Structural Damage Detection Based On The Anti-Resonant Frequency And Piezoelectric Impedance

With the rapid developing global economy and modern science and technologies, as well as the improving safety consciousness and safety demands of people, the problems concerning damage detection and health monitoring of many important engineering structures have attracted wide attention in recent years, and the study on damage detection and health monitoring has become one of the most active research fields in structural engineering community. Supported jointly by the National Natural Science Foundation of China (No.50378041), the Specialized Research Fund for Doctoral Program of Higher Education (No.20030487016) and the Excellent PhD Dissertation Foundation of Huazhong University of Science and Technology (No.2004-23), in this dissertation, the studies on structural crack identification and health monitoring based on the anti-resonant frequency information and piezoelectric impedance (or admittance) have been implemented numerically and experimentally.In this dissertation, the main research works and results include:(1) A kind of damage identification techniques based on the first anti-resonance curve and its slope and curvature is proposed. The basic idea of the proposed method is: when the driving point impedances (or admittances), and the first anti-resonance curve of the beam structure, as well as the slope and curvature of the first anti-resonance curve are acquired, the presence and the location of crack damage can be identified by monitoring the presences of the breaks or jumps of the first anti-resonance curve. Once the crack locations are determined, integrated with the identified natural frequencies and frequency equation of cracked beam, the sizes of the beam cracks can also be identified.(2) Two rotational spring models used to represent beam cracks are introduced, and the validity of the models is subsequently verified by numerical and experimental examples. The vibration characteristics analysis of a cracked concrete beam based on one of the two rotational spring models is performed, and the analytical results indicate that the presence of the beam crack results in the decrease of natural frequencies, the lower natural frequencies, however, are insensitive to the beam crack, and the crack at a position induces the same change in natural frequencies as its symmetrical position of the beam. It seems to be deduced that it is not reliable to detect damage of beams only depending on the lower natural frequencies.(3) The frequency equation of simple supported beams with one crack or two cracks is deduced. Based on the antiresonance information vibration analysis and damage identification of single-crack and double-crack and multi-crack beams are numerically implemented. The results indicate that not only the presence of the crack damage and the location of moderate crack damage are identified, but also the extent of crack damage is qualitatively evaluated by using the first anti-resonance curve; the small crack damage is located by using the slope and curvature of the first anti-resonance curve; integrated with the first two identified natural frequencies and the frequency equation, the sizes of the beam cracks are also identified with a high degree of accuracy.(4) Damage identification of single-crack and double-crack and multi-crack beams based on the first antiresonance curve vibration analysis is experimentally implemented. It is concluded that the measured first antiresonance curve is close to the caculated one in genaral; by the use of the measured first antiresonance curve the moderate crack damage is identificated, however, when the number of the cracks increase, the breaks or jumps of the measued first antiresonance curve become not visible.(5) Impedance of a piezoelectric smart steel beam with a single PZT sheet is implemented theoretically. On the basis, the piezoelectric impedance of a smart steel beam with free boundary conditions at both ends is verified numerically and experimentally, and the numerical and experimental results are comparable. Experimental study on damage identification of cracked steel beams is performed based on the piezoelectric impedance technique. The presence of the crack of a steel beam with a PZT sheet is detected via monitoring the variations of piezoelectric admittances. Further, when the steel beam is furnished with three PZT sheets in different positions, integrated the antiresonant frequency in the scaning frequency range, the crack location is also identified successfully.(6) Experimental study on a steel frame structure for health monitoring is implemented based on the piezoelectric impedance technique. By monitoring the changes in piezoelectric admittances of different PZT sheets in different damage cases, it is found that the electric admittances of the PZT bonded on the joint plate vary obviously, and those of the PZT bonded on the low diagonal support vary a little, and those of the PZT bonded on the top diagonal support hardly vary. So it is concluded that the damage of the steel frame structure can be identified by using the piezoelectric impedance technique, and the piezoelectric impedance technique has the ability to detect the local damage. Subsequently, the two damage indexes, i.e. RMSDR and RMSDI, are used to qualitatively evaluate the damage extent of the steel frame structure, and the results show that the damage extent can be preferably evaluated by the use of the RMSDR index.(7) Curing process of concrete samples is monitored based on the piezoelectric impedance technique. It is found from the experimental results that with the increase of concrete ages, the resonance of electric admittance curve of the PZT sheet bonded on the concrete block gradually increases, and the extent of increase is larger in 3-6 day ages than in the next ages. From the electromechanical coupled characteristic of PZT it is known that the resonance of piezoelectric admittance is corresponding to the natural frequency of the concrete block, so the variations of rigidity of concrete block in concrete ages can be forecasted indirectly. (8) Based on distributed sensing of multi-PZT and piezoelectric impedance technique, the experimental study on crack location of a concrete beam is performed. From the experimental results it is found that the PZT sheet near to the crack is sensitive to the crack, the PZT sheet far from the crack, however, is insensitive to the crack, which further verifies the ability of PZT to detect local damage transducer and the ability to damage location. Integrated the RMSDR index the crack locations of the concrete beam are determined in different damage cases.(9) Experimental strudy on a reinfored concrete beam for load process monitoring is conducted. The RC beam is loaded on two sites of beam by the use of a distribution beam, and several PZT sheets are bonded to the bottom surface of the RC beam with certain interval, and are used to monitor the presence of cracking and the loss of rigidity of the RC beam. The experimental results indicate that the load value of cracking of the RC beam under the vertical loads can be estimated, and the cracking propagation and the loss status of rigidity of the RC beam can also intuitively reflected based on the piezoelectric impedance technique.