PSI - Issue 5

Alejandro Carvajal-Castrillón et al. / Procedia Structural Integrity 5 (2017) 729–736 Alejandro Carvajal-Castrillón/ Structural Integrity Procedia 00 (2017) 000 – 000

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inscribed inside it, lowering the cost of complex systems, as described in Rajan (2015). Also, they are non-invasive, passive and robust, which make them suitable for monitoring applications on many different materials, specially composites. Recent researches have been done using FBGs for SHM in the aerospace field: Kim et al. (2013) performed strain measuring with damage and acoustic emission monitoring on a solar-energy powered aircraft ’s wing structure. The composite material airframe was subjected to static loads on a solar-light rich environment, where the acoustic signals generated by cracks and component strains were detected by FBGs, allowing to effectively identify and locate structural damages. Li et al. (2014) placed FBG sensors at the superior and inferior surfaces of an active variable camber wing, gathering tension and compression strains produced by changes on wing geometry. The values obtained from the sensors where validated with the strains acquired from electric gages. These experimental data allowed to generate a mathematical model for the physical behavior of this specific structure. Kim et al. (2014) designed a data acquisition system with FBG sensors. They installed it on the wing's main beam of a light aircraft for SHM purposes. Structural strain data were gathered on ground and during various flight missions, allowing to create the aircraft’s load history . Rezayat et al. (2016) developed a SHM system on composite structures. They instrumented a CFRP plate with FBG sensors and subjected it to several impact tests, measuring strain data and determining the location of the applied forces with a minimized cost function. They proposed a new method for force estimation using an impact reconstruction model based on time-domain sensor analysis, giving the capability to locate of possible delamination areas. Wireless data transmission is a technology present on many of daily life systems and devices, like cellular communications and WiFi. WLANs are designed to connect several mobile devices like computers and cellphones in a range limited by the power of the connected radios and their antennas. These systems are designed for high data rates and allow portability between the terminals. A Wireless network is integrated by a base station, also known as an access point, though which all the data traffic flows in and out, and terminal nodes or stations that communicate with each other or with the internet by connect ing to the base station’s network. The most common standard for this kind of networks is the IEEE 802.11 which comprises a series of half-duplex modulation techniques over the air using a same basic protocol. The equipment designed with this standard use either the 2.4 GHz band or the 5 GHz band, varying from one country to another, as explained by Tse and Viswanath (2005). Some works have implemented wireless data transmission for SHM applications: Hew et al. (2012) employed a low-power consumption, wireless strain measurement and transmission system, based on strain sensors and microwave energy transmission suitable for SHM on aerospace structures. The system could wirelessly send strain data to a station computer during tests performed on a cantilever beam. Bai et al. (2014) developed an in-situ fatigue monitoring bio-inspired system, using a strain wireless sensor employing an ultra-high frequency transmitter to send data to a ground computer, performing real-time fatigue monitoring on laboratory test specimens. Kilic and Unluturk (2015) installed a Supervised Event Server Health Monitoring System (SESHM) consisting on eight accelerometer-based Wireless Sensor Network (WSN) cells on the tower of a wind turbine. They evaluated sensor's data to predict the tower's strength and structural performance. The system considered the combination of GSM and local area network technologies to transmit messages permitting to have real-time sensor data transmission to various remote ground clients. Somwanshi and Gawalwad (2015) created a WSN for SHM for civil structures subjected to severe loading and progressive environmental deterioration. The network, based on accelerometers, FBG and humidity sensors, provides data to an ARM processor which sends it to a remote computer with a ZigBee Network. Testing results from a sample civil structure were processed using a MATLAB® program. The research showed this kind of networks are suitable for analyzing structural parameters to prevent accidents on civil, mechanical and aerospace structures. 2.2. Wireless data transmission

2.3. Pattern Recognition Techniques

Pattern recognition is a group of techniques implemented in SHM for damage detection (diagnostic and prognostic) based on data, which means that it is not necessary to find a model which represents the behavior of the system. As