Abstract
In modern gas turbines, efforts are being made to improve efficiency even further. This is achieved primarily by increasing the generated pressure ratio in the compressor and by increasing the turbine inlet temperature. This leads to enormous loads on the components in the hot gas region in the turbine. As a result, non-destructive testing and structural health monitoring (SHM) processes are becoming increasingly important to gas turbine manufacturers. Initial cracks in the turbine blades must be identified before catastrophic events occur. A proven method is the linear ultrasound method. By monitoring the amplitude and phase fluctuations of the input signal, structural integrity of the components can be detected. However, closed cracks or small cracks cannot be easily detected due to a low impedance mismatch with the surrounding materials. By contrast, nonlinear ultrasound methods have shown that damages can be identified at an early stage by monitoring new signal components such as sub- and higher harmonics of the fundamental frequency in the frequency spectrum. These are generated by distortion of the elastic waveform due to damage/nonlinearity of the material. In this paper, new global nonlinear parameters were derived that result from the dual excitation of two different ultrasound frequencies. These nonlinear features were used to assess the presence of cracks as well as their qualitative sizes. The proposed approach was tested on several samples and turbine blades with artificial and real defects. The results were compared to samples without failure. Numerical simulations were conducted to investigate nonlinear elastic interaction of the stress waves with the damage regions. The results show a clear trend of nonlinear parameters changing as a function of the crack size, demonstrating the capability of the proposed approach to detect in-service cracks.
Highlights
Linear ultrasound techniques are a proven method in modern component inspection
Hikata et al brought a significant development of this technique in 1965. They found that a sinusoidal ultrasonic wave distorts the fundamental frequency as it propagates in the presence of nonlinearities [4]
An ultrasonic wave propagates into a solid with the fundamental frequency. If this is disturbed during propagation in the body, harmonics are generated
Summary
Failure detection is carried out here by changes in the elastic properties such as sound velocity, damping, transmission coefficients, and reflection coefficients [1,2,3]. Hikata et al brought a significant development of this technique in 1965. They found that a sinusoidal ultrasonic wave distorts the fundamental frequency as it propagates in the presence of nonlinearities [4]. An ultrasonic wave propagates into a solid with the fundamental frequency. If this is disturbed during propagation in the body, harmonics are generated. If the amplitudes of these harmonic frequencies are measured and compared with the fundamental frequencies, these comparative values are a good indicator for detecting changes in the material [4]
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