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It happens to everyone that in some conditions of loading to your body, the body couldn’t tolerate the situation and it will be injured. Body of older persons or individuals with a previous injury are more vulnerable and they are at greater risk of injury. So how you realize that your body is in danger? The nervous system that is distributed all over the body will recognize the pain and brings all the data to the brain and then after analyzing the data your brain finds out what has happened to the body and will decide the next step and will trigger an immediate command before the failure of the body. For example, if body experiences pain inside the knee and this pain signal becomes amplified, the next step is to sit, otherwise it could cause the body to fall down.
Now consider that helicopter structure could sense what is going to happen like human body and could make decision before a complete failure. It means that the structure could sense if it has been strained to the point of damage and could reduce its load-carrying capacity, then report that information in real time before the structure’s safety is compromised or in the case of propagation of damage, pilots will be informed how much time he has left for landing. For many years, such a scenario was more like the stuff of science fiction than fact, but today, structural health monitoring (SHM) systems that can perform these tasks are closer to reality.
The damage tolerance and high strength-to-weight ratio of composites have motivated designers to expand the role of advanced materials in aircraft structures. This practice, however, further complicates inspection of flaws before it causes critical situation. As a result, the aviation industry has recognized the need for more sophisticated and more innovative ways to deploy them in situations where complex structural geometries create accessibility limitations that may impede efforts to locate and identify deeply hidden flaws. “The basic idea of SHM is to build a system similar to the human nervous system, with a network of sensors placed in critical areas where structural integrity must be maintained” says Holger Speckmann, who is the Co-Founder of the SHM-AISC (SHM – Aerospace Industry Steering Committee) (Bremen, Germany) [1].
In general, SHM is a method of integrity analysis of an object via real-time measurements of a set of parameters, which characterize the object’s integrity [2]. The main assignment of SHM system is to alert system operator in case of critical structure situation such as fibre strains or breakage and matrix cracks. The other benefit of damage detection is reduction in the cost of maintenance. Operation and maintenance which can account for more than half of a rotorcraft’s total cost, but health monitoring can reduce those maintenance costs by more than 15% and reduce unscheduled downtime by nearly 50%.
The most common measurement quantity which is used to detect the presence of damage is strain. Devices used for this measurement are commonly referred to as ‘sensors’, measuring a value linearly proportional to a quantity of interest such as deformation. Currently, most sensors employed in SHM systems are either optical fibres or piezoelectric devices [2].
Practically, fibre optic sensors form the most useful class of sensors for SHM systems. Employing an embedded FBG sensors for measurement of distributed stress and strain fields of the composite material has significant advantages. First of all, FBG sensors can be easily embedded into materials to provide damage detection or internal strain field mapping. The FBG sensors supposed to be located in fiber optic cable mounted in the sample. Furthermore, the key feature of FBGs sensors is that the information about perturbations is encoded in wavelength. Indeed, this type of sensor has fantastic compatibility with composite [3].
Although SHM is still immature, research and development has already yielded promising results. It will be hoped that in the near future this system could prevent all the accidents cause by structural failure and help to reduce the cost and time of maintenance.
References:
[1] https://www.compositesworld.com/articles/structural-health-monitoring-composites-getsmart
[2] Fedorov A, Lazarev V, Makhrov I, Pozhar N, Anufriev M, Pnev A, Karasik V, Structural monitoring system with fibre Bragg grating sensors: Implementation and software solution, Journal of Physics: Conference Series, vol. 594, issue 1 (2015) Published by Institute of Physics Publishing
[3] Guo H, Xiao G, Mrad N, Yao J, Fibre optic sensors for structural health monitoring of air platforms, Sensors, vol. 11, issue 4 (2011) pp. 3687-3705