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Recovery of microfields in fiber-reinforced composite materials: Principles and limitations.

机译：纤维增强复合材料中微区的恢复：原理和局限性。

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摘要

A detailed investigation of the limitations and errors induced by modeling a composite layer composed of straight carbon fibers embedded in an epoxy matrix as an homogenous layer with Cauchy effective moduli is performed. Specifically, the material system studied has IM7 carbon fibers arranged in a square array and bonded together with 8552 epoxy resin (IM7/8552). The finite element method is used to study the effect of free surfaces on the local elastic fields in 0°, 45° and 90° laminae, in which as many as 256 individual fibers are modeled. Through these analyses, it is shown that a micro-boundary layer, analogous to the macro-boundary layer observed in composite laminates, is developed at the microlevel. Additionally, [0/90]s and [90/0]s laminates are studied to investigate the joint action of the macro- and micro-boundary layers. Unless otherwise noted, fiber volume fractions of Vƒ=0.20 and Vƒ=0.65 are selected and the domains are subjected to uniform axial extension. Although this study is done for a highly idealized geometry (i.e. with a single material system and under a simple loading condition) the principles of periodicity, symmetry and antisymmetry used to efficiently perform a direct numerical simulation with a large number of fiber inclusions is general, and can be applied to more complicated geometries and boundary conditions. The purpose of the current work is to be the first step in a building block approach to understanding the interaction of multiple scales in fiber-reinforced composites through direct numerical simulations.;The main part of the current manuscript focuses on the characterization of a micro-boundary layer that develops in fiber reinforced composite layers. This phenomena results from the changing constraints on the constituent phases as a result of discontinuities, such as free surfaces or ply interfaces. The effect is most pronounced in laminae that have a fiber termination intersecting a free surface, and appears to be maximized in a 90° lamina. In an individual lamina, under uniform extension, the micro-boundary layer emanating from a free surface intersected by a fiber termination is analogous to the macro-boundary layer described by Pipes and Pagano.;One consequence of the micro-boundary layer is a variation of apparent moduli in this region. The use of homogeneous effective moduli, whether they are based on Cauchy elasticity, micropolar elasticity, or some other higher order theory, cannot capture the effect in composite laminae. Methods based on the use of modified effective modulus theory (e.g. GOALS or Voroni cell finite elements) are capable of capturing this effect. This is because these types of methods place a realistic representation of the microstructure in critical locations. Accuracy results when correct microstructure overlaps correct boundary conditions.;The methods used to perform direct numerical simulations on individual laminae are extended to cross-ply laminates (i.e. [0/90]s and [90/0]s). In this final study of the current manuscript, many interesting results are observed because of the joint action of the micro- and macro-boundary layers. First, more error is observed in the 0° plies than in the individual 0° laminae under uniform extension. This result is expected because of the differences in loading applied to the layers in the different configurations. In agreement with other studies, the largest amount of error in a 0° ply is observed at the free-surface ply interface with a 90° ply. In addition, a stacking sequence effect is observed in the 0° plies.;Compared to the individual laminae results, there is an increased error in the effective modulus stiffness observed in the interior of the individual plies of the cross-ply laminates. However, the largest errors in the entire laminate are observed at the free surface intersected by fiber terminations in the 90° plies. Rather unexpectedly, the overall maximum error in the 90° plies is removed from the interface with the 0° plies for both stacking sequences. For the current laminate and loading configuration, the errors in the 90° plies are bounded by the errors observed in the individual 90° laminae under uniform extension. This result indicates that for fiber-reinforced polymer-matrix composites, it might be possible to estimate the overall maximum error in a multiscale analysis from a relatively simple direct numerical simulation of a 90° layer. (Abstract shortened by UMI.).
机译：进行了详细的调查和限制，这些限制和错误是通过对复合层进行建模而造成的，该复合层是由嵌入在环氧树脂基体中的直碳纤维构成的，具有柯西有效模量的均质层。具体而言，所研究的材料系统具有以正方形阵列排列并与8552环氧树脂（IM7 / 8552）粘合在一起的IM7碳纤维。有限元方法用于研究自由表面对0°，45°和90°层中的局部弹性场的影响，其中模拟了多达256条单纤维。通过这些分析，表明在微观水平上形成了类似于在复合层压板中观察到的大边界层的微边界层。此外，对[0/90] s和[90/0] s层压板进行了研究，以研究大边界层和微边界层的结合作用。除非另有说明，否则选择纤维体积分数Vƒ= 0.20和Vƒ= 0.65，并且对畴进行均匀的轴向延伸。尽管这项研究是针对高度理想的几何形状（即在单一材料系统和简单载荷条件下进行的）进行的，但通常使用周期性，对称和反对称原理来有效地对大量纤维夹杂物进行直接数值模拟，并可以应用于更复杂的几何形状和边界条件。当前工作的目的是通过直接数值模拟来了解纤维增强复合材料中多个尺度之间相互作用的基本方法的第一步。本手稿的主要部分着重于对微结构的表征。在纤维增强复合材料层中形成的边界层。这种现象是由于不连续（例如自由表面或层界面）对组成相的约束条件发生变化而导致的。这种效应在具有与自由表面相交的纤维终端的薄片中最为明显，并且似乎在90°的薄片中被最大化。在单个薄层中，在均匀延伸下，由纤维终端相交的自由表面发出的微边界层类似于Pipes和Pagano所述的大边界层;;微边界层的一个后果是变化在该区域的视模量无论是基于柯西弹性，微极性弹性还是其他高阶理论，均质有效模量的使用都无法捕获复合层中的效应。基于修改后的有效模量理论（例如GOALS或Voroni单元有限元）的方法能够捕获这种效果。这是因为这些类型的方法可以在关键位置真实显示微观结构。当正确的微观结构与正确的边界条件重叠时，将产生准确的结果。用于对单个薄片进行直接数值模拟的方法已扩展到交叉层压板（即[0/90] s和[90/0] s）。在当前手稿的最终研究中，由于微边界层和宏观边界层的共同作用，观察到了许多有趣的结果。首先，在均匀伸展下，在0°层中观察到的误差要大于单个0°层中的误差。由于施加到不同配置中的层上的负载的差异，因此可以得到此结果。与其他研究一致，在自由曲面层与90°层的界面处观察到0°层中的误差最大。另外，在0°层中观察到堆积顺序效应。与单个层状结果相比，在交叉层状层压体的各个层内部观察到的有效模量刚度的误差增加。然而，在整个层压制品中最大的误差是在90°帘布层中与纤维终端相交的自由表面上观察到的。出乎意料的是，两个堆叠序列的90°层的总体最大误差都从0°层的界面中删除。对于当前的层压板和加载配置，在90°帘布层中的误差受在均匀延伸下在各个90°薄片中观察到的误差的限制。该结果表明，对于纤维增强的聚合物基复合材料，可以通过相对简单的90°层直接数值模拟，在多尺度分析中估算整体最大误差。 （摘要由UMI缩短。）。

著录项

  • 作者

    Ritchey, Andrew J.;

  • 作者单位

    Purdue University.;

  • 授予单位 Purdue University.;
  • 学科 Aerospace engineering.;Materials science.
  • 学位 Ph.D.
  • 年度 2012
  • 页码 203 p.
  • 总页数 203
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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