessed using standard failure analysis method. This method included analysis of chemical composition, microstructural examination using optical microscopy, hardness and tensile tests, scanning electron microscopy (SEM) fractography and stress analysis. To evaluate fatigue performance, fatigue crack growth rate (FCGR) test was performed using a sinusoidal load with a constant load amplitude. Results of this investigation suggest that the cause of failure was low cycle fatigue and the initial crack location was consistent with high stress concentration, i.e. fillet radius. From metallurgical point of view, the connecting rod was made of cast steel, not forged steel, with a considerable number of non metallic inclusions such as Al₂O₃, SiO₂ and FeO. These inclusions which were present near the surface of the rounded fillet seemed to act as stress raiser and they were responsible for crack initiation. In addition, the presence of inclusions could increase fatigue crack growth rate, da/dN (in m/cycle) as indicated by a high value of Paris’ constant (n), typically of 5.2141.     

Cracking of a Tool Steel Guide Roller in a Bar Mill

Failure analysis of a guide roller used in a bar mill of an integrated steel plant has been presented. The guide rollers are positioned at the entry of rolling stands and act as guide in the multi-grooved pass for aligning the hot bars and hold the twisted oval bar while it enters a round pass. The hot bar is at a temperature of ??1100°C, and the rollers are water cooled. The component is subjected to cyclic thermal stress depending on the mill operating conditions. The rollers are cracking longitudinally leading to failure. The investigation consists of visual inspection, chemical analysis, fractography, characterization of microstructures using optical and scanning electron microscopes (SEM), energy dispersive spectroscopy (EDS) analysis, and measurement of micro-hardness. The chemical analysis indicates the material as AISI D2 grade of cold work tool steel. Visual observation of the failed component shows multiple longitudinal cracks on the roller surface associated with a dark circular band of oxidation. The fracture surface shows a dark oxidized area propagating from the roller surface followed by flat bright appearance indicating final brittle fracture. Fractography of the dark surface from where the crack initiates shows fatigue striations. Microstructural examination under optical and SEM shows a network of coarse carbide particles at the grain boundary as well as uniformly distributed fine carbide precipitates within the martensite matrix. Multiple cracks are observed to initiate and propagate from the surface through the clusters of grain boundary carbides. EDS analysis and elemental mapping suggest the carbides to be chromium carbides. SEM shows micro-cracking of carbide particles associated with the crack. Clustered distribution of carbides deteriorates toughness and initiates cracking at the roller surface subjected to thermal cycling because of their differential thermal expansion coefficient leading to the failure of the component.     

Multiaxial fatigue life calculation model for components in rolling‐sliding line contact with application to gears

Important components such as gears, rollers, or bearings operate in rolling‐sliding contact loading conditions. Determination of their fatigue lives remains a challenging task due to complex states of stress and strain in the contact region, as well as complex contact conditions such as variable loading amplitude and complex geometry of contact. A mathematical model of rolling‐sliding line contact combined with a multiaxial fatigue life calculation model based on the Fatemi‐Socie critical plane crack initiation criterion is proposed. The developed model was applied to gears' teeth in mesh and compared with fatigue lives of gears reported in the literature. Good agreement was determined confirming the validity of the proposed model. A further advantage is obtaining locations of initiated cracks and the orientation of critical plane(s), which can subsequently be used for the estimation of crack shapes in initial phases of their growth and the damage type that they can be expected to develop into.     

Failure analysis of martensitic stainless steel bridge roller bearings

The paper is aimed at finding the likely failure mechanism of a bridge roller bearing made of high strength martensitic stainless steel. Spectroscopy and finite element stress analysis of the roller indicated that an initial radial surface crack, found at an end face of the roller and close to the contact region, was induced by stress corrosion cracking (SCC). The initial crack subsequently changed shape and increased in size under growth through fatigue and finally formed a quarter-circle radial crack centred on the end face corner of the roller. Numerically computed stress intensity factors for the final crack showed that crack loading was predominantly in Mode II. For a crack size as observed on the fracture surface, the maximum service load, as specified by the manufacturer, enhanced by a certain roller bearing misalignment effect, was sufficient for failure through fracture.     

