An air crash due to fatigue failure of a ball bearing

The failure analysis of an air crash conclusively shows that the cage of the central main bearing of the compressor region failed due to fatigue. The broken piece of the cage got struck between the bearing balls and the races and impaired the function of the bearing resulting in the crash.     

Shielding effects and residual stresses at cleavage due to pre-existing dislocations

The motion of pre-existing edge dislocations in an infinite linear elastic body is studied at initiation of crack growth and at quasi-static steady-state crack growth. Dislocation nucleation is assumed not to occur. Thus, the study concerns only dislocations that are present in the virgin material. A dislocation is assumed to glide if its driving force exceeds a critical value. Changes in dislocation density, crack tip shielding and residual stresses are obtained. The shielding of a stationary crack tip is found to be small compared with the shielding of a growing crack tip. At steady-state the residual stresses far behind the crack tip are tensile near the crack, decreasing to zero at a certain distance from the crack plane. It is shown that the shielding due to pre-existing dislocations, e.g., for cleavage in α-iron crystals may be considerable. This revised version was published online in July 2006 with corrections to the Cover Date.     

Disappearance of stress singularity at interface edge due to nanostructured thin film

The purpose of this study is to examine the stress distribution near the interface between a nanostructured thin film and a solid body. We focus on a nanostructured thin film that consists of Ta₂O₅ helical nanosprings fabricated on a Si substrate by dynamic oblique deposition. The mechanical properties of the thin film are obtained by vertical and lateral loading tests using a diamond tip built into an atomic force microscope. The apparent shear and Young’s moduli, G′ and E′, of the thin film are 2-3 orders of magnitude lower than those of a conventional solid Ta₂O₅ film. Moreover, the thin film shows strong anisotropy. A finite element analysis for two types of components with different interface edges between the thin film and an elastic solid body is conducted under uniform displacement. One has a free edge where the surface-interface angle is 90°-90°, and the other has a short interface crack. These analyses indicate the absence of not only stress singularity but also high stress concentration near the free edge and the interface crack tip. The characteristic stress distributions near the interface are due to the nanoscopically discrete structure of the thin film.     

球轴承柔性多体动力学分析与接触振动研究

结合有限元法和柔性多体动力学方法,提出球轴承柔性多体接触动力学模型和动力学分析方法。综合考虑钢球与内外圈滚道的游隙、柔性多体接触、套圈弹性变形、摩擦、离心力和转速等关键因素,建立球轴承的柔性多体接触动力学模型。研究了球轴承的柔性多体动力学特性和接触振动响应,计算出球轴承钢球公转一周时的接触力变化、套圈中心的相对振动位移、加速度、套圈弹性变形、截面接触应力和径向接触应力等接触振动响应,揭示了球轴承的支承钢球数目奇偶交替时的变柔性接触振动本质。提出的球轴承动力学模型与计算结果为以系统振动为目标的球轴承动态设计和球轴承系统设计提供理论指导和参考数据。     

Changes in the structure of hardened cement paste due to high temperature

The investigation performed was aimed at explaining explaining experimentally the changes occurring in the microstructure and phase composition of cement pastes during their exposure to heat in the temperature range 20–800°C. The investigation was performed by means of the following methods; thermal analysis, X-ray diffraction analysis, infrared spectroscopy analysis and mercury porosimetry. During the investigation observations were made regarding the behaviour of the following phases in pastes exposed to heat: Ca(OH)₂, CaCO₃, C−S−H, non-evaporable water and micropores. From an analysis of the experimental results, temperature ranges of the following changes were determined in the structure of the investigated paste: additional hydration of unhydrared cement grains, recrystallization and carbonization of Ca(OH)₂, deformation and transformation of C−S−H phases, non-linear changes in the distribution of pore diameters and total porosity.

