Accurate fatigue crack evaluation is vital for structural life management. Long-term aircraft monitoring faces challenges from heteroscedasticity—changing uncertainty distributions under varying operational conditions—which degrades conventional diagnostics’ accuracy. This study develops a heteroscedastic Gaussian process model combined with guided-wave SHM, using quantile regression to estimate crack evaluation distribution. The median regression output replaces traditional mean estimation, while quantile-specific confidence intervals explicitly represent heteroscedastic uncertainties. Validated through aircraft attachment lug fatigue tests, the method enhances evaluation quality and uncertainty quantification capabilities.

In this paper, we optimize the spectrum efficiency (SE) of uplink massive multiple-input multiple-output (MIMO) system with imperfect channel state information (CSI) over Rayleigh fading channel. The SE optimization problem is formulated under the constraints of maximum power and minimum rate of each user. Then, we develop a near-optimal power allocation (PA) scheme by using the successive convex approximation (SCA) method, Lagrange multiplier method, and block coordinate descent (BCD) method, and it can obtain almost the same SE as the benchmark scheme with lower complexity. Since this scheme needs three-layer iteration, a suboptimal PA scheme is developed to further reduce the complexity, where the characteristic of massive MIMO (i.e., numerous receive antennas) is utilized for convex reformulation, and the rate constraint is converted to linear constraints. This suboptimal scheme only needs single-layer iteration, thus has lower complexity than the near-optimal scheme. Finally, we joint design the pilot power and data power to further improve the performance, and propose an two-stage algorithm to obtain joint PA. Simulation results verify the effectiveness of the proposed schemes, and superior SE performance is achieved.

Accurate damage evaluation for an aircraft by using guided wave-based structural health monitoring (SHM) technology is in critical demand. An aircraft experiences various uncertainties during its service life, such as the time-varying environment, operational conditions, and different flight missions. Additionally, the actual aircraft structure involves complex structural forms such as varied thickness, stiffeners, ribs, and large cutouts, leading to a strong uncertainty of damage initiation and propagation. Consequently, when applied to an individual aircraft structure, the diagnosis model trained by prior data inevitably introduces errors, and the errors accumulate as the damage propagates, which brings challenges in engineering application. To achieve individual aircraft structure’s more accurate damage evaluation, an on-line updated guided wave-based Gaussian process (GP) damage evaluation method is investigated in this paper. Multi-source data from the design, service, and maintenance stages are utilized to continuously update and evolve the GP model, enabling its accurate evaluation throughout the long-term service life.

可变后掠翼飞行器通过改变其机翼后掠角,可实现在不同飞行速度条件下,达到最佳飞行状态.为了实现在单驱动下机翼后掠角变化时,同时实现机翼部分蒙皮的展收,基于平面复合连杆机构,提出一种可变后掠翼联动驱动机构.为了解决该机构函数生成与运动生成综合相结合的尺寸综合问题,将复合连杆机构拆分为3个子机构,通过矢量方程法对机构进行分析,建立复合连杆机构的矢量环路方程,并对机构的可动性进行分析.结合机构矢量环路方程,提出一种利用优化算法与运动学分析软件判断机构可动性、优化机构性能的尺寸综合方法.结合相应实例,使用所提方法对机构进行设计,获得了满足可动性要求、尺寸约束,且包络面积最小的机构尺寸构型,证明了所提方法的有效性.

目的 改善直壁方形盒制件在传统成形路径中厚度减薄严重的问题.方法 采用一种上下交替往复成形轨迹的方法对TB8钛合金直壁方形盒进行试验研究,分析不同成形轨迹下制件壁厚的变化规律,测量直壁方形盒的最大减薄情况,并在此基础上讨论减薄程度与制件口部翘曲间的相关性.结果 采用上下交替往复成形轨迹的方法能使制件壁厚减薄程度减轻、壁厚分布更加均匀且壁厚突破正弦定律,经测量制得的TB8钛合金直壁方形盒的最大减薄率为42.6％,比经自上而下成形轨迹的成形方法制得的TB8钛合金直壁方形盒的最大减薄率降低了23％,同时能有效减轻制件口部翘曲程度.结论 上下交替往复成形轨迹的成形方法是改进传统自上而下成形方法制作直壁方形盒的有效途径.