Fatigue crack propagation in rocker and roller-rocker bearings of railway steel bridges

In this work, a method is proposed for rolling contact fatigue crack propagation analysis using contact and fracture theories in conjunction with fatigue laws. The proposed method is used in the fatigue analysis of rocker and roller-rocker bearings of a railway open web girder bridge which is instrumented with strain gages. Using a contact algorithm based on the minimum energy principle for bodies in rolling contact with dry friction, the normal and tangential pressure distribution are computed. It is seen that the most critical location of a crack in bearings is at a point very close to the contact region, as expected.     

Failure Analysis of High-Speed Pinion Gear Shaft

The failure of a high-speed pinion gear shaft was investigated. The pinion gear was an integral part of a system used to compress the natural gas. It was a high-speed gear mounted on two roller bearings. An abnormal wear pattern was observed on the shaft surface, beneath the inner race of the bearing. The material from shaft was observed to be dislodged and stuck to the surface of the inner race. This transfer of material imparted an imbalance to the assembly, and abnormal sounds and fumes were observed two days before failure. The macrofeatures of the fracture surface resemble these of fatigue but electron microscopy showed brittle and mostly intergranular fracture. Fatigue features such as striations were not found on the fracture surfaces. Fatigue samples made from the same material and heat-treated to the same hardness were tested under uniaxial fatigue and the fracture features were compared with the original crack surface. The microfeatures of fracture surfaces were almost identical. The root cause of failure was determined to be fatigue, and cracks on the pinion shaft in the region beneath the inner bearing race lead to the transfer of material.     

某电传操纵系统弹簧拉杆可调叉形接头断裂原因

某飞机电传操纵系统弹簧拉杆可调叉形接头在运行过程中发生断裂,通过宏微观分析和金相检验等方法对拉杆接头的断裂原因进行了分析。结果表明:拉杆接头断裂为疲劳断裂,断裂的根本原因是其显微组织不均匀的同时出现了魏氏组织,使接头的抗疲劳性能严重降低,导致疲劳裂纹的萌生与扩展;而拉杆的受力不均匀则加速了其疲劳裂纹的扩展。     

THE SHORT CRACK PROBLEM

Abstract— The problem associated with short crack growth, defined as situations in which the intensity of the crack tip field is underestimated by linear elastic fracture mechanics analyses, is briefly reviewed.
Two cases are identified, cracks growing in plastically strained materials, such as occurs in high strain fatigue studies and at notch roots, and small cracks growing in single grains as occurs close to the fatigue limit in plain specimens.
Important mechanical and metallurgical features of short cracks are discussed with particular reference to the upper and lower bound definition of a short crack.     

Comparison of fatigue life assessment by analytical,experimental and damage accumulation modelling approach for steel SAE 1045

Fatigue life assessment for two‐phase steel SAE 1045 has been carried out by experimental and simulation techniques. Analytical approach, termed as fatigue lifetime calculation, was employed making use of a load increase testing procedure and constant amplitude tests equipped with measurement techniques – plastic strain amplitude, change in temperature and change in electrical potential difference. The predicted fatigue life has been validated by constant amplitude tests and compared with fatigue life estimation by microstructure‐based simulation. Simulation has been carried out over the complete cross section of the specimen. The simulation uses damage accumulation in the gage section of the specimen culminating in the macro‐crack propagation, taking into account the inhomogeneous fatigue resistance of the material element. The results show that at the initial intervals of high cycle fatigue range at relatively higher stress amplitudes, the experimental and simulation results are in agreement; whereas in the (high cycle fatigue) region at relatively low stress amplitudes, the simulation results were found more optimistic and the corresponding fatigue scatter is also increased. Each scatter is attributed to the relatively small number of analysed models of the material structure. Scanning electron microscope was used to determine volume fraction of the microstructure for simulation. Fatigue fracture surface analysis shows that crack initiated from internal defect of material and crack propagation is driven by silicon oxide inclusion.