Optimal maintenance—replacement policy under imperfect maintenance

During a given planning period, in order to have the system working at or below a fixed failure rate, it may be necessary to replace the system to minimize the expected total cost. Preventive maintenance (PM) or replacement should be performed whenever the system reaches the maximum failure rate. This paper formulates a cost model and a branching algorithm to determine the number of PM interventions to be performed between each replacement and the number of such replacements during the planning period to minimize the expected total cost. Adjustment of the failure rate for the system degradation after each PM intervention and inflationary trends in cost factors are also incorporated. Numerical illustration and computational times have been presented.     

Biomechanical studies on aliphatic physically crosslinked poly(urethane urea) for blood contact applications

Hydrophobic and physically crosslinked (virtually crosslinked through hydrogen bonding) aliphatic poly(urethane urea)s were developed and characterized for its biomechanical properties. The aging under induced-stress (bend samples) condition reveals resistance of poly(urethane urea) to environmental stress corrosion cracking (ESC) in hydrolytic media, Ringer's solution and phosphate buffered saline at 50 degrees C. The strain-induced (20% tensile strain) and aged polymer in hydrolytic enzyme medium, papain and in buffer reveals increase of elastic modulus in papain enzyme and papain buffer. The increase of elastic modulus is attributed to unidirectional reorganisation of chains under continually strained conditions. The polymer exposed in boiling alcoholic potassium hydroxide solution (accelerated hydrolytic chemical degradation) reveals no degradation. A comparative evaluation of poly(ether urethane urea)s reveals inferior properties. Poly(ether urethane urea)s polymer undergo hydrolytic degradation in boiling alcoholic potassium hydroxide solution. The candidate poly(urethane urea) HFL 18-PUU is more promising elastomer for long-term biomechanically sensitive blood contact applications such as heart valve and blood pump diaphragm of left ventricular assist device.     

Thermal modification of poly(vinyl chloride) and formation of dehydrochlorinated poly(vinyl chloride)–poly(methyl methacrylate) polymer blend in the process of foaming: Identification and application

Impact resistant plastic foam of dehydrochlorinated poly(vinyl chloride) (DHPVC)—poly(methyl methacrylate) (PMMA) was prepared for cryogenic insulation in space vehicle by the method of compression molding and chemical blowing. Impact resistance was achieved by the formation of the polymer blend, dehydrochlorinated poly(vinyl chloride)-poly(methyl methacrylate), during the process of foaming the mold at the temperature of 200°C. The polymer blend was separated from the plastic foam and the compatibility was investigated by ultraviolet, infrared spectral studies and differential scanning calorimetry (DSC). The compatibility of dehydrochlorinated poly(vinyl chloride) and poly(methyl methacrylate) was highlighted on the basis of allylic activation introduced in the thermally modified poly(vinyl chloride). The thermodynamic views were also correlated. The versatility of the present method for impact-resistant foam was pointed out.     

Modeling corrosion behavior of gas tungsten arc welded titanium alloy

The pitting corrosion characteristics of pulse TIG welded Ti-6Al-4V titanium alloy in marine environment were explained. Besides the rapid advance of titanium metallurgy, this is also due to the successful solution of problems associated with the development of titanium alloy welding. The preferred welding process of titanium alloy is frequently gas tungsten arc（GTA） welding due to its comparatively easier applicability and better economy. In the case of single pass GTA welding of thinner section of this alloy, the pulsed current has been found beneficial due to its advantages over the conventional continuous current process. The benefit of the process is utilized to obtain better quality titanium weldments. Four factors, five levels, central composite, rotatable design matrix are used to optimize the required number of experiments. The mathematical models have been developed by response surface method（RSM）. The results reveal that the titanium alloy can form a protective scale in marine environment and is resistant to pitting corrosion. Experimental results are provided to illustrate the proposed approach.     

