Tae-Hun Lee

THE NONLINEARITY OF GUIDED WAVE IN AN ELASTIC PLATE

The ultrasonic nonlinearity has been considered as a promising method to evaluate the material degradation, since it is sensitive to the minute variation of material properties. However, most researches were restricted to the nonlinear phenomena of bulk (longitudinal) wave propagation in the bulky medium. In the case of plate, however, the propagating elastic wave is Lamb wave of which characteristics are completely different with the bulk wave, and thus the separate study for the nonlinearity of Lamb wave is required. This paper reports the result of our basic study on the nonlinear phenomena of Lamb waves, which proposes conditions for practical application as well as for the cumulative propagation of quadratic harmonic frequency mode; (1) phase matching, (2) non-zero power flux, (3) group velocity matching, and (4) non-zero out-of-plane displacement.

Harmonic generation of an obliquely incident ultrasonic wave in solid-solid contact interfaces.

The conventional acoustic nonlinear technique to evaluate the contact acoustic nonlinearity (CAN) at solid-solid contact interfaces (e.g., closed cracks), which uses the through-transmission of normally incident bulk waves, is limited in that access to both the inner and outer surfaces of structures for attaching pulsing and receiving transducers is difficult. The angle beam incidence and reflection technique, where both the pulsing and receiving transducers are located on the same side of the target, may allow the above problem to be overcome. However, in the angle incidence technique, mode-conversion at the contact interfaces as well as the normal and tangential interface stiffness should be taken into account. Based on the linear and nonlinear contact stiffness, we propose a theoretical model for the reflection of an ultrasonic wave angularly incident on contact interfaces. In addition, the magnitude of the CAN-induced second harmonic wave in the reflected ultrasonic wave is predicted. Experimental results obtained for the contact interfaces of A16061-T6 alloy specimens at various loading pressures showed good agreement with theoretical predictions. Such agreement proves the validity of the suggested oblique incidence model.

EVALUATION OF FATIGUE DEGRADATION USING NONLINEAR ULTRASONICS

A study on the evaluation of fatigue degradation in SUS316L material using nonlinear ultrasonics was reported. The nonlinearity of ultrasonic wave was estimated by relative nonlinear parameter defined by the ratio of fundamental and second‐harmonic wave amplitude. A measurement system with contact transducers was constructed, which has a capability of reliable measurement by keeping measurement condition in consistent and reducing extra harmonics generated in the measurement system. Two types of SUS316L specimen were tested: one was a rotating bar fatigue specimen and another was a tensile fatigue specimen. Fatigue condition was high cycle fatigue. The former has cylindrical shape and was used to observe the change of nonlinear parameter while the fatigue accumulation in a specimen. The latter is a plate‐shaped specimen and was used to confirm the change of nonlinear parameter in the spot where the stress concentration was occurred. Measured nonlinear parameter showed strong correlation with damage level ...

NONLINEAR BEHAVIOR OF ULTRASONIC WAVE AT CRACK

A nonlinear model for contact acoustic nonlinearity (CAN) of closed crack was proposed and analyzed to investigate the nonlinear characteristics of transmitted wave for normal incident wave. Closed crack was modeled as an imperfectly jointed interface that has nonlinear discontinuity in displacement across its boundary. Based on the interfacial stiffness of contact interface, a nonlinear parameter was defined as the ratio of second harmonic component to first harmonic, and derived in terms of discontinuous displacements across the interface. It is found that the amplitude of second order harmonic wave is highly dependent on interfacial stiffness. This was verified by experiments for a real crack. For this, a CT fatigue specimen of Al6061 with V‐notch was prepared to initiate the crack, and the amplitude of second order harmonic was measured by scanning along the crack direction. From the results of pre‐test to evaluate the contact condition of crack interfaces by monitoring the amplitudes of echo and thro...

