Effect of Voids on Damage Development in Carbon Fiber-reinforced Polymer Composites

Nothing is perfect, and composite materials are no exceptions. In the micro- and meso-structure of Fiber-Reinforced Composites (FRCs), there are imperfections that are created during their manufacturing. Being called "manufacturing defects", these imperfections can influence the mechanical performance of FRCs. They include fiber waviness and misalignment, broken fibers, initial fiber/matrix debondings, initial delaminations, incomplete matrix cure, and voids. Voids are one of the main types of manufacturing defects. They can cause degradation of matrix- and interface-dominated mechanical properties of FRCs. Though the research on voids has started soon after the emergence of structural composites, accurate and systematic investigation of voids' effect has become possible only in the last decade, exploiting experimental and computational advancements. Still, there are remaining unknowns about how voids can affect the damage development in FRCs, especially about their effect on statistically-controlled properties such as matrix cracking. Matrix cracking is one of the first damage mechanisms occurring in multidirectional composite laminates under mechanical loading. Being sensitive to manufacturing defects, particularly voids, matrix cracks can lead to more dangerous forms of damage such as delamination and fiber breakage. Therefore, understanding of cracking evolution during loading is of high importance in investigation of damage progress and failure, and their sensitivity to voids in FRCs. Real-time analysis of matrix cracking is, however, challenging, in particular in carbon fiber-reinforced polymers due to their opacity. Thus, there is a need for improvement in in-situ detection of cracks. In addition to experimental analysis, simulation of matrix cracking is potentially an efficient and accurate way to evaluate the influence of voids. It can be efficient because it does not need production and testing of real composite specimens, so saving money, time, and energy. It can be accurate since void characteristics, such as shape, size, orientation, and spatial distribution, can be completely controlled, which is not the case in experimental analysis. Nevertheless, modeling of matrix cracking and its sensitivity to voids in composite laminates is not straightforward due to the stochastic nature of cracks and the multi-scale effect of voids. The current dissertation aims to analyze the effect of voids on the damage development, particularly matrix cracking, in carbon fiber-reinforced polymer composites. First, the relevant literature for the last 50 years is extensively reviewed and synthesized. Then, a broad characterization of voids in carbon/epoxy laminates is performed, using X-ray micro-computed tomography. The characteristics of voids, which are shape, size, orientation, and spatial distribution, are measured and their statistics are reported, providing a vast dataset useful for detection and modeling of voids. For analysis of the effect of voids on matrix cracking, two parallel methodologies are developed: one experimental and the other one computational. The experimental approach takes advantage of in-situ image acquisition and digital image correlation. Images are captured from deforming specimens at three different scales: 1) macro-scale images from the front-surface side, 2) meso-scale images from the thickness side, and 3) micro-scale images from the thickness side using electron microscopy. A semi-automatic approach is developed for detection and counting of matrix cracks in the images, processed with digital image correlation. The multi-scale approach allows analysis of the crack initiation along the length and its propagation through the width of the outer ply, the crack density evolution at the edge of the inner and outer plies, and the interactions between cracks and micro-features such as voids. The methodology is applied to a reference and an imperfect material, produced with different cure cycles. It is observed that voids can facilitate the formation of matrix cracks, therefore causing earlier start of cracking. The different cure of the imperfect material brings about higher crack density at saturation and less regular crack propagation paths. The computational methodology is performed also in a multi-scale framework. The effect of voids on certain micro-scale properties are calculated using computational micromechanics. To this end, finite element models with a random distribution of fibers and plasticity and damage for the matrix are created. The micro-scale properties are then transferred to a meso-scale model of a composite laminate, serving as input parameters. Regions with degraded properties, representing voids, are distributed randomly in the laminate. The simulation is carried out for two different void contents and void sizes. The results confirm that the initiation of cracks can be facilitated by voids, and the highest effect is for the model with the highest number of voids. The most noteworthy outcomes of the dissertation are, therefore, 1) a comprehensive review of the literature about voids in FRCs, covering their formation, characterization and effects, which is of great value to the composites society, 2) a full-scale characterization of voids, using X-ray micro-computed tomography, which provides an extensive dataset on voids' characteristics, and 3) approaches for identification of the effect of voids on matrix cracking. The latter, beside revealing results about the influence of voids on damage development in FRCs, help to address the challenges in currently-available approaches for analysis of cracking evolution, which is crucial for understanding of the damage mechanics in composites.

Publication year:2018

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