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The production of concrete leads to substantial carbon emissions (~8%) and includes reinforcing steel which is prone to corrosion and durability issues. Carbon-fiber-reinforced concrete is attractive for structural applications due to its light weight, high modulus, high strength, low density, and resistance to environmental degradation. Recycled/repurposed carbon fiber (rCF) is a promising alternative to traditional steel-fiber reinforcement for manufacturing lightweight and high-strength concrete. Additionally, rCF offers a sustainable, economical, and less energy-intensive solution for infrastructure applications. In this paper, structure–process–property relationships between the rheology of mix design, carbon fiber reinforcement type, thermal conductivity, and microstructural properties are investigated targeting strength and lighter weight using three types of concretes, namely, high-strength concrete, structural lightweight concrete, and ultra-lightweight concrete. The concrete mix designs were evaluated non-destructively using high-resolution X-ray computed tomography to investigate the microstructure of the voids and spatially correlate the porosity with the thermal conductivity properties and mechanical performance. Reinforced concrete structures with steel often suffer from durability issues due to corrosion. This paper presents advancements towards realizing concrete structures without steel reinforcement by providing required compression, adequate tension, flexural, and shear properties from recycled/repurposed carbon fibers and substantially reducing the carbon footprint for thermal and/or structural applications. [ABSTRACT FROM AUTHOR]

Carbon fiber-reinforced concrete as a structural material is attractive for civil infrastructure because of its light weight, high strength, and resistance to corrosion. Ultra-high performance concrete, possessing excellent mechanical properties, utilizes randomly oriented one-inch long steel fibers that are 200 microns in diameter, increasing the concrete's strength and durability, where steel fibers carry the tensile stress within the concrete similar to traditional rebar reinforcement and provide ductility. Virgin carbon fiber remains a market entry barrier for the high-volume production of fiber-reinforced concrete mix designs. In this research, the use of recycled carbon fiber to produce ultra-high-performance concrete is demonstrated for the first time. Recycled carbon fibers are a promising solution to mitigate costs and increase sustainability while retaining attractive mechanical properties as a reinforcement for concrete. A comprehensive study of process structure–properties relationships is conducted in this study for the use of recycled carbon fibers in ultra-high performance concrete. Factors such as pore formation and poor fiber distribution that can significantly affect its mechanical properties are evaluated. A mix design consisting of recycled carbon fiber and ultra-high-performance concrete was evaluated for mechanical properties and compared to an aerospace-grade and low-cost commercial carbon fiber with the same mix design. Additionally, the microstructure of concrete samples is evaluated non-destructively using high-resolution micro X-ray computed tomography to obtain 3D quantitative spatial pore size distribution information and fiber clumping. This study examines the compression, tension, and flexural properties of recycled carbon fibers reinforced concrete considering the microstructure of the concrete resulting from fiber dispersion. [ABSTRACT FROM AUTHOR]

In this paper, the degradation of mechanical properties of marine structural aluminum alloy AA5083 and onset of lattice plastic anisotropy subsequent to fire exposure is investigated. For virgin and fire-exposed samples, microstructural characterization is carried out for the first time and changes in lattice specific strain responses under step and continuous axial deformation are studied using novel bulk neutron diffraction. Peak fitting of raw data yields d-spacing values of reflections of interest and allows for lattice specific strain calculations. A reduction in yield stress from 260 MPa in the virgin material to 120 MPa in the fire-exposed sample is observed. Virgin material exhibits dynamic strain aging during plastic deformation; this is captured in neutron diffraction measurements. Larger peak broadening for virgin material indicates possible presence of Type II and III residual stresses due to intergranular stress or dislocation stress fields. Stress vs. lattice strain plots show large deviations from linearity post-yield for fire-exposed samples. This is due to strain redistribution among grains as well as grain reorientation. After fire exposure dynamic strain aging does not occur, but individual lattice planes reveal plastic anisotropy in their response to plastic deformation. Results from neutron diffraction, combined with electron backscatter diffraction characterization, provide insight into the yielding mechanisms of AA5083 and effects of fire exposure. [ABSTRACT FROM AUTHOR]

