PSI - Issue 5

K.S.C. Kuang et al. / Procedia Structural Integrity 5 (2017) 1168–1175 Dong Yang et al. / StructuralIntegrity Procedia 00 (2017) 000 – 000

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et al., 1998, Sohn et al., 2002, Farrar and Worden, 2007, Fan and Qiao, 2010). Of all the various types of damage and defect, in engineering structures, crack is arguably a key threat to the safety of structures. Crack induced by fatigue loading or corrosion is a very common problem in aged structures, and should be detected and monitored or repair early. Thus, many crack monitoring techniques which should provide early warning is highly desirable. Conventionally, numerous methods has been used to detect cracks such as ultrasound, acoustic emission (AE) and eddy current techniques and others. Ultrasound method is one of the most popular methods for crack inspection. Tua et al. (Tua et al., 2004) used Lamb wave which was actuated by a piezoelectric transducer (PZT) to detect crack in plate structures. Ohara et al. (Ohara et al., 2008) introduced a method to investigate the initiation and propagation of crack using a sub-harmonic phased array ultrasonic testing, while Roberts and Talebzadeh (Roberts and Talebzadeh, 2003) studied the use of AE technique to monitor fatigue crack propagation in steel, welded steel compact tension specimens and T-section girders. Weekes et al. (Weekes et al., 2012) established the probability of detection for eddy therm inspection of small fatigue damage in laboratory-type metal beam specimens. In recent years, sensors such as optical fibres have been increasingly applied to monitor various engineering structures and the potential of opt ical fibre sensors to monitor a variety of structural health parameters such as strain, temperature in various engineering structures has been published by many scholars. The advantages of optical fibre-based sensors are well-known which include their light weight, being spark-free, immunity from electromagnetic interference, minimal intrusiveness due to their relatively small size and they do not corrode. In the literatures, a number of optical fibre sensors have been proposed to monitor the crack in structures, and reviews of these approaches have been given elsewhere. For example, Rossi and Maou (Rossi and Maou, 1989) embedded optical fibres in concrete to detect cracks based on the principle that light signal can be interrupted by fibre breakage due to the appearance of cracks. Bao (Bao, 2012) monitored the crack by fibre Bragg grating sensors. Bao et al. (Bao et al., 2013) proposed a on-line fatigue crack detection method based on FBG crack monitoring test bed. The intensity-based optical fibre (IOF) sensor used in crack monitoring is very attractive especially in the view of its cost and simplicity in installation and maintenance. It is easy to construct using simple and low cost instrumentation such as LEDs and photodiodes. The IOF sensor represents one of the earliest and perhaps the most basic type of optical fibre sensor that has been used for SHM (Kuang and Cantwell, 2003). In applications the IOF sensor has showed excellent ability to monitor the oscillatory response under dynamic loading in real time(Kuang et al., 2004). POFs, compared to their glass-based counterparts, offer simplicity in handling, ease of termination and lower cost. Moreover, POFs are much less fragile, making it easier to install in harsh environments. Indeed, recent research using POFs has highlighted their potential as versatile and highly cost-effective sensors for SHM (Kuang et al., 2009, Peters, 2011, Bilro et al., 2012). In this paper, a integrating intensity-based POFs sensor setup is applied for real-time monitoring initiation and propagation of fatigue crack. IPOFs sensor is introduced for the first time to monitor crack initiation and propagation at the same time. This system is validated through a laboratory experiment where its efficiency and accuracy are evaluated by comparing its results to those obtained through the