Stochastic analysis of nonlinear vehicle systems using a generalized harmonic linearization technique

The discrete harmonic linearization method which is applicable to stochastic systems could not provide an equivalent stiffness coefficient for a nonlinear restoring element. In this paper, this technique is generalized to obtain linear representations of both nonlinear restoring and damping elements, based on a principle of energy similarity of dynamic elements. A numerical iterative procedure is presented to compute the local linear coefficients of nonlinear dynamic elements. A nonlinear system is then represented by a set of its complex frequency response function matrices, as functions of the excitation frequency. Stochastic analysis of general multi-DOF nonlinear vehicle systems is established in terms of response PSD characteristics.     

Selection of optimal set of weights in a layered network using genetic algorithms

Genetic algorithms represent a class of highly parallel robust adaptive search processes for solving a wide range of optimization and machine learning problems. The present work is an attempt to demonstrate their effectiveness to search a global optimal solution to select a decision boundary for a pattern recognition problem using a multilayer perceptron. The proposed method incorporates a new concept of nonlinear selection for creating mating pools and a weighted error as a fitness function. Since there is no need for the backpropagation technique, the algorithm is computationally efficient and avoids all the drawbacks of the backpropagation algorithm. Moreover, it does not depend on the sequence of the training data. The performance of the method along with the convergence has been experimentally demonstrated for both linearly separable and nonseparable pattern classes.     

Identification of factors that lead to perceived learning improvements for female students

Past research has shown that females have more negative attitudes toward engineering and technology than do males. These negative attitudes may explain the decreasing number of females choosing technical careers. Past studies have shown that a change in learning environments and the methods by which learning takes place might foster a change in this situation. A multimedia case study incorporating a real-world engineering and technical problem faced by a power plant was developed in order to provide a new learning environment for engineering and business students. This research investigates whether the use of this material by female and male students led to differences in perceived higher level cognitive skills and, if so, seeks to identify the factors that cause the difference. The results suggest that when designing new learning environments, it is important for the female students to be challenged and have opportunities both to learn by themselves and to learn from others. These results have implications for teaching programs, such as the provision of opportunities for group learning, especially for female students.     

Comments on `Enumeration of all minimal cutsets for a node pair ina graph'

The commenters point out that the algorithm of S.A. Kumar and S.H. Lee for the enumeration of all minimal s-t cutsets is defective (see ibid., vol.R-26, p.51-5, April 1979). It is illustrated through an example that their algorithm misses several minimal cutsets. The commenters cite two reasons for this: the adjacency problem and the back-vertex problem     

Tensile creep study of copper with alumina dispersions prepared by high rate physical vapour deposition

Mott has suggested that the ideal creep-resistant material will be one with a fine grain size in which the grain boundaries are filled with some substance, say a refractory oxide, to inhibit the motion of grain boundaries. Such a system, alumina-dispersed copper, was prepared by high rate physical vapour deposition. The process parameters and their effect on structure and texture have been studied. The room temperature mechanical properties have also been reported. This paper deals with a high temperature mechanical property, i.e. tensile creep. Tests were made on a constant-stress vacuum creep rig with a Ferrometic feedthrough to ensure a zero leakage rotary seal. A vacuum of 1.33 X 10^?3 Pa was maintained. The test temperatures were 500°C (0.57T_m) and 700°C (0.72T_m). The stresses applied were 2.07, 3.45, 4.14 and 6.89 X 10⁷ Nm^?2. Tests were made on as-deposited films and on cold-rolled condensates.Minimum creep rate curves showed the effect of the alumina content in raising the creep resistance of copper. Cold rolling also reduced the minimum creep rate markedly. Varying the temperature and stress affected the shape of these curves. Stress-rupture plots were used to summarize the data.Grain refinement together with a fine stable dispersion seem to give improved creep strength. The critical barrier of the Orowan stress was noted. The stress exponent for a low alumina deposit (0.21 vol.%) was about 8, and the apparent activation energy for creep was about 202 kJ mol^?1 (0.13 vol.%). There was an inverse relationship between the rupture life and the minimum creep rate, their product being constant (about 0.2).     

Electronic absorption spectrum of Cu2 ions doped in lithium ammonium sulphate crystal

Single crystals of Lithium Ammonium Sulphate doped with Cu²⁺ ions were grown by slow evaporation technique. Electronic absorption spectra have been recorded at room and liquid nitrogen temperatures on Cary-2390 spectrophotometer. The spectrum is characteristic of a Cu²⁺ ion in octahedral symmetry. From the magnitude and and relative position of the bands a successful interpretation of all the observed bands has been made. The crystal field and tetragonal field parameters are derived. The observed and calculated energies are in good agreement.     

A computer program for automatic generation of stiffness and mass matrices in finite-element analysis

A computer program to generate automatically the stiffness and mass matrices in finite element analysis is introduced. By using programs like this, researchers only need to derive the expressions for the energies (strain or kinetic) and the expressions for the displacements or stresses and their derivatives in terms of nodal variables. Tedious calculations to obtain the matrices are avoided. Such programs would facilitate checking of existing matrices and would encourage researchers to try out new finite element models.     

Experimental investigation of oil retention in air conditioning systems

In air conditioning and refrigeration systems a small amount of oil is carried with the refrigerant and is retained in the system components. Oil retention characteristics in the condenser, evaporator, liquid and suction lines were measured and are presented and discussed here. Refrigerants R22, R410A, and R134a with miscible and non-miscible lubricants were considered to investigate oil retention physics in the widest possible range of transport properties. A parametric analysis in the suction line showed that oil retention depends on the oil mass fraction, vapor refrigerant mass flux, mixture viscosity ratio and orientation of the pipe. In the suction line, an increase in mixture viscosity of about 55% caused a rise in oil retention in the range of 50%, depending on the oil mass fraction. Oil retention in the upward vertical suction line is about 50% higher than in the horizontal line at similar conditions.