Some Problems Of Damage Identification For Straight Pipes

In view of the fact that the modal parameters of structures are insensitive to local damage in structural health monitoring, this paper discusses the method that can identify the damage location and extent according to the concept of wave motion. For the damage identification of the uniform straight steel pipe, this paper adopts the methods that combine theory analysis, numerical simulations and physical experiments to discuss the feasibility of this research direction. The details are shown as follows:1. By numerical simulation, the condition under which the wave motion in slender and straight pipe can be described with 1-D fixed shape wave, when there is a knock on a point of cross section at the end of pipe, was illuminated. The research results indicate that at a distance from the knocked end, the 3-D dispersed wave motion can be described approximately with 1-D fixed shape wave. Based on 1-D fixed shape wave, two methods which separate full wave field into one way wave field are proposed. The first one is using the two surveying records at the same cross section of the pipe (selecting arbitrarily two from the four physical records including displacement, velocity, acceleration and stress (strain)), to derive the one way wave formulae in the neighborhood of surveying point. The other is using the same physical records at two nearby surveying points to derive the decomposition equation of wave field.2. Based on the characteristics of steel pipe damage, two simplified models that are matched with 1-D fixed shape wave are proposed. The first one is continuous damage model, which is used to describe the change of straight pipe stiffness with limited length. The other is simplified crack model, which is used to describe the traverse crack, estimate approximately the free face farther field effect and match with the 1-D fixed shape wave. Based on the two models and the concept of one way wave, a simplified identification method of straight pipe damage which uses the records at two surveying points on the pipe to detect the existence and the degree of damage, is proposed. The characteristics of this method are that it is only related to the pipe between the points, and is not related to the other part of the structure. By some examples, the feasibility and sensitivity of the reverse method are demonstrated.3. A series of tap experiments which include slender straight pipe, jointed straight pipes with hoop which is relevant to continued damage model, straight pipes with traverse cut and so on, were conducted. The research results of the experiments include: First, the feasibility of 1-D fixed shape wave model was validated, and the effect of the method in which one way wave was separated from experiment records was tested. Second, the characteristics of the reflected wave from the end of pipe, joint hoop, and traverse cut were analyzed, and the damage locating methods were tested. Third, the preliminary tries for damage extent identification were made, and the future works were proposed. In addition, the HHT etc., signal processing methods were used to de-noising and damage locating.4. Combining with the damage identification of structure, the theories, methods and problems of HHT, which is a new time-frequency analysis method, were discussed systemically. By using modified FFT to make the effective bandwidth up to twice Nyquist frequency, the time-frequency analysis effect of HHT was improved.