Reflection and Transmission Characteristics of Oblique-Incidence Ultrasonic Waves at Solid-Solid Contact Interfaces

In order to evaluate the characteristics of solid-solid contact interfaces, reflection or transmission techniques involving normal-incidence longitudinal waves are generally used. However, these normal-incidence techniques are of limited use in field applications such as in the inspection of welded parts. The oblique-incidence ultrasonic technique may be an alternative for overcoming these problems. However, in this technique, the mode conversion at the contact interfaces should be taken into account along with the normal and tangential interface stiffness. In this study, we have suggested a theoretical model for obliqueincidence ultrasonic waves at the contact interfaces and analyzed their reflection and transmission characteristics. Experimental results showed that the measured reflection coefficient and second harmonic wave agreed well with the suggested theoretical model. Consequently, the oblique-incidence technique can be a promising method for evaluating the characteristics of the contact interfaces.

Evaluation of Fatigue Degradation in SUS316L Using Nonlinear Ultrasonics

초록:본논문에서는비선형초음파기법을이용하여SUS316L재료시편에서의피로열화를평가한연구의결과를보고한다.초음파의비선형성은입사주파수성분의크기와2차고조파성분의크기비로정의되는상대적비선형파라미터에의해측정된다. 실험을위해접촉식탐촉자를이용한계측시스템을구성하였으며, 안정된비선형파라미터의측정이가능하도록계측시스템자체에서발생되는고조파성분을억제하고계측조건이일관되게유지되도록하였다. 시편으로는SUS316L재료의두가지피로열화시편이사용되었다. 하나는봉형으로회전굽힘피로를가한시편이고, 다른하나는판형으로인장피로를가한시편이다. 피로조건은모두고주기피로에해당한다. 전자는피로가누적되는과정에서비선형파라미터가어떻게변화하는지를관측하는데이용되었고, 후자는피로를가한후피로응력이집중되는위치와그외위치에서의비선형파라미터의차이를관측하는데이용되었다.측정결과비선형파라미터는두가지시편모두에서피로도와강한상관성을갖는것으로나타났다.Abstract: This study evaluated the fatigue degradation in a SUS316L specimen using the nonlinear ultrasonicmethod. Thenonlinearityof theultrasonicwavewasestimatedbyarelativenonlinearparameterdefinedastheratioof theamplitudesforthefundamental wavetothesecondharmonicwave. Intheexperiment, ameasurement systemwith contact transducers was constructed; reliable measurements were assured by keeping measurement conditionsconsistent and reducing extra harmonics generated in the measurement system. Two types of SUS316L specimenwere used in experiments; a rotating bar fatigue specimen and a tensile fatigue specimen. The fatigue conditionused was high cycle fatigue. The former specimen had a cylindrical shape and was used to observe the change inthe nonlinear parameter after fatigue accumulation in a specimen. The latter was a plate-shaped specimen and wasused to confirm the change in the nonlinear parameter at the position where the fatigue stress was concentrated.Themeasurednonlinearparametershowedastrongcorrelationtothedamagelevel inbothfatiguetests.

Acoustic Nonlinearity of Narrow-Band Surface Wave Generated by Laser Beam with Line-Arrayed Slit Mask

We examined the mechanism of generation of higher harmonics by theoretically analyzing the frequency characteristics of a narrow-band surface wave generated by a laser beam with line-arrayed slit masks. We experimentally analyzed the effects of slit opening width and laser intensity on the acoustic nonlinearity of aluminum 6061-T6 alloy by using single-slit and line-arrayed slit masks. The magnitude of the harmonic wave depended on the slit opening width. In our experiment, we generated a 1.75-MHz surface wave by using an arrayed slit with intervals of 1.67 mm. The magnitude of the second harmonic component decreased about by 80% when the slit opening width was increased from 0.5 mm to 1.0 mm. In addition, the relationship between the magnitudes of the fundamental and the second harmonic wave showed good linearity, which agreed well with the typical behavior of acoustic nonlinearity.