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Résumé Les changgements de la microstructure et de la composition des phases résultant de l'effet de température dans le domaine de 20 à 800°C ont été expliqués par la voie expérimentale. Les investigations ont été réalisées en utilisant les méthodes de TG, rayons X, absorption infrarouge et porosimétrie au mercure. Les phases de Ca(HO)₂, CaCO₃, C−S−H, l'eau non évaporable et la microporosité ont été étudées. A partir des résultats obtenus, on a conclu qu'il se produit les changements suivants: hydratation additionnelle des grains de ciment restant anhydre, recristallisation et carbonatation de Ca(OH)₂, déformation et transformation de la structure du gel de la phase C−S−H, accroissement non linéaire de la porosité totale et de la distribution de ses dimensions. Il a été possible d'expliquer que l'hydratation additionnelle du ciment restant anhydre et la recristallisation du Ca(OH)₂ sont dues à l'effet de la vapeur sur la phase environnante (la vapeur libérée lors du processus d'hydratation). Suite à la libération de l'eau chimiquement liée de la phase C−S−H, celle-ci subit une transformation. Pour des températures inférieures à 600°C, elle est instable; pour des températures supérieures, cette transformation est stable, irréversible et similaie à celle de βC ₂ S. L'absence de corrélation entre la qualité d'eau libérée de la pate de ciment et de la porosité totale dans la gamme des températures de 100 à 300°C résulte du retrait qui accompagne le départ de l'eau de la pate de ciment. L'accroissement de la porosité totale se produit dans le régime de 300 à 700°C et en particulier augmente le nombre de pores de grande dimension, d'où la formation de microfissures résultant de phases du ciment et de Ca(OH)₂. La réduction de la porosité totale à 800°C pourrait résulter du début de fusion de la pate de ciment durcie.

     

Ultrasonic evaluation of spall damage accumulation in aluminum and steel subjected to repeated impact

This paper shows that the spall damage accumulated in target plates during repeated impact tests can be effectively evaluated by invoking ultrasonic techniques. The experimental data obtained for commercially available, pure aluminum and medium carbon steel subjected to three times repeated plate impact test, serve as a proof of efficiency and accuracy of our method. The spall damage was evaluated using a low frequency scanning acoustic microscope (C- and B-scan images) as well as measurement of ultrasonic velocity, attenuation (amplitude change of B2-B1 echo) and backscattering intensity. We provide here a detailed account of the damage history in essentially ductile aluminum. This proves that the voids nucleated during the first impact form the defect array undergoing coalescence when the stress of second impact is lower than the level applied in the first one. The case of aluminum contrasts the behavior of carbon steel, governed by the development of microcracks generated already during the initial impact. Our conclusions are based on the output of B- and C-scan images consistent with the results of ultrasonic measurements. While providing insights into the evolution of the spall damage, the non-destructive ultrasonic technique promoted by the present authors offers advantages of efficiency, direct applicability to the components of commercial structures, and for early damage prediction.     

Delamination due to interaction between curvature induced interlaminar tension and stresses at terminating plies

Curved unidirectional glass fibre-epoxy specimens with some of the plies on the tension side cut across the complete width have been tested in four point bending. Two sets of tests were carried out with the cut plies in different positions through the thickness to change the ratio between the overall interlaminar tensile stresses induced by the curvature and the local stresses at the cut. A failure envelope based on a linear interaction between overall and local stresses is shown to be a reasonable fit to the experimental data.     

Identification of the temperature field and stresses in a thermosensitive cylinder according to the surface strains

We identify the time dependence of temperature of a surface of a long hollow thermosensitive cylinder and its thermal and thermal stressed states according to the known temperature and strains on the other surface. The posed problem is reduced to the inverse problem of thermoelasticity. By using the solution of the direct problem of thermoelasticity, we perform the numerical verification of the proposed procedure of solution of the inverse problem. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 6, pp. 55–61, November–December, 2007.     

