PSI - Issue 22

Sophia Metaxa et al. / Procedia Structural Integrity 22 (2019) 369–375 Sophia Metaxa/ Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction From antiquity to the present day, the science of engineering was based on the concept of "prediction". It was always asked by engineers to design and construct a technical project that would be able to meet the operating requirements and withstand the worst possible conditions (environmental, operating etc.), at the lowest possible cost. Thus, the engineer, among other things, had to know all the elements of the environment and the function of the construction prior to even starting the design. Nevertheless, the thousands of construction failures and mishaps that have occurred to date, prove that this is not always possible. A number of elements can go wrong in one or more steps of the manufacturing process (incorrectly predicted loads, inadequate design, manufacturing defects, etc.) resulting in inability to perform before the expiry of the intended operating limit of the construction. As is easily understood, this would have been particularly difficult without accurate knowledge of the structural state of the construction, no longer on a "forecast" basis (that is, as it would have been theoretically stressed, according to the original design data), but on a realistic basis, according to the actual data in the current period. In this context, Non-Destructive Testing (NDT) of structures, such as radiography, ultrasound inspection, liquid penetrants etc. began to be introduced, that is, it was recognized that the aforementioned "forecasting" process could not in any case be 100% successful, and any attempt to approximate this percentage has led to a large increase in the cost of construction [1]. Owing to the above, a new trend was gradually introduced, which included the implementation of a regular structure inspection procedure in order to examine their status, and subsequently, on the basis of this examination, to decide on a case-by-case basis. NDTs provide the reliable (high probability of detection) and accurate (in terms of dimensions) 'periodic control' of potential structural errors created in a construction. The engineer can accurately detect the presence or absence of cracks, corrosion, delamination (in the case of composite materials) or general degradation of the material at points of interest, at a time when they deem appropriate, essentially updating the original forecast, in accordance with the actual construction data. The development of new scientific fields, such as the design of structures under the assumption of structural failures (Damage Tolerance Design Philosophy) [10], played an important role in achieving this goal. 2. Structural Health Monitoring In some cases, however, it is necessary to suspend construction for a reasonable period of time in order to enable the NDTs to be carried out, while the cost of equipment required and other costs for investigation is not low. In addition, their execution can be continuous throughout the construction period, but only periodically, and in most cases the control cannot be performed during the application of operating loads. Aided in this effort of "forecasting" by the development of electronics and optics, through which advanced, reliable and low-cost sensors were created, which were able to be permanently installed in the area of interest. In this way, the engineer from the construction phase (or later, in the case of older structures) is given the opportunity to place a sensor network at selected sectors/segments of a construction so that the end user can continuously, reliably and accurately obtain data on with its behavior when subjected to operating loads. This allows the engineer to reduce the uncertainty encountered during the initial design phase of the construction (and therefore the safety factors - and the added costs). In addition, it is possible to diagnose (immediately after loading) any manufacturing errors (failure to comply with material specifications, poor construction, etc.) which could subsequently develop into premature failure factors. And all this, without requiring construction to be withdrawn from use, as in the case of NDTs, and at relatively low direct costs. Understandably, when a deviation from the actual behavior of the construction with respect to the standard is found, it is usually necessary to implementation a nondestructive investigation to identify the problem in detail, to proceed in structural repairs or restrictions on the operation of the structure. If repairs or operating restrictions are deemed impossible or economically unprofitable, then the construction should be permanently withdrawn. In order to ensure the structural integrity of the structures and thus maintain the safe operation of a structure, various techniques for monitoring the "health" and use - behavior of the construction are applied in many areas of engineering. The integrity of a structure is directly related to the functionality and performance of the structure and is, therefore, one of the key factors in its safe operation. Accordingly, a possible change in the structural health of a structure may affect its structural behavior to the extent that repairs are necessary. Structural operations in a construction impose additional costs in two ways: • Immediately due to the cost of design and implementation of the repair. • Indirectly, due to the requirement to temporarily suspend construction for repair, which means loss of revenue due to shut down.