ULTRASONIC GUIDED WAVES NONDESTRUCTIVE DAMAGE DETECTION FOR SQUARE STEEL PIPE BASED ON SEMI-ANALYTICAL FINITE ELEMENT METHOD

Ultrasonic guided-wave (UGW) NDT technology is an efficient means for damage identification and has been applied widely in the field of detection for pipelines, railway tracks, ships and aircrafts. Besides, the dispersion curves of the guided waves in a square steel pipe are indispensable references for the integrity test of continuous structural components, which represent the frequency dependence of guided wave velocities. Unfortunately, the complete dispersion curve of ultrasonic waves in square steel pipes cannot be solved by the traditional finite element modal analysis method. To address the problem, the semi-analytical finite element (SAFE) method was used to obtain the ultrasonic guided wave dispersion curves in a square steel pipe, on this basis, a UGW-based NDT strategy is proposed. Firstly, triangular elements are adopted to perform the finite element discretization on the cross-section of the square steel pipe, and the guided wave is assumed to be in a harmonic motion along the wave propagation direction. Then, the wave equation of the ultrasonic guided waves propagating in the square steel pipe is deduced theoretically, through solving the characteristic equation, the wave number and frequency can be obtained, and the relation between the frequency and phase velocity & group velocity is obtained; thus, the dispersion curves can be plotted, which can be used to analyze the vibration characteristics of the guided wave modes. Afterwards, the optimal excitation frequency, excitation direction and excitation location are selected based on dispersion property for the different damage modes of the square steel pipe. Lastly, the proposed damage identification method is validated through numerical simulation. The results show that the dispersion curves of square steel pipe solved with the semi-analytical finite element method are in good agreement with the simulated result, besides, for the damage on the square steel pipe surface, the reflected guided wave package can identify the damage location effectively under the selected excitation.