Ultrasonic Modulation Technique Research Articles

Creep detection of Hastelloy X material for gas turbine components with nonlinear ultrasonic frequency modulation

Creep damage is one of the main failure modes in hot-gas‑leading components in gas turbines, which results from high temperatures along with mechanical loads. The aim of this study is to clarify the metallurgical creep behaviour of the Hastelloy X material and detect and evaluate creep damage at an early stage with a nonlinear ultrasonic modulation technique. For this purpose, multiple samples were examined to demonstrate that pores and microcracks in grain boundaries spread from the outside to the inside. Inside the specimen, molybdenum was identified as the main precipitation element. In addition, the chromium diffusion in the outer areas led to the depletion of this element and favoured the formation of pores and microcracks. Failures were proven with nonlinear dual-frequency ultrasound technology. Moreover, two different longitudinal waves were sent into the samples to use harmonic and modulated response frequencies for evaluation. As a result, harmonic frequencies offered a favourable prediction of pore sizes, whereas defined sideband frequencies reacted very sensitively to the damage density and area distribution of the failures. This study offers a method for detecting creep damage with nonlinear ultrasound techniques at an early stage as well as for differentiating between pores, microcracks, dislocations and precipitation. Therefore, the design of future gas turbine components made of Hastelloy X can be adapted with regard to the shown metallurgical behaviour and damage signatures.

Nondestructive assessment of microcracks detection in cementitious materials based on nonlinear ultrasonic modulation technique

The generation and expansion of concrete microcracks have a significant influence on the durability of special concrete structures such as nuclear power plants and reservoirs. Based on the theory of nonlinear ultrasonic modulation, the edge-band peak counting method and the concept of damage index (DI) are proposed to evaluate the development of concrete microcracks. The cementitious materials specimens with different damage degrees can be obtained by alkali-aggregate reaction, and the results of cumulative and continuous micro damages in concrete specimens show that the DI increases with the development of microcracks, which is consistent with the theoretical research. The peak value of DI increases with a higher water-cement ratio in an appropriate range, indicating a higher alkali-aggregate reaction degree. The nonlinear ultrasonic modulation method could be a good indicator of the damage in cementitious material structures.

Effect of Crack Closure on Magnitude of Modulated Wave

Fatigue cracks generated by repeated loads cause structural failures. Such cracks grow continuously and at an increasing speed owing to the concentration of stresses near the crack tips. Therefore, the early detection of fatigue cracks is imperative in the field of structural-health monitoring for the safety of structures exposed to dynamic loading. In particular, the detection of those cracks subjected to compression is known as a challenging problem in the nondestructive inspection area. The nonlinear ultrasonic modulation technique is effective for the detection of microcracks smaller than the size of a wavelength because this technique uses the deformation of waves passing through the crack surfaces. However, the technique has not been thoroughly verified for detecting cracks subjected to external forces. In this study, nonlinear ultrasonic modulation tests are performed on two types of crack specimens under compressive forces. The results show that in fatigue-cracked specimens, the cracks can be detected using modulated waves even under strong compressions. With artificial cracks, buckling occurs at a relatively low compression, and the amounts of modulated waves rapidly increase due to the bending of the specimen before buckling failure takes place. In this study, the crack detection methodology under compression is proposed and experimentally verified. The proposed method might be beneficial to find cracks under compression in various structural components.

Detection of Micro-Cracks in Metals Using Modulation of PZT-Induced Lamb Waves.

The ultrasonic modulation technique, developed by inspecting the nonlinearity from the interactions of crack surfaces, has been considered very effective in detecting fatigue cracks in the early stage of the crack development due to its high sensitivity. The wave modulation is the frequency shift of a wave passing through a crack and does not occur in intact specimens. Various parameters affect the modulation of the wave, but quantitative analysis for each variable has not been comprehensively conducted due to the complicated interaction of irregular crack surfaces. In this study, specimens with a constant crack width are manufactured, and the effects of various excitation parameters on modulated wave generation are analyzed. Based on the analysis, an effective crack detection algorithm is proposed and verified by applying the algorithm to fatigue cracks. For the quantitative analysis, tests are repeatedly conducted by varying parameters. As a result, the excitation intensity shows a strong linear relationship with the amount of modulated waves, and the increase of modulated wave is expected as crack length increases. However, the change in the dynamic characteristics of the specimen with the crack length is more dominant in the results. The excitation frequency is the most dominant variable to generate the modulated waves, but a direct correlation is not observed as it is difficult to measure the interaction of crack surfaces. A numerical analysis technique is developed to accurately simulate the movement and interaction of the crack surface. The crack detection algorithm, improved by using the observations from the quantitative analyses, can distinguish the occurrence of modulated waves from the ambient noises, and the state of the specimens is determined by using two nonlinear indexes.