Thermal stresses due to internal friction of the tube of a pneumatic tire clutch

A method is examined of determining the design thermal stresses due to friction of the material during operation of a pneumatic tire clutch. An example is given of the calculation of the thermal stresses in the tube of a PM 300 X 100 clutch.     

Some studies on stresses and strains of aluminium alloy during extrusion-forging at room temperature

The present investigation has been undertaken to evaluate some aspects on extrusion-forging during cold upsetting using a suitable die using aluminium alloy (H9-6063) solid cylinders subjected to different geometrical conditions such as approaching angles namely, 35°, 45°, 50° and 60°, with two different initial protrusion heights namely 8 mm and 10 mm. During the experiments, three geometries such as barreled cylinder, truncated cone part and protruded part or extruded parts were observed. The calculations were made on the assumption that the volume constancy principle holds on and the curvatures of the barrel were in the form of a circular arc. Further, it is established that the protrusion height increases with the increase in the approaching angle for a given extrusion load. The relationship was also established between the various bulge parameters namely the hoop stress, the hydrostatic stress and the stress ratio parameters. In this work an attempt has been made to establish a relationship between the approaching angle and protrusion height and to establish a relationship among various height strains and stress ratio parameters.     

Temperature stresses due to a heat source located on one side of a straight wedge

A heat source acts on one side of a straight wedge with adiabatic boundaries. The temperature stresses are determined in terms of the displacement potential and Airy stress function.     

Modelling ferroelastic domain switching at a stationary crack tip in a single crystal with account of transformation stresses due to domain reorientation

The present paper deals with ferroelastic domain switching around a stationary crack tip in a single crystal for potential applications in the context of crack toughening. The main focus is directed towards the effect of transformation stresses on the domain. The transformation stresses themselves are coupled to the extent and the shape of the domain. Therefore, they and the domain are unknown a-priori and have to be determined simultaneously. The model is applied to a crack in barium titanate and indicates a major effect of the transformation stresses on the observed needle-like domain shapes. Comparison with experimental observations shows a reasonable agreement although essential properties of barium titanate as ferroelectricity and piezoelectricity are neglected yet.     

On the effect of the release of residual stresses due to near-tip microcracking

This article provides a comprehensive theoretical treatment of the interaction effects associated with the release of a general residual stress field as a result of the formation of microcracks in the vicinity of a main crack. The theoretical formulations are based upon the use of the complex potentials of Muskhelishvili and an appropriate superposition procedure. The induced stress intensity factor at the main crack is obtained in a series form and the leading order closed form solution was utilized to elucidate two interesting features of the study. The first is concerned with shielding and amplification effects associated with the release of a uniform residual stress field from a single microcrack. The second is concerned with shielding effects due to the formation of a microcracking zone in the vicinity of a stationary and a steadily growing main crack.The results for the case of a single microcrack reveal that, depending upon the location and orientation of the mirocrack, shielding and amplification effects may become prevalent. In the case of a microcracking zone, equivalence between the current interaction model and the contiuum mechanics approach was established based upon similar microcrack nucleation criteria.     

An expert system for the design and selection of ball bearing parameters

Artificial Intelligence (AI) is an emerging technology. Research in Al is focused on developing computational approaches to intelligent behaviour.The computer programs with which AI could be associated are primarily processes associated with complexity, ambiguity, indecisiveness, and uncertainty. One of these computer programs is referred to as Knowledge-based Expert System (KBES) as it represents knowledge acquired from various experts in a particular field of interest to the user. The expert system emulates human behaviour in solving problems thought to require experts for their solution by utilizing computer programs that incorporate experts' heuristic reasoning. In this paper, the application of KBES to aid the design of ball and roller bearing is discussed. The precision rolling-element bearing of the twentieth century is a product of exacting technology and sophisticated science. A bearing supports radial and axial loads, at the same time allowing relative motion between two elements of a machine. Various requirements and steps in the design of ball and roller bearings are discussed. Equations are developed for the relevant design parameters and input into the expert system shell called VP-Expert. The expert system rules are also provided.