Cost optimization of maintenance scheduling for a system withassured reliability

Systems which have to work at or below a maximum acceptable failure rate should be maintained at predetermined points such that the failure rate does not exceed the acceptable level. As the system ages, the post-maintenance failure rate of the system drops to some newer one, unless the system has been replaced, but does not restore the system to the original state. A branching algorithm with effective dominance rules that curtail the number of nodes created is presented; this algorithm determines the number of maintenance interventions before each replacement in order to minimize the total cost over a finite time horizon. The model considers inflationary trends. A numerical example and computational experience are presented. The authors treat the maintenance cost as constant and successive simple-maintenance intervals as decreasing. Though the cost/maintenance is assumed constant, any increasing maintenance cost function could be incorporated. The optimum solutions depend on the constant improvement factor, first simple-maintenance point, rate of increase in acquisition cost, maintenance cost factor, and planning period     

Statistical analysis of the fracture strengths of aluminum alloy–alumina (Al<Subscript>2</Subscript>O<Subscript>3</Subscript>) particulate composites

The mechanics of composite materials and their “fracture behaviors” are relatively complex phenomena to analyze and establish due to their inconsistent process stability and reliability, combined with production and related processing problems. In this work, an attempt has been made to statistically analyze the tensile behavior of metal matrix composites. Composites of aluminum alloy containing 5–20% volume fraction of Al₂O₃ particles of 15 μm size were prepared by adding alumina particles to a vigorously agitated semi-solid aluminum alloy. Prior to this, alumina particles were subjected to preheating at 800 °C for 5 h. Particles were then added to the aluminum alloy and further heated to 850 °C by using a mixer in a nitrogen medium. A total of 20 tension tests were performed for each volume fraction according to ASTM Standards B557 and using these test data, the initial estimators for an empirical model were obtained. Using this empirical model, the reliability of the composite characteristics in terms of its tensile strength was assessed. Another significant implication of the present study is proving the ability and utility of the Weibull statistical distribution for describing the experimentally measured data on the tensile strength of metal matrix composites, in a more appropriate manner.     

Prediction and Optimization of Pulsed Current Gas Tungsten Arc Welding Process Parameters to Obtain Sound Weld Pool Geometry in Titanium Alloy Using Lexicographic Method

In this article the weld pool geometry of pulsed current gas tungsten arc (GTA)-welded titanium alloy was analyzed. Increase in use of pulsed current process creates dependancy on the use of mathematical equations to predict the weld pool geometry. Hence, the development of mathematical models using four factors, five levels, central composite design was attempted. The developed models were checked for their adequacy. Lexicographic method was used for optimizing the process parameters of pulsed current GTA welding technique. Optimizing the process parameters has resulted in bringing out strong weld pool geometry, which was later confirmed by conducting confirmation tests.     

A new algorithm for optimum tolerance allocation of complex assemblies with alternative processes selection

Allocating tolerance to sub-components of a complex assembly with alternative processes selection by using Lagrange’s multiplier method is tedious as well as difficult. The present work is aims to solve the problem with simple effort in three stages. In the first stage, the maximum of two processes are selected from the alternative processes of each component and these two processes correspond to the smaller sum of difference in manufacturing cost. A hybrid optimum tolerance allocation method is developed in a second and third stage by combining Tabu search (TS) and heuristic approach. Application of the proposed algorithm is demonstrated on complex tolerancing products like knuckle joint and wheel mounting assembly. For the same manufacturing conditions, compared with tolerance synthesis by Singh method, the proposed method saved nearly $74,880 and $479,520, respectively, per year in manufacturing costs of knuckle joint and wheel mounting assembly.     

Newtonian behaviour of sheared aqueous carboxymethylcellulose solution on aging

The effects of microbiological attack on the rheological properties of sodium carboxymethylcellulose (NaCMC) solutions are described. The effect of aging on the viscosity of aqueous NaCMC solution was studied by means of a Brookfield viscometer. An aqueous NaCMC solution was allowed to age and was subjected to shearing at different time intervals. Dilatant rheological properties were observed in contrast to the usually pseudoplastic behaviour of a normal NaCMC solution. This is attributed to the effect of shear-induced recombination of NaCMC macromolecular crystallites produced as a result of chain scission of NaCMC molecules during aging. The recombination of the crystallites gives higher values of viscosity at higher shear rates. A five months aged sample when subjected to shearing for a period of 20 h exhibits different behaviour. After prolonged post-shearing of this sample, the viscosity measured at shear speeds of 10 rpm and 20 rpm at 72 h and 102h approaches a common value for post-shearing period. The approach to the Newtonian character is attributed to the shear-stress-induced breakdown of the crystalline aggregates. These studies reveal that the aging effects on NaCMC solution have a drastic influence on the rheological properties.     

Process Parameter Optimization of the Pulsed Current Argon Tungsten Arc Welding of Titanium Alloyin process parameter selection of pulsed current gas tungsten arc welding of titanium alloy

The selection of process parameters for obtaining optimal tensile properties in the pulsed current gas tungsten arc welding is presented. The tensile properties include ultimate tensile strength, yield strength and notch tensile strength. All these characteristics are considered together in the selection of process parameters by modified taguchi method to analyse the effect of each welding process parameter on tensile properties. Experimental results are furnished to illustrate the approach.     

Analytical studies on surface damage of PVC sheets

Analytical studies on surface damage of PVC sheets due to weathering were carried out. PVC sheets which contain embossed and smooth surface which were subjected to cyclic weathering conditions of humidity and temperature were analysed. PVC sheets prepared with di-2-ethylhexyl phthalate as plasticizer undergo leaching of plasticizer; the leaching being high in smooth and less in embossed surface. The presence of plasticizer on the surface was detected by Attenuated Total Internal Reflectance Infrared Spectrophotometry (ATIR). The higher the leaching in smooth surface, the higher the surface damage in the embossed surface. The higher damage is ascribed to surface brittleness whereas the lower damage is ascribed to the surface plasticization. The surface damage was assessed by photomicrography.     

Particle swarm optimization for minimizing assembly variation in selective assembly

Quality of a product is based on the quality of the mating parts. When the parts are assembled interchangeably, the assembly variation will be the sum of the component tolerances. If the assembly variation is to be less than the sum of the component tolerances, selective assembly is the only solution. In conventional selective assembly, the corresponding selective groups are assembled. In this paper, selective group combinations for assembling the mating parts is obtained using particle swarm optimization (PSO). The combination obtained has resulted in an appreciable reduction in assembly variation. The proposed algorithm has been demonstrated for a linear assembly, which consists of three components having equal dimensional distributions. The assembly variation obtained by interchangeable assembly is 36 μm. By implementing the proposed method, the assembly variations are reduced from 36 to 7.2 μm. However, this algorithm can be extended for assemblies with more number of components and with different dimensional distributions.