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A View on The Ways Design of Reliability Criteria in Structural Mechanics

Kopecký M.

Machine Dynamics Research

2010

Vol. 34, No. 3

124-132

Contemporary intensive development of technology puts ever-increasing demands on the reliability of products. The increase in the reliability level is emphasised also in transport machines and equipment. This all requires a further improvement of the method of designing and strength checking of a construction. The methods described in this paper are the ways to reach the solution goals with the maximum use of computer technology. A practical example of loading system analysis in presented which demonstrates use the special instrument to measurement of distribution the force and torsion moment in card an-joint for control purposes and uses the special instrument to generally measurement of distribution random loading parameter. In some machinery and equipment, or their elements, the problem of strength reliability is due to the present-day regulations conditioned by a fatigue process and by knowledge of a fatigue curve. A considerable part of dynamically loaded components in mechanical engineering is loaded with time-variable strength. In the case of the means of machinery , competitiveness leads to weight reduction savings and extreme operational situations cause high overloading. It is probable that the most significant cause of this unfortunate situation is the fact that so far there has not been a single universal theoretically and experimentally proved fatigue theory which would consider all the factor that have an influence on the phenomena. Apart from this, there is not a generally accepted methodology of fatigue tests. A demand to guarantee strength reliability is more important predominantly in case when a failure-free operation of a constructional element can influence the safety of human lives or when an eventual failure can bring about considerable economic losses. The variability of fatigue mechanisms in the Wohler curve belongs among the questions so far not clearly specified. It has to be said, that insufficient information about the values of the fatigue curve can lead to errors sometimes higher than 100%. Also, because of economic and others reasons, it is impossible to carry out sufficient amount of fatigue tests on a finished product in order to get reliable information about the fatigue curve. It is further connected with a choice of a suitable commutation of a fatigue failure which can also lead to different results. Random operational loading creates a stochastic process of excitation forces. A successful reproduction of the response of this random loading depends on the technical facilities. With regard to a probability character of the fatigue curve, where its exponents p, q, have a character of a random quantity, a total service life is also a random quantity characterized by a probability density. The probability that a failure might occur before a required servicelife Lp ends is given by a hatched surface. The methods described in this paper are the ways to reach the solution goals by means of a characteristic curve of strength reliability and reduced fatigue curve with the maximum use of computer technology. The application of this methodology shortens knowledge of the time to failure of mobile machine components and contributes to the safety and economy of mechanical systems. The results of its application would be presented to mobile facility elements.