Circular fiber geometries are predominant in commercial carbon fiber material systems, but the use of this fiber shape has numerous limitations. Circular geometries have continuous symmetry, which is helpful for various processing considerations, but also have the largest possible maximum diffusion thickness for a given fiber area. This characteristic means that circular carbon fibers always have the highest material processing cost and lowest production throughput compared to any other fiber shape with the same area and tow count. To quantify material cost and other benefits for non-circular carbon fiber geometries, process models for polyacrylonitrile based carbon fiber production are developed in relationship to the carbon fiber shape, size, and tow count. For a given fiber shape, precursor production costs are shown to favor maximizing fiber size while conversion costs are minimized by the smallest fiber size. These competing cost trends result in a numerically optimal fiber size for a given shape and tow count, while both cost components are decreased by increasing tow count. Cost-performance tradeoffs for three lobe fiber geometries are studied by supplementing the cost trends with a numerical failure model to predict compressive strength for discrete shape variants. The shape selection is shown to be more sensitive to variations in cost than compressive strength while suboptimal shape designs can improve manufacturing robustness and achievable fiber volume fractions. An optimal three lobe carbon fiber is identified that balances the set of considerations while reducing costs and embodied energy and increasing production throughput compared to a commercial carbon fiber.

Carbon fiber provides opportunity to reduce weight in structural composites, including wind turbine blades, due to the material’s superior specific stiffness and specific strength compared to alternatives. Despite these advantages, cost and compressive performance are considered weaknesses for carbon fiber products available today. Studies to produce low-cost carbon fiber alternatives, including the use of textile-derived precursor systems, have shown progress and merit through the DOE/ORNL low-cost carbon fiber initiatives. This work focuses on enabling increases in compressive strength through design of the carbon fiber geometry, applicable to both textile and conventional precursor systems, while also providing opportunities to reduce carbon fiber processing costs. Fiber-resin interface and fiber alignment are among the most frequently cited factors controlling composite compressive performance. However, it is believed that there is opportunity in traditionally unexplored routes to increasing compressive strength through alteration of the carbon fiber geometry by increasing the fiber area moment of inertia and/or the fiber perimeter and interfacial area. This paper presents initial results from manufacturing carbon fiber materials to assess the impacts of carbon fiber size on tested composite compressive performance with projected neutral or even beneficial impact on fiber and composite manufacturing economics. Carbon fiber systems with increasing size illustrate a favorable correlation for compressive performance greater than predicted from a micromechanical failure model. The manufacturing and mechanical test results support the hypothesis of this work that alterations to fiber geometry can be used to produce improvements of the compressive strength of carbon fiber reinforced polymers and provide incentive for related work in designing alternative shapes to further enhance compressive performance.

Carbon fiber composites offer superior mechanical performance compared to nearly all other useful materials for the design of structures. However, for cost-driven industries, such as with the wind energy and vehicle industries, the cost of commercial carbon fiber materials is often prohibitive for their usage compared to alternatives. This paper develops an approach to optimize fiber geometries for use in carbon fiber reinforced polymers to increase the compressive strength per unit cost. Compressive strength is a composite property that depends on the fiber, matrix, and interface, and an exact analytic expression does not exist that can accurately represent these complicated relationships. The approach taken instead is to use a weighted summation between the fiber cross-sectional area moment of inertia and perimeter as a proxy for compressive strength, with different weightings explored within the paper. Analyses are performed to identify optimal fiber geometries that increase the cost-specific compressive strength based on various assumptions and desired fiber volume fraction. Robust optimal shapes are identified which outperform circular fibers due to increases in area moment of inertia and perimeter, as well as decreases in carbon fiber processing costs.

