Key Engineering Materials Vols. 321-323

Abstract: Wedge waves (WW) are guided acoustic waves propagating along the tip of a wedge, with energy tightly confined near the apex. Like Lamb waves, wedge waves with displacement field anti-symmetric about the mid-apex-plane are called anti-symmetric flexural (ASF) modes. This study is focused on exploring the dispersion behaviors of ASF modes propagating along a bilinear wedge (BW). A BW is wedge with a cross section of two apex angles, compared with a linear wedge (LW) having a single apex angle. In the literature, many studies regarding to the dispersion behaviors of ASF modes are reported for LW, but not for BW. In this study, a laser ultrasonic technique and finite element simulations are used to investigate the dispersion behavior of BW-ASF modes. It is found out that a BW-ASF mode is a result of mode coupling between the two LW-ASF modes of the same order corresponding to the two apex angles of the BW.

Abstract: Scattering analysis of Lamb waves in perpendicularly bent plates with and without a crack is presented. Single cracks on the inner or outer surface of a bent part are considered. The scattering behaviors of Lamb waves in the bent plate with or without a crack are discussed in order to investigate the potential of Lamb wave ultrasonic method for characterizing cracks in a bent part. Experimental study is also carried out to verify the numerical results.

Abstract: For the long range inspection of structures in nuclear power plant using array transducers, it is necessary to focus waves on defects under interrogation. To take care of such a need, in this study we adopt a time reversal technique that is claimed to be very robust to focus ultrasonic waves on defects. Specifically, we calculate the appropriate time delay using the time reversal technique and re-generate ultrasonic guided waves that are focusing to an interrogated defect with the calculated time delay. In this paper, we describe the principle of the time reversal technique briefly and present the performance enhancement obtained by the time reversal techniques.

Abstract: The leakage of a pipe in nuclear power plants is a significant concern from the point of view of nuclear safety. Because of the geometrical complexity of a pipe and an inaccessibility due to a high radiation, it is difficult to inspect it by the conventional ultrasonic methods. The guided ultrasonic method can be useful for the inspection of a pipe in those harsh environments. Based on the analysis of the dispersion curves for a pipe, a torsional vibration mode, T(0,1) was selected for the detection of cracks. The T(0,1) mode has many advantages, such as a higher sensitivity for a crack from the viewpoint of its non-dispersion characteristics and its wave structure. The torsional mode can be generated by using either an array of electromagnetic acoustic transducers (EMATs) technique or a magnetostrictive sensor technique. The detectability of the cracks was estimated through a series of experiments with artificial notches on a pipe.

Abstract: Rail represents one of the most important means of transportation. Many nondestructive testing methods have been used to find defects in rail. The guided wave technique is the most efficient because of its long propagation characteristic along the rail. Potential for detecting transverse cracks exists whereas standard bulk wave technique could miss the cracks. The wave structure of the rail cross-section for a particular loading condition of modes and frequencies is an important feature. In this paper, the propagation and scattering patterns of guided waves in a rail are studied using finite element methods. The wave structures are also examined. Various multiple defect situations and rail boundary conditions can also be studied.

Abstract: Rail represents one of the most important means of transportation. Many nondestructive testing methods have been used to find defects in rail. The guided wave technique is the most efficient because of its long propagation characteristic along the rail. The wave structure of the rail cross-section for a particular loading condition of modes and frequencies is an important feature. The wave structures are examined at different modes and frequencies. The propagation scattering patterns of guided waves from various multiple defects in a rail are also studied using finite element method.

Abstract: Presented in this paper is a numerical approach to extract useful features of SH guided wave scattering in a Plate-like structure with thickness variation. The problem statement is chosen to establish a physically based guideline for mode selection and data analyses in power industry application. The hybrid BEM technique with absorbing boundaries on the structure cross-sections is proposed to calculate reflection and transmission from thickness variation. Sample numerical data is presented as a function of incident mode, frequency and defect geometry. Mode optimization is proposed to enhance penetration power and quantify geometry change based on various interesting features including reverberation pattern of scattering fields.

Abstract: This study presents a feasibility of using guided waves for a long-range inspection of pipe through investigation of mode conversion and scattering pattern from edge and wall-thinning in a steel pipe. Phase and group velocity dispersion curves for reference modes of pipes are illustrated for theoretical analyses. Predicted modes could be successfully generated by controlling frequency, receiver angle and wavelength. The dispersive characteristics of the modes from and edge wall-thinning are compared and analyzed respectively. The mode conversion characteristics are distinct depending on dispersive pattern of modes. Experimental feasibility study on the guided waves was carried out to explore wall thinning part in pipe for data calibration of a long range pipe monitoring by comb transducer and laser.

Abstract: Conventional non-destructive techniques for inspection of weld in pipelines require significant test time and high cost. In this paper, a study in the application of ultrasonic guided waves to long range inspection of the pipeline is presented. The characteristics and setup of a long range guided wave inspection system and experimental results in pipes of various diameters are introduced. The experimental results in mock-up pipes with cluster type defects show that the limit of detectable wall thickness reduction with this guided wave system is 2~3% in the pipe cross section area and the wall thickness reduction of 5% in cross section area can be detected when actual detection level is used. Therefore, the applicability of the ultrasonic guided wave technique to long range pipeline inspection for wall thickness reduction is verified.

Abstract: For efficient NDE of pipes, essential components of power plant facilities, ultrasonic guided waves were generated and received applying an air-coupled transducer and comb one as non-contact technology. Mode generation and selection were predicted based on theoretical dispersive curve and the element space of a comb transducer. In addition, a receiving angle of the air-coupled transducer was determined to acquire the predicted modes by theoretical phase velocity of each mode. Theoretical dispersive curve was compared with the results of the time-frequency spectroscopes based on the wavelet transform and 2D-FFT to identify the characteristics of the received mode. The received modes show a good agreement with the predicted ones.