Toshihiro Ohtani

Ultrasonic attenuation peak during fatigue of polycrystalline copper

Abstract We studied microstructure evolution in a 4N polycrystalline copper subjected to zero-to-tension fatigue through in situ monitoring of shear-wave attenuation and velocity using electromagnetic acoustic resonance (EMAR). Contactless transduction based on the Lorentz force mechanism is the key to establishing a continuous monitor for the microstructural change in the bulk of metals with a high sensitivity. In a short interval, between 20 and 40% of the total life in the order of 104–105 cycles, attenuation experiences a large peak and ultrasonic velocity shows a depression, being independent of the cyclic stress amplitude. This novel phenomenon is interpreted in terms of drastic change in dislocation mobility and rearrangement, which is supported by the replication for slip bands and TEM observations for dislocation structure. At this particular period, the dense dislocation structure starts to transform to cells, which temporally accompanies long, free dislocations absorbing much ultrasonic energy to produce the attenuation peak. The possibility of remaining-life prediction is discussed.

Ultrasonic attenuation monitoring of fatigue damage in low carbon steels with electromagnetic acoustic resonance (EMAR)

We studied microstructure evolution during tension-compression fatigue in low carbon steels, containing C: 0.15 mass%. In situ monitoring of axial-shear-wave attenuation and velocity was achieved with electromagnetic acoustic resonance (EMAR), which is a combination of the resonant spectroscopy technique and a noncontacting electromagnetic acoustic transducer (EMAT). Transduction occurs with the magnetostriction effect and is the key to establish a continuous monitoring of microstructural changes in the surface region of the metals with high sensitivity. We found for the first time that the attenuation is highly sensitive to the accumulated fatigue damage, showing a minimum around 20% of the whole life. This phenomenon is interpreted in terms of drastic change of dislocation mobility and rearrangement, which is supported by TEM observation for dislocation structure. This technique has the potential to assess the damage advance and to predict the fatigue life of metals.

Line-focusing electromagnetic acoustic transducers for the detection of slit defects

This paper describes the design principles of a line-focusing electromagnetic acoustic transducer (LF-EMAT) and the results of a feasibility test for detecting slit-type defects in metals. The LF-EMAT excites shear vertical (SV) elastic waves and focuses them to a line in a metal body. It consists of a permanent magnet block and a meanderline coil, whose spacing is continuously varied so that the excited SV waves become coherent on a focal line after traveling oblique paths. The measured directivity of generation and reception show a sharp peak at the designed focal line. The LF-EMATs are then applied to detecting slit defects in the bottom surface of steel blocks, on which the focal lines are located. Portions of the scattered defect signals are received by the same EMAT. When operated at 4 MHz, the LF-EMATs are capable of detecting slits deeper than 0.05 mm. The sensitivity decreases with liftoff and the LF-EMATs are usable with liftoff up to 0.6 mm.

Nonlinear Acoustic Evaluation of Creep Damage in Boiler Heat Exchange Tubes

In this study, we evaluated creep damage accumulation in steel welds using the nonlinear acoustic method and microstructural observation. The welds used for the experiment had been employed in boiler exchange tubes of a fossil fuel power plant for 20 years. Samples were cut from the welded region so that they had two planes parallel to the longitudinal direction of the tubes. For the nonlinear acoustic method, we used the immersion technique. Burst waves of 18 MHz were transmitted to the samples, and harmonic waves were detected in the reflection to obtain the images of amplitudes. Increased amplitudes were measured around the heat-affected zone (HAZ), which corresponded to the areas of high creep void density observed by microscopy. The nonlinear acoustic method has been confirmed to have the potential to assess the creep damage.

Noncontact Evaluation of Surface-Wave Nonlinearity for Creep Damage in Cr–Mo–V Steel

A nonlinear acoustic measurement is studied for creep damage evaluation. An electromagnetic acoustic transducer (EMAT) magnetostrictively couples to a surface-shear-wave resonance along the circumference of a cylindrical specimen during the creep of Cr–Mo–V steels. The excitation of the EMAT at half of the resonance frequency caused a standing wave to contain only the second-harmonic component, which was received by the same EMAT for determining the second-harmonic amplitude. This measured surface-wave nonlinearity showed a peak at 30% and a minimum at 50% of the total life. We interpreted these phenomena in terms of dislocation mobility and restructuring, with support from scanning electron microscope (SEM) and transmission electron microscope (TEM) observations. This noncontact resonance-EMAT measurement can monitor the evolution of surface-shear-wave nonlinearity throughout creep life and has a potential to assess damage advance and predict the creep life of metals.

