The authors have been studying the strain sensitive materials which are based on conductivity change resulting from structural change in percolation system. In this study, we have developed a maximum strain memory sensor, which enables to detect damage to structures easily even after a large earthquake. To confirm the performance as the sensor, tensile tests embedded into concrete specimen have been conducted. As a result, it is discovered that this sensor is sufficiently effective to diagnose cracks in the concrete structure.
In recent years, the importance of Structural Health Monitoring has been recognized but an SHM system still confronts serious problems related to complexity and cost in practical use. To solve these problems, the authors have developed the simple and smart SHM system by integrating self-diagnosis material and a wireless data measurement device. By installing this SHM system, it is possible to detect damage to structures easily even after a large earthquake or other disaster and also to inspect possible deterioration of a structure in a short time. As a practical matter this SHM system is expected to be very reliable, and when it is mass-produced it should have a low cost. To confirm the utility of the damage detection of a building after a large earthquake, the pre-production system was installed in a specimen simulating the beam-to-column connection part in a mid-size conventional reinforced concrete building, and a loading test was performed on the specimen. The effectiveness of the proposed system is demonstrated by the test results.
The authors have been continuously conducting a series of research works on the development of the fiber reinforced composites as self-diagnosis materials. The function to detect damage is based on the property of carbon materials as a conductor of electricity. The conductive fiber reinforced composite, which is the glass fiber reinforced plastics added carbon particles for electrical conductivity, has been confirmed to possess excellent sensitivity as a self-diagnosis materials. In this study, a self-diagnosis material with the ability to memorize damage history has been applied. Irreversible resistance changes dependent on the strain histories of the composites were utilized to achieve this ability. The authors have also developed an electrically conductive film sensor bonded on the concrete surface to detect cracks and measure crack width. The specimens of the reinforced concrete bridge pier columns were tested under quasi-static cyclic lateral loading. The performance of the proposed self-diagnosis materials to detect damage to concrete structures is evaluated through confirmation of the relationship between the extent of damage and the variation of electrical conductivity of self-diagnosis materials. On the basis of the obtained experimental results, the applicability of self-diagnosis materials to structural health monitoring for concrete structures are discussed in detail, and the practical monitoring techniques for structures are proposed.
Health monitoring techniques that utilize structural materials with the ability to diagnose their own condition, so-called self-diagnosis materials, have been under development. The authors have developed two types of electrically conductive fiber reinforced composite to diagnose cracks in concrete structures: a high sensitivity detection sensor and maximum strain memory sensor. Three points bending tests on pre-notched reinforced concrete beam under the cyclic loading is presented using these two self-diagnosis materials, with attention towards the relationship between crack width of the concrete beam and electric resistance. Moreover, effects of volume fraction of carbon particle on memorizing maximum strain are investigated. It has been proved that both self-diagnosis materials are highly effective to detect the cracks in the concrete. And present strain can be obtained by the proposed fiber reinforced plastic composites. Although volume fraction of carbon particle has significant influence on the characteristics of memorizing maximum strain, maximum strain of the concrete structures can be memorized using the appropriate self-diagnosis materials.
Electrically conductive fiber-reinforced composites have been designed in order to develop self-diagnosis materials with the ability to memorize damage histories. Irreversible resistance changes dependent on the strain histories of the composites were utilized to achieve this ability. Conductive fiber-reinforced plastics for memorizing maximum strain were prepared by adding carbon fibers or particles into the composites. Pre-tensile stresses in composites containing carbon fibers were found to effectively enhance their residual resistance and to significantly improve the limit of smallest detectable strains. The residual resistances of composites containing carbon particles connected by a percolation structure were found to depend strongly on the volume fractions of carbon particles; composites with high volume fractions of carbon displayed remarkable residual resistance without application of a pre-tensile stress. In order to memorize cumulative damage, composites consisting of a brittle titanium nitride ceramic wire laminated with glass fiber reinforced plastics were prepared. These composites were found to exhibit remarkable residual resistances that increased in proportion to the logarithm of the number of tensile cycles. These results suggest that a simple and low cost monitoring technique without real-time measurement system will be available in wide range of applications using these composites.
