Experimental observations and finite element analysis of the initiation of fiber microbuckling in notched composite laminates

A better understanding of the factors that affect the semi-circular edge-notched compressive strength is developed, and the associated failure mode(s) of thermoplastic composite laminates with multidirectional stacking sequences are identified. The primary variables in this investigation are the resin nonlinear shear constitutive behavior, stacking sequence (orientation of plies adjacent to the 0 degree plies), resin-rich regions between the 0 degree plies and the off-axis supporting plies, fiber/matrix interfacial bond strength, and initial fiber waviness. Two thermoplastic composite material systems are used in this investigation. The materials are the commercial APC-2 (AS4/PEEK) and a poor interface experimental material, AU4U/PEEK, designed for this investigation. Notched compression specimens are studied at 21, 77, and 132 C. Geometric and material nonlinear two-dimensional finite element analysis is used to model the initiation of fiber microbuckling of both the ideal straight fiber and the more realistic initially wavy fiber. The effects of free surface, fiber constitutive properties, matrix constitutive behavior, initial fiber curvature, and fiber/matrix interfacial bond strength on fiber microbuckling initiation strain levels are considered

Thermophysical ESEM and TEM Characterization of Carbon Fibers CTE, Spectroscopy and Roughness Studies at High Temperatures

Accurate determination of the transverse properties of carbon fibers is important for assessment and prediction of local material as well as global structural response of composite components. However the measurements are extremely difficult due to the very small diameters of the fibers (few microns only) and must be conducted within a microscope. In this work, environmental scanning electron microscope (ESEM) and transmission electron microscope (TEM) are used to determine the transverse coefficient of thermal expansion of different carbon fibers as a function of temperature

Computational Assessment of a Modular Composite Wind Turbine Blade Joint

Wind energy is one of the most promising and mature alternatives to satisfy the global demand for energy as the world population and the economic activity surge. The wind energy market has grown rapidly in the last couple of decades, boosting up the size of wind turbines to generate higher power output. Typically, the larger/longer blade designs rely on hybrid material systems such as carbon and/or glass fiber (CF/GF) reinforced polymers to improve specific stiffness/strength and damage tolerance. Herein, we propose a computational design concept for a modular hybrid composite wind turbine blade that maintains its structural integrity and serviceability requirements. The modular configuration will simplify manufacturing-assembly processes and reduce expenses both in transportation and facilities requirements. The 80 m blade in this study is composed of two sections that are joined together with an innovative compression joint. Our results when compared to a single continuous blade, showed no significant alterations to its structural response. It is concluded that the proposed computational design concept that allow two modular blades to create full-length blade with robust joints is achievable. This modular concept can be easily extended for further multi-section modular blade configurations

Multifunctional Hybrid Carbon Foams: Integrating Processing and Performance

Finite element models based on X-ray tomography of the carbon foam morphology are developed to characterize response of multifunctional carbon foams. These models incorporate ligament anisotropy as well as coating layers and provide comprehensive information for adapting processing and performance parameters. Illustrations at multiple scales are presented to explore the range of properties in coupled-field environments that are crucial for the emerging multifunctional devices. The results highlight the importance of anisotropy and the impact of common processing defects in tailoring properties. Specifically, coating coverage and quality interfaces are critical in delivering the desired thermoelectrical response

Mechanical Properties of Copper-Coated Carbon Foams

The mechanical properties of copper-coated carbon foam were investigated. Reticulated vitreous carbon cell type foams, with 97% porosity and 10 ppi pore size, were electroplated with copper for different periods of time to obtain coatings with different thicknesses and foams with different porosities. Compression tests were performed to determine the Young’s modulus and the plateau stress. The copper electroplating technique improved these two properties, with the modulus increasing from 4.5 to 8.6 MPa for the sample electroplated for 40 min and the plateau stress increasing from 54 to 171 kPa for the foam coated for 80 min. The relationships between the measured properties and the copper weight ratio were determined

Fracture Toughness of Fiber Metal Laminates: Carbon Nanotube Modified Ti–Polymer–Matrix Composite Interface

Multifunctional hybrid composites are proposed as novel solutions to meet the demands in various industrial applications ranging from aerospace to biomedicine. The combination of carbon fiber and/or fabric, metal foil, and carbon nanotubes is utilized to develop such composites. This study focuses on processing and fracture toughness characterization of the carbon fiber-reinforced polymer–matrix composites and the carbon nanotube modified interface between the polymer–matrix composite and titanium foil. Vacuum Assisted Resin Transfer Molding (VARTM) process is used to fabricate the laminate. Double cantilever beam tests at both room temperature and high temperature are conducted to assess the mode I interlaminar fracture toughness. The experimental and characterization efforts suggest that carbon nanotubes improve bonding at the hybrid interface. Simple computational models are developed to assist the interpretation of experimental results and further investigate the damage modes. The numerical results agree well with the limited experiments at crack initiation and furthermore support the absence of mode mixity

Development of a Hydroxyapatite-Poly (d, l-lactide-co-glycolide) Infiltrated Carbon Foam for Orthopedic Applications

Reticulated vitreous carbon (RVC) foams are of interest in orthopedic applications due to their porous, honeycomb-like structure, biocompatibility, and bio-inert nature. Despite these desirable properties, RVC foams lack the strength necessary for orthopedic applications. Specifically, orthopedic biomaterials, whether bone scaffolds, plates or screws, must be able to withstand normal bone tissue loading at the time of implantation. This manuscript focuses on developing a composite RVC foam infiltrated with a hydroxyapatite (HA)-reinforced poly(d,l-lactide-co-glycolide) (PLGA) polymer. The HA/PLGA filler is envisioned to increase initial RVC foam mechanical stability while enabling osteoblasts to invade and deposit new tissue within the foam as the filler resorbs, providing long-term strength and osseointegration. Herein, a facile processing technique is developed which results in HA/PLGA-infused RVC foams with good internal interfacial bonding and increased modulus and strength relative to pure RVC foams. As anticipated, in vitro hydrolytic degradation studies indicate that the porous network of the RVC foam becomes progressively more accessible as the PLGA filler degrades and that the RVC foam may support improved structural integrity of the resorbing filler. Initial cell studies also demonstrate that this material system allows for robust osteoblast adhesion. These results indicate the proposed composites warrant further investigation for orthopedic applications