• Journal of the Korean Society for Nondestructive Testing
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• v.36 no.2
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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.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.30 no.3
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• pp.215-222
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• 2017

In order to evaluate a stress state of concrete according to the change of tensile force of prestressed beam, improved nonlinear resonant ultrasonic spectroscopy(NRUS) method is proposed. This technique is advantageous to evaluate the stress state in initial state because the method shows much higher sensitivity than existing linear ultrasonic methods. The NRUS technique measure a nonlinearity parameter, which is calculated from the resonant frequency shift of ultrasonic wave related to the medium state, and the result is also closely related to the stress state of concrete. In this study, the nonlinearity parameter was measured with the change of tensile force to verify the close relationship between the two factors, and the effect of repetitive load cycle on the change of nonlinearity parameter was analyzed. In addition, sensitivity comparison with the linear ultrasonic pulse velocity method was performed. Through the experimental results, the possibility of NRUS technique for the evaluation of stress state in prestressed beam was confirmed.

• Journal of the Korean Society for Nondestructive Testing
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• v.32 no.4
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• pp.369-375
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• 2012

In order to overcome the detection limit by the current nondestructive evaluation technology, a nonlinear resonant ultrasound spectroscopy(NRUS) technique was applied for detection of micro-scale cracks in a material. A down-shift of the resonance frequency and a variation of normalized amplitude of the resonance pattern were suggested as the nonlinear parameter for detection of micro-scale cracks in a materials. A natural-like crack were produced in a standard compact tension(CT) specimen by a low cycle fatigue test and the resonance patterns were acquired in each fatigue step. As the exciting voltage increases, a down-shift of resonance frequency were increases as well as the normalized amplitude decrease. This nonlinear effects were significant and even greater in the cracked specimen, but not observed in a intact specimen.