P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-C2-Drv January 12, 2002 1:0 296 CURE AND HEALTH MONITORING Resin Resin Resin Base (a) Transmission type (NIRS-based) Base Optical fiber Stripped fiber (High index) Optical fiber (b) Evanescent type (NIRS-based, Fluorimetry-based, Transmission-type for Index-based) (c) Distal end type (Fluorimetry-based, Reflection type for Index-based) Optical fiber Figure 6. Constructions of sensing parts of NIRS-based, fluorimetry-based, and index-based fiber- optic sensors (a) transmission type; (b) evanescent type; (c) distal end type. to determine the properties of the polymer’s optical absorp- tion to select a proper measuring instrument. For instance, monitoring the cure of the epoxy–amine resin system re- quires the 1500 to 1700-nm range, which includes the ab- sorption bands of epoxy, amine, C–H, and O–H groups (4,5). The sensing part of the optical fiber is fabricated so that the light propagates through the polymer. Two configu- rations of fiber-optic sensors are suggested (4–8). One is a transmission-type sensor, and the other is an evanescent- type sensor. The transmission-type sensor has a simple structure, in which the sensor has a gap, as shown in Fig. 6a. A configuration that uses a bore metal capillary is proposed to fix the input and output fibers (6). When the sensor placed in liquid polymer, the gap is filled by it. Then, light propagates through the polymer in the gap. The evanescent-type sensor consists of a fiber, which has a stripped cladding region, as shown in Fig. 6b. An evanes- cent wave is light transmitted in the cladding of the fiber. In the stripped region of the evanescent-type sensor, the evanescent wave transmits in the polymer instead of in the silica cladding of the fiber. The refractive index of the fiber core must be larger than that of the resin to propagate light in the stripped region (4). An example of the applica- tion of an NIRS-based sensor is shown in Fig. 7. The figure shows that the absorption peak of epoxy decreases due to a decrease in epoxy molecules from cross-linking in the epoxy–amine resin system in the curing process. Note that the behavior of absorption peaks is sometimes complex due to the overlaps of peaks related to different molecules. The use of neural network analysis has been proposed to im- prove the difficult quantitative analysis of spectra (9). Fluorimetry is an optical spectroscopic technique that measures the molecular or atomic composition of a liquid, gas or solid by using ultraviolet (UV) light or X rays. This technique is based on the photoluminescent phenomenon that incident light irradiates fluorescent materials. The fluorimetry-based fiber-optic sensor uses this phenomenon for monitoring the cure of the resin (7,8,10). When UV light is incident on a liquid resin mixed with a fluores- cent curing agent, the curing agent absorbs the UV light and emits short-wavelength visible light (400–600 nm). The fluorimetry-based sensing system has UV light source and two wavelength-scanning filtersfortheexcitatorylight and the emission light, and a photo detector (Fig. 8). The emission spectra are scanned by fixing the excitatory wave- length at the absorption wavelength of the fluorescent ma- terial, and the excitation spectra have a fixed emission wavelength, which has maximum emission intensity. In the curing process, the peak position and the intensity of 1500 Absorbance 0.8 0.9 1.0 1.1 1.2 1520 Wavelength (nm) 1540 1580 Figure 7. Overlaid optical fiber evanescent wave spectra obtai- ned during the cure of Epikote 828 and hexanediamine at 40 ◦ C (6). P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-C2-Drv January 12, 2002 1:0 CURE AND HEALTH MONITORING 297 Grating Grating UV light Lens Detector Molding Optical fiber PMC product Absorbed light PMC Emitted visible light Optical fiber Figure 8. Schematic of fluorimetry-based fiber-optic sensor system for monitoring cure. the spectrum are changed due to changes in the chemical structure of the fluorescent curing agent. The peak shifts of the spectra provide a quantitativemeasurementof the cure state. Fluorimetry-based sensors have an evanescent-type sensor and a distal end-type sensor as shown in Fig. 6 b,c (7,8,10). The construction of an evanescent-type sensor is similar to that of the NIRS-based sensor. A distal end-type sensor has a flat end where the light leaks out. An example of cure monitoring by using a fluorimetry-based fiber-optic sensor is shown in Fig. 9 (8). The figure shows the peak of excitatory spectra shifts in the curing process. The use of a sapphire optical fiber for the evanescent-type sensor has also been reported (7). A refractive-index-based fiber-optic sensor measures changes in the refractive index of a polymer from the in- tensity of light. There are two types of construction for the sensor, a transmission-type sensoranda reflection-type sensor, as shown in Fig. 6b,c (5). The transmission-type sensor used in the index-based sensor is similar to the evanescent-type sensor used in the NIRS-based sensor and the fluorimetry-based sensor (4,5). The transmission- type sensor that uses a polymer core fiber has also been proposed since the late 1980s (11). A light propagates in the 1. 