Engineering Computations

Comparative analysis on a redundant repairable system with different configurations : Table of Contents

Hsin-I Huang, Jau-Chuan Ke — Sun May 24 14:15:05 BST 2009

Abstract:
Purpose – The purpose of this paper is to examine the cost/benefit (C/B) analysis of four configurations for a repairable system with two primary components/units and one standby. Design/methodology/approach – The four configurations are set to the status of the detection and switching failure of standby, as well as the possible reboot of failed units. The time to failure for each of the primary and standby is assumed to follow an exponential distribution. The time to repair and the time to reboot is assumed to have a k-stage Erlang distribution. The paper develops the explicit expressions of the mean time to failure (or MTTF) and the steady-state availability (or A) for four various configurations and performed some comparative analysis. Based on the C/B criterion, comparisons are made for specific values of distribution parameters and of the costs of the units. The four various configurations for a repairable system are ranked by using MTTF, A and C/B, where B is either MTTF or A. Findings – Although it is uncertain which configuration is the optimal one among the four ones, the paper provides much comparative information to manager and manufacturers. Managers can use these results to choose the best configuration according to the used data of parameters and selections of the weight of MTTF or Cost/MTTF. Originality/value – This paper shows a comparative analysis for a two-unit online repairable system with one standby under four different configurations. It is the first discussion of comparable work on reliability and availability models for redundant repairable systems in which the units are characterized by detection, switching failure and reboot.

Natural neighbor Galerkin method for deflection analysis of inflatable structures : Table of Contents

Farhang Daneshmand, Narges Dialami — Sun May 24 14:15:05 BST 2009

Abstract:
Purpose – The purpose of this paper is to extend the application of natural neighbor Galerkin Method (NNGM) to deflection analysis of inflatable structures such as new and modern textile structures under arbitrary conditions. Design/methodology/approach – Inflatable structures have a proper mechanical strength when they are inflated at different pressures. NNGM or natural element method (NEM) is defined as a new meshfree method based on the natural neighbor interpolation to analyze the deflections of these structures under arbitrary pressures, load and support conditions. The whole interpolation is built with regard to the natural neighbor nodes and Voronoi tessellation of the given point. Findings – The performance of NNGM is investigated in the deflection analysis of inflatable tubes and panels. The excellent agreement between the presented modeling and analytical results and also finite element solutions and experiment are observed. Originality/value – Despite the wide usage of NNGM in many engineering problems, this comparison shows the other aspect of application of NNGM in the deflection analysis of inflatable structures, not previously examined.

A study on flow characteristics of electrorheological fluid in a damper model : Table of Contents

Hiroshi Yamaguchi, Xin-Rong Zhang, Daisuke Inoue — Sun May 24 14:15:05 BST 2009

Abstract:
Purpose – The purpose of this paper is to investigate flow behavior of electrorheological (ER) fluid in a closed piston–cylinder system. Design/methodology/approach – A basic study of flow characteristics of ER fluid in a damper model is conducted experimentally and numerically. The electric field is applied between inner wall of the cylinder and outer wall of the piston, and the pressure difference between upper and lower chamber of the cylinder is measured. A numerical prediction of ER fluid flow in the damper model system is performed in order to study the ER fluid flow characteristics. Visualization experiment is also made and used to qualitatively verify the numerical formulation. Findings – The agreement between the numerical predictions and experimental results is encouraging, and the ER fluid flow patterns under different piston aspect ratios, movement speeds and applied electric field strengths are presented. The results show that the piston aspect ratio has much smaller influence on the ER flow pattern than other influencing factors. Increasing piston movement speed or reducing the electric field applied is helpful to reduce the pressure response time period, which is an important indicator showing sensitiveness of the damper. It is also seen that the pressure difference between the upper and lower chamber of the cylinder increases with the electric field strength and the piston movement speed. Originality/value – First time the detailed investigation into the hydrodynamics behavior in such working models of engineering applications for ER fluid.

An element-free Galerkin method for metal forming simulations : Table of Contents

Rodrigo Rossi, Marcelo Krajnc Alves, Hazim Ali Al-Qureshi — Sun May 24 14:15:05 BST 2009

Abstract:
Purpose – The purpose of this paper is to investigate the application of the element-free Galerkin (EFG) method to the simulation of metal forming processes and to propose a strategy to deal with volumetric locking problem in this context. Design/methodology/approach – The J2 elastoplastic material model, employed in the work, assumes a multiplicative decomposition of the deformation gradient into an elastic and a plastic part and incorporates a non-linear isotropic hardening response. The constitutive model is written in terms of the rotated Kirchhoff stress and the logarithmic strain measure. A Total Lagrangian formulation of the problem is considered in order to improve the computational performance of the proposed algorithm. The imposition of the essential boundary conditions and also of the unilateral contact with friction condition are made by the application of the Augmented Lagrangian method. Here, aspects related to the volumetric locking are investigated and an F-bar approach is applied. Findings – The results show that the proposed approach presents no volumetric locking phenomenon when using the mean dilation approach. Moreover, differently from finite element approximations, no hour-glass instabilities in the deformation pattern are observed, avoiding in this way the need to devise additional stabilization procedures in the proposed procedure. Originality/value – This paper demonstrates the implementation and validation of the mean dilation approach, in the scope of the EFG, which was successful in coping with the volumetric locking phenomena and presented no hour-glass instabilities in the problem cases considered in this work.

