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Longitudinal torsional ultrasonic-assisted grinding (LTUG) is one of the main methods to achieve high-quality and high-efficiency machining of high-performance ceramic materials. However, it isn't easy to accurately characterize the three-dimensional (3-D) surface topography due to multiple random factors during LTUG. Aiming at the complex surface features caused by multiple random factors in the LTUG of Si3N4 ceramics, a probabilistic algorithm for the height of residual material on the surface (HRMS) in LTUG of Si3N4 ceramic was proposed, and the prediction model for the 3-D surface topography and 3-D surface roughness parameters of Si3N4 ceramics in LTUG was established by using this algorithm. Simulation and experimental results show that the prediction model of 3-D surface topography and 3-D surface roughness established by the HRMS algorithm can more realistically predict the general characteristics of 3-D surface topography in LTUG under different process parameters, and the error range of the 3-D surface roughness parameter is 0–14.07%, which realizes the high-precision and high-reliability prediction of the 3-D surface topography and 3-D surface roughness parameters of the Si3N4 ceramic under LTUG. [ABSTRACT FROM AUTHOR]

Abstract: The fast spin echo (FSE) triple-echo Dixon (FTED) sequence uses bipolar triple-echo readout during each echo-spacing period of FSE to collect all the images necessary for Dixon water and fat separation in a single scan. In comparison to other FSE implementations of the Dixon technique, the triple echo readout used in FTED incurs minimal deadtime in the pulse sequence design and thus greatly enhances the overall scan efficiency. A potential drawback of FTED is that the time dependence of the gradient moment along the frequency encode direction becomes more complicated than in FSE and flow compensation based on the gradient moment (GM) nulling is difficult to achieve. In this work, the first order GM along the frequency encode direction of FTED was examined and two different methods to minimize the GM were proposed. The first method nulls the GM at all the locations of the refocusing radiofrequency pulses so that the Carr-Purcell Meiboom-Gill condition is always maintained. The second method minimizes the GM of the spin echo component of the FSE signal at the echo locations. The efficacy of both methods in reducing the first order GM and flow-related artefacts was demonstrated both in phantom and in images in vivo. [Copyright &y& Elsevier]

Carbon fiber reinforced plastic (CFRP) is widely used in the aerospace field because of its excellent mechanical properties. To ensure that the structure has a certain bearing capacity in multiple directions, MD-CFRP is generally used in engineering applications, which makes the anisotropy and inhomogeneity more complex. Therefore, it is easy to produce defects when machining CFRP. Additionally, MD-CFRP is recognized as the superposition of UD-CFRP, but this has not been verified. To solve these problems, the fracture mechanism evolution model for milling CFRP is established and verified by a series of experiments. The results show that the fiber cutting angle is an important factor for the fracture mechanism in orthogonal cutting. Upon increasing the fiber cutting angle from 0° to 180°, the fracture mechanism changes from interface separation to shear fracture, and then to bending fracture. Because the instantaneous fiber cutting angle of the surface is equal to the fiber orientation angle, the fiber orientation angle is the decisive factor in milling CFRP. After the slot milling of UD-CFRP and two types of MD-CFRP, the cutting force, surface morphology, and surface roughness are analyzed in detail. The results also show that each layer of MD-CFRP maintains the mechanical properties and fracture mechanism of UD-CFRP. Therefore, some of the existing research results of UD-CFRP can be directly applied when designing and processing MD-CFRP with different laying orders. This provides an important basis for the research on the machining mechanism, damage mechanism, and engineering application of MD-CFRP. [ABSTRACT FROM AUTHOR]