The effect of braking energy on the fatigue crack propagation in railway brake discs

Thermal fatigue cracks can often be found on the friction surface of brake discs used in railway vehicles after a period of usage and include crackle, radial and circumferential patterns. These cracks typically exhibit different initiation and propagation behavior under different braking conditions. In this paper, the effect of braking energy on fatigue crack evolution is analyzed by using experimental testing and numerical simulations. Macro observations show that a significant number of radial cracks appear on the surface of brake discs which operate at 300 km/h, while crackles typically appear after repeated emergency braking (EB) at 200 km/h. No crack growth was observed on disc surfaces after routine braking. The cyclic load that leads to the fatigue crack propagation consists of compressive stress during braking and residual tensile stress after cooling. Simulation results show that the depth of cracks correlates well with the residual tensile stress distribution in brake discs. Breaking tests exposed that the fracture surface of fatigue cracks which were covered by oxides shows nearly elliptic-type. Higher braking energy leads to a hardened layer on the friction surface and oxide generation near the crack edges, which are also important factors that contribute to accelerating crack propagation.     

34Mn2V钢的热加工缺陷

34Mn2V钢坯经火焰切割下料，再经热冲压制成氧气瓶，因氧气瓶底凹面出现热撕裂而造成大量废品。通过试验跟踪和解剖分析，研究了下料坯端面裂纹和半成品瓶底部相应缺陷的本质。结果表明，切割下料时在切割面热影响层发生部分奥氏体化；当切割热消失后，奥氏体化的金属即受到周围冷金属的冷却作用形成马氏体组织。因此，瓶底的局部热撕裂是由下料坯切割时产生的温度梯度和相变引起的。采用预热切割、切割后缓冷和下料坯温度降至环境温度前立即入炉退火都能避免热影响层的开裂。     

300M钢锻件中的凹坑缺陷及其形成机理

对300M钢制锻件中的凹坑缺陷进行了观察与分析，并对凹坑缺陷的形成机理进行了分析与研究，结果表明，锻件中的凹坑缺陷是在锻造过程中产生的，它是在锻件裂纹延性开裂与扩展时，伴随锻造裂纹的产生，扩展以及微裂纹与空洞形成，连通，圆钝与氧化而形成的。     

ELECTRICAL POTENTIAL MONITORING OF CRACK FORMATION AND SUBCRITICAL GROWTH FROM SMALL DEFECTS

Abstract —An experimental method was developed to quantify the formation and subcritical propagation of small cracks emanating from artificial surface defects. Continuous crack depth information was obtained from dc electrical potential measurements employing an analytical model. Fatigue experiments were conducted to evaluate the crack monitoring procedures for conditions relevant to the elevated temperature defect tolerance of superalloys. Cracking progressed uniformly and predictably from small surface defects (0.1 mm deep and 1.4 mm long) in A286, 304 and 10Ni steels and in René 95 and MP–159 superalloys. Crack depths, computed based on analytical calibration of measured electrical potentials, agreed to within ±18% of corresponding values measured optically. Similar results were obtained for a penny-shaped defect, 75 μm deep. Applications of the technique were investigated, including characterizations of fatigue crack formation and growth in René 95.     

1240MPa级Ti-38644高锁螺栓的拉伸疲劳增寿机理

用扫描电镜和能谱观测分析Ti-38644高强钛合金高锁螺栓的拉伸疲劳断口,揭示了高锁螺栓的疲劳裂纹萌生和扩展的微观特征和疲劳增寿机理。结果表明,Ti-38644高强钛合金高锁螺栓的疲劳断口包括疲劳裂纹萌生区、扩展区和瞬断区:疲劳裂纹从螺栓头下圆角滚压薄弱部位表面萌生,随后在基体中呈放射性扩展;进入扩展区后裂纹的尺寸由微观扩展至宏观,以疲劳条带扩展机制为主,同时也存在解理断裂。头下圆角处的变形层对Ti-38644高锁螺栓的疲劳寿命有显著的影响,变形层使Ti-38644钛合金高锁螺栓的疲劳寿命明显提高。通过微观组织与疲劳寿命的对比,探讨了Ti-38644钛合金高锁螺栓疲劳强化的作用机理。     