0 361.28 2. 7 366.08 3. 13 372.8 4. 25 377.6 5. 44 380.48 6. 156 383.36 t(min) λ(min) 22 nm DGEBA/DDS Cured at 180°C Wavelength (nm) Normalized intensity 328 .100 .300 .500 .700 .900 344 360 12 3456 376 392 Figure 9. Excitatory spectra of DGEBA-DDS epoxy obtained in situ at 180 ◦ C as a function of cure time (spectra plotted without regard to intensity) (8). fiber core by reflecting at the boundary between the fiber core and the resin in the stripped region. The reflection- type sensor uses Fresnel reflection at the cut end of the fiber, which contacts the polymer (5,12,13). The changes in the intensity of light result from changes in the reflection rate at the boundary between the fiber core and the resin. The reflection-type sensor requires a simple, low-cost op- tical system that uses a silica fiber for communication, so the cost is much lower than that of spectroscopic monitor- ing methods. However, note that the long-term stability of optical devices that include a light source and a detector is essential for stable and low S/N measurement during cure. Figure 10 shows the experimental measurements of the two types of refractive-index-based sensors during cure (6). The figure shows that the curve of the reflection-type sensor is inversely proportional to that of the transmission- type sensor. Because most fiber-optic strain sensors are sensitive to temperature, they can also be used for measuring tem- perature. Several kinds of fiber-optic strain/temperature sensors are discussed later. An extrinsic Fabry–Perot in- terferometer (EFPI) sensor, a fiber Bragg grating (FBG) sensor, and an interferometric sensor are commonly used for in situ monitoring. These sensors were developed orig- inally for health monitoring, and therefore, they can be used after the manufacture of products. An EFPI sen- sor is constructed from two optical fibers that are fixed in a capillary tube and have half-mirrors at the ends of the fibers (Fig. 11). The two mirrors comprise a multiple ray interferometer in the capillary tube, which is called a Fabry–Perot interferometer. There are two measurement systems for EFPI sensors. One uses a narrowband light source, and the other uses a broadband light source. The former is cheaper and is used for high-speed measurement but is stronglyaffected by the optical powerlossin the fiber- optic guide. The loss is a problem for cure monitoring be- cause high pressure is applied to PMCs in the manufactur- ing process. The latter is independent of the optical power loss due to the capability for absolute measurement of the cavity length in the wavelength domain (14). Therefore, the latter system is more suited to monitoring in the manufac- turing process. Most of the commercial EFPI strain sensors have low thermal sensitivity because the gauge length is about 20 times as long as the cavity length. Then, the ther- mal effect on EFPI strain sensors is sometimes negligible for strain measurement. There are several applications for monitoring strain or temperature in the curing process. The residual strains in a pultruded composite rod in the P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-C2-Drv January 12, 2002 1:0 298 CURE AND HEALTH MONITORING Figure 10. Cure data obtained from sin- gle-wavelength back-reflection (reflection- type) and stripped cladding (transmis- sion-type) optical fiber sensors during the cure of Epikote 828 and hexanediamine at 45 ◦ C (6). 0 120 90 60 30 0 70 60 50 Resin temperature (°C) Sensor signal 40 30 50 100 Cure time (min) 150 200 250 Single wavelength back- reflection sensor (1310 nm) Resin cure temperature from embedded thermocouple Stripped cladding single wavelength sensor (1310 nm) pultrusion molding process were evaluated in (14). It ap- peared that the strain measured in FRPs by using EFPI sensors could be used for cure monitoring in an autoclave molding and an FW molding (15,16). The thermal sensi- tivity of the sensor for temperature measurement can be maximized by bonding the capillary tube to a high CTE (coefficient of thermal expansion) material such as alu- minum (17). An FBG sensor has a longitudinal periodic variation in its refractive index in the core of a single-mode fiber (Fig. 12a). The wavelength shift of the reflected light from the Bragg grating is proportional to the strain varia- tion. This absolute measurement technique is affected by strain and temperature change. The effect of temperature on strain measurement by an FBG sensor cannot be negli- gible during cure at high temperature. It was reported that FBG sensors embedded in CFRP and GFRP composites can