Accurate evaluation of fatigue crack by using structural health monitoring (SHM) methods is very important to the life management of aircraft structures. However, fatigue crack propagation is always a complicated process for individual aircraft structures during their long-term service. And the monitoring has to be performed under different environmental and operational conditions. These factors result in the uncertain distribution of the damage index obtained by SHM. The distribution usually changes along with the service time, which can be defined as heteroscedasticity. The heteroscedasticity characteristic of the uncertain distribution of damage index has an important negative impact on the SHM based diagnostics algorithms if not considered during the evaluation. However, till now, few researchers have considered this important aspect. To improve the evaluation accuracy under different environmental and operational conditions for individual aircraft, this paper proposes a new guided wave-based heteroscedastic Gaussian process method. Gaussian process quantile regression is adopted to estimate the conditional quantiles of the damage index distribution during the service to deal with the heteroscedasticity. The method is validated on an attachment lug fatigue test, an important aircraft structural component. The experimental results demonstrate that the proposed method can quantify the heteroscedastic uncertainty associated and obviously improve the quality of crack evaluation. For the serious heteroscedastic specimen, the maximum evaluation is only 0.7 mm reduced from the original 7.4 mm, which is an order of magnitude.

在金属旋压工艺中,回弹是一种无法避免的成形缺陷,为了减小旋压制件的回弹,在电流辅助旋压的基础上,以AZ31B镁合金旋压件为研究对象,通过正交实验探究了电流强度、主轴转速、旋压轮进给速率与回弹角的关系,对实验结果进行了极差分析和方差分析,得到了工艺参数对回弹角的影响规律及最小回弹角的工艺参数组合.以实验数据作为训练样本,建立了BP神经网络模型进行回弹角预测,将实验得到的工艺参数组合作为输入,进行回弹角预测及实验验证,结果表明:BP神经网络模型的预测结果与实验结果的相对误差小于3％,能够较准确地预测回弹角,为实际生产和进一步的实验研究提供了理论指导.

This paper investigates the sum rate optimization of uplink massive multiple-input multiple-output (MIMO) system with imperfect channel state information (CSI) and maximum ratio combining (MRC) under the constraints of maximum power and minimum rate, and power allocation (PA) schemes are developed to improve the rate. With the help of concave-convex procedure (CCCP) method, a near-optimal PA scheme is proposed to transform the no-concave maximization problem into a concave one. Considering that both small-scale and large-scale fading information are required in near-optimal PA scheme, which will result in high complexity, a suboptimal PA scheme under the case of large number of receive antennas is presented, which only needs large-scale fading information without real-time estimation and frequent feedback. Moreover, it has the rate close to that of near-optimal scheme but with lower complexity. Simulation results show that the sum rate obtained by the near-optimal PA scheme can match that offered by the benchmark scheme well, and the suboptimal scheme can obtain the rate close to that of near-optimal scheme, especially for large number of receive antennas, which verifies the effectiveness of the proposed schemes.

星载抛物面天线在轨运行时由于姿态调整或环境因素变化,可能产生振动,从而降低其工作性能.为了减弱振动的影响,需采取加装张紧绳索的方法来提升天线刚度.提出了一种新型张紧绳索多层设计方法,通过将抛物面天线划分为多层,调整各层绳索的张紧力,从而提高天线结构刚度,同时尽量降低张紧力引起的形面误差.为了验证所提方法的有效性,建立了天线的有限元模型.在此基础上进行了有限元分析,研究了不同张紧力参数配置下的结构刚度和形面误差,并以提升结构刚度的同时降低张紧力引起的形面误差为目标,对张紧力参数进行了优化设计.为了提高计算效率,采用响应面法建立代理模型参与优化迭代计算.采用非劣排序遗传算法(N S GA-Ⅱ)完成优化迭代计算,优化后的张紧力参数使天线的性能得到了进一步的提升,为构架式可展开抛物面天线的设计提供理论指导.

In this paper, the cross-layer design (CLD) performance of uplink massive MIMO system is studied by combining the discrete rate adaptive modulation with truncated automatic repeat request over composite Rayleigh fading channel. As shown in the performance analysis, each user's effective signal-to-noise ratio (SNR) and the corresponding conditional probability density function are, respectively, derived. Furthermore, we deduce the switching thresholds for CLD by means of the approximation of the PER with the constraint of the target packet error rate (PER). Based on switching thresholds, using the appropriate numerical integration method, average PER (APER) and overall average spectral efficiency (ASE) of massive MIMO with CLD are derived, and resultant closed-form expressions of APER and ASE can be achieved. The theoretical results above can agree with the corresponding simulations, and thus the system performance can be feasibly assessed. Simulation results illustrate that the CLD system is able to increase the ASE while maintaining the target PER, and thus the theoretical analysis is valid. Besides, with the increase of the maximum retransmission number, the ASE increment will decrease while the PER will increase. Moreover, the system performance will become better with the number of receive antennas increases, as expected.