Study on stress evolution in SiC particles during crack propagation in cast hybrid metal matrix composites using Raman spectroscopy

The evolution of stress in the SiC particles during crack propagation under monotonic loading in a cast hybrid MMC was investigated by micro Raman spectroscopy. The experiment was carried out in situ in the Raman spectroscopy. Experimental results showed that cracks due to monotonic loading propagated by the debonding of the particle/matrix interface and particle fracture. Secondary cracks those formed in front of the main crack tip coalesced with the main crack in subsequent loading and final failure occurred. A high decrease in stress (several hundreds in MPa) was observed with the interfacial debonding at the interface and with the particle fracture on the particle. Moreover, the critical tensile stresses for particle–matrix interface debonding and particle fracture developed in hybrid MMC were also estimated during the crack propagation.     

热暴露对γ-TiAl合金表面缺陷损伤容限的影响

研究了热暴露(700℃,10 000 h)对高强度全片层γ-Ti Al合金Ti-44Al-4Nb-4Hf-0.2Si-1B表面缺陷损伤容限的影响,采用扫描电子显微镜研究了热暴露导致γ-Ti Al合金的显微组织变化,并将之与在交变载荷下的表面短裂纹行为和长裂纹扩展行为联系起来.研究发现,在热暴露后,该合金的疲劳强度提高,且长疲劳裂纹启裂门槛值改善,但热暴露导致该合金短裂纹效应的尺寸范围明显增大.采用Kitagawa-Takahashi线图的形式总结和分析了实验结果,分析了热暴露引起的疲劳强化、疲劳失效的非安全短裂纹的尺寸变化以及长裂纹的启裂门槛值的变化,定量确定了热暴露对表面缺陷的损伤容限.长期热暴露所导致的材料内部应力释放、偏聚缓解、缺陷钝化显著影响裂纹尖端的应力状态,更有利于增大长裂纹的启裂抗力并减缓长裂纹的扩展速率.     

Analysis of dynamic characteristics of through‐wall cracks between 2 boreholes in the directed fracture controlled blasting

Through‐wall cracks between 2 boreholes in the directed fracture controlled blasting have been always concerned by researchers. Dynamic characteristics of through‐wall cracks between 2 boreholes and lateral crack propagation of boreholes in the double‐borehole slot mode and the synchronous blasting of boreholes were examined using the explosive loading digital dynamic caustics experiment system. And the effects of borehole loading mode and borehole clearance on through‐wall cracks between boreholes were examined using distinct lattice spring model numerical analysis, based on the experiment model. Findings show that the tips of through‐wall cracks between boreholes did not meet directly but staggered, continuously propagated after meeting, and moved closer to the existing anisotropic crack direction. The velocity and acceleration of crack propagation fluctuated. K_I rapidly decreased from the maximum value, then gradually increased after a repeated volatility, and began to decrease after it reached the second peak. During the process of crack propagation, K_II was basically smaller than K_I. The dynamic energy release rate rapidly decreased from the maximum value, reached the second peak after the volatility, and gradually decreased again. The borehole loading mode and borehole clearance had significant effects on through‐wall cracks between boreholes.     