detect the onset of vitrification of the resin during cure (18). An FBG sensor for temperature measurement can be man- ufactured, so that it is sensitive only to temperature, by making a sensing part free from strains, as shown in Fig. 12b (19,20). Simultaneous measurement of temper- ature and strain by FBG sensors are of major interest, and the studies are described in the section on health monitoring. Figure 11. Schematic of an EFPI fiber-optic sensor. Gauge length Reflected light from first mirror Adhesion Reflected light from second mirror Cavity length First mirror Second mirror Incident light Optical fiber Capillary tube Dielectric Sensors for Cure Monitoring Most polymers are nonconductive but have a little con- ductivity in the liquid state. Therefore, the electric prop- erties of polymers provide useful information about the cure state. Dielectric measurement techniques for poly- mers have been investigated since the 1960s. The appli- cation to monitoring cure started in the 1980s, and micro- dielectric sensors have been developed especially for in situ cure monitoring. This measurement technique is based on the method for measuring the complex dielectric constant of conductive materials. The real part ε  and the imagi- nary part ε  are called relative permittivity and loss fac- tor, respectively. The basic components of dielectric sensing are a voltage source and two electrodes. A micro dielec- tric sensor has an electrode pattern printed on a small, thin base plate, as shown in Fig. 13 (21). When the sen- sor is covered by resin, it can be assumed that the sensor and the resin comprise an equivalent RC electric circuit. Consequently, when a sinusoidal voltage is applied to the circuit, the sinusoidal current generates with a lag of phase angle δ. Then, the resin capacitance C, the resin resistance R, and tanδ can be obtained simply from the current out- put. The complex dielectric constant is represented by the P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-C2-Drv January 12, 2002 1:0 CURE AND HEALTH MONITORING 299 Broadband light Reflected light λ λ λ Bragg grating (a) Core Optical fiber Transmitted light Schematic view of an FBG sensor Adhesion FBG sensor (stress-free) Capillary tube Schematic of an FBG temperature sensor (ref. 17,18) (b) Figure 12. Schematic of an FBG fiber-optic sensor (19,20). following simple form: ε  = C/C 0 , and ε  = 1/RωC 0 , where C 0 is the capacitance of a free space capacitor and ω is the angular frequency of the voltage source. The previous relationship indicates that the loss factor depends on the frequency. The loss factor consists of both a dipole orien- tation and a free charge migration. Hence, the loss factor is expressed as a linear combination of the contribution of dipole polarization (ε r − ε u )(ετ)/(1 + ω 2 τ 2 ) and the con- tribution of free charge migration σ/ωε 0 . Here, ε r is the relaxed permittivity, ε u is the unrelaxed permittivity, ε 0 is the permittivity in vacuum, τ is the relaxation time, and σ is the ionic conductivity defined as σ = ε 0 G/C 0 . The con- tribution of dipole polarization is negligible when ωτ  1 at low frequency which is generally less than 1 kHz (22), and then the ionic conductivity can be calculated from the equation σ = ωε 0 ε  . The ionic conductivity is conve- nient for estimating the cure state because it has a strong relationship to the mobility of ions in polymers. The resis- tance 1/σ is called the ion viscosity, and the logarithmic value is used also for the estimate. The behavior of the ion viscosity is similar to that of the viscosity before the gel point. Figure 14 shows that the behavior of the log ion vis- cosity of a graphite/epoxy composite is qualitatively similar Area : 3 mm × 3 mm W : 0.24 mm GAP : 0.15 mm A A′ Electrode (Cu) W Gap t Cu = 35 µm t Si = 0.2 mm Substrate (Si-varnish) < AA' section > Figure 13. Schematic of a micro dielectric inter- digital sensor (21). to that of the mechanical viscosity up to the gel point, but the difference increases after the point (23). A comparison of the DOC data from DSC and the dielectric measurement of an epoxy resin is shown in Fig. 15. It is evident that the DOC from the dielectric measurement does not have a linear relationship to that obtained by DSC measurement. The dielectric measurement of polymers is described in de- tail in the paper by Mijovic et al. (23). Several new systems, new sensors, and new applica- tions have been proposed in recent years for in situ cure monitoring by dielectric sensors. A comparative study of three types of commercial dielectric sensors was conducted (24). It was demonstrated that the dielectric sensors used for monitoring the cure of a polymer coating can moni- tor the degradation of performance properties during use in acid, at high temperature, and in water (25). This implies the feasibility of using embedded dielectric sen- sors in both cure and use. The dielectric parameters were measured at a high-frequency range (kHz–MHz) to mon- itor dipole rotational mobility (25,26). The new parame- ter was introduced to estimate the DOC from the mea- sured dielectric parameters; the experimental data agreed well with simulation from using an analytical model and P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-C2-Drv January 12, 2002 1:0 300 CURE AND HEALTH MONITORING Figure 14. Measured resistivity and vis- cosity as a function of time during the cure of a graphite/epoxy composite (23). 