Abstract
The testing of KMN steel bending fatigue with different cycles was carried out using a nonlinear ultrasonic detector to obtain its nonlinear coefficient. The experimental results show that the nonlinear coefficient first increases and then decreases with an increase in fatigue cycles. The relationship between the propagation of the micro-cracks inside the material and the nonlinear coefficient was researched by microscopic analysis in the dangerous position of the specimens. As the fatigue cycles increase, the microstructure of the specimen gradually deteriorates and cracks occur, which proves that nonlinear ultrasonic detection can be used to characterize the initiation of micro-cracks in the early fatigue stages of the material and that the nonlinear coefficient β of the material can be used to reflect the fatigue damage degree and fatigue life of the interior of the material. An analysis of the numerical statistics of the fatigue cracks inside the specimens was carried out, and the extreme value of fatigue cracks was calculated using the Gumbel distribution. An empirical formula for the nonlinear coefficient and crack growth size of KMN steel was established and then a method for estimating the fatigue life of KMN steel based on nonlinear ultrasonic testing was proposed.
Highlights
Since only a few small areas are selected for statistics when observing the microstructure of the specimen section, the main crack may not be in the observational field of view, so we must find the value of all the cracks in the dangerous area of the specimen
After the nonlinear ultrasonic testing of the blades in service, the corresponding non-linear coefficients are obtained and substituted into the empirical formula to obtain the equivalent crack size inside the blades at this time
Perform nonlinear ultrasonic testing of the impeller in service to obtain the non-linear coefficient of the impeller at a particular point in time; Use the β–ae empirical formula to obtain the corresponding equivalent crack size; Combined with the fatigue crack growth life formula, the fatigue crack growth life of the impeller at this time is obtained, and the residual life of the impeller is obtained
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Conducted non-linear, ultrasonic, in situ observations of the ultra-high cycle fatigue characteristics of aluminum alloys and found that the ultrasonic nonlinear coefficients are more sensitive to fatigue damage (such as crack initiation and growth). They found that there is a good correlation between the ultrasonic nonlinear coefficient and the stiffness and plastic strain of the specimen. The current research has not established the mapping relationship between the nonlinear coefficients and the fatigue damage and fatigue crack size and lacks data on the use of nonlinear ultrasonic testing to quantitatively estimate the fatigue life of a material
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