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In order to obtain a high gain, a Huygens' metasurface was proposed as the phase shift surface (PSS) for the resonant cavity antenna (RCA) to uniformize the field phase distribution over the RCA's aperture. The metasurface consists of two sets of split ring resonators (SRRs), which are parallel and perpendicular to the incident electromagnetic wave, respectively. The SRRs resonate with the magnetic and electric field of the incident wave, and thus the metasurface is able to manipulate the reflection and transmission of the incident wave. By configuring the dimensions of the SRRs independently, the magnetic and electric resonances can be adjusted respectively, and thereby a fast design of the PSS is achieved. To verify the proposed method, a PSS sample with an operating frequency of 5.8 GHz was designed and applied to a cylindrical RCA with a diameter of 180 mm. The RCA with the PSS was fabricated and the measurement agrees will with the simulation. The simulated and measured results demonstrated that the PSS had a phase shift range of 360° with a minimum transmission magnitude of 0.99 and the RCA with the PSS obtained a more uniform field phase distribution over the antenna's aperture. The RCA's gain was enhanced from 14.90 dB to 18.31 dB, and the aperture efficiency was enhanced from 25.9% to 56.7%. |
The DZ125 alloy has excellent mechanical, fatigue, and high-temperature behaviors. It has been widely used in high-temperature gas turbine components. Although a lot of researches have been carried out on the fatigue problem of the DZ125 alloy, there are still few systematical studies on the fatigue crack initiation mechanism and its failure mechanism during very high cycle fatigue (VHCF) by microscopic fracture analysis. It is found that the scatter of fatigue data increases as fatigue stress decreases, and the fatigue life does not increase significantly even at a low fatigue stress (<220 MPa). The above characteristics are directly related to the change of micro-crack initiation mechanism. Under a high fatigue stress, micro-crack tends to initiate from surface or sub-surface of specimen, and the large secondary crack is the main feature of its fracture morphology. At a low fatigue stress, micro-crack is likely to initiate from interior material defects, and the existence of these defects can seriously affect the fatigue life. Persistent slip bands (PSB) lead to the formation of rough surfaces, which is the main feature at the crack tip. The main crack competes with other secondary cracks and eventually causes fatigue fractures. The results of the Murakami equation show that the average stress intensity factor at micro-crack initiation stage is 3.15 MPa·m1/2, the average stress intensity factor at the onset of unstable crack propagation is 7.7 MPa·m1/2, and the average fracture toughness (KIC) is 15.70 MPa·m1/2.
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