0 0 2 0 30 60 90 120 150 180 4 6 8 10 6 7 8 9 10 12 11 60 120 180 Time, t (min) Log viscosity, η (poise) Log inverse ionic conductivity 240 η 300 360 420 480 540 Temperature, T (°C) σ 1 (ohm cm) σ 1 DSC data from the various temperature profiles (21). The Dielectric sensing technique was applied to process moni- toring in the SMC/BMC industry and involved cure mon- itoring and quality assurance/quality control (27). As for the impregnation process in liquid molding, it was shown that the dielectric sensors can be applied to monitoring the impregnation in resin infusion molding (28) and in RTM molding (22,29). The prediction method for the DOC using finite-element analysis from the results of dielectric mea- surement was also studied (30). Control of a curing process that had a dielectric sensing system was tried by using ar- tificial intelligence (31). Piezoelectric Sensors for Cure Monitoring Piezoelectric ceramics wafers have been employed as sen- sors/actuators for monitoring and controlling structural vi- bration. Cure monitoring using a piezoelectric wafer actua- tor/sensor started in 1997. This cure monitoring technique uses the phenomenon that the piezoelectric wafer becomes 1.0 0.8 0.6 0.4 0.2 0.0 α DSC Dielectric measurements 0 40 80 120 Time (min) Figure 15. A comparison of the degree of cure from DSC and from dielectric measurements (normalized log resistivity) as a function of time during the cure of an epoxy resin at 200 ◦ C (23). constrained by resin in the solidification during cure. Two types of measurement concepts were proposed. One is the measurement of viscosity using a PZT (lead zirconate ti- tanate) laminate that sandwiches two PZT thin films in three insulating tapes (32). Another is the impedance mea- surement of an equivalent electromechanical circuit com- posed of a piezoelectricwafer and resin (33).Theformer has individual PZT sensor and PZT actuator parts, whereas the latter uses a piezoelectric wafer as both sensor and actua- tor. The former PZT sensor was applied to monitoring the cure of GFRP laminates in autoclave molding (32). The ex- perimental results show that the output curve of the PZT sensor reflects the viscosity qualitatively and that gelation can be monitored. For impedance measurement, the system composed of a piezoelectric wafer and resin can be modeled by a series of mass–spring–damper systems that comprise equiva- lent electric circuits (Fig. 16). In the process of curing the resin, changes in the shear modulus (spring) and viscos- ity (damper) affect the electric response of the piezoelec- tric wafer. The measurement of electric response in the resonant frequency regionis carried out to monitorchanges in electric admittance at the resonant frequency and the antiresonant frequency. The increase in the modulus and viscosity of the resin reduces the amplitude of the trans- fer function, which is the peak-to-peak value. An example of transfer functions of an epoxy resin measured at differ- ent curing times is shown in Fig. 17 (34). The tempera- ture influences the capacitance of piezoelectric wafers and consequently, the magnitude of the transfer function. How- ever, the peak-to-peak amplitude of the transfer function is more sensitive to changes in the mechanical properties of Liquid Z l Z l l x Mass-spring- damping systems Figure 16. A simplified model of a piezoelectric wafer in a viscous liquid (34). P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-C2-Drv January 12, 2002 1:0 CURE AND HEALTH MONITORING 301 0.0 0.2 0.4 0.6 Curing time in air 50 minutes 59 minutes 64 minutes 67 minutes 0.8 1.0 Frequency (Hz) Tr H 20 29000 32000 35000 38000 41000 Figure 17. Transfer functions of an piezoelectric wafer embedded in epoxy taken at different curing times (34). a liquid-state resin (35). Therefore, the measurement be- fore the gelation of the resin is available. It is found that the resonance peak amplitudes of the transfer function of a piezoelectric wafer have a good relationship to the viscosity of the resin before gelation, whereas the resonant response is suppressed after gelation of the resin. Therefore, this sensor can be used only as an internal temperature sensor after gelation. This technique has the advantage that em- bedded piezoelectric ceramics can be used in operation as well as in cure. Because the peak-to-peak amplitude of the transfer function changes