Construction of closed-form equations and graphical representation for optimal tolerance allocation

The Lagrange multiplier method (LM) is currently used to allocate tolerance for optimum manufacturing cost. This is a tedious iterative process and sometimes it allocates a component's tolerance outside its process tolerance limits. The present work develops a graphical representation which can help the process engineer to visualize the minimum and maximum values for assembly tolerances. The graphical representation developed can also help the process engineer to determine the exact total manufacturing cost of the assembly and help to fix the tolerance, which would not fall outside the limits prescribed. A simple C program is developed to construct the closed-form equations (CFE), and a single EXCEL graphical representation is derived for assembly tolerance, allocated tolerance, and total manufacturing cost. The developed algorithm has been demonstrated on a two- to five-component linear assembly, to help the process/manufacturing engineer's visualization before determining the tolerance specification on component dimensions. The test results show a maximum percentage deviation of 0.09% of assembly tolerance and 0.33% of total manufacturing cost between the LM and the newly developed CFE method.     

Infiltration and sustenance of viability of cells by amphiphilic biosynthetic biodegradable hydrogels

Amphiphilic biosynthetic hydrogels comprising natural polysaccharide alginate (I) and synthetic polyester polypropylene fumarate (II) units were prepared by crosslinking the copolymer of I and II with calcium ion and vinyl monomers viz, 2-hydroxyethyl methacrylate (HEMA), methyl methacrylate (MMA), butyl methacrylate (BMA) and N,N′-methylene bisacrylamide (NMBA). Three fast degradable hydrogels, ALPF-MMA, ALPF-HEMA and ALPF-BMA and one slow degradable hydrogel ALPF-NMBA were prepared. These hydrogels are amphiphilic and able to hold sufficient amount of proteins on their surfaces. All these hydrogels are found to be hemocompatible, cytocompatible and genocompatible. ALPF-NMBA promotes infiltration of L929 fibroblasts and 3D growth of H9c2 cardiomyoblasts and long-term viability.     

Injectable biomaterials for minimally invasive orthopedic treatments

Biodegradable and injectable hydroxy terminated-poly propylene fumarate (HT-PPF) bone cement was developed. The injectable formulation consisting HT-PPF and comonomer, n-vinyl pyrrolidone, calcium phosphate filler, free radical catalyst, accelerator and radiopaque agent sets rapidly to hard mass with low exothermic temperature. The candidate bone cement attains mechanical strength more than the required compressive strength of 5 MPa and compressive modulus 50 MPa. The candidate bone cement resin elicits cell adhesion and cytoplasmic spreading of osteoblast cells. The cured bone cement does not induce intracutaneous irritation and skin sensitization. The candidate bone cement is tissue compatible without eliciting any adverse tissue reactions. The candidate bone cement is osteoconductive and inductive and allow osteointegration and bone remodeling. HT-PPF bone cement is candidate bone cement for minimally invasive radiological procedures for the treatment of bone diseases and spinal compression fractures.     

A Review on Kombucha Tea—Microbiology,Composition, Fermentation,Beneficial Effects,Toxicity, and Tea Fungus

Fermentation of sugared tea with a symbiotic culture of acetic acid bacteria and yeast (tea fungus) yields kombucha tea which is consumed worldwide for its refreshing and beneficial properties on human health. Important progress has been made in the past decade concerning research findings on kombucha tea and reports claiming that drinking kombucha can prevent various types of cancer and cardiovascular diseases, promote liver functions, and stimulate the immune system. Considering the widespread reports on kombucha, we recognized the need to review and update the research conducted in relation to kombucha tea, its products and tea fungus. Existing reports have suggested that the protective effects of kombucha tea are as good as those of black tea, however, more studies on kombucha tea and its composition are needed before final conclusions can be made.     

PAPR Reduction in OFDM Systems Using Modified SLM with Different Phase Sequences

Wireless Personal Communications - High Peak-to-average power ratio (PAPR) is one of the main issues in multicarrier modulation systems. PAPR is mainly caused due to the summation of various...