Nguyen Van Pho

The general interference model in the fuzzy reliability analysis of systems

The interference model of the fuzzy reliability analysis has been the focus of some researches. In this paper, the author proposed a general interference model for fuzzy reliability analysis, which based on the safety condition, not use the stress-strength interference model. Therefore, the proposed model can apply for every reliability problems of the systems.

Formulation of the membership function and determination of the input of fuzzy loads in the structural fuzzy analyzing problem

The fuzzy analyzing process consists of different steps. In this paper, the author considers only the method for formulation of the membership function of fuzzy loads acting on the structure. Based on the membership function of fuzzy loads, the combinations of deterministic of the regression analyzing process will be determined . The membership function of fuzzy loads is selected by the triangular membership funct ion. It is in conformity with the concept on selection of loads in the design standards. The combination of inputs for thEt analyzing process will be determined, based on the number of present times of the value of input parameters (including the deterministic parameters, fuzzy parameters and the random ones) in the schema of analysis . The number of present times of input parameters is either proportional to value of the corresponding membership function or to the value of the probabilistic density function. A method for determining the appropriate combination of deterministic inputs so that each input parameter will present only one time in each combination is proposed. To illustrate the proposed method, an example on the determination of input combinat ions of tornados velocity in Vietnam is presented .

On the reliability problems of structures subjected to dynamical loads

Mechanically, the assessment of the structures safety relates to three aspects: strength, stability and oscillation. In the oscillation problems of structures, the safety conditions are conditions of frequency, amplitude, resonance, maximum displacement,etc ... In case that the structure itself contains random parameters and subjects to external loads that are also the random parameters (or random processes), the assessment of the structures safety according to the deterministic inequalities of structural mechanics will be insignificant. Therefore, this should be assessed according to the probabilistic point of view, namely, according to the reliability. The determination of reliability of the oscillation problems of structures encounters many difficulties because the outputs of the structural analysis problem are the random processes (or the random field). Meanwhile, up to now, the determination of a probability according to which a random process will belong to a given domain by mathematical method has not been sufficiently studied yet. In this paper, the authors, originating from a general definition on the reliability of a system by V. V. Bolotin, assess the reliability of oscillating structure, by determining the upper and the lower bounds of the reliability. The upper and the lower bounds of the reliability are recommended to be determined by determining the probability depending on only an inequality instead of determining the it depending on a system of inequalities. Thus, the determination is very favourable.

A new method for determination of the reliability index of distributed parameter systems

In this paper, a method to determine the reliability - index of distributed systems is proposed. By the new method, instead of finding jointly probability depending on multi - conditions of inequalities one can find probability depending on only one inequality and no requiring to know jointly density function of basic variables. Therefore, it is very favourable for the calculation of the reliability - index of mechanical systems with distributed parameter. For illustrating the proposed method, a simple example is considered

A variable separation method for the calculation of the reliability of construction

In this paper, a variable separation method for determining the reliability of construction is proposed. Therefore, the reliability is a multiflication of two probabilities: The probability of the occurrance of load and the probability of construction in the condition the load has a determined value.

Reliability index of distributed parameter system

In this paper, a method to determine the reliability index of distributed parameter systems by using a method of approximation the multi-condition dependent probability by a one-condition dependent probability is proposed. Therefore, the problem of the reliability index of distributed parameter system is transformed into the defined case. To illustrate for the method, the reliability index of air tube is considered.

On the problem of optimal structure design subjected to regular lifetime criterion

The objective function of problem of optimal structure design can be chosen differently. For example the objective function can be the weight, the net cost, the regular resistance, the regular reliability, an so on. In this paper the problem of optimal structure design, subjected to regular lifetime condition for all elements is considered. The author sets up the problem, proposes an iterative method for solving and apply the obtained results to a simple example.

An extension of the E. Melan's shakedown theorem of the elasto-plastic structure

by the lighter conditions. Here Prj(X) stands for the time independent of residual stress field (satisfying the equilibrium equations for vaniBhing loads), Q is the domain of loads; Vthe domain occupied by the body; i = {x;}the space variables [3[. In thiB paper the author shall ext and the obtained results by looking at the extensions of E. Melans theorem under an unifield point of view, and then we prove a more extended shakedown critere, and its consequences. A numerical example is given, in virtue of illu:5t.ration and comparison.

On a method for solving the reliability problem of the mechanical system

In this paper the problem of reliability is formulated in stochastic programming forms and corresponding algorithms are given. In some ordinary conditions of moebanical system, stochastic programming problems can change by that ones or the deterministic mathematical programming.