In this study, continuous lignin fibers from switchgrass have been successfully synthesized via multifilament melt-spinning and converted to carbon fibers using optimized stabilization and carbonization techniques. Unidirectional lignin carbon fiber reinforced composites are produced using a vacuum assisted resin transfer molding process for the first time. Produced lignin carbon fiber is evaluated in mechanical, interfacial, and microstructural areas. Single fiber mechanical properties are characterized using a unique MTS Bionix Nano-UTM. Interfacial properties and resin/fiber behavior are evaluated using single fiber fragmentation. Microstructural properties are determined using wide-angle x-ray diffraction. Mechanical results indicated an initial tensile modulus along fiber axis of 36 GPa and a failure stress of 600 MPa for single carbon fibers. Lignin carbon fibers demonstrate a nonlinear increase in modulus with applied tensile strain similar to recently observed for commercial poly-acrylonitrile (PAN) carbon fibers. Microscopy revealed few defects within and along the lignin carbon fibers, and the processed lignin fibers demonstrated minimal crystalline regions and crystallite alignment when compared to commercial PAN based fibers. Interfacial shear strength with epoxy resin was found to be 17 MPa with a fiber fracture length of 228 μm. Unidirectional composite coupons achieved tensile modulus of 9 GPa and a failure strength of 85 MPa at 1% failure strain. Lignin carbon fiber and composites produced are targeted for non-structural applications as mechanical properties are currently less than PAN carbon fibers and therefore are not suitable to replace PAN carbon fiber at this stage. Relatively low composite strength is the likely result of fiber layup, fiber fusing, and non-continuous fibers across the gage length of the composite. Nano-tensile and interface shear strength results presented here have important implications for lignin carbon fiber composite development to optimize fiber/resin bonding. In addition, composite manufacturing procedures detailed here are a major step forward in the development of sustainable carbon fiber composite production. [ABSTRACT FROM AUTHOR]

Modeling the mechanical behavior of discontinuous fiber composites has long been a challenge for the composites community and is of increasing interest due to recent interest for automotive lightweighting. Micromechanical models typically predict the stiffness accurately, but reliable strength models have been largely elusive due to the highly stochastic microstructure. This paper presents a stiffness and strength laminate analogy model for discontinuous fiber composites using stochastic fiber orientation, length, fiber volume fraction, and void volume fraction. For the first time, the master ply invariant approach has been applied to discontinuous fiber composites and shows good agreement in modeling the stiffness. A strain energy density-based laminate failure condition successfully predicts the tensile strength of these composites. This paper demonstrates accurate, first-order strength and stiffness predictions for discontinuous fiber composites using constituent properties readily obtained from material datasheets. [Display omitted] [ABSTRACT FROM AUTHOR]

A new criterion, the limit stress, is presented for characterizing the mechanical performance of carbon fibers via the impregnated tow method. A description of the method and its underlying connection to actual single carbon fiber mechanical performance is given and an initial demonstration of the technique is implemented. For implementation, a commercial 24k tow carbon fiber was interrogated mechanically, and prepared using two independent approaches, resulting in two cross-sectional geometries; flat (ribbon) and round. The average ultimate failure stress and average limit stress from round to flat were reduced by 5.76% and 16.8%, respectively. Hence, the limit stress approach is between 2–3x more sensitive to induced variations in tow performance compared to the peak load approach. This method gives critical insight to the nuances of carbon fiber performance, important to the understanding of fiber types (for example low-cost carbon fibers) and coupled fiber surface optimization. [ABSTRACT FROM AUTHOR]

Carbon fiber reinforced polymer composites are highly desirable for automotive and wind energy applications due to advantages associated with weight reduction, high stiffness and strength, durability, and recyclability. The high cost of carbon fiber has been a limiting factor in its widespread adoption in non-aerospace applications. A low cost (estimated < $11 per kg) wide tow (450-600k) carbon fiber derived from textile grade polyacrylonitirile precursor, and hence called Textile Grade Carbon Fiber (TCF) is introduced in this paper. Fundamental aspects of the TCF are discussed along with a detailed characterization of its mechanical properties. Two manufacturing processes relevant to automotive and wind energy applications are considered, namely-compression molding and resin infusion. At various stages the TCF has been compared to commercial non-aerospace 50k carbon fiber composite. Detailed physical and mechanical properties including tensile, flexural, compression, and interlaminar shear properties are reported and compared to non-aerospace carbon fiber composite. The results provide a means for designers and end-users in the automotive and wind energy sector to consider different forms of economical non-aerospace carbon fibers. Image 1 [ABSTRACT FROM AUTHOR]