Modeling creep in a rotating disc with linear and quadratic composition gradients : Table of Contents

V.K. Gupta, Vijay Kumar, S. Ray — Sun May 24 14:15:05 BST 2009

Abstract:
Purpose – The purpose of this paper is to investigate the effect of imposing linear and quadratic composition gradients on the steady state creep behavior of a rotating functionally graded Al-SiCP disc operating under a radial thermal gradient. Design/methodology/approach – Mathematical model to describe steady state creep behavior in rotating discs made of isotropic aluminum composite containing linear and quadratic distributions of Silicon Carbide (SiCP) in the radial direction has been formulated. The discs are assumed to operate under a radial thermal gradient originating due to braking action as estimated by FEM analysis. The steady state creep behavior of the discs under stresses developing due to rotation has been determined following Sherby's law. Based on the developed model, the distributions of stresses and strain rates have been obtained and compared for various functionally graded material (FGM) discs containing the same average amount (20?vol per cent) of dispersoid. The creep response of a composite disc with uniform SiCP content of 20?vol per cent and operating under a radial thermal gradient has also been computed for comparison with the results obtained for FGM discs. Findings – The study reveals that the distribution of stresses and strain rates in a rotating composite disc operating under a radial thermal gradient are significantly affected by different particle distributions with in the disc. The creep stresses and steady state creep rates in a rotating FGM disc can be significantly reduced by employing more SiCP particles in the middle compared to the inner and the outer radii. Originality/value – The study provides an understanding of the required tailoring of composition in order to control creep stresses and creep rates in a rotating FGM disc operating under a radial thermal gradient.

The pressure angle effects of spur gears on stress concentration factor : Table of Contents

Fuad M. Khoshnaw, Namam M. Ahmed — Sun May 24 14:15:05 BST 2009

Abstract:
Purpose – The purpose of this paper is to investigate the effect of pressure angle, and module of spur gear teeth on stress concentration factor, using photoelasticity method, and numerical MSC/NASTRAN finite element package. Design/methodology/approach – The stress concentration factor is determined as a ratio between maximum stress (determined in the fillet radius by photoelastic and finite element methods), and nominal stress (calculated by a common standard formulas). In order to specify the geometric parameters (height and thickness) of gears, both standard Deutsches Institut für Normung (DIN)/Japanese Gear Manufactures Association (JGMA), and five other non-standard approaches are used. Findings – The results show that the stress concentration factor increases by decreasing the pressure angle. In addition, the values which are obtained by finite element analyses exhibit more uniformity than photoelastic method. Practical implications – An accurate determination of stress concentration factors will limit both over and under design of the gears. Originality/value – The results show that one of the suggested non-standard approaches gives the highest stress concentration factor than the standard approaches.

Heuristic design procedure for structures with displacement-dependent damping devices : Table of Contents

Wenshen Pong, Zu-Hsu Lee, Chong-Shein Tsai, Bo-Jen Chen — Sun May 24 14:15:05 BST 2009

Abstract:
Purpose – The use of supplemental damping to dissipate energy is one of the most economical and effective ways to mitigate the effects of earthquake on structures. For practicing engineers, the ideal design procedure for buildings with supplemental damping should not be too complex to implement in practice. Building on the existing theoretical frame, the purpose of this paper is to develop simple and heuristic methods for the above design procedure. Design/methodology/approach – Passive displacement-dependent devices are considered in this paper. Based on the theoretical results for added damping and added stiffness (ADAS) devices, the paper first analyzes the generated forces and the effects of ADAS devices on structures under seismic impact. We identify design parameters and variables are identified, and present the procedure of how the values of the variables (e.g. column shear force, ductility ratio) are finalized so that the design requirements can be met is presented. A four-story six-bay steel building frame and a ten-story, one-bay steel building frame, equipped with ADAS devices, are used to demonstrate the performance of the design procedure. Findings – Empirical results show that the arrangement of damping devices based on the proposed procedure not only significantly reduces earthquake-induced energy, but also accomplishes the goal of being cost-effective by the control of ductility ratio. Originality/value – The proposed step-by-step procedure is easy for practicing engineers to apply for structures equipped with displacement-dependent dampers, although the modeling requirements may be complex. It will also allow practicing engineers to effectively design economic seismic dampers in the preliminary design phase and further explore the cost factors by comparing different building seismic performance objectives throughout design.