Generalized age-dependent model for estimation of mean survival time in the presence of two competing risks

The paper is motivated towards developing a generalized probability model describing the longevity of a system exposed to paired risks R₁ and R₂ which are dependent. The bivariate exponential model of Freund (1961) with failure times X and Y under risks R₁ and R₂ with a time-independent hazard rate set-up has been generalized by incorporating an additional age factor, t, as a variable. The hazard rates due to R₁ and R₂ have been changed from a to α(t) = αt^α?1, and from β to β(f) = βt^β?1 where α,β > 0 which are Weibull hazard functions for α,β > 1. Further conditions are imposed such that α is changed to α' when R₂ is off and β is changed to β' when R₁ is off. The trivariate distribution of Freund so generalized has again been doubly truncated in the range a  t  b, for a, b > 0; and the conditional distribution of X and Y given t has been used to study the role of the component's age in the context of the system's survival under paired dependent risks in the finite age range.     

EFFECT OF FOOD-PROCESSING METHODS UPON SURVIVAL OF THE TREMATODE HETEROPHYES SP. IN FLESH OF MULLET CAUGHT FROM BRACKISH EGYPTIAN WATERS

SUMMARY— Mullet caught from brackish-water lakes such as Manzala Lake in Egypt were found to serve as an intermediate host for Heterophyes parasite. Infection of man with this parasite may be very heavy. causing severe symptoms. This study revealed that some food-processing methods or treatments may not be adequate to kill the parasite in the fish muscle tissues. It was found that Heterophyes could survive for 30 hr when infected fish were stored at −10 or −20°C. Viability was greater, i.e., 9 days, when stored et refrigerator temperature of 6°C. On the other hand, when fish were dipped in chlortetracycline solution (30 pbm) for 2 hr, the parasite's viability was not affected. Moreover, ice treated with oxytetracycline used for fresh fish preservation had no marked influence on the viability of Heterophyes When infected fish were grilled in a manner quite similar to that adopted by Egyptian fisherman during their fishing trips, about 20-30% of the fish contained living metacercariee of Heterophyes owing to the insufficiency of the heat treatment. Further, in mullet kept at 50 and 100°C, the parasite lived for 180 and 10min. respectively. Thus, insufficient processing or treatment of brackish-water mullet does not completely destroy the parasite, thereby rendering such fish hazardous to the health of persons consuming them.     

Modelling and implementation of a sinusoidal-pulse-width-modulated DC drive system

The increasing availability of controlled on-and-off power switching devices enables self-commutated AC-to-DC pulse width modulated converters to be efficiently and economically employed in DC drive systems. In this paper, a mathematical model for a sinusoidally pulse width modulated DC drive system is developed. The model is presented in a generalized analytical form that is applicable for any number of pulses per output period of symmetry. The limitation of the input voltage to match the requirements of the sinusoidal pulse width modulated control strategy is pointed out. The developed model and its solution procedure are verified using a transistorized self-commutated testing model, implemented for this purpose. The effects of the chopping-to-supply frequency ratio and the armature inductance-to-resistance ratio on the nature of the armature current are investigated. The general performance characteristics are also presented and investigated.     

Stability evaluation of a power network in connection with a unit regulating exciter based on system frequency characteristics

This paper presents a mathematical model (algorithm) suitable for calculating the transfer function of a practical power network. For this network the steady-slate stability limits are derived and the non-linear characteristics of the transfer function are considered. The computational results obtained by using the proposed model differ from the corresponding ones obtained previously by approximate methods, whereas they are in close agreement with similar results obtained from experimental measurements. The oscillation coefficients of first- and second-order derivatives of the generator current highly characterize the performance of the locus diagram of the generator transfer function. The proposed accurate technique is applied to a part of the Egyptian power system consisting of a power station, a substation and a 500 kV double circuit transmission line.     

EFFECT OF OXYTETRACYCLINE AND CHLORTETRACYCLINE ON SURVIVAL OF THE TREMATODE Heterophyes sp. IN THE FLESH OF MULLET CAUGHT IN BRACKISH EGYPTIAN WATERS

SUMMARY– Mullet, Mugil cephalus, caught from brackish Egyptian waters, serve as an intermediate host for Heterophyes parasite which renders such fish hazardous to health. The objective of this work was to show the effectiveness of antibiotics, used to prolong the storage life of fresh fish, as parasiticides. To achieve this, both whole mullet and small portions of infected mullet tissue were dipped in oxytetracycline (OTC) and chlortetracycline (CTC) solutions of different concentrations ranging from 30–4000 ppm. The rate of parasite destruction, very slow at low concentrations of either antibiotic, increased at much higher concentrations of antibiotic. However, a concentration of antibiotic sufficiently high to destroy the parasite completely cannot be used for the preservation of fresh fish because of the high concentration of antibiotic in the fish tissue after treatment. Although these findings are negative, they have a place in the literature.     

Practical stabilization of a class of uncertain time-varying nonlinear delay systems

In this paper we deal with a class of uncertain time-varying nonlinear systems with a state delay. Under some assumptions, we construct some stabilizing continuous feedback, i.e. linear and nonlinear in the state, which can guarantee global uniform exponential stability and global uniform practical convergence of the considered system. The quadratic Lyapunov function for the nominal stable system is used as a Lyapunov candidate function for the global system. The results developed in this note are applicable to a class of dynamical systems with uncertain time-delay. Our result is illustrated by a numerical example.     

Effect of welding heat input and post-weld aging time on microstructure and mechanical properties in dissimilar friction stir welded AA7075–AA5086

The effects of welding heat input and post-weld heat treatment on the mechanical and microstructural aspects of dissimilar friction stir welds of age-hardened AA7075-T6 and strain hardenable AA5086-H32 aluminium alloys were investigated. X-ray diffraction (XRD) residual stress analysis and tensile testing together with optical metallography and transmission electron microscopy (TEM) were performed to assess the effects of process parameters on welded joints. It was discovered that joints produced without heat sink exhibited more homogeneous stir zones than other joints. Of the natural aging time studied, higher amount of solid solution during rapid cooling of welds produced higher driving force for increase in hardness in the AA7075 side during natural aging. Natural aging within stirring zone and thermo-mechanical affected zone of AA7075 side resulted in a 10 to 25 MPa reduction in the residual stress in these zones; its effect decreased considerably in the welds performed without heat sink. In addition, natural aging had no noticeable effect on the joint strength.