closed crack
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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.

As an important rotating component, the flexible annular thin disk is widely used in mechanical engineering. Cracks may occur in some weak disk parts, which will greatly shorten the equipment service life and even cause equipment failure. Due to the centrosymmetric structure of the flexible annular disk, two typical cracks are studied in this paper; one is radial crack parallel to diameter, including radial closed crack (RC-crack) and radial opening crack (RO-crack); the other one is vertical crack perpendicular to diameter, including circumferential crack (CF-crack) and tangential crack (TG-crack). The effect of crack parameters, such as crack length, direction, and position, on disk vibration characteristics are studied through theoretical simulation and experimental verification. The research shows that the effect of cracks on vibration characteristic gets more obvious with cracks extending in most cases, RO-crack decreases the natural frequency obviously, and vertical cracks would affect mode shapes. In addition, the bigger the nodal diameter is, the more obvious the effect gets. Meanwhile, the most obvious effect appears in the mode of a nodal diameter locating on the crack. The research possesses some guiding significance in industrial production; by comparing with the vibration characteristics of the flawless disk, the integrity of the rotating flexible disk can be judged to prevent possible equipment damage.

The reliability of micro-damage detection of metal materials plays a crucial role in the safe and reliable operation of large equipment. In recent years, nonlinear ultrasonic nondestructive testing technology has achieved good development in closed cracks detection, but the problem of nonlinear ultrasonic detection of closed cracks is weak response signal and vulnerable to external interference. This paper realizes the modulation of the ultrasonic wave by electromagnetic loading at the closed cracks, which can effectively enrich the frequency components of nonlinear ultrasonic and magnify the amplitude. This lays a foundation for further research on nonlinear ultrasonic detection of closed cracks under electromagnetic loading.

AbstractIn this study, an experimental investigation was conducted to assess the flexural tensile strength and crack-closing performance of mortar beams containing short shape memory alloy (SMA) fibers, randomly distributed only on the tensile side. The SMA fibers were mainly composed of titanium (Ti), nickel (Ni), and niobium (Nb). In addition, the effect of tensile steel wires on the flexural strength and crack-closing performance was evaluated. A four-point bending test was performed to evaluate the post-cracking tensile strength. This study also suggested a proper model to calculate the ultimate flexural moment of the SMA fiber–embedded beams. Subsequently, a heating plate that could be installed at the bottom of the beam was used to induce the shape memory effect and measure the closed crack width. This study assessed the crack-closing performance induced by the SMA fibers at the bottom side of the beams and the resistance of the tensile wires in the beams.

<p>        In the present work, a foam rubber sheet installed between two transparent thick flat glasses was used as a physical model of a permeable oil reservoir. The elastic properties of foam rubber and its coefficient of friction on glass are supposed to be measured in separate experiments. In the center of the foam sheet there is a round hole, which is a model of the end face of the well in the oil reservoir. Before the experiment, cuts are made from the hole in opposite directions and to a certain length, simulating a previously closed crack. Using a vacuum pump it is possible to change the pressure of glasses per layer and thereby simulate the increase in "rock pressure" on a productive oil reservoir . A fluid is pumped through the hole in the end of the well. Under the action of fluid filtration, the surface of the walls along the cut of the foam layer are moved apart, forming a gap.The dependence of the pressure gradient on the length of the crack formed was obtained. The overall picture of the growth of hydraulic fracturing is recorded by camera. Continuous physical observations of the formation of a fracture in time allow subsequently predict the optimal fracture geometry.</p><p>The reported study was funded by RFBR, project number №. 20-35-80028 and state task 0146-2019-0007</p>