A New Method And Its Application On Structural Heath Monitoring

Structural Health Monitoring is becoming a more and more important topics. Monitoring the performance of the structures in a periodic schedule under long-term excessive loadings can be used in the prudent allocation of emergency response resources after earthquakes for the government. Up to now, there are many different procedures available for engineers for Structural Health Monitoring and damage detection. They can be classified into local methods based on non-destructive detection and global methods based on vibration signal. However, the problems are that damage region must be known prior to damage detection and costly nondestructive detection instrument must be used for local method and that the parameters of undamaged structure are regarded as Structural Health Baseline (SHB) for global method. Such baseline may result in identification error due to inconsistent noise between pre-damage and after damage. So, research in this paper aims to build a new SHB not depending on the information of initial undamaged structure for structural damage detection. The results are summarized as following, 1. Definition and Establishment of Structural Health Baseline (SHB) SHB is a signal which represents perfect state of structure and can be employed in determining the existence and location of damage within a structure. The new SHB based on symmetrical principle is proposed and established. In structural design, symmetrical principle is widely used. A pair of elements which is symmetrical in geometry will behave symmetrically on dynamic characteristics (Frequency or mode shape) under undamaged state. If either of them is damaged, the two elements will behave differently on dynamic characteristics. Thus it can be seen from this phenomena that damage is located in one of the two elements. Because stiffness loss will make the frequency decrease, damage should situate in the element with lower frequency. The SHB is not based on the original parameters of the structure and only uses the on-site structural responses damage within the structure can be identified and located. 2. Verification of Structural Health Baseline (SHB) In order to prove the validity of SHB based on symmetrical principle, three steel truss test models are designed and fabricated. In one of the below pair bars, a notch with different depth is sawed as man-made damage. Strain response and local acceleration response of midpoint of the pair bars and global acceleration response of the model are measured under the pulse and sine wave excitations. Then local frequencies of the two symmetrical bars and global frequencies of the model are identified. From the local and global frequencies under different damage extent, it can be concluded that the global frequency varies very slowly but the local frequency of one of the two symmetrical elements varies relatively fast with the damage increasing. The experimental results show that damage detection is feasible by the SHB based on the symmetrical principle. 3. Crack Model and Frequency Equation in a Simple-Supported Beam Free vibration of an elastic simple beam with an opening notch crack located any where of the beam is investigated. The notch crack is simulated by an elastic torsion spring whose stiffness is taken to be finite and is determined from fracture mechanic theory. On the assumption that the crack is always open when the beam vibrates transversely, the motion equation and the boundary conditions of a simple-supported beam with a crack located anywhere of the beam is deduced. The first-order to the sixth-order frequencies varying with the crack depth and the crack location are calculated. The results show that the changes in vibration frequencies of damaged simple beam depend on the crack depth and location, but the modal frequency lies on the inflections in the corresponding modal shape because the frequency doesn't vary with damage extent at the modal shape inflection. 4. Wireless Transmitting System for Structural Health Monitoring The expensive nature of traditional structural health monitoring systems is a direct result of the high installation and maintenance costs associated with system cables. Installation of the system can represent up to 25 percent of the total system cost with over 75 percent of the installation time focused only on the installation of the system wires. In addition, in outdoor applications potentially awful environmental conditions require excess efforts spent on installing cables in weatherproof conduits thereby raising the installation costs. In order to reduce the installation costs and time, wireless transmitting system is suggested which make installing thousands of sensors on a structure become possible. 5. Casing Pipe Damage Detection Method Based on Elastic Wave The elastic wave reflections which take place at the fixed or free end of uniform medium may be considered as special cases of the general reflection and refraction phenomena occurring at any discontinuity in the medium properties. In this paper, the medium is a long steel pipe connected with many short pipes. The conditions of equilibrium and compatibility which must be satisfied at all points along the bar require that additional reflected and refracted waves be generated at the juncture or notch between steel pipes of different properties in response to the action of any given incident wave. On the basis of elastic wave reflection and refraction theory in well-proportioned medium, identifying the time difference ofthe acceleration waveforms in the steel pipes recorded damage location can be detected. Length of no damaged steel casing pipe, location of notch and compress damage are identified respectively. The following conclusions can be found from the experiment, (1) the incidence and reflection wave of acceleration waveform for short steel pipes connected with about five pipes can be seen very clearly; (2) reflection wave of damaged pipes with notch and compression is also completely identified; (3) using wave reflection method to detect damage in steel casing pipe, the length identified can reach 1000 meters; (4) the length identified depends on frequency band of incidence wave, because higher frequency decay faster than lower frequency; (5) higher resolution of distance interval relies on digital acquisition instruments whose sample frequency must be achieved at least to tens of Mega-Hz.