Integrated Risk of Progress-Based Costs and Schedule Delays in Construction Projects

This article aims to explore the impact of the integration of risk factors into delayed milestones for construction projects. A simulation model was developed to determine the impact of schedule variability on cost estimation. To generate random scenarios a Monte Carlo Simulation (MCS) technique was applied. The developed model computes the cost impact of delayed milestone in the expected budget. Using a risk integration approach revealed the critical time frame that may lead to a budget deficit for a project. As a result, a number of cost-sensitive risk factors and schedule delays were identified for the critical time period where the risk of budget deficit increases. The method of integration proposed in this article highlights the priority of risk factors and schedule delays for construction contracts involving Payments at Event Occurrences (PEO). Consequently, the developed method can be useful for practitioners in anticipation of potential increase of costs, hence, prevention of failure due to budget deficit.     

Warranty servicing for discretely degrading items with non-zero repair time under renewing warranty

This paper considers a repair-replacement strategy for a special class of discretely degrading and repairable products under renewing free replacement warranty. Each product may experience N different working states with different exponential hazard functions, before the warranty contract expires. Once the item enters working state i (i = 1, 2, … , N), it can either fail or move to any of the subsequent working states with different probabilities, given that a transition has been made. In the former case, the best rectification action, replacement or minimal repair, regarding the failure status, i.e., the product degradation level at the failure time and the remaining warranty time, should be conducted to put the item into operation. It is assumed that replacement of the faulty item occurs instantly and the product warranty is renewed, but non-zero repair time has been included in the model. We derive the optimal replacement-repair policy to minimize the manufacturer’s expected warranty servicing cost per item sold. The Adomian decomposition method is used to find an analytic approximate solution for a special case with two working states. For N > 2, a simulation based optimization method has been developed to analyze the expected warranty cost. Some numerical examples in each section are given to clearly demonstrate the application of this model.     

Microwave-assisted synthesis and sintering of mullite

Mullitization behaviour of a mixture of clay and alumina as the starting materials was examined by microwave heating of (a) mixed powder and (b) compacted powder samples for different soaking times. X-ray diffraction results showed that in compacted samples mullitization process was completed after 20 min heating with a density of about 87%. Densification and microstructure of samples with different green densities heated in a microwave oven and conventional electric furnace were compared. Results showed that the grain growth of mullite was restricted by microwave heating.     

Effect of SiC particles on rehological and sintering behaviour of alumina–zircon composites

The effect of additions of SiC particulates on rheological and sintering behaviour of slip-cast alumina–zircon composites has been investigated. Finely divided alumina, zircon and silicon carbide powders were first processed into slips, using polyacrylite dispersant (0.5 wt.%) to create highly concentrated, stable aqueous suspensions at 40 vol.% loadings, from which test specimens which were then slip cast and dried. They were subsequently sintered in air for 2 h at 1650 °C. Rheological properties of the prepared slips were evaluated and related to the amount of added SiC. After sintering, the resultant porosities, fractional densities, crystallographic phases present, and microstructures were determined.     

Production of Al–20 wt.% Al2O3 composite powder using high energy milling

High energy ball milling was used to produce a nanostructured Al matrix composite reinforced by submicron α-alumina particles. Scanning electron microscopy analysis as well as tap and green density measurements were used to optimize the milling time needed for the completion of the mechanical milling process. Results show that addition of alumina particles as the reinforcement has a drastic effect on the size, morphology and pressability of the powder. Scanning electron microscopy shows that distribution of alumina particles in the Al matrix reaches a full homogeneity after steady state. This would increase the hardness of powder due to a nano-structured matrix and oxide dispersion strengthening.     

In vitro characterization of a novel tissue engineered based hybridized nano and micro structured collagen implant and its in vivo role on tenoinduction,tenoconduction, tenogenesis and tenointegration

Surgical reconstruction of large tendon defects is technically demanding. Tissue engineering is a new option. We produced a novel tissue engineered, collagen based, bioimplant and in vitro characterizations of the implant were investigated. In addition, we investigated role of the collagen implant on the healing of a large tendon defect model in rabbits. A two cm length of the left rabbit’s Achilles tendon was transected and discarded. The injured tendons of all the rabbits were repaired by Kessler pattern to create and maintain a 2 cm tendon gap. The collagen implant was inserted in the tendon defect of the treatment group (n = 30). The defect area was left intact in the control group (n = 30). The animals were euthanized at 60 days post injury (DPI) and the macro- micro- and nano- morphologies and the biomechanical characteristics of the tendon samples were studied. Differences of P < 0.05 were considered significant. The host graft interaction was followed at various stages of tendon healing, using pilot animals. At 60 DPI, a significant increase in number, diameter and density of the collagen fibrils, number and maturity of tenoblasts and tenocytes, alignment of the collagen fibrils and maturity of the elastic fibers were seen in the treated tendons when compared to the control ones (P < 0.05). Compared to the control lesions, number of inflammatory cells, amount of peritendinous adhesions and muscle fibrosis and atrophy, were significantly lower in the treated lesions (P < 0.05). Treatment also significantly increased load to failure, tensile strength and elastic modulus of the samples as compared with the control ones. The collagen implant properly incorporated with the healing tissue and was replaced by the new tendinous structure which was superior both ultra-structurally and physically than the loose areolar connective tissue regenerated in the control lesions. The results of this study may be valuable in the clinical practice.     

MODELING OF MEMBRANE FOULING AND FLUX DECLINE IN MICROFILTRATION OF OILY WASTEWATER USING CERAMIC MEMBRANES

One of the major problems in pressure-driven membrane processes is reduction of flux far below the theoretical capacity of the membrane. The results of an experimental study of fouling mechanisms of ceramic membranes in separation of oil from synthesized oily wastewaters are presented. Mullite microfiltration (MF) membranes were synthesized from kaolin clay as MF ceramic membranes. The rejection of total organic carbon (TOC) for the synthetic feeds was found to be more than 94% by these membranes. Hermia's models were used to investigate the fouling mechanisms of membranes. The effect of pressure, cross flow velocity (CFV), temperature, oil concentration, and salt concentration on flux decline were investigated. The results showed that the cake filtration model can well predict the flux decline of mullite ceramic membranes; average error of this model is less than 7%. The results show that by increasing pressure from 0.5 to 4 bar, porosity of the cake layer on the mullite membranes decreases from 25.68% to 14.98%. After the cake filtration model, the intermediate pore blocking model was found to well predict the experimental data with an average error less than 10.5%.     

Theoretical and experimental analysis of machining errors on small arced corners during WEDM finishing stages

Abstract

Accurate machining in small-radius paths is a challenge associated with Wire Electrical Discharge Machining (WEDM). This article experimentally and theoretically analyzes the machining errors of the arced paths through successive machining stages. The machining errors of a three-stage WEDM on both straight and arced paths are first experimentally analyzed. Mathematical expressions are derived to relate new theoretical concepts, including spark angle and spark density, for each finishing stage on both straight and arced paths. Then, the effects of these concepts on machining errors of the finishing stages are determined. The causes of the machining errors of the first and second finishing stages on male and female arced paths are theoretically analyzed, and a novel mathematical methodology for the prediction of these errors is developed. The experimental machining errors of the first and second finishing stages on the different arced paths are compared and evaluated with related theoretical ones. Results reveal that the mathematical methodology predicts and compensates the machining errors of the first finishing stage with the accuracy of 78% and of the second finishing stage with the accuracy of 83%. There is a good improvement which can be employed in WEDM applications and to increase the wire electrical discharge (WED) machine capability.