Minoru Tagami

Ultrasonic testing with advanced signal processing for concrete structures

This paper proposes an ultrasonic test method for detecting flaws inside concrete structures. The ultrasonic testing can be valuable in locating flaws in materials. However, there are lots of coarse aggregates inside concrete and reflections from these aggregates influence ultrasonic test results. To reduce noise due to aggregates, an advanced signal processing method using the wavelet transform was applied to the ultrasonic signals. As-received ultrasonic signals were decomposed into several frequency components, which vary in width. From the decomposed result, significant frequency components, which include flaw signals, were chosen. To emphasize the flaw signals, products of chosen frequency components were calculated. The components were weighted according to these products, and signals were reconstructed. Flaws in the 3-dimensional image become increasingly evident by processing chosen frequency components. This signal processing using the wavelet transform could be a valuable method for increasing the reliability and should help in the interpretation of the test results.

Ultrasonic Creep Damage Detection by Frequency Analysis for Boiler Piping

Boiler piping of fossil-fuel combustion power generation plants are exposed to high-temperature and high-pressure environments, and failure of high-energy piping due to creep damage has been a concern. Therefore, a precise creep damage assessment method is needed. This paper proposes a nondestructive method for creep damage detection of piping in fossil-fuel combustion power generation plants by ultrasonic testing. Ultrasonic signals are transformed to signals in a frequency domain by Fourier transform, and a specific frequency band is chosen. To determine the creep damage, the spectrum intensities are calculated. Calculated intensities have a good correlation to life consumption of the weld joints, and this method is able to predict the remaining life of high-temperature piping, which has been already installed.

Towed Marine Dipole Source for Shear Wave Generation

In this paper, we focused on the generation of the shear wave using a dipole source in water. We first made a scaled model of a deep-tow seismic survey using solid material with a set of reflectors and an overlain water layer and conducted an exploration experiment as a feasibility study. For the model, we performed both an acoustic experiment as well as numerical simulations to see if the shear reflection could be generated by a source in water without any contact to the solid surface. As a result, we confirmed that shear waves are generated at the liquid-solid interface and are reflected at the interface of material heterogeneities even with a hydrophone streamer. Numerical simulation indicates that the amplitudes of the S-to-S or S-to-P reflected would be emphasized if we use horizontal seismic sensors on the liquid-solid interface. Our experiment has encouraged us to fabricate a mock-up of Marine Seismic Vibrator (MSV) to confirm seismic signal generation and to verify that the source could be distended to multi-pole setup for shear wave generator.

Study of the exploration focused on shear waves generated by a dipole source

In this paper, we focused on the generation of the shear wave using a dipole source in water. We made a scaled model of a deep-tow seismic survey using solid material with a set of reflectors and an overlain water layer. Two dilatational sources with the opposite phase to each other are simultaneously excited to form a point force in the water parallel to solid-water interface. The sound field generated by the dipole in water creates a stress field tangential to the solid-water interface so that the shear wave is generated in the solid. For the model, we performed both acoustic experiment and numerical simulations to see both compressional and shear reflection profiles. Numerical simulation indicates that the amplitudes of the S-to-S or S-to-P reflected would be emphasized if we use horizontal seismic sensors on the fluid-solid interface. Although the signal-to-noise ratio of the acoustic experiment is not sufficiently high, we confirmed that shear reflection profile could be produced. It should be noted that shear waves could be produced in water using multipolar sources fired close to the seafloor, and that some improvements to increase shear wave energy more than the compressional wave need to be pursued for practical use of multipolar sources.