Multiscale modeling of mechanical behavior and failure mechanism of 3D angle-interlock woven aluminum composites subjected to warp/weft directional tension loading

The mechanical behavior and progressive damage mechanism of novel aluminum matrix composites reinforced with 3D angle-interlock woven carbon fibers were investigated using a multiscale modeling approach. The mechanical properties and failure of yarns were evaluated using a microscale model under different loading scenarios. On this basis, a mesoscale model was developed to analyze the tensile behavior and failure mechanism of the composites. The interfacial decohesion, matrix damage, and failure of fibers and yarns were incorporated into the microscopic and mesoscopic models. The stress–strain curves and fracture modes from simulation show good agreement with the experimental curves and fracture morphology. Local interface and matrix damage initiate first under warp directional tension. Thereafter, interfacial failure, weft yarn cracking, and matrix failure occur successively. Axial fracture of warp yarn, which displays a quasi-ductile fracture characteristic, dominates the ultimate composites failure. Under weft directional tension, interfacial failure and warp yarn rupture occur at the early and middle stages. Matrix failure and weft yarn fracture emerge simultaneously at the final stage, leading to the cata-strophic failure of composites. The weft directional strength and fracture strain are lower than the warp directional ones because of the lower weft density and the more serious brittle fracture of weft yarns.     

珠海一号高光谱影像云检测算法

珠海一号高光谱卫星具有高空间、高光谱、高时间分辨率等特点,有效推动了高光谱遥感数据在农林环境、自然资源探测等领域的广泛应用,其中高精准的云检测是遥感数据预处理的关键步骤。如何对高光谱图像有效特征提取并克服传统云检测方法特征复杂、算法参数多、计算量大、鲁棒性差等缺陷,是高光谱云检测研究的关键问题。为此,提出了一种多尺度特征融合的U型结构网络,模型首先利用残差模块进行特征编码,并将编码进行多尺度融合,在网络的跳跃连接处引入了坐标注意力机制提取有用信息,最后通过残差解码得到输出结果。实验前首先利用主成分分析降维,将高光谱数据重构为4维影像数据,然后通过数据标注与数据增强,建立珠海一号高光谱影像云检测数据集。采用了38-Cloud云数据集训练初始网络参数,随后利用构建的数据集进行迁移学习。实验结果表明,对于所建立的珠海一号高光谱云检测数据集,所提方法的像素准确率达到92.28%,可以实现高精度的高光谱遥感影像云检测。     

Fractal signatures for multiscale processing of hyperspectral image data

A multiscale approach to hyperspectral image data analysis using fractal signatures was proposed and implemented in the Interactive Data Language (IDL). For 2-D hyperspectral curves, fractal signature measures the changes in curve length with changing scale. Using NASA’s Earth Observing-1 (EO-1) Hyperion image from a study area near Denton, Texas, USA, the capabilities of fractal signatures in discriminating different land cover types were presented in three different ways: (1) fractal signature curves, (2) distances between fractal signatures, and (3) fractal signature images. The asymmetry in length measurement was found to be effective in handling hyperspectral curves obtained from Hyperion radiance data. The contribution of fractal signature images was shown through comparison of image classification results. The results from the Hyperion radiance data suggest that fractal signatures at certain scales can reveal important differences in land cover types.     

An efficient analytical homogenization technique for mechanical-hygrothermal responses of unidirectional composites with applications to optimization and multiscale analyses

The elasticity-based Locally Exact Homogenization Theory (LEHT) is extended to study the mechanical-hygrothermal behaviors of unidirectionally-reinforced composites. Based on the framework developed previously, thermal and moisture effects are incorporated into the LEHT to study the homogenized and localized responses of heterogeneous materials, which are validated using available analytical and numerical techniques. The LEHT programs are then encapsulated as subroutines with Input/Output (I/O) interfaces, to be readily applied in different computational scenarios. In order to illustrate the efficiency of the LEHT, the theory is firstly coupled to the Particle Swarm Optimization (PSO) algorithm in order to minimize the axial thermal expansion mismatch in hexagonal and square fiber arrays by tailoring the fiber volume fraction. The LEHT is then implemented into the lamination theory to study fabrication-induced residual stresses arising during the cool-down process which introduces local laminate stresses owing to thermo-mechanical property mismatch between plies. Both of these applications illustrate the efficiency and accuracy of the LEHT in generating effective properties and local stress distributions, making the theory a golden standard in validating other analytical or numerical techniques as well as a reliable tool in composite design and practice for professionals and non-professionals alike.     

具有理论阈值的子波域滤波方法

本文在对多尺度滤波和子波域滤波综述的基础上，用奇异性和自相似性理论对信号与噪声的不同性质进行了分析，揭示了信号与噪声的多尺度数学机理，并对基于此机理的Ｍａｌａｔ去噪声算法和子波域波波（ＷＴＤＦ）算法进行了研究，首次给出了ＷＴＤＦ的阈值选取方法和理论证明，完善和推广了ＷＴＤＦ，通过仿真计算表明，具有理论阈值的ＷＴＤＦ较之传统的试凑阈值ＷＴＤＦ性能有明显的改善。     

基于小波变换的运动点目标检测方法

在运动点目标的检测问题中，当目标轨迹产生随机偏移时已有检测算法的效果很不理想。为解决这一难题，本文定义了一种新的小波变换——方向小波变换，提出了一种基于方向小波变换的多尺度点目标检测方法，能够稳定、有效地检测出各种作近似匀速直线运动的点目标轨迹     

Experimental and numerical investigation of mechanical behavior of plain woven CFRP composites subjected to three-point bending

The mechanical behavior of plain woven Carbon Fiber-Reinforced Polymer (CFRP) composites under Three-Point Bending (TPB) is investigated via experimental and numerical approaches. Multiscale models, including microscale, mesoscale and macroscale models, have been developed to characterize the TPB strength and damages. Thereinto, Representative Volume Elements (RVEs) of the microscale and mesoscale structures are established to determine the effective properties of carbon-fiber yarn and CFRP composites, respectively. Aimed at accurately and efficiently predicting the TPB behavior, an Equivalent Cross-Ply Laminate (ECPL) cell is proposed to simplify the inherent woven architecture, and the effective properties of the subcell are computed using a local homogenization approach. The macroscale model of the TPB specimen is constructed by a topology structure of ECPL cells to predict the mechanical behavior. The TPB experiments have been performed to validate the multiscale models. Both the experimental and numerical results reveal that delamination mainly appears in the top and bottom interfaces of the CFRP laminates. And matrix cracking and delamination are identified as the significant damage modes during the TPB process. Finally, the quasi-static and dynamic behaviors of plain woven composites are discussed by comparing the results of Low-Velocity Impact (LVI) and TPB simulations.