ultrasonic testing
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Pipes of various shapes constitute pipelines utilized in industrial sites. These pipes are coupled through welding, wherein complex curvatures such as a flange, an elbow, a reducer, and a branch pipe are often found. Using phased array ultrasonic testing (PAUT) to inspect weld zones with complex curvatures is faced with different challenges due to parts that are difficult to contact with probes, small-diameter pipes, spatial limitations due to adjacent pipes, nozzles, and sloped shapes. In this study, we developed a flexible PAUT probe (FPAPr) and a semi-automatic scanner that was improved to enable stable FPAPr scanning for securing its inspection data consistency and reproducibility. A mock-up test specimen was created for a flange, an elbow, a reducer, and a branch pipe. Artificial flaws were inserted into the specimen through notch and hole processing, and simulations and verification experiments were performed to verify the performance and field applicability of the FPAPr and semi-automatic scanner.

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.

Abstract Nondestructive evaluation of closed cracks is one of the most challenging problems in ultrasonic testing. Here, we propose a novel closed-crack imaging technique combining ultrafast phased-array with pump excitation. The pump excitation with kHz frequency can effectively induce the crack opening/closing behaviors since it can generate a large displacement of 1000 nm order. At the same time, ultrafast phased-array imaging, i.e. plane wave imaging, with MHz frequency stroboscopically captures the high-speed crack dynamics induced by pump excitation. We successfully proved the concept in a closed fatigue crack specimen.

To study the antifreezing durability of internal coating silicon fume concrete with different external coatings, fast freeze-thaw (FT) cycle testing was performed for three types of external coatings applied to the internal coatings of silicon fume concrete. Using ultrasonic testing and compressive strength tests, we analysed the relationships between the ultrasonic pulse velocity and the mechanical and physical properties of concrete under freeze-thaw action. The results show that the compressive strength and pulse velocity of the studied concrete changed little before the first 100 FT cycles but varied significantly after being subjected to 100 FT cycles and diminished linearly with increasing FT cycles. The dynamic elastic parameters of the concrete were inferred using pulse velocity calculations, and the dependence on FT cycles was very similar to that of ultrasonic pulse velocity. The concrete strength was strongly and positively correlated with ultrasonic pulse velocity. The linear regression model of between ultrasonic pulse velocity, kinetic coefficient, and compressive strength of concrete was also established. The damage incurred to the external coating material (XT-HPA + XT-SS and XT-HPS) was small, and the good performance of the concrete with the added inorganic coating after freeze-thaw cycles indicates good frost resistance.