Line-focusing of ultrasonic SV wave by electromagnetic acoustic transducer

An electromagnetic acoustic transducer has been developed for line-focusing the shear-vertical (SV) wave in a metal. The EMAT consists of a permanent magnet to supply the bias magnetic field normal to the surface, and a meanderline coil to induce the dynamic field and eddy currents in the surface region of the sample. The meanderline spacing is continuously changed so that the generated SV waves from all segment sources become coherent on the focal line after traveling oblique paths. The printed circuit technique enables the fabrication of such a functionally spaced meanderline coil within 1-μm accuracy. The directivity is measured using a half-cylindrical sample of an aluminum alloy, which shows a much sharper radiation pattern of the focusing EMAT than the EMAT having a meanderline coil of constant spacings. This EMAT is then tested through detecting a shallow notch. The scattered signal from the notch is received by the same EMAT, which shows enough strength even for the notch of 0.2-mm depth.

Electromagnetic Acoustic Resonance to Assess Creep Damage in Cr–Mo–V Steel

Electromagnetic acoustic resonance (EMAR) is a contactless resonance method using an electromagnetic acoustic transducer (EMAT). In this study, EMAR was applied to detect the creep damage process in Cr–Mo–V steel, which is an important structural material for thermal energy plants. The material was exposed to temperatures up to 923 K at various stresses. Two types of EMAT were used: bulk-wave EMAT for plate samples and axial-shear-wave EMAT for cylindrical samples. We measured ultrasonic attenuation in the frequency range between 1 and 7 MHz as creep progressed. Attenuation coefficient exhibits a much larger sensitivity to damage accumulation than velocity. It shows a maximum peak at approximately 30% and a minimum peak at 50% of the creep life, independent of the applied stress and the type of EMAT used. EMAR has the potential for assessing damage progress and for predicting the creep life of metals.

Creep-Induced Microstructural Changes and Acoustic Characterization in a Cr-Mo-V Steel

We studied the evolution of microstructure in a Cr–Mo–V steel (JIS-SNB16) during creep by monitoring ultrasonic attenuation. After obtaining a series of creep samples with various strains under a tensile stress of 25 MPa at 923 K, we removed small samples from the creep samples and measured free vibration resonance frequencies and attenuation coefficients with electromagnetic acoustic resonance (EMAR). EMAR is a combination of the resonant acoustic technique with a non-contact electromagnetic acoustic transducer (EMAT). The attenuation measurement is inherently free from any energy loss, resulting in pure attenuation in a metal sample. Furthermore, we observed the evolution of microstructure with electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The result from the small samples shows the same trend as our previous result from larger sample. We propose a non-destructive method using EMAR to evaluate creep damage in small specimens sampled from structural metals in-service.

Creep-Induced Microstructural Change in 304-Type Austenitic Stainless Steel

We studied microstructure changes of 304-type austenitic stainless steel subjected to a tensile stress at 973 K. We monitored the shear-wave attenuation and velocity using electromagnetic acoustic resonance (EMAR). The attenuation peaks at 40% to 50% and a minimum value at 70% of the creep life, being independent of the applied stress. A drastic change in dislocation mobility and arrangement interrupted this novel attenuation phenomenon, as supported by SEM and TEM observations. The relationship between attenuation change and microstructure evolution can be explained with the strings model. EMAR demonstrates a potential for assessing damage advance and predicting the remaining creep life of metals.

Nonlinear Resonant Ultrasound Spectroscopy (NRUS) applied to fatigue damage evaluation in a pure copper

We studied a monitoring technique of fatigue damage in 3N polycrystalline copper under a cyclic zero-to-tension loading by the nonlinear resonant ultrasound spectroscopy (NRUS). In NRUS, the resonant frequency of an object is studied as a function of the excitation level. As the excitation level increases, the elastic nonlinearity is manifest by a shift in the resonance frequency. We used an electromagnetic acoustic transducer (EMAT) to monitor NRUS of bulk-shear-wave propagating in the thickness direction of the plate sample. NRUS exhibits much larger sensitivity to the damage accumulation than the velocity. It rapidly increases from 70% of fatigue life to the fracture. The attenuation shows the peak at 80% of the life. This NRUS novel phenomenon is interpreted in terms of dislocation mobility. This technique has potential to assess the damage advance and to predict the fatigue life of metals.