The electrical properties of fiber reinforced plastics (FRP) have been investigated in order to develop structural materials with a damage diagnosis function. Electrical conductivity was achieved by adding carbon particles or carbon fiber as a conductive phase into the FRP. The composites containing carbon particles connected by a percolation structure were found to have advantages in terms of response of conductivity to small strains and the size of the detectable strain range, compared to composites containing carbon fiber. A part of the resistance change in the elongated composites containing carbon particles remained after unloading despite deformation being predominantly elastic. This residual resistance was found to depend largely on morphology of the carbon particles and orientation of the glass fiber. A distinct residual resistance was observed in composites containing spherical carbon particles (carbon black) and glass fibers aligned at an angle of 0 degrees with respect to the tensile direction. Electrical time domain reflectometry (ETDR) was used to locate the damaged region in multilayer composites containing CFRP and GFRP. The position of local damage in the multilayer composites was clearly located to a precision of within 20 mm.
We have successfully developed the computer simulation technique of modeling and design for continuous conductive structures in the self-diagnosis composite. The Monte Carlo (MC) method has been used for the simulations of the microstructures at the array of two or three dimensional lattices. The simulation results were analyzed and discussed in relation to microstructural parameters such as particle size, content, aspect ratio, etc. The computer simulation gave us important and quantitative information to obtain continuous structure of the particles dispersed in a matrix phase.
The electrically conductive fiber reinforced plastics (FRP) and ceramics matrix composites (CMC) have been designed and fabricated in order to introduce the self-diagnosis function which means the combination of reinforcement and damage diagnosis function into structural materials. The electrical conductivity was achieved by adding conductive fiber or particles into these composites. The composites with percolation structure consisting of carbon particles were found to have the advantages in response of conductivity to a small strain and in detectable strain range, comparing to the composites containing carbon fiber. A part of resistance change in the elongated composites with carbon particles remained after unloading despite its elastic deformation. The residual resistance increased with increasing applied maximum strain, showing that the composite possesses the function to memorize the previous maximum strain. The CMC materials containing TiN particles as a conductive phase indicated not only the fine response of resistance to slight deformation but also the increase in residual resistance during cyclic deformation at a constant load, suggesting that the composite have the ability to diagnose a cumulative damage through measurements of the residual resistance. These results suggest that the self-diagnosis functions peculiar to these composites are suitable for health monitoring techniques for many structural materials.
The function and performance of the self-diagnosis composites embedded in mortar/concrete blocks and concrete piles were investigated by bending tests and electrical resistance measurements. Carbon powder (CP) and carbon fiber (CF) were introduced in glass fiber reinforced plastics composites to obtain electrical conductivity. The CP composite has commonly good performances in various bending tests of block and pile specimens, comparing to the CF composite. The electrical resistance of the CP composite increases in a small strain to response remarkably micro-crack formation at about 200 (mu) strain and to detect well to smaller deformations before the crack formation. The CP composite possesses a continuous resistance change up to a large strain level near the final fracture of concrete structures reinforced by steel bars. The cyclic bending tests showed that the micro crack closed at unloading state was able to be evaluated from the measurement of residual resistance. It has been concluded that the self- diagnosis composite is fairly useful for the measurement of damage and fracture in concrete blocks and piles.
The function and performance of the self-diagnosis composites embedded in concrete blocks and piles were investigated by bending tests and electrical resistance measurements. Carbon powder (CP) and carbon fiber (CF) were introduced in glass fiber reinforced plastics composites to obtain electrical conductivity. The CP composite has commonly good performances in various bending tests of block and pile specimens, comparing to the CF composite. The electrical resistance of the CP composite increases in a small strain to response remarkably micro-crack formation at about 200 μ strain and to detect well to smaller deformations before the crack formation. The CP composite posses a continuous resistance change up to a large strain level near the final fracture of concrete structures reinforced by steel bars. It has been concluded that the self-diagnosis composite is fairly useful for the measurement of damage and fracture in concrete blocks and piles.
The electrical characteristics of fiber reinforced plastics (FRP) composites have been investigated in order to develop the self-diagnosis function suitable for health monitoring of structural materials. The electrical conductivity was achieved by adding carbon particles or fiber as a conductive phase into FRP. The self-diagnosis function of the composites was evaluated by the measurement of change in electrical resistance as a function of stress or strain in tensile tests. The resistance of carbon fiber in the composite slightly changed at a small strain level and increased nonlinearly with the applied stress due to the fracture of carbon fiber. The conductive FRP composite containing carbon particles had high sensitivity and linear response of the resistance in a wide strain range. In the cyclic loading tests, the phenomenon of residual resistance was observed at an unloading state in the composites with carbon particles. The residual resistance increased with an applied maximum strain, showing that the composite with carbon particles possesses the function to memorize the applied maximum strain or stress. These results indicate that the FRP composite containing carbon particles has a promising possibility for simple diagnosis of dynamic damage and for damage hysteresis with high sensitivity.