膨胀剂对混凝土变形性能的影响

膨胀剂在我国被广泛地应用于补偿大体积混凝土收缩。不同品种的膨胀剂对混凝土的补偿收缩效果有所不同。氧化钙类和硫铝酸钙类(AF t)可补偿混凝土的早期收缩,但分别在28和60 d后仍会出现后期收缩现象。水泥品种、养护条件对膨胀剂的膨胀效果有不同的影响。掺氧化镁类膨胀剂的混凝土没有出现后期收缩现象,比较适合水工大体积混凝土。在做好基础混凝土温度控制的同时,选择适宜品种的膨胀剂,可有效地控制混凝土的变形,减少收缩开裂,提高混凝土的结构耐久性能和建筑物的质量。     

基于机体振动的直升机桨毂阻尼故障诊断

研究了仅用机体振动信号诊断旋翼减摆器阻尼失效、轴向铰卡涩和水平铰卡涩3种桨毂阻尼故障的可行性。在旋翼试验台上分别设置上述3种阻尼故障,测取机体振动响应,利用快速傅立叶变换分析其频谱特征。分析表明,3种阻尼故障引起的机体振动谱图互不相同。采用概率神经网络实现了3种阻尼故障的正确分类。研究表明,仅用机体振动响应实现旋翼阻尼故障诊断是可行的。     

固体火箭发动机多层结构壳体的导波频散特性分析

应用全局矩阵法,推导了多层结构壳体的导波频散方程,并分别针对固体火箭发动机四种不同层数的壳体导波频散曲线进行了求解,发现当结构层数和第一层厚度增加时,各模式曲线的间隔缩小．曲线数目增加,并有向零频方向靠拢的趋势。同时研究了粘接质量变化对频散特征的影响,随着胶层质量相对变差,频散曲线总体向左漂移。     

A variational Bayesian-based robust adaptive filtering for precise point positioning using undifferenced and uncombined observations

In the application of precise point positioning (PPP), especially in the dynamic mode, the classical Kalman filter (KF) usually produces a large number of estimation errors or diverges when there are gross errors in the observation data or unexpected turbulences occur in target motion state or both of them. For such problem, a variational Bayesian (VB)-based robust adaptive Kalman filtering (VB-RAKF) is proposed in this paper. This filter introduces a classification robust equivalent weight function to resist observation gross error and the inverse Wishart prior to model inaccurate process noise covariance matrix (PNCM). To improve the instantaneous accuracy of state estimation, the VB approach is used to obtain better estimations of inaccurate PNCM. Several sets of observation data collected by IGS reference stations and vehicles are employed to check the robustness and positioning accuracy of the VB-RAKF model. The results show that the VB-RAKF algorithm is more robust than the KF, and can effectively resist the gross error in observation data and control state disturbance. In the IGS reference station tests, when compared to the KF, the static positioning accuracies of the VB-RAKF in the north, east and up directions are improved by 13%, 8% and 22%, respectively, and the simulated dynamic positioning accuracies of the VB-RAKF in the north, east and up directions are improved by 19%, 9% and 21%, respectively. The in-vehicle dynamic test verifies that the VB-RAKF outperforms the KF, and shows that the VB-RAKF has better performance than the KF when dealing with observation data which has obvious gross errors, and similar performance as the KF when gross errors are small.     

An improved real-time cycle slip correction algorithm based on Doppler-aided signals for BDS triple-frequency measurements

Doppler, which is an instantaneous GNSS observable signal, has been proven effective in determining velocity and acceleration due to its high availability and accuracy. We propose a real-time triple-frequency cycle slip correction (CSC) method based on Doppler-aided signals because Doppler shift is time-independent and immune to cycle slips. When the sampling interval is less than 1 s, cycle slips on triple-frequency can be detected and repaired using pure Doppler data with high reliability; however, this method cannot be used when the sampling interval exceeds 1 s because the integral cumulative error of Doppler increases significantly. For such cases, a modified triple-frequency CSC approach has been developed based on the raw phase and smoothed code data that was refined using the Doppler signal. To suppress the effect of the integral Doppler error, a balance factor is introduced to adjust the contributions of the raw code and Doppler observables. After the refinement of the GNSS data, three independent combinations need be selected to detect and repair cycle slips with triple-frequency observations. Four constrained criteria have been proposed to select optimal combinations that can reduce the residual ionospheric delay (RID) and measurement noise to a low level. Finally, experiments were carried out to test the performance of the new method using real triple-frequency BDS observations (GPST: 3:15:00–5:55:00, March 23, 2018). The results show that pure Doppler can detect and repair cycle slips effectively with small intervals, and modified Hatch-Melbourne-Wübbena (HMW) method based on Doppler-aided signals can achieve 99.7% success rate in cycle slip correction with large intervals (up to 30 s).     

