from other tested beams. Four cracks were observed on the tested beam but only three TL signal jumps were detected. The first TL signal (0.14 V) was observed at time 109 s when the beam was loaded to 50 kN (about half its strength). The second TL signal (0.09 V) was detected at time 123 s at a load value of 57 kN. Shortly before failure at time 207 s and at a load of 95 kN, the highest TL signal value of 0.18 V was detected. The beam eventually failed 30 s later at time 236 s and at a load of 105 kN. The continued loading of the beam after the occurrence of the third TL signal resulted in increase in the crack width thereby permitting more ambient light to be detected as indicated by the gradual increase in the TL noise level from time 206 s (Fig. 7.9a). Visual inspection of the sample after the test showed that only two of the four cracks passed through the sensitized section of the integrated ITOF sensor (Fig. 7.9b). In addition, the second TL signal (with the least TL signal value of 0.08 V) must have been caused by the non-contact excitation of the ITOF sensor by a crack that propagated about 20 mm away from the end of the nearest coated section. The fourth crack generated no detectable TL signal because it propagated across the ITOF sensor about 81 mm from the end of the nearest coated section. The above points to the viability of the ITOF sensor for structural health monitoring. Localized damage (cracks) that are indicators of the commencement of structural degradation were successfully detected as shown by the TL signals detected during loading. Furthermore, warning of impending structural failure was given by the integrated ITOF sensor. There was a big jump in the TL signal to 0.18 V at time 207 s before the failure of the beam. There was also an increase in the noise level indicative of crack opening that will lead to structural failure. This observation holds promise for crack width monitoring which is critical for corrosion prevention. 7.5 Fiber Reinforced Composites with In-Situ Damage Monitoring Capability Fiber reinforced polymer (FRP) has many desirable properties such as high strength, high modulus, and low density [27]. These have fueled a steady growth in the use of these composites in critical engineering structures like air crafts and wind blades [28, 29]. FRP are however very prone to barely visible impact damage (BVID) and this necessitates the need for insitu and distributed damage monitoring system. 7.5.1 Experimental 7.5.1.1 ITOF-CFRP Panel Fabrication and Low Velocity Impact Test ITOF sensors with the entire length coated with TL materials were fabricated and used for this study. The ITOF sensors were then integrated into carbon fiber reinforced polymer during the composite fabrication to create the ITOF-CFRP composite parts as highlighted in Fig. 7.10. Six layers of carbon fiber 12 K plain weave (Fiber Glast Development Corp., USA) fabric Fig. 7.9 (a) Multiple TL signals from multiple cracks providing early warning before structural failure of RC beam, (b) Base (tension side) of tested beam with cracks not being detected by strain gage but detected by integrated ITOF (black horizontal lines indicate position of integrated ITOF sensor) [26] 62 D.O. Olawale et al.
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