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Random and periodic representations of composite microstructures are inherently different both in terms of the resultant range of stresses that each phase carries as well as the total load over the entire volume comprising both matrix and fiber phases. In this study, an algorithm was developed to generate random representative volume elements (RVE) with varying volume fractions and minimum distances between fibers. The random microstructures were analyzed using finite element models (FEM) and the results compared to those for periodic microstructured RVEs in terms of the range of stress values, maximum stress, and homogenized stiffness values. Using a large number of random RVE analyses, a meaningful estimation for range and average maximum stress in the matrix phase was achieved. Results show that random microstructures exhibit a much larger range of stress values than periodic microstructures, resulting in an uneven distribution of load and distinct areas of high and low stress concentration in the matrix. It is shown that the maximum stress in the matrix phase, often responsible for failure initiation, is largely dependent on the random morphology, minimum distances between fibers, and volume fraction. Moreover, it is shown that the predicted overall load-carrying capacity of the matrix changes depending on the use of random or periodic microstructures.

Resin-rich areas in composite laminates can occur as inter-laminar resin 'layers' between plies or as intra-laminar resin 'pockets' within a single layer. In this work, numerical methods are used to study the effects of resin pockets on the transverse stiffness and failure initiation of carbon-epoxy composites. Random, or non-uniform, representative volume elements (RVE) with and without embedded resin pockets were studied. Three different types of samples with predefined volume fractions (V f) were analyzed, and data relating to the influence of resin pockets on homogenized stiffness and the strain at which failure initiates was collected and reported. Based on a control sample for each volume fraction, two methods were used to create RVE samples with resin pockets. In one, the distances between fibers were maintained and fibres removed to create the resin pocket, with a corresponding decrease in (V f). In the second method, the V f was maintained and fibers were moved to create the resin pocket, with a corresponding reduction in the distance between fibers. It is shown that intra-laminar resin pockets can reduce both the stiffness and the failure strain of composite materials. Stiffness was reduced in samples where the resin pocket resulted in a reduced volume fraction. For samples with the same volume fraction, particularly for high V f composites (e. g. 60%), the failure initiation strain in the matrix was, on average, 20% lower for samples with resin pockets compared to samples without resin pockets.

Fiber steering is one of the promising capabilities of Automated Fiber Placement (AFP) technology in manufacturing of advanced composite structures with spatially tailored properties. The so-called variable stiffness (VS) composites have considerable scope to outperform their traditionally made constant stiffness (CS) counterparts. However, there are several design and manufacturing challenges to be addressed before practically using them as structural components. In this work we demonstrate the design, manufacturing and testing procedure of a variable stiffness (VS) composite cylinder made by fiber steering. The improved bending-induced buckling performance is the objective of the VS cylinder to be compared with its CS counterpart. The experimental results show that the buckling capacity of the VS cylinder is about 18.5% higher than its CS counterpart.

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