In this paper, spectrum efficiency (SE) and energy efficiency (EE) performances of uplink massive multiple-input multiple-output (MIMO) systems with continuous-rate adaptive modulation and imperfect channel state information (CSI) are investigated over Rayleigh fading channels. Given a target BER, closed-form SE and EE expressions are derived. With these results, a constrained non-concave optimization problem of power allocation (PA) to maximize the SE is firstly formulated subject to the maximum transmit power per user. A near-optimal PA scheme with the concave-convex procedure (CCCP) method is then developed, and it can offer nearly optimal performance close to that of the optimal scheme with exhaustive search, but require lower complexity. Since this scheme needs more iterations, we propose a suboptimal scheme with reduced complexity and slight performance loss, which utilizes the property of a large number of receive antennas to approximate the objective function as a concave one, thereby reducing the number of iterations. Secondly, the PA problem for EE maximization under the same constraints is addressed. Based on the CCCP method and fractional programming theory, we propose a near-optimal PA scheme to solve this problem, and it has the EE performance very close to the optimal exhaustive search scheme. However, multi-layer iterations make this scheme have relatively higher complexity. For this reason, we propose a low-complexity suboptimal scheme with small EE loss by using variable transformation and a large number of antennas. Based on the above results, we simultaneously optimize both EE and SE. A multi-objective optimization problem (MOP) subject to the maximum power constraint and proportional data rate constraint is formulated. By means of the weighted sum method, the complicated MOP is transformed into a simple single-objective optimization problem. Then a PA scheme is proposed by using the Lagrange multiplier method to balance the EE and SE effectively. Simulation results validate the effectiveness of the proposed PA schemes in term of high SE and EE, and illustrate the fundamental tradeoff between EE and SE for different parameter settings.

A cross-layer design (CLD) scheme for uplink massive MIMO system in the presence of imperfect channel state information (CSI) is studied by combining adaptive modulation and automatic repeat request, and the corresponding system performance is investigated well. The received signal to noise ratio and conditional probability density function of each user based on imperfect CSI over Rayleigh fading channel are derived respectively. Furthermore, the switching thresholds of CLD scheme with the constraint of instantaneous packet error rate (PER) are calculated by using approximate PER. Based on the numerical integration formulas, closed-form theoretical average spectral efficiency (ASE) and average PER are obtained. The simulation results illustrate that the theoretical analysis is valid because it matches the corresponding simulations. Besides, with the increase of the maximum retransmission number, the ASE increment will decrease while the PER will increase, and the performance can be improved with the number of receive antennas increases or the estimation error variance decreases.

An essential procedure for fatigue crack evaluation based on structural health monitoring is to establish the mapping relationship between structural health monitoring signal features and the crack length. However, crack evolution in aerospace structure is a complicated process affected by many condition factors, such as the temperature, mission load, structural boundary condition and material characteristic. These factors are coupled with each other and cause serious uncertainties to the crack evolution in the structure. Ordinary methods by mapping or calibration using a set of same structural specimens or simulation to achieve an average mapping result cannot provide accurate results to a specific structure monitored, since this structure inevitably has deviations from the average result. Aiming at solving this problem, an on-line updated guided wave Gaussian Process based evaluation method is proposed in this paper, which differs from the current Gaussian Process research in the way of model dynamic updating. The proposed method is verified on attachment lug structures, which is a kind of important connection part commonly used in aircrafts. The effectiveness of the proposed method is well demonstrated.

The particle filter (PF) has shown great potential for on-line fatigue crack growth prognosis by combining crack measurements from structural health monitoring (SHM) techniques. In this method, a key problem is to construct the mapping between the feature extracted from SHM signals and the crack size. However, this mapping may be inaccurate since the data used for establishing the mapping is affected by uncertainties from sources like damage geometries, sensor placements, and boundary conditions. To deal with this problem, this paper proposes an on-line updating Gaussian process (GP) measurement model within the PF based crack prognosis framework. The GP measurement model outputs the mean and variance of the crack length corresponding to the feature of SHM signals, which are input into the PF for evaluating the posterior estimation of the crack length for crack prognosis. Then, this posterior estimation is sequentially appended to the GP dataset for updating the measurement model. Moreover, once crack inspection data is obtained, it is combined with existing SHM data for additional updating of the GP measurement model. Validations are performed on the fatigue test of attachment lug structures, in which the guided wave based SHM technique is applied for crack monitoring. As the cyclic load may cause intricate influences on the guided wave propagation, it is more difficult to quantify the crack length. The validation result shows that the on-line updating GP measurement model can effectively map the feature of SHM signals to the crack length, and result in accurate crack growth prognosis with the PF based method. (C) 2020 Elsevier Ltd. All rights reserved.