On the mechanism of squat formation on train rails – Part II: Growth

Longitudinal cross-sectioning of squats reveals characteristic features of internal crack front propagation. Leading crack planes propagate over longer lengths and greater depths as compared to more superficial trailing crack planes. A favourite depth of crack propagation occurs in the subsurface (2–3 mm), is related to the residual longitudinal stress profile, and may lead to an internal crack ‘terrace’. Especially during deeper crack propagation and branching oxidation processes are found to be metallurgical drivers of crack growth. Contact surface modification during squat growth can be distinguished between phases of transient local stress redistribution and of dynamic wheel–rail contact. If the hypothesized shearing wedge in the failure mechanism loses its load bearing capacity, this gives rise to a redistribution of normal stresses within the actual contact ellipse and the formation of a hardness envelope along the crack pattern. This may partially explain why maturing squats show decoloured and hardened surface areas bordering the surface-breaking cracks. A second effect occurs for contact patches not matching the failure ‘envelope’: due to the Poisson effect the surface overlying the crack planes settles slightly, experiences reduced contact, and corrosive products, ‘pumped’ from inside the cracks, may accumulate on the surface (as confirmed by SEM-EDX analysis). During progressive growth of the defect the harder and decoloured envelope as well as the original wedge is pressed into the deeper elastic material, accompanied by a gradual expansion of the contact band and a bilateral bridging of the defect. This may cause high-frequency impact, resulting into progressive internal crack growth affecting the global stress response and rail fracture.     

Ermittlung von Umformgrenzen beim Freiformschmieden

Determination of forming limits during open die forging The fundamentals of determining the forming limits during open die forging have been investigated in the research project “Vermeidung von Oberflächenfehlern beim Freiformschmieden” (“Prevention of surface defects during open die forging processes”) which was supported by the AiF (“Arbeitsgemeinschaft industrieller Forschungsvereinigungen”). The industrially produced material was analysed in the form of cast ingots of two high‐alloyed steels (1.2367 and 1.6957) by compression, tension and torsion tests. The tests showed that the sampling point has a small influence on the materials’ plasticity for the present dimensions of the ingots. Furthermore transformation and precipitation behaviour of the both steels have been determined and additional metallographic investigations have been made to find out reasons of variation of measurements for low forming temperatures. Forming limit diagrams are generated to predict the forming limits of open die forging processes. For this purpose, compression tests and numerical simulations of the compression tests and forging processes were made for crack‐critical temperatures. The points in time of crack initiation of the samples during the compression tests were determined by the acoustic emission analysis and show a high variation of measurements. Thus the forming limit diagrams also have high scatter, an unique forming limit cannot be determined. Instead of that a maximum allowed height‐reduction was determined and checked in open die forging tests in the laboratory under industrial conditions. The results show that the determined limits are very safe because no cracks were generated during the forging of both materials.     

Fatigue life assessment of nodular cast iron containing casting defects

The fatigue behaviour of a nodular cast iron containing casting defects has been investigated in the high-cycle fatigue regime. In this paper, we propose a fatigue life assessment model for flawed materials based on a fracture mechanics approach which takes into account the position and size of the defect, short crack behaviour and the notch effect introduced by the defect. The fatigue behaviour of smooth samples, and long and short crack behaviour have been experimentally determined in order to identify the relevant mechanical parameters; these being introduced into the model. An experimental study has been made both in air and in vacuum in order to account for the position of the defect, noting that internal defects are supposed to be under vacuum conditions. Experimental results, which are based on a two-crack front-marking technique specially developed for this study, show that the propagation of natural cracks is controlled by the effective stress intensity factor in air as well as in vacuum. The K calculation for a short crack in the stress field of a notch is analysed using numerical elastic–plastic results. Comparison between experimental results and the computation of fatigue life for fatigue lives less than 10⁶ cycles shows that the fatigue behaviour of nodular cast iron is controlled by a propagation process. The model proposed is thus relevant for fatigue lives less than 10⁶ cycles so that the defect can be considered as a crack and the initiation stage neglected. Closer to the fatigue limit, this study shows that the initiation stage should be considered in the assessment of fatigue life of nodular cast iron, because a single macroscopic propagation assessment is not enough to describe the whole fatigue life. The defect cannot be considered as a pre-existent crack in the high-cycle fatigue range (>10⁶ cycles), and the initiation stage that contains microcrack propagation around the defect should be evaluated when assessing the high-cycle fatigue life of nodular cast iron.     

