Failure of aircraft propeller assembly

This paper analyses the causes of the incident of a Cessna trainer whose propeller was separated due to the cracking of the propeller blade hub during the take off roll. Beach marks and fatigue striations, typical of fatigue cracks, were observed on the fracture surface and corrosive oxides were detected in the center of beach marks that are considered to be the crack origin. The stress acting on the fracture surface under a corrosive environment forms corrosive oxides, such as mud cracks. By analyzing the fractography and metallography of the failed parts, it is found that the propeller blade hub nucleated stress corrosion cracking (SCC) as a result of residual stress and corrosive environment and the SCC was the cause of the fatigue crack. Moreover, a fatigue crack reaches its critical length by repeated cyclic stress, which occurs during the rotation of the propeller blade and then, the rest of the fracture occurred instantaneously.     

Failure of a 17-4 PH stainless steel sailboat propeller shaft

In this study, a 17-4 PH precipitation hardening stainless steel propeller shaft failed in use when installed in a sailboat working in a marine environment. Failure analysis was conducted on the propeller shaft. Results indicate that the failure was caused by the fracture of the propeller shaft by torsional fatigue and stress corrosion cracking (SCC). SCC progressed transgranulary in the martensitic matrix.     

Failure analysis of induction hardened injector body

The failure analysis of induction hardened injector body was carried out to identify surface defects. Producer revealed the defects using nondestructive testing method and requested a detailed analysis to determine the cause of their origin. Metallographic and fracture analysis were performed to study the material microstructure and fracture surface. Metallographic analysis proved the existence of a crack, initiated from the front face of component. Microstructure of the crack vicinity as well as hardness was significantly different with increasing distance from the face of component. Microstructure near the front face consisted of coarse martensite, while finer martensitic structure was observed with increasing distance from the front face. Hardness showed decreasing tendency with increasing distance from the front face. Fractographic analysis revealed the intergranular cleavage fracture near the front face of component. With increasing distance from the front face, quasicleavage fracture was observed with increasing areal fraction characterized by ductile fracture. Due to significant difference in microstructure and corresponding difference in hardness within a small area of component can be assumed, that the crack initiation occurred due to internal stress of the material caused by heat treatment. It is necessary to optimize the parameters of induction hardening process with respect to the different thicknesses of the product.     

Investigation of Failed Pin Carrier Plate for the Anti-Rotation Device in a Motor

Parts of the anti-rotation device expelled from a power plant motor were analyzed. The “anti-rotation” device prevents the pump from rotating backwards when there is pressure in the discharge line and the motor is off. Parts that were recovered include about 2/3 of the spring compression ring and an “ear” that apparently came from the missing segment of the spring compression ring. Results of this investigation revealed that a nearly simultaneous overloading occurred on both sides of the pin carrier plate as indicated by almost equal size fracture. At least two crack initiation sites were identified on both sides of the pin carrier plate. Under tensile loads, surface irregularities can lead to surface cracks resulting in the fracture of the plate. Areas rich in aluminum were identified (using energy dispersive spectroscopy) in the vicinity of surface irregularities. A number of suggestions on avoiding such failure are proposed.     

一种高阻尼螺旋桨在宽带激励力下的振动特性研究

提出一种新型高阻尼螺旋桨结构形式,通过将螺旋桨桨叶简化为弯曲梁,对常规桨叶和新型桨叶的随机振动特性进行对比分析。首先利用基于均匀湍流统计模型的相关分析方法,求取了作用在桨叶表面上的压力谱空间分布;然后运用基于模态叠加法的随机振动理论,讨论了系统参数对桨叶叶尖速度响应功率谱和脉动推力功率谱的影响规律。研究表明新型螺旋桨结构形式具有降低叶片固有频率处峰值响应的效果。     