Performance assessment of real-time multi-GNSS integrated PPP with uncombined and ionospheric-free combined observables

For precise position services, the real-time precise point positioning (PPP) is a promising technology. The real-time PPP performance is expected to be improved by multi-system combination. The performance of real-time multi-system PPP needs to be periodically investigated, with the increasing number of available satellites and the continuously improved quality of real-time precise products of satellite clocks and orbits. In this study, a comprehensive performance assessment is conducted for the four-system integrated real-time PPP (FSIRT-PPP) with GPS, BDS, Galileo and GLONASS in both static and kinematic modes. The datasets from 118 stations spanning approximately a month are used for analysis, and the real-time stream CLK93 is employed. The superior performance of FSIRT-PPP is validated by comparing with the results of GPS/BDS, GPS/Galileo, GPS/GLONASS, GPS-only, BDS-only, Galileo-only and GLONASS-only cases. The FSIRT-PPP using ionospheric-free (IF) combined observables can achieve a convergence time of 10.9, 4.8 and 11.8 min and a positioning accuracy of 0.4, 0.5 and 0.7 cm in the static mode in the east, north and up directions, respectively, while the derived statistic is 15.4, 7.0 and 16.4 min, and 1.6, 1.2 and 3.4 cm in the kinematic mode in the three directions, respectively. Moreover, we also compare the position solutions of real-time PPP adopting IF combined and uncombined (UC) observables, and prove the mathematical equivalence between the two PPP models in the converged stage, provided that there are no external ionospheric corrections or constraints given to the estimated ionospheric delays in the UC model. The difference between the fully converged positioning accuracy of IF-based and UC-based real-time PPP is marginal, but the UC-based real-time PPP has longer convergence time due to the influence of the significant unmodeled time-varying errors in the real-time precise products as well as the different parameterization between them. For completeness, the real-time kinematic PPP results in harsh environments and the post-processed PPP results are also presented.     

Acceptance sampling plan of accelerated life testing for lognormal distribution under time-censoring

Lognormal distribution is commonly used in engineering.It is also a life distribution of important research values.For long-life products follow this distribution,it is necessary to apply accelerated testing techniques to product demonstration.This paper describes the development of accelerated life testing sampling plans(ALSPs)for lognormal distribution under time-censoring conditions.ALSPs take both producer and consumer risks into account,and they can be designed to work whether acceleration factor(AF)is known or unknown.When AF is known,life testing is assumed to be conducted under accelerated conditions with time-censoring.The producer and consumer risks are satisfied,and the size of test sample and the size of acceptance number are optimized.Then sensitivity analyses are conducted.When AF is unknown,two or more predetermined levels of accelerated stress are used.The sample sizes and sample proportion allocated to each stress level are optimized.The acceptance constant that satisfies producer and consumer risk is obtained by minimizing the generalized asymptotic variance of the test statistics.Finally,the properties of the two ALSPs(one for known-AF conditions and one for unknownAF conditions)are investigated to show that the proposed method is correct and usable through numerical examples.     

Cracking and Mechanical Properties of Airport Concrete Pavement with Fiber and Expansive Agent

Polypropylene fiber and expansive agent are used in airport concrete to improve its shrinkage cracking resistance and mechanical properties. The concrete specimens with amount content of polypropylene fiber or expansive agent or both of them are prepared. The morphology of specimens is observed by scanning electron microscope, the time when the first crack occurred is recorded through slap test, and the mechanical properties such as compressive strength and impact energies of concrete are measured. The results show that polypropylene fiber in concrete can reduce the shrinkage and delay the first crack, improve the impact resistance obviously, and improve the compressive strength slightly. Expansive agent can compensate the shrinkage and reduce cracks of concrete pavement markedly, and improve the mechanical properties of concrete pavement slightly. The study provides recommendations for cracking control of airport concrete pavement in the future.