Fatigue crack detection using dual laser induced nonlinear ultrasonic modulation

In this study, a nonlinear ultrasonic modulation technique based on dual laser excitation is proposed for fatigue crack detection. Two pulse lasers are shot on the target specimen for ultrasonic generation. The corresponding ultrasonic responses are measured by a laser Doppler vibrometer (LDV) and analyzed to extract the crack induced nonlinear ultrasonic modulation. First, the effect of the pulse laser beam size on the frequency content of the generated ultrasonic waves is numerically and experimentally investigated. Then, this finding of the laser beam size effect is utilized to generate wideband (WB) and narrowband (NB) ultrasonic waves by adjusting the laser beam sizes of the two pulsed excitation lasers. Nonlinear ultrasonic modulation results from the interaction of WB and NB ultrasonic waves when a fatigue crack exists in the target specimen. The fatigue crack is then detected by comparing the spectral responses obtained under a single WB input and both WB and NB inputs. In the end, a fully noncontact dual laser ultrasonic system is developed and used to detect micro fatigue cracks in aluminum and steel plate specimens.

Development and field application of a nonlinear ultrasonic modulation technique for fatigue crack detection without reference data from an intact condition

In this study, a fatigue crack detection technique, which detects a fatigue crack without relying on any reference data obtained from the intact condition of a target structure, is developed using nonlinear ultrasonic modulation and applied to a real bridge structure. Using two wafer-type lead zirconate titanate (PZT) transducers, ultrasonic excitations at two distinctive frequencies are applied to a target inspection spot and the corresponding ultrasonic response is measured by another PZT transducer. Then, the nonlinear modulation components produced by a breathing-crack are extracted from the measured ultrasonic response, and a statistical classifier, which can determine if the nonlinear modulation components are statistically significant in comparison with the background noise level, is proposed. The effectiveness of the proposed fatigue crack detection technique is experimentally validated using the data obtained from aluminum plates and aircraft fitting-lug specimens under varying temperature and loading conditions, and through a field testing of Yeongjong Grand Bridge in South Korea. The uniqueness of this study lies in that (1) detection of a micro fatigue crack with less than 1 μm width and fatigue cracks in the range of 10–20 μm in width using nonlinear ultrasonic modulation, (2) automated detection of fatigue crack formation without using reference data obtained from an intact condition, (3) reliable and robust diagnosis under varying temperature and loading conditions, (4) application of a local fatigue crack detection technique to online monitoring of a real bridge.

비선형 초음파 변조 기법을 이용한 열손상 콘크리트의 미세균열 평가

이번 연구에서는 비선형 음향효과를 기반으로 한 비선형 초음파 변조 기법을 통해 열손상 콘크리트의 미세균열 정도를 평가할 수 있는 방법을 제안하였다. 화재 시 콘크리트 구조물은 물리적, 화학적 변화에 따른 콘크리트 내 미세균열이 발생하므로, 기존 초음파 비파괴 기법의 민감도 한계를 극복한 비파괴 기법의 도입이 필요하다. 비선형 초음파 기법은 초음파와 저주파의 변조파로부터 열손상 평가 인자인 비선형인자를 측정하며, 이는 열손상 콘크리트의 미세균열에 적합한 민감도를 가진다. 이 연구에서는 SEM 관측, 열손상 전후 콘크리트의 투수공극량 변화 측정으로부터 수열온도에 따라 미세균열이 급격하게 발생함을 보였으며, 수열온도별 콘크리트의 초음파 전파속도 측정을 통해 제안된 방법의 민감도를 검증하였다. 추가적으로 열손상에 따른 미세균열이 콘크리트의 성능저하에 미치는 영향을 파악하고자 열손상 콘크리트 시편의 압축강도 측정을 수행하였다. 측정값 및 실험값의 연관성을 파악하여 비선형 초음파 변조 기법이 열손상 콘크리트의 미세균열 평가에 적합함을 보였으며, 향후 압축강도 추정에 대한 적용 가능성을 확인하였다.

The ultrasonic modulation technique of determining strain effects in ESR

By replacing the magnetic modulation in an ESR experiment with strain modulation it is possible to determine strain effects in ESR simply and directly. The apparatus producing the strain modulation consists of two piezoelectric quartz crystals and a specimen vibrating in a longitudinal mode at 40 kHz. The stress applied to the specimen is longitudinal ( sigma /sub /// only) while the strain amplitude can range from a thermal limit of 10-13 to a fracture limit of 10-3. Because ultrasonically modulated electron spin resonance (UMER) enables a spectrum change due to strain to be measured directly rather than as a small difference between two large magnetic fields, it has been found to be more sensitive than other methods. The equipment for conducting UMER experiments is described and the theoretical basis for the method is discussed.