In recent years, the prevalence and application of composite materials has exploded. Due to the demands of commercial transportation, the aviation industry has taken a leading role in the integration of composite structures. Among the leading concepts to develop lighter, more fuel-efficient commercial transport is the Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) concept. The highly integrated structure of PRSEUS allows pressurized, non-circular fuselage designs to be implemented, enabling the feasibility of Hybrid Wing Body (HWB) aircraft. In addition to its unique fabrication process, the through-thickness stitching utilized by PRSEUS overcomes the low post-damage strength present in typical composites. Although many proof-of-concept tests have been performed that demonstrate the potential for PRSEUS, efficient computational tools must be developed before the concept can be commercially certified and implemented. In an attempt to address this need, a comprehensive modeling approach is developed that investigates PRSEUS at multiple scales. The majority of available experiments for comparison have been conducted at the coupon level. Therefore, a computational methodology is progressively developed based on physically realistic concepts without the use of tuning parameters. A thorough verification study is performed to identify the most effective approach to model PRSEUS, including the effect of element type, boundary conditions, bonding properties, and model fidelity. Using the results of this baseline study, a high fidelity stringer model is created at the component scale and validated against the existing experiments. Finally, the validated model is extended to larger scales to compare PRSEUS to the current state-of-the-art. Throughout the current work, the developed methodology is demonstrated to make accurate predictions that are well beyond the capability of existing predictive models. While using commercially available predictive tools, the methodology developed herein can accurately predict local behavior up to and beyond failure for stitched structures such as PRSEUS for the first time. Additionally, by extending the methodology to a large scale fuselage section drop scenario, the dynamic behavior of PRSEUS was investigated for the first time. With the predictive capabilities and unique insight provided, the work herein may serve to benefit future iteration of PRSEUS as well as certification by analysis efforts for future airframe development. / PHD

This research aims to investigate the influence of 3D through-thickness stitching on the gas permeability and transverse microcracking of cryogenically cycled carbon/epoxy composites. 3D through-thickness stitching can be used to improve the interlaminar properties of polymer matrix composites (PMCs) and produce lightweight, unitized structures for cryogenic storage tanks. To fully utilize stitched composite structures for these applications, their inherent gas permeability challenges must be understood. Therefore, in this study, the stitched composites' damage evolution and gas permeability was experimentally characterized under a) pure thermal stress, b) thermal and uniaxial mechanical stress, and c) thermal and biaxial mechanical stress. Helium gas permeability was measured for each specimen at room or cryogenic temperatures under a mechanically strained state following the thermo-mechanical cycles. Optical microscopy was used to measure microcrack densities and monitor their evolution through the thickness of the composite specimens. Thin plies, graphene nanoplatelets (GNP) modified resin, and a hybrid barrier layer comprising of both were incorporated in the stitched specimens as barrier layers to reduce their gas permeability. The dependence of gas permeability of stitched composites on the mechanical strain, test temperature, and load history was evaluated and correlated to microcrack density. A significant reduction in permeability and damage evolution (transverse microcracks and delaminations) was obtained for all thermo-mechanical cases using the hybrid barrier layer laminate. Additionally, the permeability was several orders of magnitude lower than the allowable. Overall, the hybrid barrier layer shows tremendous promise as a viable barrier layer for stitched/unstitched composites undergoing thermo-mechanical fatigue involving a cryogenic environment.

The purpose of this research is to evaluate the influence of through-the-thickness reinforcements on the fracture behavior of stitched sandwich composites and to develop predictive methodologies to aid in simulating their damage-tolerant capability. Sandwich composites are widely used for their high stiffness-to-weight ratio due to their unique material architecture, which is composed of two rigid, outer facesheets that are bonded to a light-weight internal core. However, sandwich composites are limited by their low interlaminar strengths and can develop core-to-facesheet separation when subjected to low out-of-plane loads. In this study, sandwich composites were manufactured with through-the-thickness reinforcements, or stitches, to act as crack-growth inhibitors and to improve interlaminar properties. Stitch processing parameters, such as the number of stitches per unit area (stitch density) and stitch diameter (linear thread density), have considerable influence on the in-plane and out-of-plane behavior of composite structures. A design of experiments (DoE) approach is used to investigate stitch processing parameters and their interaction on the fracture behavior of stitched sandwich composites. Single cantilevered beam (SCB) tests are performed to estimate the required energy to propagate crack growth, or Mode I fracture energy, during the separation of the facesheet from the core. Additionally, embedded optical fibers within the SCB test articles are used to determine the internal crack front variation. During testing, unique fracture morphologies are obtained and show dependency on stitch processing parameters. Furthermore, embedded optical fibers indicate that the internal crack front is approximately 10% greater than visual edge measurements, which is primarily attributed to Poisson’s effect. The DoE approach is then used to develop a statistically informed response surface model (RSM) to optimize stitch processing parameters based on a maximum predicted fracture energy. Novel analytical formulations are developed for estimating the mode I fracture energy using the J-integral approach. The DoE approach is then used to inform and validate finite element models that simulate the facesheet-to-core separation using a discrete cohesive zone modeling approach. The predicted load and crack growth response show good agreement to experimental measurements and highlights the capability of stitching to arrest delamination in stitched sandwich composites.

The purpose of this research is to investigate the influence of stitching architectures by using different stitching periodic patterns on the in-plane and out-of-plane mechanical properties. By using the inherent periodic architecture of these composites, their mechanical properties may be tailored for specific applications. Composite structures are extensively used in several industries such as aerospace, automotive, sports, and construction due to their many advantages, which include tailorable mechanical properties, high strength-to-weight ratios, and high specific stiffness. However, due to their low interlaminar tensile strength, composites are prone to delaminations, which can degrade the overall mechanical performance of the structure. Through-thickness stitching provides the third-direction reinforcement to enhance the interlaminar tensile and shear strengths. In this study, quasi-isotropic composite test articles were manufactured and stitched through-thickness using different chain stitch patterns. Full-field surface strain measurements were collected through the non-contact digital image correlation (DIC) technique. A design of experiments (DoE) approach was used to investigate the stitch parameters, such as stitch density (number of stitches per unit area), stitch angle (stitch seam orientation), and linear thread density (thread diameter), and their interactions on the in-plane and out-of-plane mechanical properties. Experimental results are then used to develop a statistically informed response surface model (RSM) to find optimal stitching parameters based on a maximum predicted tensile strength, tensile modulus and flexural strength.