TOP > Report & Column > The Forefront of Space Science > 2015 > Forefront of the Materials Science and Engineering in Rocket Engine Development
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Nondestructive Inspection and Residual Life Assessment of the Combustion Chamber Wall Since the damage accumulation caused by creep-fatigue on the combustion chamber wall is not able to be avoided, we have to quantitatively measure and evaluate the damage of each combustion cycle with nondestructive methods. If we stop using the engine when a sub-millimeter-order crack, which indicates the final stage of the creep-fatigue, is detected, we can prevent big accidents. Additionally, if we are able to inspect the growth of the grain boundary voids less than the sub-micrometer level at earlier stages, we can evaluate the remaining use cycles (residual life) of the engine. As for the nondestructive inspection of the cracks, since the combustion chamber wall possesses cooling grooves, the flow echo from the reverse side (LH2 side) detected on the inspection surface (combustion gas side) is buried under the shape echo of the cooling grooves. Currently we are examining considering nonlinear ultrasonic testing technique as well as the laser ultrasonic visualization technique. Especially the former, nonlinear ultrasonic testing technique can detect the waveform distortion in higher harmonic regions reflected by abnormal parts inside the material when they are shaken to large amplitudes by a burst wave. Since it does not utilize the acoustic impedance difference, the signals are not buried under the shape echo, we hope it can successively detect sub-micrometer cracks on the reverse side. As for the residual life assessment, since it is necessary to inspect the grain boundary voids and micro-cracks under the sub-micrometer level, it is extremely difficult. Currently, we have just started collaborative research on the positron annihilation method and the electromagnetic inspection together with specialists from universities of our country. Through the study of this special creep-fatigue phenomenon, we have become able to develop a rocket engine of the next generation launch vehicle with high reliability in designing with the basis of the damage accumulation mechanism. In addition, the nondestructive inspection of cracks and the residual life assessment of damage accumulation are not only necessary to improve the safety and reliability of the engine, but also an essential breakthrough to realize the reusable engines in the future. We should try our best to put them into practice. (Eiichi Sato, Shusuke Hori)
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