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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A key problem in the process of predicting and detecting failures in a structure is to measure field parameters worthy of being associated with the failure occurring and derive its characteristics. In the case of such operating systems, the predominant field component capable of describing the above is the difference in deformation. Of course, how a failure is associated with the field parameters depends directly on the monitoring technique used and the desired result. According to current developments in the field of structural integrity monitoring and safe structure operation, two main techniques are identified: the local monitoring technique and the universal monitoring technique. Local techniques are usually optical or based on local field measurements using X-rays, strain gauges, optical fibers, etc., and require prior knowledge of both the nature of the failure and the monitoring area, these require time and effort when applied to large scale projects. On the other hand, universal techniques (oscillation monitoring techniques) can monitor changes in the dynamic characteristics of a structure under the influence of dynamic loads and detect any failures in both small and large structures, but with a lesser precision compared to local techniques. It is therefore common practice to make additional use of these techniques when monitoring the structural health of a large-scale construction [2]. Constructions in the area of civil engineering are usually large-scale, spatially and structurally distributed and consist of several main building blocks. These features often make it difficult to implement local health monitoring systems and serve to implement systems based on universal techniques. The implementation of an operating monitoring system, whether universal or local, is based on the following stages/levels which are distinguished on the basis of the desired accuracy in detecting a failure: • Level 1: Confirmation of structural failure Recognizing a failure in a structure, as mentioned above, is essentially a pattern recognition problem, where a set of features of the structure, e.g. its operating stresses in a healthy state, which can be measured by appropriate techniques. Both of the above techniques (local or universal) have a common philosophy of integrating them into an SHM system. In particular, both techniques: • Are based on integration within the construction of sensing and recording capabilities of the measured field parameters, using appropriate sensors. • Require the existence of reasonable structural integrity to link them to failure and to further exploit the measured values. 4. Structural Integrity Monitoring Systems The structures’ mechanical properties deteriorate in service due to continuous loading, fatigue, aging and unforeseen loading. As the modern engineer recognizes the need for maintenance on structures, diagnostic inspections have been introduced into these structures to assess the impairment of their structural integrity. This evaluation is difficult and not fully standardized as yet [2]. The following types of sensors, which can be used to monitor structural integrity, will be briefly analyzed: • Level 2: Spatial Failure T racking • Level 3: Quantification of Failure • Level 4: Calculation of the remaining operational life Each system is evaluated according to the level it meets. Given the large surface area of the components, it is assumed that extremely large numbers of sensors will be required for SHM. Fiber optic sensors have been identified as the main technology to meet this requirement with the significant advantage of lower weight. Numerous sensor regions can be combined along a single optical fiber, mitigating the complexity and weight with the wiring required for a large number of autonomous sensors. The overriding requirement for a sensible construction is to detect the measured quantity as quickly and accurately as possible (max. Voltage, temperature, torque) while transmitting it to the data acquisition unit. At the same time, the measurement should be independent of external factors such as electromagnetic radiation, etc., having a satisfactory resolution and ease of adjustment (internal or external) to the The discretization of the optical sensors is based on the configuration of the optical sensor in relation to the carrier optical fiber [3]. Optical sensors incorporated internally into the carrier fiber are called intrinsic sensors while sensors incorporated externally or into the interface between optical fibers and other device by suitable methods (such as by fusion splicing, with soldering or other mechanical wiring), called extrinsic sensors. 1) Fiber optic networks; 2) Acoustic emissions. 4.1. Fiber optic networks