The interaction between the wellbore and pre-existing fractures

Down-hole imaging techniques have revealed that natural fractures can be reactivated in the vicinity of a wellbore. Fracture reactivation, however, has never been analysed in detail. In particular, no one has quantified the influence of in-situ state of stress, fracture friction angle, wellbore pressure, mud invasion in the fracture plane and fracture location on the short and long term stability of the fracture. We have therefore little knowledge of the importance of the phenomenon and of the conditions which lead to fracture reactivation.This work aims to quantify the influence of various controlling parameters on fracture instability. For this purpose, we have extended a method which was previously used to analyse the interaction of tensile cracks with the wellbore. The fluid invasion into the fracture is taken into account by assuming a length of invasion along which the fluid pressure is equal to the wellbore pressure.This work shows that fracture reactivation due to near wellbore effects is likely to occur although the fracture movement may not be easily detected. However the occurrence of this mechanism is quite sensitive to the fracture location with respect to the wellbore and to the fracture orientation. This work has also revealed the possibility of two length scales in fracture instability: the first length scale is related to the interaction of the wellbore with the fracture and the second one is related to fracture instability with respect to the far-field state of stress.     

In-situ cathodoluminescence spectroscopy of silicon oxide thin film under uniaxial tensile loading

In this paper, in-situ cathodoluminescence (CL) stress analysis of a silicon oxide (SiO(x)) thin film prepared by wet thermal oxidation is described. The specially-developed uniaxial tensile loading jig was used to apply tensile displacement to the SiO(x) film specimen. CL spectra of the specimen during tensile loading were obtained, and the peak position of around 1.85 eV emission band was monitored for tensile stress analysis. The peak position gradually shifted towards higher/lower energy side when tensile displacement increased/decreased. The tensile stress-to-emission energy ratio of 6.21-8.97 x 10(2) GPa/eV was estimated on the basis of linear elastic theory, which demonstrated that CL is able to provide information on stress induced in the film. Scanning electron microscopy (SEM) revealed that the fracture of SiO(x) and SCS laminated structure occurred at the vicinity of SiO(x) film surface.     

Object Grating Method Application in Strain Determination on CTOD Tests

Abstract:  Standard fracture toughness tests require standard specimens with the presumption that mechanical properties are uniform in the crack growth direction. Standards for crack tip opening displacement (CTOD) fracture tests prescribe remote crack mouth opening displacement, which can lead to inadequate results in the case of heterogeneous materials properties. This paper describes the application of an object grating method (OGM) on the fracture behaviour of a heterogeneous specimen. Fracture behaviour is described by measuring deformation on the surface of a specimen, in terms of CTOD and, consequently, by strain determination. An OGM is advantageously used when measuring modified CTOD tests on two specimens with an initial crack in a macroscopic heterogeneous welded joint. Results significantly show that fracture behaviour depends on the material in the vicinity of the crack tip concerning the direction of crack propagation.     

Copper spall fracture under sub-nanosecond electron irradiation

Ultra-short powerful electron beam is a suitable tool for producing of high rate deformation in substance. In paper we present a new model of high rate fracture and use this model for numerical investigation of fracture of copper target at irradiation by sub-nanosecond electron beam. In this model, fracture is considered as a time-dependent process of nucleation and growth of opening mode cracks. The nucleation and growth rates are controlled by specific free energy of crack surface which is sole fitted parameter. Plastic deformations, both in cracks vicinity and total in substance, are described in frames of dislocation theory. For verification of the model, we performed simulations of spall fracture at plate impact and at irradiation by high-current electron beam with pulse duration of tens of nanoseconds, and reasonable agreement with experimental data has been demonstrated. Simulations of the sub-nanosecond electron beam action on target indicate that spall fracture of the irradiated target surface is possible. This fracture takes place at the enclosed energy density slightly below the value, which is sufficient for melting of irradiated substance. Fracture threshold energy density does not depend on the origin dislocation density and it increases with the increase of pulse duration. As a result, at long pulse durations (more than ten nanoseconds) the substance melts before fracture.     

