Influence of casting defects on fatigue behaviour of A356 aluminium alloy

Abstract

The fatigue behaviour of A356 cast aluminium alloy under 10⁴ cycles was investigated by a servo-hydraulic machine at the temperature of 150°C. The specimens for the fully reversible tension compression tests were cut from cylinder head castings of automobile engines. The testing data were treated by S–N curve and the staircase method and the mean values of fatigue property coincided with each other by these two methods, though the data scattered with different casting defects. The fracture surfaces of fatigue specimens were examined by OM, SEM and EDS. It was found that in most of the specimens two or more fatigue crack initiations appeared at a fracture surface. Compared with casting defects such as oxide film, inclusion and blowhole, shrinkage porosity is more detrimental to the fatigue behaviour of aluminium alloy castings.     

核电常规岛用超高液压螺栓拉伸器关键结构的数值分析

针对核电常规岛用螺栓拉伸器具有超高压、超小空间作业工况的特性,提出一种基于数值分析与实验相接合的设计方法,运用ANSYS软件对螺栓拉伸器关键结构——缸体与螺帽进行有限元校核,并与测定的应变实验数据进行对比;同时分析参数Rm、D对螺帽结构的优化特性。研究结果表明:初设的缸体强度和刚度满足材料的要求,但螺帽在"L型"截面拐点处出现"负"安全系数;优化螺帽结构尺寸,当Rm=8 mm,D=60 mm时与初设参数相比应力减小了121.2%,应变减小了58%,安全系数得到极大提高,这为该设备的可靠性和安全性提供了设计依据。     

发动机气缸孔压板镗珩加工工艺研究

作者在发动机气缸加工中采用压板镗珩工艺，模拟实际装配状态，用工艺缸盖螺栓将工艺压板(模拟缸盖)、工艺缸垫与气缸体压紧，然后对气缸孔进行精镗、珩磨等精加工，对气缸在实际装配时所产生的变形进行校正，从而保证发动机在装配缸盖后气缸孔的圆度和锥度，有效减少气缸孔的早期不均匀磨损。     

Failure analysis of a diesel engine cylinder head based on finite element method

Macro fatigue cracks are expected to occur in valve bridges of cylinder head when engine is operating in normal working condition. In order to determine the causes of these failures, stress analysis is carried out using finite element method with a concern of temperature dependency of material properties. Mechanical and thermal properties of material tested at high temperatures are applied to the finite element analysis. Furthermore, temperatures of the cylinder head in actual working condition are measured to validate the simulation results of finite element analysis. After that, stress computation is performed and the regions of stress concentration on the flame deck surface are obtained. The analysis results of stress show that the regions of stress concentration are in agreement with the actual failure regions of the cylinder head. When analyzing the failures on the flame deck surface of a cylinder head by evaluating stress concentration, temperature’s effect on mechanical strength of material should not be ignored. The methodology of failure analysis proposed in this paper is time-saving and also relatively accurate and predictive in actual engineering practice.     

An investigation of the repetitive failure in an aircraft engine cylinder head

Cylinder head (CH) failures in aircraft piston engine may have serious or fatal consequences to the safety of the crew and the aircraft. Moreover, when failure becomes undoubtedly repetitive and critical resulting in loss of aircraft, destruction of properties, and first and foremost loss of human lives, the cause of the failure requires to be investigated using a scientific approach. Therefore, the aim of this study is to investigate and identify the root cause of a repetitive premature failure in an aircraft engine CH.The piston engine of the training aircraft Utva-75 has malfunctioned during the flight due to the cracking of its aluminum cast CH. It has been the second engine failure of this type of aircraft due to the cracking in the CH in a very short span of time. From the visual examination of the mating fracture surfaces, it has been possible to observe typical beach and ratchet marks indicating the occurrence of fatigue failure. The crack has initiated from multiple origins located on the inner flange fillet on the exhaust side of the CH. Further examinations by using scanning electron microscopy as well as energy dispersive spectroscopy and metallography have shown that the fatigue had promoted from pre-existing material defect due to a high concentration of shrinkage pores at the initiation crack site and can be most likely associated with the manufacturing process of casting. The stress analysis of the cylinder assembly, carried out by means of finite element analysis, has also confirmed that the crack origin was located at the most stressed area of the cylinder assembly i.e. on the inner flange fillet of the exhaust side of the CH.This case study, together with the other recently reported, has definitely confirmed the repetitive and therefore systematic problems with the CH of air cooled, horizontally opposed, aircraft piston engines.     

Modeling of program, erase and retention characteristics of charge-trap gate all around memories

This paper describes the effect of geometry in charge-trap (CT) memory devices. We first theoretically analyze the impact of the curvature radius on the behavior of the gate current in Gate-All-Around devices, and then describe the change to make to planar model in order to fit the cylindrical devices characteristics. This model is used to simulate Nanocrystal and SONOS program, erase and retention behaviors. The dynamics enhancement during program/erase due to the bending of the active region in such cylindrical devices is explained. The scaling perspectives conclude this paper.     

各国压力容器标准中外压圆筒的计算及数值计算屈曲和弹塑性分析

通过比较GB150-1998、ASMEⅧ-1、ASMEⅧ-2(2007)、EN13445-3(2009)、AD-2000和гост14249中外压圆筒的计算,利用Ansys软件做屈曲和弹塑性分析。结果表明,GB150-1998中外压圆筒的计算方法偏于保守。     

MELTING OF ICE AROUND A HORIZONTAL ISOTHERMAL CYLINDRICAL HEAT SOURCE

Processes during melting from a horizontal cylindrical heat source of uniform surface temperature embedded in ice have been studied experimentally. The volume of the melt and its shape were photographed at different times for various constant temperatures of the heat source. At early times and under all conditions, the melt occupied a cylindrical annulus. At later times free convective motion caused pear-shaped melt contours which pointed downward when the temperatures of the heat source were below 7°C and upward when the temperatures were above 8°C. Instabilities in cellular natural convection motion resulted in waviness of the interface. The location and magnitude of these ripples were found to depend on the temperature of the heat source and the melt layer thickness. Shadowgraph techniques were used to determine local heat transfer coefficients at the heat source surface.     

R108A型柴油机的开发研制

本文就由R105系列柴油机经扩缸、加长行程为R108A系列柴油机的研制开发过程作了介绍,较为详细的说明了零部件的结构设计及整机的性能和可靠性。     

Effect of a Magnetic Field on a Micropolar Fluid Flow in the Vicinity of an Axisymmetric Stagnation Point on a Circular Cylinder

The effect of a magnetic field on a micropolar fluid flow in the vicinity of an axisymmetric stagnation point on a circular cylinder is studied numerically. The governing conservation equations of continuity, momentum and angular momentum are partial differential equations which are transformed into a system of ordinary differential equations by using the usual similarity transformations. The resulting system of coupled non‐linear ordinary differential equations is solved numerically by using the shooting method. The numerical results indicate the velocity, angular velocity and pressure distributions for different parameters of the problem including Reynolds number, magnetic parameter and dimensionless material properties, etc. In addition, the effect of the pertinent parameters on the local skin friction coefficient and the couple stress are discussed numerically and illustrated graphically.