Fatigue crack growth prognosis is a fundamental task for ensuring structural integrity. Recently, attentions have been gradually paid to methods that combine on-line measurements of structural health monitoring (SHM) with the particle filter based prognostics. However, most studies used the basic particle filter algorithm, which has the intrinsic particle impoverishment problem. There are different kinds of improvements made to the basic particle filter in the field of particle filtering. Few studies have been carried out to discuss their applicability to on-line fatigue crack growth prognosis combining with the SHM technique. Therefore, this paper compares four different improved particle filter algorithms for the on-line application of fatigue crack growth prognosis by integrating the guide wave based SHM. Studies are carried out on the basis of fatigue tests performed on the attachment lug, which is a key structural element in engineering structures. Two cases are involved that the SHM measurement has relatively large or small errors, which are influenced by whether the measurement equation is accurately trained with historical data. The prognostic accuracy of these improved particle filters is discussed. Moreover, effects of the particle number on the performance and computational cost are analyzed.

The numerical simulation of three dimensional free bending process of copper complex axis hollow component was investigated by using the finite element analysis method based on ABAQUS software.The effects of the key process parameters including the offset between tube axis and bend die axis (u),the distance between the center of the bending die and the head of guide sleeve (M) and the Z axial push velocity (v) on the forming quality of the 3D tubular bending components were studied respectively.In addition,the stress and strain states of the forming zone were analyzed and the optimal 3D free bending forming process parameters of the complex bending component were obtained.Finally,the free bending test of copper tube was carried out based on the optimal simulation results.The results indicate that the dimensions of the bending components are similar to the simulation results and close to the design size under the forming condition with u value of 0.25 mm,M value of 26.5 mm and v value of 20 mm · s-1.There are no obvious cross section distortion,excessive thickening and thinning of wall thickness.The formed components have good quality and the simulation results provide a technique reference for the forming of the copper complex axis hollow component.

利用有限元模拟和试验相结合的方法,研究了拉形辅助的渐进成形回弹问题.以回弹角为主要指标,研究了不同的工艺参数对拉形辅助渐进成形的成形精度的影响.分析了成形角度、拉形高度、进给量、进刀方式对拉形辅助的渐进成形回弹量的影响规律,并且通过正交试验法,分析了拉形辅助的渐进成形回弹影响因素.

进行了室温下的单向拉伸试验;采用数字图像相关测量方法(DIC方法),测量了AA5754和Q235B的塑性应变比随着试样塑性变形的变化规律.分析了应变水平和材料厚度对两种材料塑性应变比、平均塑性应变比和凸耳参数的影响.结果表明两种金属板料的塑性应变比均不是恒定的,随着塑性变形量的变化而变化.在材料刚进入塑性变形状态时,塑性应变比急速上升至最大值,在随后阶段则呈现下降;对于不同厚度的试样,其塑性应变比随应变水平的变化趋势基本一致;材料的凸耳参数受应变水平的影响比较小.

In order to improve the wall thickness uniformity of incremental forming components for sheet,the incremental forming process of aluminum alloy AA5754 sheet was numerically simulated by finite element simulation software ABAQUS/Explicit. Then,the finite ele-ment model suitable for incremental forming was established,and the incremental forming experiment was carried out. Furthermore, the accuracy of model was verified by comparing simulation results and experimental results. Meanwhile, the influences of clamping opening size,forming speed and back pressure on the thickness uniformity of side wall of incremental forming components were analyzed. The re-sults show that the clamping opening size and forming speed have great influence on the wall thickness uniformity of incremental forming components,and the smaller the clamping opening size and the greater the forming speed are,the better the wall thickness uniformity is. In addition,the existence of back pressure is also beneficial to improve the wall thickness uniformity. However,more serious bulging de-fects are produced when the back pressure is over a certain value.

目的 研究材料的渐进成形翻边性能.方法 使用一种硬铝合金AA2024-O,进行渐进成形圆孔翻边实验和异形孔翻边实验,并且研究了不同成形路径的多道次渐进成形对异形孔翻边零件的影响.结果 AA2024-O渐进成形翻边的极限翻边系数是0.89,材料渐进成形的翻边成形性能高于冲压翻边成形性能;异形孔翻边零件各段的高度和减薄率存在差异.结论 压缩类翻边区域和直线段翻边区域的翻边高度较高,减薄率较小,不易发生破裂;而内曲圆弧部分属于伸长类翻边,在厚度方向减薄严重,容易破裂;内曲圆弧半径越小,变形越大,越容易破裂;成形角改变的多道次异形孔翻边件破裂严重,而成形深度改变的多道次异形孔翻边件质量比单道次翻边的翻边件质量要好.