SiCw/MB15复合材料断裂行为研究

扫描电镜下采用动态拉伸法,原位观察SiCw/MB15复合材料复合材料在动态受载条件下裂纹形成、扩展至断裂的全过程,并对断口进行了分析。结果表明,裂纹主要在折断的晶须处萌生,在主裂纹前端应力集中区内长大、连结,在与主应力垂直方向上形成宏观裂纹。     

A comprehensive approach for modeling fatigue and fracture of rails

A comprehensive approach is developed for studying the fatigue phenomena (crack initiation and propagation) induced by repeated rolling or rolling-sliding contacts between wheel and rail. Cracks initiate and propagate in the rail head in a complex varying multiaxial stress regime due to Hertzian or non-Hertzian contacts generating 3D residual stress pattern. This paper presents the main steps of such an approach devoted to the modeling of defects induced in the rails by the traffic. Special attention is paid to some of the principal difficulties met as well as to the proposed solutions. Examples of applications for the prediction of initiation as well as propagation of some defects are presented. It is shown that numerical simulations predict very well the locus of crack initiation as well as its propagation in the rail. Our approach presents at least three main originalities: first, it is a global approach starting from the evaluation of the initial state of the rail to the simulation of the crack propagation under complex loading including multiaxial residual stresses. Second, special and original numerical methods for the evaluation of the initial states, the overloads and the elastoplastic state under service loading have been developed. Third, a new concept based on a “structural Paris law” has been developed and used in the crack propagation simulations.     

Orientation-dependent crack-tip blunting and crack propagation in a single crystal BCC iron

Atomistic simulations of cracks with four different orientations in body-centered cubic single crystal iron are presented using molecular dynamics. Crystal orientation has considerable effect on the activation and evolution of crack propagation mechanisms. The results reveal that (a) crack-tip blunting depends on the crystallographic orientation, (b) continuous generation of dislocations form crack tip occurs for large crack-tip blunting, and (c) absence of deformation activities like dislocation generation, twin formation, etc. at the crack tip results in crack propagation in a brittle manner.     

On the opening of a class of fatigue cracks due to thermo-mechanical fatigue testing of thermal barrier coatings

The evolution of fatigue cracks observed in thermal barrier coatings (TBCs) subjected to an accelerated test scheme is investigated via numerical simulations. The TBC system consists of a NiCoCrAlY bond coat and partially yttria stabilized zirconia top coat with a thermally grown oxide (TGO) between these two coatings. The cracks of interest evolve in the bond coat parallel and near the interface with the TGO during thermo-mechanical fatigue testing. In their final stage, the cracks lead to partial spallation of the TBC. This study focuses on why the cracks open to their characteristic shape. To this end, finite element simulations are utilized. The crack surface separation is monitored for a range of material properties and oxidation rates. The simulations show that the inelastic response of the bond coat and the oxidation rate of the TGO govern the crack surface separation. Most interestingly, permanent separation of the crack surfaces is caused by a structural ratcheting in the vicinity of the crack.     

Assessment of defect size in brittle fibers

Fiber defects, modeled as inner circular or part-circular surface cracks in planes normal to the fiber axis, are analyzed for computing the relationship between the maximum defect size and the fiber fracture stress. Linear elastic fracture mechanics is applied to compute the critical tensile stress for both surface and inner cracks, taking into account that when an inner crack reaches the surface it can stop growing or become unstable. The minimum critical tensile stress is correlated with defect size when defects are randomly located, and the numerical computations are compared with some available experimental results.     