with respect to the contact area with liquid, it can be used for controlling the impregnation process in liquid molding such as RTM (35). Ultrasonic Measurement for Cure Monitoring The monitoring technique using an ultrasonic wave prop- agating in a material is a traditional nondestructive tech- nique for measuring modulus, density, and viscosity. This technique is also widely used for nondestructive testing of products in inspection. The ultrasonic monitoring tech- nique has been applied to in situ cure monitoring of poly- mers since the late 1980s. This cure monitoring tech- nique is based on measuring the velocity and attenuation of an ultrasonic wave propagating in a viscoelastic and anisotropic material (36). Elastic wave propagation is af- fected by changes in the modulus, density, and viscosity of a resin in the curing process. In most cases, the size of the re- inforcement of a composites is smaller than the wavelength of propagating elastic waves, so that the composites can be treated as homogenous materials. There are two meth- ods for generating ultrasonic waves in composites during the molding process. One locates ultrasonic transmitters and receivers in or on the mold, and therefore, this con- figuration has the advantage that internal sensors are not needed (37). Another method uses an acoustic waveguide that propagates an ultrasonic elastic wave (38). The wave velocity, the attenuation, and the reflection factor can be used to estimate the DOC. Sound velocity increases as the elastic modulus of a resin increases from liquid to solid in the curing process, whereas the attenuation decreases by the viscoelastic relaxation and the scattering factor. Sound velocity is convenient for evaluating the DOC because the influence of molding pressure on sound velocity is small. 0 2000 2100 2200 2300 2400 2500 2600 2700 50 Longitudinal sound velocity (m/s) 100 Sound velocity Relative attenuation 150 Processing time (s) 200 Relative attenuation (neper/mm) 250 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Figure 18. Longitudinal sound velocity and relative attenuation as a function of processing time of a phenolic-formaldehyde mold- ing compound PF31 (37). Figure 18 shows an example of measurements of longitu- dinal sound velocity and relative attenuation in the cure of a thermoset resin (37). HEALTH MONITORING Like the human body, structures deteriorate or are dam- aged in long-term use. The damages are generated by the initial defects, overload, and impacts. Structural perfor- mance such as modulus, strength, and damping is de- graded by moisture, acid, and high temperature. The dam- age and the deterioration of structures are significant problems because they often cause catastrophic accidents. However, unlike the human body, the health of structures cannot be recovered automatically. Therefore, periodic in- spection is essential to ensure the safe operation of struc- tures. The most common inspection method is visual in- spection by human eyes. It involves specimen inspection by microscopy and easiervisibleinspection techniques suchas inspection by using a fluorescent dyestuff. However, dam- ages generated in opaque materials cannot be found by vi- sual inspection techniques. In addition, undetected small damages trigger accidents due to the rapid development of damages that result from their interaction. Nondestruc- tive evaluation (NDE) techniques have been developed to detect internal or invisible damage. Traditional NDE tech- niques are ultrasonic scan, an eddy current method, X radiography, an acoustic emission method, and passive thermography. These NDE techniques are effective in detecting damages in materials and structures, but it is dif- ficult to use them in operation due to the size and weight of the devices. This means that traditional NDE techniques require field operators and transporters for heavy, large testing machines. Then, the operation must be interrupted during traditional NDE testing. Because these facts in- crease operating cost, speedy and simple inspection tech- niques are desired. Health monitoring is an attractive approach to solving the problems that occur in aged and degraded structures. The damage and performance degration are checked for maintaining the health of materials and structures. The mechanical, thermal, and chemical states in and around structures provide useful information for predicting the P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-C2-Drv January 12, 2002 1:0 302 CURE AND HEALTH MONITORING Figure 19. Concept of a smart vehi- cle that has a health monitoring sys- tem. Temperature change Damage Aerodynamic load Smart vehicle Integrated sensing system Sensors, data analyzer Remote reporting Health of structures Damage Degradation State of material Impact damage Fatigue damage Modulus Density Corrosion Strain, stress temperature service life. These values are remotely monitored