PSI - Issue 22

S.V. Belodedenko et al. / Procedia Structural Integrity 22 (2019) 51–58 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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The mean lifetime of the BF casing in the tuyere zone is 30-40% lower than in the shaft area. However, the guaranteed lifetime T 98 in the tuyere zone is slightly higher than in the shaft area (Fig.2). This is a consequence of calculating of the casing shaft as the only large element (monoblock), and the tuyere zone, in fact, is estimated as a separate compact element.

Fig. 2. Guaranteed residual lifetimes of the BF casing areas without taking into account the variation of operative time (solid lines) and taking into account the variation of operative time (dotted lines). 5. Amalgamation of elements reliability indicators of large structures (account of the BF casing safety) One of the amalgamation algorithms is the method of resource safety index β P (RSI-method). Its feature is that at first the required level of reliability P is selected, after which the corresponding lifetime T P to it is found. Traditionally, a logarithmic scale is used for lifetime. Therefore, the initial (at the beginning of operation) safety index is β 0P = lgT P . The RSI-method aims at combining individual safety indices β Рік of elements i or degradation processes k of a complex system into a general RSI β РΣ . Almost all amalgamation rules for the reliability indices of series connected powertrain systems are sensitive enough to increase the number of elements i + k. As a result, general system reliability P Σ decreases excessively in comparison with the reliability of elements P ik . The opposite picture is for systems with parallel working elements. Large structures consist of a many number of bearing elements that duplicate each other. But at the same time as the powertrain system it has signs of a series connected structure. It is difficult to develop so structural scheme, which is suitable for forecasting reliability and safety. Therefore, a large complex structure is considered as a monoblock E, on which the damaging process D operates (Fig.3, a). Variation of the parameters of such a process v D is wide enough. This leads to a decrease of the guaranteed lifetime T P , as well as safety β Р . In this aspect, monoblock E to regard as a complex of elements Ei it is perspective. On them, the processes Di are operate, the variation of which parameters v Di << v D (Fig.3, b). As a result, the guaranteed resources of the elements T Pi are projected to be larger than the total resource T P . Therefore, individual β Рі > β РΣ . However, further amalgamation of individual reliability indicators again returns us to a situation where the reliability of the system will be too reduced in relation to its elements. It was precisely to prevent such a situation and the RSI-method was developed. Although in the original version it was an alternative to the rule of multiplication of reliability for series connected systems, further studies have shown that the model of regulator in the form of criticality u ik allows the method to propagate to systems of any configuration. In addition, for systems with a much number of elements, the β PΣLND model is developed, which is a modernization of the model β PΣ = β PΣEXP . The problem of the use of RSI-method in practice lies in the ambiguity of the division of a monoblock E into the elements of E i . The situation is aggravated by multifocal (multiple sources of damage) damages of large structures. In the opinion of the authors, the such division is rationally implemented on the principle of the minimum size of the element to be repaired. Also worthy of note is the approach in which the number of elements of Ei is equal to the number of sources (points) of destruction.