The objective of this study is to assess the commercial viability to develop cost-competitive carbon fiber composites specifically suited for the unique loading experienced by wind turbine blades. The wind industry is a cost-driven market, while carbon fiber materials have been developed for the performance-driven aerospace industry. Carbon fiber has known benefits for reducing wind turbine blade mass due to the significantly improved stiffness, strength, and fatigue resistance per unit mass compared to fiberglass; however, the high relative cost has prohibited broad adoption within the wind industry. Novel carbon fiber materials derived from the textile industry are studied as a potentially more optimal material for the wind industry and are characterized using a validated material cost model and through mechanical testing. The novel heavy tow textile carbon fiber is compared with commercial carbon fiber and fiberglass materials in representative land-based and offshore reference wind turbine models. Some of the advantages of carbon fiber spar caps are observed in reduced blade mass and improved fatigue life. The heavy tow textile carbon fiber is found to have improved cost performance over the baseline carbon fiber and performed similarly to the commercial carbon fiber in wind turbine blade design, but at a significantly reduced cost. This novel carbon fiber was observed to even outperform fiberglass when comparing material cost estimates for spar caps optimized to satisfy the design constraints. This study reveals a route to enable broader carbon fiber usage by the wind industry to enable larger rotors that capture more energy at a lower cost.

A novel thermoplastic composite preforming process has been developed. This thermoplastic programmable powdered preforming process (TP-P4) uses commingled glass and polypropylene yarns, which are chopped to a desired length and deposited onto a vacuum screen. The as-placed fibers are then heat-set for improved handling, before potential preconsolidation, and final conversion with preheating and press forming. This work investigated the effect of using either a double belt lamination preconsolidation stage or using an improved heat-setting stage. Polymer degradation was examined using gel permeation chromatography analysis, and the void content evolution tracked using image analysis techniques from the heat setting stage until the final part. It was shown that without preconsolidation, preforms prepared for a 2 mm thick final pressed part could be compression molded into a substantially void free, non degraded part. By using the lamination route, this limit could be increased to 4 mm, but without allowing local thickness changes in the preform. POLYM. COMPOS., 31:653-665, 2010. (C) 2009 Society of Plastics Engineers

A novel thermoplastic programmable preforming process, TP-P4, has been used to manufacture preforms for non-isothermal compression molding. Commingled glass and polypropylene yarns are deposited by robot onto a vacuum screen, followed by a heat-setting operation to stabilize the as-placed yarns for subsequent handling. After an optional additional preconsolidation stage, the preforms are molded by preheating and subsequent press forming in a shear edge tool. The in- and out-of-plane flow capabilities of the material were investigated, and compared to those of 40 (wt)% Glass Mat Thermoplastics (GMTs). Although the TP-P4 material has a fiber fraction of 60 (wt)%, the material could be processed to fill 77 mm deep ribs with a thickness of 3 mm, indicative of complex part production. The pressure requirements for out-of-plane flow were shown to depend on the fiber length and fiber alignment. Segregation phenomena were found to be less severe with TP-P4 than with GMT material.

A coupled load frame and x-ray micro-tomography unit was used to observe damage processes occurring within model composite specimens subjected to mechanical loading. These specimens consisted of two tows of glass fiber in the form of bound loops within an epoxy matrix of cylindrical form with 3-mm diameter cross-section. Computed tomography reconstruction was used to investigate internal structure of the specimens under load. This reconstruction revealed internal cracking and progressive failure in several experiments.

The objective of our work is to enable the optimum design of lightweight automotive structural components using injection-molded long fiber thermoplastics (LFTs). To this end, an integrated approach that links process modeling to structural analysis with experimental microstructural characterization and validation is developed. First, process models for LFTs are developed and implemented into processing codes (e.g. ORIENT, Moldflow) to predict the microstructure of the as-formed composite (i.e. fiber length and orientation distributions). In parallel, characterization and testing methods are developed to obtain necessary microstructural data to validate process modeling predictions. Second, the predicted LFT composite microstructure is imported into a structural finite element analysis by ABAQUS to determine the response of the as-formed composite to given boundary conditions. At this stage, constitutive models accounting for the composite microstructure are developed to predict various types of behaviors (i.e. thermoelastic, viscoelastic, elastic-plastic, damage, fatigue, and impact) of LFTs. Experimental methods are also developed to determine material parameters and to validate constitutive models. Such a process-linked-structural modeling approach allows an LFT composite structure to be designed with confidence through numerical simulations. Some recent results of our collaborative research will be illustrated to show the usefulness and applications of this integrated approach.

