The appeal of hybrid composites is the ability to create materials with properties which normally do not coexist such as high specific strength, stiffness, and toughness. One possible application for hybrid composites is as backplate materials in layered armor. Fiber reinforced composites have been used as backplate materials due to their potential to absorb more energy than monolithic materials at similar to lower weights through microfragmentation of the fiber, matrix, and fiber-matrix interface. Composite backplates are traditionally constructed from graphite or glass fiber reinforced epoxy composites. However, continuous alumina fiber-reinforced aluminum metal matrix composites (MMCs) have superior specific transverse and specific shear properties than epoxy composites. Unlike the epoxy composites, MMCs have the ability to absorb additional energy through plastic deformation of the metal matrix. Although, these enhanced properties may make continuous alumina reinforced MMCs advantageous for use as backplate materials, they still exhibit a low failure strain and therefore have low toughness. One possible solution to improve their energy absorption capabilities while maintaining the high specific stiffness and strength properties of continuous reinforced MMCs is through hybridization. To increase the strain to failure and energy absorption capability of a continuous alumina reinforced Nextel" MMC, it is laminated with a high failure strain Saffil® discontinuous alumina fiber layer. Uniaxial tensile testing of hybrid composites with varying Nextel" to Saffil® reinforcement ratios resulted in composites with non-catastrophic tensile failures and an increased strain to failure than the single reinforcement Nextel" MMC. The tensile behavior of six hybrid continuous and discontinuous alumina fiber reinforced MMCs are reported, as well as a description of the mechanics behind their unique behavior. Additionally, a study on the effects of fiber damage induced during processing is performed to obtain accurate as-processed fiber properties and improve single reinforced laminate strength predictions. A stochastic damage evolution model is used to predict failure of the continuous Nextel" fabric composite which is then applied to a finite element model to predict the progressive failure of two of the hybrid laminates. / Ph. D.

<p>Carbon-fiber reinforced polymers (CFRPs) are often used in the aerospace and automotive  industries for their high strength-to-weight ratios and corrosion resistance. A new class of  composites – known as Prepreg Platelet Molded Composites (PPMCs) – offers further  advantageous such as high forming capabilities with modest compromises in strength and stiffness.  One such property of PPMCs that have garnered interest over the years is their apparent  insensitivity to notches. Previous studies have researched the effect of specimen size and platelet  length on its effect on the open-hole performance of PPMCs. Research however has focused on  thinner samples with smaller hole sizes and neglected thicker samples with larger holes.  Additionally, while platelet sizes have been investigated for unnotched samples, platelet width on  notched samples is less clear from the literature. The present thesis offers some investigations to  aid in filling this knowledge gap. </p> <p><br></p> <p>The objective of this work is to study two parameters that could influence the performance of PPMCs under open-hole tension. First, thick (7.6 mm) specimens are subjected to large hole  sizes (up to 19.08 mm) to investigate their behavior in comparison to the smaller sample sizes  previously investigated in the literature. Through-thickness DIC measurements are taken to  investigate strain gradients in these thicker specimens. Second, various platelet widths are tested  to research their influence on notch insensitivity of open-hole tensile PPMC specimens. Lastly, a  finite element based continuum damage mechanics model is implemented to predict macro-level  structural properties using only material properties of the parent prepreg. It is found that large holes  in thick samples increase notch sensitivity compared to other samples of similar diameter-to-width  ratios. Narrower platelets were found to produce higher unnotched strengths, while wider platelets  offered more notch insensitivity. Lastly, the finite element model developed was found to  qualitatively replicate features and failure modes that are exhibited by PPMCs, though strength  predictions became inaccurate at larger specimen sizes. Recommendations are made for future  work on the basis of these findings.   </p>

Les composites à matrice polymère sont de plus en plus utilisés dans le secteur automobile. Afin de remplir les conditions exigeantes du cahier des charges vis-à-vis des conditions de mise en service, les pièces en composite doivent maintenir leur forme géométrique sous des conditions thermo-mécaniques parfois extrêmes. Par exemple, un assemblage de hayon composite est soumis à des contraintes mécaniques élevées associées à des variations de température importantes lors des essais de validation du cahier des charges. Les enjeux de la thèse sont axés sur l’aide à la conception dans le domaine quasi-statique de pièces industrielles injectées en thermoplastique renforcé de fibres discontinues. L’amélioration des outils numériques doit permettre un dimensionnement virtuel optimal de ces pièces en anticipant les variations rencontrées en service et les distorsions résiduelles résultantes de chargements thermo-mécaniques. Cette démarche s’appuie sur la connaissance du comportement thermo-mécanique du matériau de l’étude, celui du renfort de hayon, un polypropylène renforcé à 40% en masse de fibres de verre discontinues, et sur la modélisation du comportement de ce type de matériau. / Discontinuous fibers reinforced thermoplastic materials have been widely used for several years in the automotive industry. These parts must resist demanding service life conditions and must meet thermo- mechanical specifications. Indeed, structural automotive spare parts have to endure high temperatures, like a few tens of degrees Celsius, for a long duration, at least a few hours. As an example, a structural part of tailgate is subject to high mechanical loading, associated to strong temperature variations, during the validation test, regarding specifications. The purpose of this work is to improve the design of complex industrial parts, like the tailgate in quasi-static domain, by relying on numerical simulation. One of the challenges related to the use of such material, is to have a reliable virtual design of industrial parts by predicting the geometrical variations during service life conditions, and residual strain. Therefore, it is necessary to characterize and to model the thermo-mechanical behavior of the tailgate material, a polypropylene matrix reinforced with discontinuous glass fibers, with a given mass fraction of 40%