基于B样条的螺旋桨升力面设计

为了改善螺旋桨的空泡性能，从效率和空泡性能2个方面的权衡来确定环量分布．在升力面理论设计中计及桨毂的影响．桨叶几何用较少的B样条控制角点来表示．将升力面理论计算的控制点处法向速度的平方和作为优化目标函数，并通过最小化目标函数得到桨叶几何．     

Investigation of process zone structure in Fe–3Si alloy

Abstract

An in situ fracture experiment was carried out in a scanning electron microscope to investigate plastic deformation and strain distribution in the process zone (PZ) located in the immediate vicinity of the crack tip in an Fe–3Si alloy (wt-%) under mixed mode loading conditions. It was observed that plastic deformation occurred by successive activation of a number of slip systems. The strain distribution and shape of the PZ were strongly dependent on the crystallographic orientation of the grain containing the crack tip. The distribution differed from that predicted using near tip blunting calculations and was best expressed by an exponential equation. Additional strain concentrations created by surface defects caused slight perturbations in the overall distribution. Crack propagation started along a coarse slip band which possessed the highest strain. It was found that the maximum strains in the PZ exceeded the uniaxial tensile fracture strain.

MST/1404     

Effects of Bend–Twist Coupling on Composite Propeller Performance

An investigation of a conventional propeller, made from composite materials, was conducted in which propeller characteristics were studied under quasi-static aerodynamic loading. Emphasis was placed on understanding the effects of bend-twist coupling of composite laminates on propeller performance. The classical blade element theory of propellers was used to calculate propeller characteristics and the aerodynamic force distribution acting on the propeller. The finite-element method was used to calculate the resulting deformation of the propeller blades. A simple algorithm was used in which propeller blade deformation and resulting changes in loading conditions were calculated repeatedly until equilibrium between deformation and loading was achieved. Results show that improvements in propeller performance are possible using the bend-twist coupling of composite laminates. Additionally, this study shows that the numerical approach developed by the authors is well suited for analyzing the performance characteristics of composite propellers.     

Stress analysis of elastomeric materials at large extensions using the finite element method

A newly developed finite element method is applied to the stress and strain analyses of stress fields, at the vicinity of a primary crack surrounded by secondary cracks. The results show that the primary crack propagation deviates from the crack axis, when a secondary crack entered the stress fields of the primary crack within the distance of the diameter of the secondary crack. The fundamental unit of surface roughness, the deviation from planarity, will be the diameter of the secondary crack. The roughness generated in real elastomers strongly depends on mechanical hysteresis, and thus the fracture surface energy of the materials.     

Effect of filler surface morphology on the impact behaviour of hydroxyapatite reinforced high density polyethylene composites

Instrumented falling weight impact tests have been carried out to characterize the impact behaviour of hydroxyapatite reinforced high-density polyethylene composite (HA-HDPE) in order to use this biomaterial in skull implants. The effects of HA filler surface morphology and volume fraction on the fracture toughness were studied, and fracture mechanism investigated. Impact resistance was found to be markedly improved by using a sintered grade HA filler with smooth particle surface instead of spray dried grade HA with rough surface. SEM examination of impacted fracture surfaces revealed that the improvement of impact resistance was due to the stronger interfacial bonding between smooth HA particles and HDPE polymer matrix compared with that between rough HA and HDPE, which results in more energy absorption during impact and hence better fracture resistance.     

调距桨桨叶曲面数据和图形处理造型和消隐

本文介绍了运用计算机辅助几何设计方法,为可调螺距螺旋桨的性能论证、分析而建立的采集处理螺旋桨几何数据及图形显示系统。该系统可提供桨叶上任一叶切面和径向切面的数据、桨叶自转后新切面的数据及桨叶上任一点空间坐标,用以对桨叶的加工和检验。同时在系统中进行了桨叶曲面的造型和消隐,使人们可以从屏幕上观察到螺旋桨桨叶转到各指定位置的三维图象。     