This report describes the status of the current process modeling approaches to predict the behavior and flow of fiber-filled thermoplastics under injection molding conditions. Previously, models have been developed to simulate the injection molding of short-fiber thermoplastics, and an as-formed composite part or component can then be predicted that contains a microstructure resulting from the constituents’ material properties and characteristics as well as the processing parameters. Our objective is to assess these models in order to determine their capabilities and limitations, and the developments needed for long-fiber injection-molded thermoplastics (LFTs). First, the concentration regimes are summarized to facilitate the understanding of different types of fiber-fiber interaction that can occur for a given fiber volume fraction. After the formulation of the fiber suspension flow problem and the simplification leading to the Hele-Shaw approach, the interaction mechanisms are discussed. Next, the establishment of the rheological constitutive equation is presented that reflects the coupled flow/orientation nature. The decoupled flow/orientation approach is also discussed which constitutes a good simplification for many applications involving flows in thin cavities. Finally, before outlining the necessary developments for LFTs, some applications of the current orientation model and the so-called modified Folgar-Tucker model are illustrated through the fiber orientation predictions for selected LFT samples.

Electron beam, or E-beam, curing of composite structures has been successfully demonstrated over the past several years. Under a prior Phase I SBIR program, electron beam cured composite pressure vessels exhibited delivered strengths comparable to a conventional, thermally cured IM7 graphite/epoxy composite material system. This paper presents the results of the Phase 11 development and demonstration program to further develop and characterize E-beam curable resin systems for filament wound composite structures and to demonstrate the technology for a representative small diameter composite rocket motor case application. Filament winding and electron-beam curing process development and characterization testing involving several different E-beam resin formulations has been performed to assess the influence of factors such as resin viscosity, process temperature and E-beam process conditions on shear strength, resin content, and burst pressure. E-beam curable resin formulations offering improved performance at higher temperatures have also been evaluated. Resin systems selected for full-scale demonstration included wet-winding formulations and a higher Tg towpreg system. Hydrostatic pressure testing of full-scale motor cases has demonstrated that electron beam cured composite cases achieve delivered burst strength performance comparable to conventional thermally cured composite motor cases.

Electron beam curing of composites is a nonthermal, nonautoclave curing process offering the following advantages compared to conventional thermal curing: substantially reduced manufacturing costs and curing times; improvements in part quality and performance; reduced environmental and health concerns; and improvements in material handling. In 1994 a Cooperative Research and Development Agreement (CRADA), sponsored by the Department of Energy Defense Programs and 10 industrial partners, was established to advance electron beam curing of composites. The CRADA has successfully developed hundreds of new toughened and untoughened resins, offering unlimited formulation and processing flexibility. Several patent applications have been filed for this work. Composites made from these easily processable, low shrinkage materials match the performance of thermal cured composites and exhibit: low void contents comparable to autoclave cured composites (less than 1%); superb low water absorption values in the same range as cyanate esters (less than 1%); glass transition temperatures rivaling those of polyimides (greater than 390 degrees C); mechanical properties comparable to high performance, autoclave cured composites; and excellent property retention after cryogenic and thermal cycling. These materials have been used to manufacture many composite parts using Various fabrication processes including hand lay-up, tow placement, filament winding, resin transfer molding and vacuum assisted resin transfer molding.

A Cooperative Research and Development Agreement (CRADA) sponsored by the Department of Energy and 10 industrial partners has been established to develop high performance electron beam (E-beam) curable polymer matrix composites. Since E-beam curing is a nonthermal curing method, the use of lower cost, low temperature tools made of foam, wood, or other materials becomes possible. As part of the CRADA, E-beam irradiation/stability studies on low-cost EB tooling have been performed. Approximately 50 potential tooling materials representing a broad cross-section of both commonly used and specialty materials were tested. Coupons from each material were exposed to the equivalent of 5 E-beam curing cycles of 75 kGy each. At the conclusion of each 5-cycle exposure set, the materials were evaluated for dimensional and hardness changes. The testing continued for each material until obvious breakdown from radiation exposure or handling. The goal is ultimately to extend testing to approximately 150 simulated cure cycles (11,250 kGy) which represents a fairly large aircraft/aerospace structure production run.