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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The basic requirement of an optical sensor assembly is to measure a field parameter in an area of interest. 4.2. Acoustic Emissions Acoustic emissions are concerned with the creation of transient elastic waves generated by the redistribution of the stress field. When a structure is altered, displacements are created which lead to energy release in the form of waves propagating to the surface. Measurements with appropriate sensors are also obtained at these points. The detection and analysis of acoustic emissions offers useful information regarding the nucleation and severity of a failure. Control is almost always performed during the operation of the structure, therefore AEs focus on dynamic phenomena such as propagation of a crack or detachment. Of course, this second point implies the difficulty of taking measurements in high-noise environments, which adds to the signal noise [3]. A negative feature of AEs is that they can only qualitatively identify failures in the monitored structure. Other quantitative methods need to be used. Basically, acoustic emissions are generated by stresses. Trends which in turn cause deformations either elastic or plastic. The most detectable AEs are observed when the material undergoes plastic deformation. These shifts create elastic waves (ultrasound). The amount of energy, bandwidth, and waveform released by an acoustic transmission are related to the magnitude and speed that distinguishes failure. The detection and consequently conversion of these waves into electrical signals is the principle of the operation of AEs. The analysis of these signals gives useful information regarding the discontinuities within a monitored structure. 5. Analysis Methods The methods of analysis commonly used to calculate the remaining construction's life are distinguished from those employing conventional fatigue calculation methods and those involving complex fracture mechanic methods. The similarities between the above methods mainly include the input information (local trends and charging history, depending on the operating time of the component in question). On the contrary, the differences between them are found in the initial conditions and in the periodicity of the inspections required. For example, the initial condition in the fatigue method involves a "perfect" (no cracks or other imperfections) state of the material, while, on the other hand, fracture mechanics assumes that in each case there is an initial defect of a given size and shape [3]. In addition, a fatigue test is considered to be complete when failure of the specimen occurs, deriving two basic information: the number of cycles to fracture and the loads that the specimen went through in each cycle. In contrast, in an experiment in fracture engineering, in addition to these two, the number of cyclic loading required to extend the crack by a given length, is known. Additional information allows the researcher to calculate the maximum inspection intervals, given the crack behavior and expansion rate, depending on the use of a component. 5.1. Conventional fatigue calculation methods Conventional fatigue calculation methods are usually empirical and employ statistical techniques to obtain safe (but often conservative) calculations of material fatigue thresholds using S-N curves. A "safe" calculation is one that ensures that the probability of failure of the component over a given period of time is extremely low. These methods are usually based on material data collected by performing axial loading experiments of the same material, components under maximum loading and full-scale fatigue tests. The life span calculated using an S-N curve represents the time needed for a crack to initiate and extend to fracture. As the stress is cyclic, the stress ratios are determined as a function of the maximum and minimum stresses for a given experiment. 5.2. Fracture mechanics methods On the contrary, complex methods of fracture mechanics, which incorporate the philosophy of damage tolerance, consider both the initiation and propagation of cracks. The philosophy of damage tolerance is based on the assumption that all constructions, whether new or in operation for many years, contain infinitesimal imperfections, which under appropriate conditions can lead to fracture. These imperfections may take the form of material abnormalities, such as inclusions or gaps, or may occur during the manufacturing process. Both manufacturing processes and maintenance processes are determined taking into