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1、Applications of a distributed fiber optic crack sensor for concrete structuresK.T. Wan, C.K.Y. Leung / Sensors and Actuators A 135 (2007) 458464459sensors require knowledge of the exact crack locations 9,10,whichcannotbepredictedforconcretestructuresduetomaterialinhomogeneity.Othersensorsareonlyable
2、todetecttheintegralstrain along the gauge length 11,12. For these sensors, if theselected gauge length is too large, it is not possible to differenti-ate between one widely opened crack and many small harmlessones. If the gauge length is small, many gauges are required tocover the plausible cracking
3、 region, and this not be realistic inpractice. Brown et al. 13 and Oka et al. 14 worked on a newtechnique that measured the distributed strain along the opticalfiberwithBrillouinOpticalTimeDomainReflectometer.Cracksformedalongthefiberwillberevealedaspeaksinthestrainver-sus time plot. However, as hig
4、h local strain at a crack may leadto fiber debonding, the measured strain represents an averagevalueoveradebondedregion,thelengthofwhichishardtopre-dict (because the interfacial properties between optical fiber andconcrete is hard to characterize). Quantitative information oncrack opening, which is
5、important for durability considerations,will be difficult to obtain.Considering the limitations of various sensing approachesdiscussed above, Leung et al. 15,16 have developed a noveldistributed crack sensor based on optical time domain reflec-tometry. To detect cracks with this sensor, a-priori kno
6、wledgeof crack locations is not required. Also, a single fiber is capableto detect and monitor a number of cracks. In the following sec-tions, the sensing principle is first explained. Then, results fromrecent experimental investigations will be described to demon-strate the practical applicability
7、of the sensing concept.2. Sensing principleTheprincipleofthesensorisillustratedinFig.1.Inconcrete,a highly inhomogeneous material, it is impossible to predictexact crack locations from theoretical analysis. However, thecrack orientation can be reliably determined. An optical fibercan hence be couple
8、d to the structure in such a way that it isinclined to the crack (Fig. 1a). Once the crack opens, the opticalfiber needs to bend to maintain its continuity as shown in theinset of Fig. 1a. The bend will induce loss of light power fromthe fiber core to the surroundings. As a result, the transmittedli
9、ghtinthefibercore,aswellasthebackscatteredopticalsignal,Fig. 1. Concept of distributed sensing with the novel sensor.will both exhibit a sudden drop across the crack. By utilizingthe optical time domain reflectometer (OTDR), which measuresthe Rayleigh backscattered signal as a function of time, we c
10、anlocate the crack positions from the time corresponding to thesignal loss (distance=timelight velocity in the waveguide)andthecrackopeningfromthemagnitudeofthedrop.Atypicalplotofthebackscatteredsignalversustime,bothbeforeandaftercrack formation, is shown in Fig. 1b. Before any crack forms,the gradu
11、al declination of the backscattered signal is solely dueto attenuation. At the turning points of the fiber, power lossmay also be observed. To minimize these losses, the curvatureat the turning points should be reduced. After the cracks open,sharpdropscorrespondtothecracklocationsalongthefiberwillap
12、pear in the plot.3. Experimental verification of the sensing principleAs shown in Fig. 1b, it is possible to use a single fiber todetect and monitor a number of cracks. For sensor applications,it is important to ensure that the loss across a crack is inde-pendent to losses at other cracks or bent re
13、gions (such as thefiber turning points). Theoretically speaking, if light in the fiberpropagates in single mode, the energy distribution over the fibercross-sectionissimilarbeforeandafterabent,althoughthetotalenergy is reduced. The ratio of each drop (in decibels) shouldmerelydependonthebentcurvatur
14、eatthatposition.Inourcase,the bend radii vary from less than 1mm to infinity within veryshort distance (about half of the crack opening). To check thevalidityoftheassumption,asimpleexperimentisconductedandits schematic diagram is shown in Fig. 2. Two epoxy blocks aremade as crack simulators. Before
15、the block is cast, an inclinedsteel wire with diameter slightly larger than the fiber is placedin the mold. After the epoxy hardens, the steel wire is pulledout and a hole is left for insertion of optical fiber. The OTDR(Opto-ElectronicsOFM-20)islinkedtothecracksimulatorwitha spool of fiber between
16、the OTDR bulkhead and crack simula-tor so the very weak backscattered signals from the cracks willnot be overwhelmed by the strong pulse created at the bulkheadconnection. In this case, optical fiber 3M FS-SN 4224 is used,for which the light propagates in single mode at 850nm wave-length. The crack simulators are opened horizontally one afteranother,withthesecondcracksimulatoropensfirst.Afterwards,the first crack simulator is opening and then the fiber is bent
