On the definition of the term “dependability”

Aim. Solving the task of ensuring the dependability of flexible space structures requires an unambiguous interpretation of the term “dependability”, as there is an objective need for considering each and every of the many factors that affect the operating performance. In this case, neither the parametric, nor the functional definitions of dependability given in GOST 27.002 are acceptable. The functional definition of dependability does not require a profound knowledge of the physical principles of flexible structures operation, identification and management of the factors that can cause failures, while the parametric definition of dependability does not allow for a complete parametric description of a product, as the explanation of the term “dependability” states and assumes the presence of factors that are “impossible” or “unnecessary” to characterize based on parameters.

Methods. The contradiction between the parametric and functional definitions of dependability can be resolved by means of the hypothesis of confluence of the parametric and functional approaches to dependability that implies that if all the parameters that characterize the ability of a product to perform the required functions continuously maintain their values in time in specified modes and conditions of operation, maintenance, storage and transportation, then the composite dependability indicator of such product also maintains its values in time in specified modes and conditions of operation, maintenance, storage and transportation. Under the hypothesis of confluence of the parametric and functional approaches to dependability omissions in the parametric description of a product in operation are not allowable. As a consequence, the parametric description must take into consideration not only the parameters, but also the indicators that are not technically measurable, but can be evaluated quantitatively. E.g. the probability of an event can be evaluated within the range from 0 to 1.

Results. The parametric description of a flexible structure based on all parameters and indicators that characterize the ability to perform the require functions allows expressing all values of parameters in different units and all abstract numeric values of indicators numerically to enable the “addition” of the parameters and indicator values. For that purpose, the values of each of the parameters and indicators within the specified limits are evaluated subject to the probability of being with the specified limits over the operation time. Thus found probabilities of the parameters and indicators being within the specified ranges can be reduced to a single generalized dependability indicator by using the method of dependability structure diagram that takes into consideration the functional connection between the operation of elements with a certain reliability in a specific sequence.

Conclusions. The article shows the possibility of a uniform understanding of parametric and functional dependability that are connected in terms of meaning, concepts, definition and methodology. In order to solve the flexible structures dependability tasks when every little detail must be taken into consideration, a parametric definition of the term “dependability” can be used with the addition of just two words to the definition given in GOST 27.002. As a result, the definition of the term “dependability” required and sufficient for the purpose of flexible structures dependability can be as follows: “Dependability is the property of an object to maintain in time and within the set limits the values of all parameters and/or indicators that characterize the ability of the system to perform the required functions in specified modes and conditions of operation, maintenance, storage and transportation”.

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