by a health monitoring automated system in real time. The need for health monitoring has been growing in the fields of aircraft, space structures, and civil structures since the 1980s. The accidents and the growth of maintenance costs of aging structures motivate the need for research into health monitoring systems. The structures located in space or in the deep sea especially require real-time and re- mote monitoring systems to improving safety and relia- bility because on-site maintenance sometimes costs more than manufacturing and installing new ones. Here, we emphasize that the research area of health monitoring in smart materials and structures partially overlaps that of NDE. However, unlike NDE, a health monitoring system is naturally integrated into materials and structures by using small sensors and a powerful data analyzer. Remote monitoring is sometimes essential for practical applications. Figure 19 shows a schematic view of a vehicle that has a health monitoring system. The de- velopment of the health monitoring technique has been accelerated by advances in sensor technologies. Advanced computer technology is so powerful for analyzing moni- tored data in real-time and so small that it can be in a structure. The rapid development of the computer net- work, “Internet,” enables remote monitoring on the www (World Wide Web) using software written in a network- friendly language like JAVA. These advanced technolo- gies comprise an automated health monitoring system that can perform a self-inspection, a self-assurance of safety, and a self-report for the future. Nondestructive damage Table 3. In Situ Sensing Techniques for Health Monitoring Sensor Configuration Monitored Value Sensing Area Cost Networking Fiber-optic sensors Embed, Break, strain, Around fiber High Normal Attach vibration, (depends on temperature method) Piezoelectric sensors Embed, Dynamic strain, Middle–large Middle Easy Attach impedance Magnetostrictive No sensor Damage, static strain Large N/A N/A tagging technique (tag) Electric resistance No sensor Damage, static strain Large N/A N/A technique (electrode) detection techniques are employed for self-inspection. Safety assurance can be achieved by monitoring whether the measured values such as strain, load, or temperature go over the safety limit. In recent years, many sensors and sensing techniques have been developed for health monitoring. Representative sensing techniques are shown in Table 3. They are fiber- optic sensors, piezoelectric sensors, a magnetostrictive tag- ging technique, and an electrical resistance technique. Fiber-optic sensors and piezoelectric sensors are so small that they are embedded in materials. Fiber-optic sensors are most suited for internal measurement by embedded sensors due to their size, weight, high flexibility and long- term durability. The magnetostrictive tagging technique and the electrical resistance technique do not need any embedded sensors for in situ monitoring because the ma- terial itself acts as a sensor. These four types of sensing techniques are available for detecting internal damages. Some types of fiber-optic sensors can detect internal dam- age directly without computational identification. To de- tect damage, piezoelectric sensors use diagnostic signals, which are generated by impact or actuators. The changes in magnetic and electrical properties of conductive materi- als such as carbon-reinforced composites reflect the pres- ence and progress of damage. Note that detectable dam- age modes depend on the kind of sensors, sensing methods, and integrating configurations. Therefore, to select sensors and a sensing technique, it is important to understand the behavior of damage initiation and growth in materi- als and structures. Piezoelectric sensors can be used for P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-C2-Drv January 12, 2002 1:0 CURE AND HEALTH MONITORING 303 Table 4. Requirement and Purpose of Health Monitoring System in Engineering Fields Requirement Purpose Aircraft Light weight, reliability To maintain safe operation To reduce maintenance cost Space structure Light weight, reliability, To maintain performance insensitivity to electromagnetic field, temperature resistance, radiation resistance Civil structure Long-term durability, To reduce maintenance cost chemical resistance, moisture resistance dynamic strain measurement, and magnetostrictive tag- ging and electrical resistance techniques for static strain measurement. Fiber-optic sensors can be used to measure both static and dynamic strain. Internal temperature can be measured by using fiber-optic sensors. Here, the sensing area of the sensing techniques should be considered. Mag- netostrictive tagging and electrical resistance techniques provide large sensing areas. Size of popular piezoelectric sensors for health monitoring is several centimeters. Fiber- optic sensors have various gauge lengths