• 비파괴검사학회지
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• 제36권2호
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• pp.121-129
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• 2016

As the lifespan of concrete structures increases, their load carrying capacity decreases owing to cyclic loads and long-term effects such as creep and shrinkage. For these reasons, there is a necessity for stress state monitoring of concrete members. Particularly, it is necessary to evaluate the concrete structures for behavioral changes by using a technique that can overcome the measuring limitations of usual ultrasonic nondestructive evaluation methods. This paper proposes the use of a nonlinear ultrasonic method, namely, nonlinear resonant ultrasonic spectroscopy (NRUS) for the measurement of nonlinearity parameters for stress monitoring. An experiment compared the use of NRUS method and a linear ultrasonic method, namely, ultrasonic pulse velocity (UPV) to study the effects of continuously increasing loads and cyclic loads on the nonlinearity parameter. Both NRUS and UPV methods found a similar direct relationship between load level and that parameter. The NRUS method showed a higher sensitivity to micro-structural changes of concrete than UPV method. Thus, the experiment confirms the possibility of using the nonlinear ultrasonic method for stress state monitoring of concrete members.

• Smart Structures and Systems
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• 제25권3호
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• pp.301-310
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• 2020

The laser ultrasonic technique is gaining popularity for nondestructive evaluation (NDE) applications because it is a noncontact and couplant-free method and can inspect a target from a remote distance. For the conventional laser ultrasonic techniques, a pulsed laser is often used to generate broadband ultrasonic waves in a target structure. However, for crack detection using nonlinear ultrasonic modulation, it is necessary to generate narrowband ultrasonic waves. In this study, a pulsed laser is shaped into dual-line arrays using a spatial mask and used to simultaneously excite narrowband ultrasonic waves in the target structure at two distinct frequencies. Nonlinear ultrasonic modulation will occur between the two input frequencies when they encounter a fatigue crack existing in the target structure. Then, a nonlinear damage index (DI) is defined as a function of the magnitude of the modulation components and computed over the target structure by taking advantage of laser scanning. Finally, the fatigue crack is detected and localized by visualizing the nonlinear DI over the target structure. Numerical simulations and experimental tests are performed to examine the possibility of generating narrowband ultrasonic waves using the spatial mask. The performance of the proposed fatigue crack localization technique is validated by conducting an experiment with aluminum plates containing real fatigue cracks.

• Nuclear Engineering and Technology
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• 제52권3호
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• pp.621-625
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• 2020

Cast austenitic stainless steel (CASS) is used for fabricating different components of the primary reactor coolant system of pressurized water reactors. However, the thermal embrittlement of CASS resulting from long-term operation causes structural safety problems. Ultrasonic testing for flaw detection has been used to assess the thermal embrittlement of CASS; however, the high scattering and attenuation of the ultrasonic wave propagating through CASS make it difficult to accurately quantify the flaw size. In this paper, we present a different approach for evaluating the thermal embrittlement of CASS by assessing changes in the material properties of CASS using a nonlinear ultrasonic technique, which is a potential nondestructive method. For the evaluation, we prepared CF8M specimens that were thermally aged under four different heating conditions. Nonlinear ultrasonic measurements were performed using a contact piezoelectric method to obtain the relative ultrasonic nonlinearity parameter, and a mini-sized tensile test was performed to investigate the correlation of the parameter with material properties. Experimental results showed that the ultrasonic nonlinearity parameter had a correlation with tensile properties such as the tensile strength and elongation. Consequently, we could confirm the applicability of the nonlinear ultrasonic technique to the evaluation of the thermal embrittlement of CASS.

• 대한기계학회논문집A
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• 제38권10호
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• pp.1133-1137
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• 2014

초음파 비선형 파라미터 ${\beta}$는 재료의 미세한 변질을 평가하는데 효과적인 인자로 알려져있다. 그런데 지금까지 대부분의 연구는 접촉식 방법을 이용하여 측정하는 연구가 수행되어왔다. 그러나, 접촉식 방법은 탐촉자와 시편사이의 접촉상태가 측정결과에 큰 영향을 미치기 때문에 측정의 재현성을 보장하기 위해 탐촉자의 접촉상태를 일정하게 유지시키기 위한 부수적인 장치가 필요하다. 이러한 불편함을 해소할 목적으로 본 연구에서는 비접촉식 기법을 도입하였다. 다만 초음파의 송신은 접촉식으로 하고 수신만 비접촉식을 적용함으로써 동일한 조건에서 접촉식으로 수신한 경우와 비접촉식으로 수신한 경우를 비교하고자 하였다. 시편은 시효 처리된 알루미늄 합금을 사용하였고, 두 방식의 측정결과 비선형 파라미터 ${\beta}$의 변화는 유사하게 나타났다. 이로부터 초음파 비선형 특성의 변화를 비접촉식 수신방법으로 용이하게 평가할 수 있음을 확인하였다.

