The focus of this paper is on estimating the stress concentration factor of circular hollow section KK-joints with different geometric parameters and subsequently assessing the effectiveness of carbon fiber-reinforced polymer (CFRP) wrapping for repairing joints with cracks. Different geometric parameters, such as θ (brace inclination angle), γ (the ratio of the outer diameter to the wall thickness of the chord), and τ (the thickness ratio of the brace to the chord), were studied to investigate changes in stress concentration using numerical simulation. The results indicated that the stress concentration factor was most sensitive to changes in θ, followed by γ. Subsequently, the effect of crack length and depth was analyzed to simulate cracks in joints subjected to reciprocating load. The results showed that changing D from T/16 to T/2 (where T is the thickness of the chord) can cause more stress concentration, with an average of 8.37%. Next, damaged joints were wrapped in carbon fiber-reinforced polymer as a repair. Analysis of the effects of different layers and directions of polymer wrap revealed that even six layers of wrapping effectively reduced the stress concentration compared to the initial model. Finally, based on the results of parametric analysis and nonlinear fitting, a calculation formula for the stress concentration factor suitable for KK-joints under axial loads is proposed.
A finite element (FE) analysis of corroded circular hollow section T-joints repaired using grouted clamps is presented in this study. To ensure an accurate simulation, experiments on a T-joint with uniform corrosion and an intact T-joint with grouted clamp are reproduced separately using an FE model. The experimental and FE modeling results correlate closely, displaying similar failure behaviors and load–displacement responses. Subsequently, a total of 56 FE T-joint models with varying degrees of corrosion on the chord, repaired by grouted clamp, were then analyzed numerically. The corrosion was artificially applied to the chord’s outer surface at depths of 10%, 20%, and 30% of the chord’s thickness. The models also account for variations in joint geometry, dictated by parameters β and γ, which range from 0.565 to 0.678 and from 21 to 28, respectively. Furthermore, grouted clamp’s properties were examined, including sleeve length, thickness, strength, as well as the thickness and strength of the grout, in relation to the complexity of the repair. The grouted clamp demonstrated significant repair capability, increasing the ultimate strength of the corroded joint by up to 2.23 times. Reinforcements that are both thicker and longer substantially enhance the joint’s ultimate strength. However, inappropriate repair construction results in an abrupt termination of the load–displacement curve and a brittle failure phenomenon outside reinforced chord region. Additional weight from the grouted clamp requires reasonable control, and sleeve overlapping ratio should be guaranteed to be greater than 0.7. The joint bearing capacity can be efficiently increased by thicker grouts and sleeves only in ductile failure cases. The confinement effect and mechanism of the grouted clamp in joint deformation were visually analyzed as stress distribution from FE analysis results. Finally, a prediction equation is proposed to estimate the static strength of the repaired joint through regression analysis.
This study presents a general outline of the Qitai radio telescope (QTT) project. Qitai, the site of the telescope, is a county of Xinjiang Uygur Autonomous Region of China, located in the east Tianshan Mountains at an elevation of about 1800 m. The QTT is a fully steerable, Gregorian type telescope with a standard parabolic main reflector of 110 m diameter. The QTT has adopted an um-brella support, homology-symmetric lightweight design. The main reflector is active so that the deformation caused by gravity can be corrected. The structural design aims to ultimately allow high-sensitivity observations from 150 MHz up to 115 GHz. To satisfy the requirements for early scientific goals, the QTT will be equipped with ultra-wideband receivers and large field-of-view mul-ti-beam receivers. A multi-function signal-processing system based on RFSoC and GPU processor chips will be developed. These will enable the QTT to operate in pulsar, spectral line, continuum and Very Long Baseline Interferometer (VLBI) observing modes. Electromagnetic compatibility (EMC) and radio frequency interference (RFI) control techniques are adopted throughout the system design. The QTT will form a world-class observational platform for the detection of low-frequency (nanoHertz) gravitational waves through pulsar timing array (PTA) techniques, pulsar surveys, the discovery of binary black-hole systems, and exploring dark matter and the origin of life in the universe.
This study examines the dynamic mechanical properties of square tubular T-joints with impact loads acting on the chord surface in the joint area. The study first verified the failure modes and behaviors of the specimens under a brace axial force and impact, respectively, where the simulation results demonstrated good agreement with the experimental results. A total of 138 square hollow section tubular T-joint finite element models were divided into T1, T2, and T3 groups based on different tube diameter ratios. The failure modes, displacement-time history curves, and impact force-time history curves were obtained. The results revealed that the joint deformation modes were primarily characterized by significant local indentation at the impact site and junction of the chord and brace, as well as a certain degree of deformation at both ends of the chord. Within a certain range, the preloaded axial force could mitigate the development of plastic deformation, whereas an increased ratio of the drop hammer length to chord diameter exacerbated it. Finally, theoretical analysis was simplified by defining the plastic element set, and the energy dissipation coefficient ψ was proposed to evaluate the impact resistance of square tubular T-joints by analyzing the specific energy changes in the intersecting region (El) and at the ends of the chord (Ee).
4.2 K photoluminescence (PL) and 77K standard Hall-effect measurements were performed for In0.52Al0.48As∕InxGa1−xAs metamorphic high-electron-mobility-transistor (HEMT) structures grown on GaAs substrates with different indium contents in the InxGa1−xAs well or different Si delta-doping concentrations. It was found that electron concentrations increased with increasing PL intensity ratio of the “forbidden” transition (the second electron subband to the first heavy-hole subband) to the sum of the “allowed” transition (the first electron subband to the first heavy-hole subband) and the forbidden transition. And electron mobilities decreased with increasing product of the average full width at half maximum of allowed and forbidden transitions and the electron effective mass in the InxGa1−xAs quantum well. These results show that PL measurements are a good supplemental tool to Hall-effect measurements in optimization of the HEMT layer structure.
In this study, 90 finite-element models are used to explore the behaviour of fibre-reinforced polymer (FRP) reinforced joints under combined in-plane bending (IPB) and axial load (AX). The effects of joint geometry, FRP layer count, and AX levels of the chord or brace are considered. Three typical failure modes are observed: chord plastic failure, brace plastic failure, and brace buckling failure. Increasing the number of FRP layers can ensure that failure is chord-related failure in a ductility manner rather than the unexpectedly brace-related brittle failure. Depending on the stress distribution of fibres, FRP reinforcement can restrict the deformation of joints subjected to complex loading patterns. Moreover, added FRP layers efficiently reduce the effect of brace AX on the IPB resistance. Finally, a modified strength equation is established, including the influence of FRP reinforcement, chord AX, and brace AX.
A step-graded InAlAs buffer layer and an In0.52Al0.48As/In0.53Ga0.47As metamorphic high electron mobility transistor (MM-HEMT) structures were grown by molecular beam epitaxy on GaAs (001) substrates, and rapid thermal annealing was performed on them in the temperature range 500–800 °C for 30 s. The as-grown and annealed samples were investigated with Hall measurements, and 77 K photoluminescence. After rapid thermal annealing, the resistivities of step-graded InAlAs buffer layer structures became high. This can avoid leaky characteristics and parasitic capacitance for MM-HEMT devices. The highest sheet carrier density ns and mobility μ for MM-HEMT structures were achieved by annealing at 600 and 650°C, respectively. The relative intensities of the transitions between the second electron subband to the first heavy-hole subband and the first electron subband to the first heavy-hole subband in the MM-HEMT InGaAs well layer were compared under different annealing temperatures.
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