according to the kind of sensor, but the sensing area is limited to the neigh- borhood of the optical fiber. Applied studies of health monitoring techniques con- centrate on aircraft, space structures, and civil structures. These fields have individual purposes for health monitor- ing systems, as shown in Table 4. The increase of aging aircraft motivates the development of health monitoring systems for aircraft to maintain safety and provide quick, low-cost maintenance. In the field of civil engineering, a heath monitoring system is expected to reduce mainte- nance cost, which grows as large civil structures increase. The health monitoring of spacecraft is an approach to com- pensate for performance when the craft is damaged. As shown in Table 4, the requirements of the sensing tech- niques are different for each of the applied fields. Aircraft and space structures require lightweight sensors and mea- surement systems because of the additional weight intro- ducing by the sensing system, which increases operating cost. A health monitoring system for aircraft and space Table 5. Fiber-Optic Sensors for In Situ Health Monitoring Multiplexing/ Monitored Value Distributing Gauge Length Sensor Cost System Cost Intensity-based Break, microbend, OTDR Short/Long Cheap Cheap a strain, vibration Interferometric Strain, temperature, Switching Long Cheap Middle vibration Polarimetric Strain, temperature, Switching Long Cheap Middle vibration EFPI Strain, temperature, Switching / Frequency Short High Middle-High vibration domain FBG, LPG Strain, temperature, Frequency domain Short High High chemical property (Easy multiplexing) Raman scattering Temperature OTDR (ROTDR) Variable Cheap High b Brillouin scattering Strain, temperature OTDR (BOTDR) Variable Cheap High b a Not including OTDR. b Includes OTDR. structures must be reliable because these engineering fields are conservative. The sensing techniques used for space structures must be insensitive to electromagnetic fields, temperature, and radiation. For civil structures, long-term durability, chemical resistance, and moisture re- sistance are required because of the long lifetimes of the structures. In this section on health monitoring, four types of sens- ing techniques are described from the viewpoint of sensor technology. In additions, the application of health moni- toring techniques to aircraft, space structures, and civil structures are also described. Fiber-Optic Sensors for Health Monitoring Fiber-optic sensors are the sensors most promising for monitoring the internal state of materials. Early studies of health monitoring of composites by using optical fibers can be seen in papers published in the 1980s (39–42). The sim- ple sensing method in these studies was based on an optical power loss by a break in an optical fiber. The quantitative monitoring of internal strain and temperature using em- bedded fiber-optic sensors started in the early 1990s. There are many kinds of fiber-optic sensors for in situ health monitoring, including intensity-based sensors, interfero- metric sensors, polarimetric sensors, EFPI sensors, FBG sensors, long-period grating based (LPG) sensors, Raman scattering sensors, and Brillouin scattering sensors, as shown in Table 5. The sensors, except for Raman scattering P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-C2-Drv January 12, 2002 1:0 304 CURE AND HEALTH MONITORING Optical switch in time domain Light source Detector Optical switching technique (a) Demultiplexing system Light source Optical fiber sensors Serial multiplexing technique (b) Distributed measurement system Distributing technique Length (c) Figure 20. Configurations of distributing and multiplexing techniques. sensors, can measure the static strain. It is difficult to apply sensing systems in the frequency domain such as absolute EFPI sensors, FBG sensors, LPG sensors, and Brillouin scattering sensors to measuring high-speed vibration. Most of the sensors can also measure temper- ature because the reflective index is sensitive to tem- perature. The distributing or multiplexing techniques for fiber-optic sensors are key techniques in making a health monitoring system practical. Three configurations, optical switching (parallel multiplexing), serial multiplexing, and distributing, are available, as shown in Fig. 20. The optical switching system is the common method of measurement that uses multiple fiber-optic sensors, but the system is not ideal due to the low switching speed, (Fig. 20a). The serial multiplexing technique is ideal for short-gauge sen- sors such as intensity-based strain sensors, EFPI sen- sors, and FBG sensors (Fig. 20b). The total weight and cost of a serial multiplexed fiber-optic sensor system can be re- duced compared to that of a system using optical switching devices dueto the simpleconfiguration andthe short length [...]... 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