Laboratory demonstration of geopotential measurement using transportable optical clocks

We report an experimental demonstration of geopotential difference measurement using a pair of transportable ^(40)Ca^(+) optical clocks(TOC-729-1 and TOC-729-3)in the laboratory,each of them has an uncertainty of 1.3×10^(−17) and an instability of 4.8×10^(−15)/√τ.Referenced to a stationary clock of TOC-729-1,the geopotential difference measurements are realized by moving TOC-729-3 to three different locations and the relevant altitude differences are measured with uncertainties at the level of 20 cm.After correcting the systematic shifts(including gravitational red shift),the two-clock frequency difference is measured to be–0.7(2.2)×10^(−17),considering both the statistic(1.0×10^(−17))and the systematic(1.9×10^(−17))uncertainties.The frequency difference between these two clocks is within their respective uncertainties,verifying the reliability of transportable ^(40)Ca^(+) optical clocks at the low level of 10^(−17).

Suppression of servo error uncertainty to 10^(-18)level using double integrator algorithm in ion optical clock

A universal locking model for single ion optical clocks was built based on a simple integrator and a double integrator.Different integrator algorithm parameters have been analyzed in both numerical simulations and experiments.The frequency variation measured by the comparison of two optical clocks coincides well with the simulation results for different second integrator parameters.According to the experimental results,the sensitivity of the servo error influenced by laser frequency drift with the addition of a double integrator was suppressed by a factor of 107.In a week-long comparison of optical clocks,the relative uncertainty of the servo error is determined to be 1.9×10^(-18),which is meaningful for the systematic uncertainty of the transportable single^(40)Ca^(+)ion optical clock entering the 10^(-18)level.

Effects of Prior Corrosion with and Without Stress on the Mechanical Properties of 7475-T761 Aluminum Alloy

The effect of prior corrosion on the mechanical properties of 7475-T761 aluminum alloy was investigated by immersion test, stress corrosion test, cathode charge method and electrochemical polarization test. Results show that prior corrosion in the solution with 3 wt% NaC1 and 0.5 wt% H202 leads to mechanical properties deterioration of 7475-T761 aluminum alloy. Moreover, the elongation decreases significantly. This is mainly attributed to electrochemical corrosion and hydrogen embrittlement, in which corrosion plays a major role. Tensile stress promotes the degradation of the mechanical properties by accelerating the pitting corrosion and hydrogen embrittlement.

Corrosion Fatigue Behavior of 7A85 Aluminum Alloy Thick Plate in NaCl Solution

The S–N curves of 7A85-T7452 aluminum alloy in laboratory air and in neutral 3.5 wt% NaCl solution were obtained by axial fatigue tests. Results show that the detrimental effect of the aggressive solution was not noticeable at high-cyclic-stress regions, but the effect was significant at low-stress region. Corrosion fatigue mechanism was discussed by corrosion morphology analysis, fracture surface analysis and microstructure characterization. It was found that the corrosion fatigue crack commonly initialed at the localized intergranular corrosion site. TEM analysis showed that the microstructures of 7A85-T7452 aluminum alloy were characterized by fine and homogeneously distributed matrix precipitates, as well as continually distributed anodic grain boundary precipitates. The types of microstructures are the reason for its intergranular corrosion susceptibility. The corrosion fatigue process of 7A85 aluminum alloy in 3.5 wt% NaCl solution can be divided into four stages： the crack initiation stage, the stable growth stage with low and high growth rate and the final rupture stage. The sodium chloride solution mainly affected the crack initiation stage and the stable growth stage with low growth rate, and when the crack growth rate reached a threshold, the effect was reduced.

A Comparison Study of Wear and Fretting Fatigue Behavior Between Cr-alloyed Layer and Cr-Ti Solid-solution Layer

This work reported a comparison between the wear and fretting fatigue（FF） behaviors of a Cr-alloyed layer and a Cr-Ti solid-solution layer.The hardness and toughness of both layers were evaluated to support this comparison.The results showed that the Cr-alloyed layer had high surface hardness but poor toughness,while the Cr-Ti solid-solution layer had excellent toughness but low hardness.The FF properties of the modified Ti6A14 V alloy depended on the trade-off between two factors：wear resistance and fatigue resistance.Although the Cr-alloyed layer could effectively resist the wear in fretting areas,its poor toughness caused the fatigue resistance to drop sharply and hence led to a premature failure in FF test.Due to the relatively good fatigue resistance,the Cr-Ti solid-solution layer had slightly higher FF life than that of the Cr-alloyed layer;however,its low hardness resulted in severe wear in correspondence with the fretting area and thus a failure to improve the FF properties of Ti6A14 V alloy.When combined with shot peening post-treatment,the FF life of both layers increased by about three times compared to that of the Ti6A14 V alloy.A further study showed that the poor toughness or low hardness still exerted negative influence on combination-treated samples.