Studies On Application Of Wavelet Transform In Damage Detection For Pipes

Pipeline which is one of the five leading transportation tools plays an important role in transiting liquid and gas. The wall of pipes will be thinner and thinner due to the effect of scouring and corrosion in long time, even breaks, that not only brings the enormous economical losses, but also contaminates the environment and effete the balance of ecology. So, many researchers focus on the damage detection for pipes economically and effectively.The main work of this thesis includes three espects:(1) The Damage Location Assurance Criterion (DLAC) is extended to locate and assess damage in a circular cylindrical shell based on natural frequencies and mode shapes. Frequency sensitivities computed from a defect-free finite element model are applied to calculate the theoretical frequency changes. The axial position of the damage can be easily obtained by comparing the theoretical and measured frequency changes due to damage. For the shell is axisymmetric, additional information of mode shapes is introduced to locate the exact damage position. The damage extent can be estimated by the first order approximation method. The feasibility and practicality of the damage detection scheme are evaluated for several damage scenarios by locating and sizing damage in the free-free, simply-supported and free-clamped shells, respectively. Results from simulation examples show that the proposed detection scheme can confidently identify the single or multiple cracks.(2) Wavelet transformation is a time-scale and time-frequency analysis.