A tip-sensitive fibre-optic Bragg grating ultrasonic hydrophone (TSFBGUH) with high spatial resolution for measuring high-intensity focused ultrasound (HIFU) fields is reported. When measuring a HIFU field, the sensitive axis of the TSFBGUH with a flexible fibre-optic sensor holder should be in parallel with the acoustic axis of the HIFU transducer to ensure the measured ultrasound impinges on the tip of the TSFBGUH. The TSFBGUH system has been established and tested. The experimental results show that the acoustic pressure sensitivity of the TSFBGUH system is about 31.3 mV/MPa within the measurement range of 10 MHz and the noise equivalent pressure is about 10.4 kPa.Introduction: High-intensity focused ultrasounds (HIFUs) have been used to cure cancers non-invasively by destroying tumours through heating and acoustic cavitations. To ensure the treatment safety, the acoustic pressure distributions and the size of the focal regions of HIFU fields need to be measured and characterised accurately. At present, HIFU fields are commonly measured and characterised by piezoelectric needle hydrophones, fibre-optic hydrophones etc. [1][2][3][4]. The fibre-optic hydrophones possess the merits of simple configuration, miniaturisation and immunity to electromagnetic interference. The fibre-optic Bragg gratings (FBGs) have been developed into various sensors for measuring static pressures [5], ultrasonic pressures [6-13] and other physical parameters. The FBG hydrophones, possessing the capability of multiplexing and withstanding high acoustic pressure, have attracted much interest [9-13]. All the above-mentioned FBG hydrophones sensed ultrasounds through the lateral side of FBGs, which could break the hydrophones by the ultrasonic wave and limit the spatial resolution [2,8]. Up to now, we have not found the literature about the tip-sensitive FBG hydrophones for measuring HIFU fields.In this Letter, a tip-sensitive FBG ultrasonic hydrophone (TSFBGUH) for measuring HIFU fields is described. The TSFBGUH system and the experimental setup have been built to measure the HIFU fields generated by a 0.965 MHz HIFU transducer in continuous mode.
Stacking fault tetrahedron (SFT) is a kind of detrimental three-dimensional defect in conventional face-centered cubic (FCC) structural metals; however, its formation and anisotropic mechanical behavior in a CoCrFeNiMn high-entropy alloy (HEA) remain unclear. In this work, we first performed molecular dynamics simulations to verify the applicability of the Silcox-Hirsch mechanism in the CoCrFeNiMn HEA. The mechanical responses of the SFT to shear stress in different directions and that of the pure Ni counterpart were simulated, and the evolutions of the atomic structures of the SFTs during shear were analyzed in detail. Our results revealed that the evolution of the SFT has different patterns, including the annihilation of stacking faults, the formation and expansion of new stacking faults, and insignificant changes in stacking faults. It was found that the effects of SFT on the elastic properties of Ni and HEA are negligible. However, the introduction of SFT would reduce the critical stress, while the critical stress of the CoCrFeNiMn HEA is much less sensitive to SFT than that of Ni.
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