Evaluating Reliability Of Complex Mechanical Systems And Developing Corresponding Software

For the most systems serving in the engineering, their parts work in the same random load environment, so their failures are not independent generally. Dependent failure is one of the most important issues for system reliability. For mechanical systems, dependent failure is an universal feature. If the dependence of system failure is ignored, the analysis and calculation of system reliability often lead an excessive error, sometimes even come to a wrong conclusion on the condition that the parts’failures of the system are independent. The impact on system reliability caused by dependent failure shoud be considered in the modeling of the mechanical system reliability.The paper proposes the method of normalizeing load used in the reliability interference model based on the principle of sampling random variables on condition that the stress and strength of all the parts of the system are both subjected to the Weibull distribution. Static system reliability model is developed based on the principle of normalizeing load.The time-dependent system reliability model is developed base on the static system reliability model with two factors namely the load frequency and strength degradation being considered. The research ideas are as follows:(1) The impact on system reliability caused by strength degradation is merely considered. Strength degradation caused by corrosion is subjected to power fuction or exponential function law, and time-dependent system reliability model can be deduced based on static model with strength degradation being considered only.(2) The impact on system reliability caused by the load frequency is merely considered. The cumulative distribution function and probability density function of equivalent load are developed with the order statistic theory when the random load acts for many times. The time-dependent reliability model of the system on the condition that random load acts for many times and the strength doesn’t degenerate or doesn’t degenerate obviously can be deduced based on the static model.(3) The impact cuased by the load frequency and strength degeneration is both considered. Under the load environment of constant amplitude cyclic stress, Strength degradation is subjected to logarithm law, and residual strength and the number of stress cycles are assumed to obey a certain equation. If working load is a constant amplitude cyclic stress, load anplitude’s probability density function describes the random of load’s magnitude. The reliability when load acts for n times is equal to the probability of system being reliable as the nth time load acts on the system. However, most mechanical structures serve under the random load. The paper uses the distribution of the equivalent value(equivalent stress amplitude) of a sample course of the random load instead of load amplitude’s distribution under the constant amplitude stress,and with the order statistic theory being used, system strength distribution can be denoted by parts’strength distribution. Time-dependent system reliability model can be deduced based on load-strength interference model with the factors the load frequency and strength degradation being considered.All the models are validated using Monte Carlo method,and the corresponding applied software which can be used to analyze the reliability of complex systems is developed.