A monitoring system for contact fatigue crack testing

A new monitoring system is developed to detect crack initiation and propagation in the surface layer during contact fatigue running. The magnetic head of a commercial tape recorder is used as a probe. A combination of leakage and eddy current methods is ussed for the contact roller specimen on the JPM-1 machine. Experimental results show that this system is very sensitive to artificial defects and can distinguish defects on the surface from those beneath the surface. The defect signals agree well with their positions. Testing of an ion-nitrided specimen shows that a tiny subsurface crack (∼ 1 μm), which can only be examined under a microscope with 200 × magnification, is observable with this system.     

TEM observations of dislocation emission at crack tips in aluminium

Direct observations were made of the propagation of ductile cracks and associated dislocation behaviour at crack tips in aluminium during tensile deformation in an electron microscope. In the electropolished area, the cracks propagated as a Mode III shear-type by emitting screw dislocations on a plane coplanar to the crack plane. A zone free of dislocations was observed between the crack tip and the plastic zone. As the cracks propagated into thicker areas, the fracture mode changed from Mode III to predominantly Mode I. The crack top of the Mode I cracks was blunted by emitting edge dislocations on planes inclined to the crack plane. The blunted cracks did not propagate until the area ahead of the crack tip was sufficiently thinned by plastic deformation. The cracks then propagated abruptly, apparently without emitting dislocations. The stress intensity factor was measured from the crack tip geometry of Mode III cracks and it was found to be in good agreement with the critical value of the stress intensity factor required for dislocation generation.     

On propagation of short rolling contact fatigue cracks

Recent accidents involving railway rails have aroused demand for improved and more efficient rail maintenance strategies to reduce the risk of unexpected rail fracture. Numerical tools can aid in generating maintenance strategies: this investigation deals with the numerical modelling and analysis of short crack growth in rails. Factors that influence the fatigue propagation of short surface‐breaking cracks (head checks) in rails are assessed. A proposed numerical procedure incorporates finite element (FE) calculations to predict short crack growth conditions for rolling contact fatigue (RCF) loading. A parameterised FE model for the rolling‐sliding contact of a cylinder on a semi‐infinite half space, with a short surface breaking crack, presented here, is used in linear‐elastic and elastic–plastic FE calculations of short crack propagation, together with fracture mechanics theory. The crack length and orientation, crack face friction, and coefficient of surface friction near the contact load are varied. The FE model is verified for five examples in the literature. Comparison of results from linear‐elastic and elastic–plastic FE calculations, shows that the former cannot describe short RCF crack behaviour properly, in particular 0.1–0.2 mm long (head check) cracks with a shallow angle; elastic–plastic analysis is required instead.     

FATIGUE BEHAVIOUR OF STEEL CASTINGS CONTAINING NEAR-SURFACE DEFECTS

Manufacturing defects in the near-surface region of 2¼% Cr 1% Mo castings were investigated in a joint research programme. After ultrasonic testing and X-ray inspection, large push-pull specimens (cross section 1000 mm²) were taken from castings in regions with indications of defects and tested under fully reversed loading in the elastic and elastic plastic region up to predetermined levels of defect growth. The defects are micropores, pin holes, slag and oxide layers or inclusions, hot tears and small micro cracks in welds. The initiation of crack growth was analysed by two methods, namely (a) the local strain concept with idealization of defects as three-dimensional elliptical notches and (b) by the fracture mechanics concept with idealization of defects as two-dimensional elliptical cracks. The main results can be summarized as follows: 1. The local strain concept describes the crack initiation potential of the defects more accurately than the fracture mechanics concept. 2. Interpretation of casting defects as cracks produces very conservative estimates in many cases. 3. Analytical evaluation of crack growth behaviour using the fracture mechanics approach for defects experiencing crack growth of at least 1 mm is much more accurate than when defects having a smaller extent of crack growth are included in the analysis. Additional evaluations of crack growth behaviour of hot tears by fracture mechanics show that crack growth starting from open and partially closed hot tears must be subjected to a larger number of cycles than normal fatigue cracks of sound material. Partially closed hot tears in the vicinity of the crack front additionally delay crack propagation.