PSI - Issue 48

Snežana Stojičić et al. / Procedia Structural Integrity 48 (2023) 104 – 112 Stojičić et al/ Structural Integrity Procedia 00 ( 2023) 000–000

106

3

unsafe procedures or unsafe conditions. Engineering uncertainty states can be seen as the combined application of forensic procedures for past events and risk management for events that may occur in the present or future. Unwanted events, failures in the functioning of technical systems, or the occurrence of accidents occur most often due to design failures, errors in the realisation and/or construction of the system, inadequate materials, failure in maintenance, and errors in performing the procedures and processes of the operational use of technical systems. According to the literature, distribution of these states is: 60% design failures, 25-30% construction of the system, 10-15% inadequate materials, and 5-10% failure in maintenance (Delic et al, 2014). Errors that cause failure or inadequate functioning in technical systems represent one of the modalities of an unwanted event or condition. Undesired conditions, on the other hand, are the basis for identifying possible risks and planning responses to reduce the likelihood of their occurrence. The reliability of technical systems represents the probability at a certain level of confidence that the system will successfully function according to its purpose without failure and within the defined performance limits, taking into account the previous time of using the system, during an observed period of time. One of the most important indicators of reliability is the frequency of failures, which is most often defined as the expected number of failures in a certain time interval. In the case of a large number of technical systems, the failure frequency distribution curve can be presented in Fig. 1. The intensity of failure in relation to the time of exploitation of technical systems that are exposed to constant stress over time can be observed through three basic periods of system operation (Fig. 1). Namely, early failures due to run-in, a period of stable operation, that is, normal exploitation, and a period of increased failures, that is, an increased risk of failure. The periods of system functioning in relation to failure can be described in more detail as follows:  "Early failures" are failures that occur during the installation process and the start of production use of the system. They arise during the construction/ installation of the system, errors in production, during assembly and/or assembly, and different standards for the production of elements, especially in the part of the system operation due to running-in;  Normal exploitation, production use of the system, represents a relatively stable period of exploitation of the system. Failure of the functionality of technical systems in this phase may occur due to heavy loads, dynamic instability, and other conditions affecting them;  Increased risk of failure, due to wear and tear, occurs due to fatigue and degradation of the material, the intensity of wear, which is more pronounced in this phase.

Fig. 1. Failure intensity in relation to exploitation time: (a) early failures due to run-in; (b) period of normal exploitation; (c) terminations due to wear and tear

The causes of system failure can be classified in relation to the life cycle of the system. At the same time, the causes of failures can be classified into the periods of design, production, and use of the technical system. In terms of design, problems can arise as a result of inadequate design or structural weakness. The causes of failure in production arise as a result of manufacturing errors, while the causes of failure in use are the result of ageing, incorrect use, and incorrect handling. As a result of the inappropriate design of some elements of the system, the so-called design failure. Failure due to weakness in the structure itself, when subjected to loads, is another cause of failure in the period of early failures.