• 비파괴검사학회지
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• 제27권6호
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• pp.576-581
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• 2007

Ultrasonic nonlinearity has been considered as a promising method to evaluate the micro damage of material; however, its magnitude is so small that its measurement is not easy. Especially, when we use contact PZT transducer, if the contacting pressure is not kept in constant during the measurement then there exists extraneous fluctuation in the measured nonlinearity caused by the unstable contact condition, In this paper, we developed a pneumatic control system to keep the contacting pressure of transducer in constant during the measurement and analyzed the effect of contacting pressure to the ultrasonic nonlinearity measurement As a result, we found that the pressure of transducer in our measurement system should be greater than 170 kPa to measure the ultrasonic nonlinear parameter in stable with no dependency on the contacting pressure.

• 한국정밀공학회지
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• 제13권4호
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• pp.53-60
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• 1996

In the conventional linear elasticity, sound speed is determined by only elastic modulus and density of the medium. In actual, however, sound speed depends on the stress and this dependency becomes nonlinear as the stress increases. These phenomena can be introducing nonlinear elastic modulus. In this paper, relationships between nonlinear elastic modulus up to 4th order and the internal status of materials are discussed through computer simulations and experiments. For the measurement of sound speed, a new type of measurement system using ultrasonic wave is proposed on the basis of ultrasonic pulse echo method which has been generally used in nondestructive ultrasonic test equipment. In order to confirm the stress dependency of sound speed, several experiments are carried out for alumina specimen.

• 비파괴검사학회지
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• 제37권2호
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• pp.115-121
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• 2017

The nonlinear ultrasonic technique is a potential nondestructive method to evaluate material degradation, in which the ultrasonic nonlinearity parameter is usually measured. The ultrasonic nonlinearity parameter is defined by the elastic nonlinearity coefficients of the nonlinear Hooke's equation. Therefore, even though the ultrasonic nonlinearity parameter is not equal to the elastic nonlinearity parameter, they have a close relationship. However, there has been no experimental verification of the relationship between the ultrasonic and elastic nonlinearity parameters. In this study, the relationship is experimentally verified for a heat-treated aluminum alloy. Specimens of the aluminum alloy were heat-treated at $300^{\circ}C$ for different periods of time (0, 1, 2, 5, 10, 20, and 50 h). The relative ultrasonic nonlinearity parameter of each specimen was then measured, and the elastic nonlinearity parameter was determined by fitting the stress-strain curve obtained from a tensile test to the 5th-order-polynomial nonlinear Hooke's equation. The results showed that the variations in these parameters were in good agreement with each other.

• Structural Engineering and Mechanics
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• 제89권3호
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• pp.301-308
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• 2024

Concrete carbonation is a continuous and slow process from the outside to the inside, in which its penetration slows down with the increased depth of carbonation. In this paper, the results of the evaluation of the measurement of concrete carbonation depth using a non-destructive ultrasonic testing method are presented. According to the results, the relative nonlinear parameter caused more sensitivity in carbonation changes compared to Rayleigh's fuzzy velocity. Thus, the acoustic nonlinear parameter is expected to be applied as a quantitative index to recognize carbonation effects. In this research, combo diagrams were developed based on the results of ultrasonic testing and the experiment to determine carbonation depth using a phenolphthalein solution, which could be considered as instructions in the projects involving non-destructive ultrasonic test methods. The minimum and maximum accuracy of this method were 89% and 97%, respectively, which is a reasonable range for operational projects. From the analysis performed, some useful expressions are found by applying the regression analysis for the nonlinearity index and the carbonation penetration depth values as a guideline.

• 대한기계학회논문집A
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• 제25권2호
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• pp.214-219
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• 2001

In this paper, the nonlinear behavior of ultrasonic wave in partially degraded material is considered. For this aim, FDM(finite difference method) model for the nonlinear wave equation was developed with the restriction to the 1-D longitudinal wave motion and how the partial degradation in material contributes to the detected nonlinear parameter was analyzed quantitatively. In order to verify the rightness of this simulation method, the relation between the detected nonlinear parameter and the continuous distribution of degradation obtained from simulation was compared with experiment results and the simulation and experiment results showed similar tendency. It can be known from simulation result that the degree of degradation, the range of degradation and the continuous distribution of degradation have strong correlation with the detected nonlinear parameter. As it was possible in these simulations that only special part is assumed as degraded one, the quantitative evaluation of partially degraded material may be obtained by using this method.

• 대한기계학회논문집
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• 제17권8호
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• pp.1971-1979
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• 1993

In this paper, a new method to estimate stress status in metal nondestructively by using nonlinear dependency of sound speed on stress is proposed. For the purpose, equivalent nonlinear elastic constants up to fourth-order are introduced and a new characteristic parameter given as a function of these constants is presented. And a concrete system to measure the characteristic parameter is constructed by electromagnetic pumping wave and ultrasonic probing wave system. Some experimental results for Al alloy showed that the estimation of stress status in metal is possible by the proposed method.