水下观测机器人推进器的动态结构仿生设计研究

目的 为了更好地实现水下机器人在海洋牧场观测环境中的低速稳定航行,设计一种水下观测机器人的仿生推进器。方法 结合结构仿生原理提出动态结构仿生设计方法,具体过程包括选择仿生对象、动态结构特征提取、生物特征与产品匹配、产品集成创新设计及设计评价。以水下观测机器人推进器为例,重点模仿蓝斑条尾魟胸鳍波动推进的形态与结构,分步骤完成仿生推进器造型和功能的动态结构仿生设计。结果 通过流体动力学仿真评估,得到波动鳍表面压力分布情况,结果表明波动鳍推进能够产生一定的推力,验证了动态结构仿生设计方法的可行性。结论 该仿生推进器具有低速稳定航行的特点,能够有效提升海洋牧场的观测能力,动态结构仿生设计方法对仿生推进器的结构设计具有借鉴意义。     

锯齿形叶梢螺旋桨高负荷水动力和减振降噪特性试验研究

本文用涡动力学理论分析了锯齿形叶梢对螺旋浆的梢涡的影响，在拖曳水池中分别对常规、后倾且倾斜和后缘锯齿形叶梢的螺旋浆模型进行了水动力特征试验。在大型空泡水洞中的模拟伴流场中分别对这三种形状叶梢的螺旋浆模型进行了高负荷水动力时的脉动压力和噪声测试，试验结果表明，用锯齿形叶梢螺旋浆在高负荷时效率可提高2．5％，激振力降低44．2％，噪声下降3－5dB。而这一切是在用后倾侧斜叶梢螺旋浆已经取得了明显增效和减振降噪效果的基础上又增加的效果，试验验证了用涡量汇的理论分割浆叶梢涡的设计思想是正确的和可行的。     

Casting defects induced fatigue damage in aircraft frames of ZL205A aluminum alloy – A failure analysis

Two different types of aircraft frame components, which had collapsed respectively in their former vibration-fatigue performance tests, were submitted for failure analysis. The two failed frames were both made of aerial material ZL205A, a high-strength cast Al–Cu–Mn–Ti alloy. According to a series of experimental procedures including visual observations, X-ray detections, fractography inspections, microstructure examinations, mechanical tensile tests, hardness measurements and fluorescent penetrating inspections, it was indicated that the fracture was attributed to fatigue cracks which were induced by casting porosity defects at the external surfaces of frames. Numerous fine fatigue striations presented in the vicinity of casting porosities. Especially, it was observed of a special appearance of latitude–longitude crossed fatigue striations on the fracture surface due to the coupled stresses supplied by the former multi-directional vibration tests. The overload fast-rupture regions on fracture surfaces suggested the typical cleavage fracture mode, which was characterized by a number of river patterns and cleavage steps. The intergranular spatial dendrite-shaped casting porosities largely contributed to the local stress-concentrations in matrix materials. Triangular grain boundaries induced by the former casting burnt implied that the intergranular melting phenomenon had occurred. Furthermore, the effect of groove-shaped structure at roots of spatial convex-bodies on the edge of casting porosity was especially analyzed. And the influence of the casting porosity size on fatigue cracks was briefly discussed.     

Synthesis-microstructure-mechanical properties-wear and corrosion behavior of an Al-Si (12%)—Flyash metal matrix composite

In the present investigation, Aluminium based metal matrix composite containing up to 15% weight percentage of flyash particulates were successfully synthesized using vortex method. The properties like density, hardness, microhardness, ductility and ultimate tensile strength were investigated. The MMC produced was also subjected to corrosion, dry sliding wear and slurry erosive wear test to investigate its behavior under different material wearing conditions. The results of microhardness revealed higher hardness of the matrix material in the immediate vicinity of flyash particle. The addition of flyash particles reduces the density of composite while increasing some of their mechanical properties. The results of wear studies have shown that the resistance to wear increases with increase in percentage of flyash. Corrosion resistance decreases with increase in flyash content. The macrostructural and microstructural characteristics of the MMC were investigated with particular emphasis on the distribution of flyash particles in the matrix. Macrostructural studies have shown near uniform distribution of flyash particles in matrix. Analysis of fractured surface of tensile test specimen is also made which revealed brittle fracture behavior of MMCs.