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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It is clear that early diagnosis of a failure requires lesser-scale corrective actions and has lower direct and indirect implementation costs. Alternatively, if, in addition to a timely diagnosis, there is a detailed prediction of failure progression, it may be possible to suspend the repair temporarily so that the timing of application coincides with other events which would, in any case, require the temporary suspension of the operation of a construction (eg for scheduled maintenance or seasonal work) thus significantly reducing indirect repair costs. Structural health or, equivalently, the extent of the possible structural deterioration of a structure can be ascertained either directly or indirectly. Immediately, it is done by checking for the type and extent of damage locally. Indirectly, by comparing the response of the construction to the loads on the expected construction behavior. Understandably, the application of one method does not preclude the application of the other - on the contrary, the above methods are usually complementary. At this point, it should be emphasized that in both cases the sensitivity, the probability of detection and the reliability with which the diagnosis is achieved in each case play a very important role. The above are determined by the "physical limitations" of each method, the equipment and software used, as well as the personnel involved in each diagnostic methodology. What should be emphasized is that the total cost of constructing and operating a Life Cycle Cost (LCC) is steadily decreasing to the end users advantage. This ambitious field of engineering is called "Structural Health Monitoring" (SHM) and summarizes the following steps: Step 1: Selection of locations for sensor network development. This is achieved by thoroughly calculating and simulating the expected (standard) behavior of the structure and identifying its 'sensitive areas' (eg stress areas, points of maximum deformation, etc.). It also specifies the type of sensors to be used, depending on the type of sizes measured, the precision required and the operating environment [1] (a different type of sensor will be used on the exterior surface of an aircraft to the one applied on a reinforced concrete structure). Step 2: Take measurements from the sensors. This phase takes place during the construction operation and requires, inter alia, detailed knowledge of the loads required (i.e. usually expected to receive) Step 3: Compare the measurements taken by the sensors to the standard expected and, in some cases, the maximum [2] behavior of the inspected structure. The most difficult phase that requires the integration of all theoretical and experimental data in order to evaluate the results for accurate failure detection. Initial measurement is usually performed immediately after the sensors are installed, which is compared to the corresponding magnitudes resulting from the design and calculation of the construction. The initial measurement is also the "reference measurement" against which the subsequent measurements will first be compared in order to decide whether further analysis of the results is required to draw conclusions on the test integrity of the construction. 3. Structural Integrity Monitoring Systems Due to the current economic developments in all areas of our society and particularly in the creation and subsequent implementation of complex and costly constructions, there is an attempt to maximize the cost effectiveness while maintaining or extending the originally planned lifetime. Therefore, the effort to monitor the structural integrity / optimal operation of structures both in the area of modern aeronautics and in the field of civil engineering, shipbuilding, etc. has emerged. Structural integrity monitoring systems can be divided into two main categories [2]: • Load spectrum monitoring systems: These systems record the load spectrum during operational use and calculate the cumulative fatigue damage effect using pre-existing analytical / numerical models. These systems are called Operational Load Monitoring Systems [10]. • Health monitoring systems: These systems record accurate field parameter values (reduced deformation, temperature, etc.) and are integrated within the structures, with the possibility of determining a failure event, its size, location and impact. These systems are further divided into: • Integrated, consisting of sensors, built -in logic of structural integrity and feedback capabilities to external stimuli. • Partially integrated, incorporating only sensory capabilities and logical struc tural integrity. 3.1. Monitoring Systems- The Challenges Of particular importance for the subsequent understanding of such monitoring systems is the understanding and definition of health which is the ability to operate a construction, during its service life, while maintaining its structural integrity.