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This paper investigates the mechanical properties and microstructure of additively manufactured, continuous fiber reinforced composites made by the Continuous Fiber 3D Printing (CF3D®) process. We specifically investigate composites of GF-2, a high temperature thermosetting acrylic polymer, and T-1100, a high strength carbon fiber, with a 41.5% fiber volume fraction. The resulting composites have a longitudinal tensile modulus of 122 GPa and a strength of 1599 MPa, which is 89% and 55% of the theoretical value by rule of mixtures, respectively. This appears to be the highest strength additively manufactured composites to date, largely due to the high mechanical properties of the T-1100 fiber, low defects, and substantial fiber volume fraction. Comparable results are obtained for a cationic cure resin, CATPRO14. We optically measured the fiber–fiber contact surface area fraction of GF-2 and employing the unidirectional strength model of Karam to predict a strength efficiency of 0.6, which is in good agreement with mechanical estimate of 0.55. Comparing results with the efficiency factors of high fiber volume prepreg laminates, we determine the printing process yields composites with 88%–95% of the anticipated longitudinal modulus and 65%–70% of the anticipated longitudinal tensile strength of those processed by traditional methods. To extend the printing to multifunctional microvascular composites, we consider the mechanical implications of including sacrificial filaments of 100–300 μm within each tow. A modest decrease in longitudinal strength in proportion to the microchannel volume fraction is predicted, where the decrease is mainly attributed to the increased fiber-to-fiber interactions in more tightly packed tows.

Organosheet composite scrap made from polyphenylene sulfide reinforced with long recycled carbon fibers was reprocessed to produce compression molding compounds. No additional polymer was added to the process, making this a demonstration of closed-loop recyclability in composites manufacturing. The recyclate, produced by hammer-milling organosheet trimmings, was sieved and the resulting particulate geometry was measured to predict the fiber length in the molded composites. Tensile testing of the composites revealed that high stiffness parts (tensile modulus greater than 13 GPa) can be achieved using particulate molding compounds, but that tensile strength was significantly degraded. Still, the isotropic molded composites exhibited a greater than 18.8% increase in tensile strength over the neat polymer. Evaluation of the composite microstructure via optical microscopy revealed that fiber packing played a significant role in the tensile strength of the particulate composites, indicating that microstructural heterogeneity should be avoided to maximize the properties of composites made of recycled organosheet waste.

The integration of repurposed and recycled carbon fibers into high-performance composites is essential to the adoption of composites for automotive structures due to their low-cost, high formability, and reduced environmental impact. When high areal density nonwovens of these fibers are infused with a semi-crystalline thermoplastic resin, organosheets offering competitive mechanical properties can be produced. This study examined the optimization of such composites through multiscale material characterization and post-process annealing. Single fiber tensile tests were used to characterize repurposed and recycled fiber formats. The thermomechanical properties of the polyphenylene sulfide matrix and resulting composites subjected to different post-process annealing conditions were characterized using differential scanning calorimetry, dynamic mechanical analysis, and nano-indentation. Single fiber push-in testing was conducted to evaluate the fiber–matrix interface as a function of annealing. It was shown that statistical methods based on the bootstrap principle successfully identify the effects of post-process annealing, which are otherwise masked by material inhomogeneity. Post-process annealing was shown to be an effective method of improving the resulting mechanical properties of repurposed and recycled carbon fiber organosheet composites, thereby optimizing their properties for use as a high-performance automotive structural material.

The thermoplastic resin infiltration process in compression molded recycled/repurposed carbon fiber organosheet composites was studied experimentally to develop an understanding of optimal processing conditions for improved cycle time and infiltration quality. A custom permeameter employing highly viscous polydimethylsiloxane fluid to mimic a thermoplastic melt was built, and the measured permeability was compared to water and air measurements. The infiltrating fluid played a significant role in the measurement. A novel x-ray radiographic imaging system and measurement protocol were developed to evaluate the in-situ infiltration behavior during compression molding. Dual-scale permeability was observed as the highly viscous thermoplastic melt preferentially infiltrated interbundle spaces before filling intrabundle regions. Optical microscopy showed that infiltration pressure strongly impacts the microstructure of organosheets. Despite the decrease in infiltration time resulting from increased molding pressure, it was found that moderate pressures were preferable, yielding lower void content and a largely homogeneous fiber volume fraction.

Microvascular networks can provide host materials with many functions including self-healing and active cooling. However, vascular networks are susceptible to blockage which can dramatically reduce their functional performance. A novel optimization scheme is presented to design networks that provide sufficient cooling capacity even when partially blocked. Microvascular polydimethylsiloxane (PDMS) panels subject to a 2000 W m−2 applied heat flux and 28.2 mL min−1 coolant flow rate are simulated using dimensionally reduced thermal and hydraulic models and an interface-enriched generalized finite element method (IGFEM). Channel networks are optimized to minimize panel temperature while the channels are either clear (the O 0 scheme), subject to the single worst-case blockage ( O 1 ), or subject to two worst-case blockages ( O 2 ). Designs are optimized with nodal degree (a measure of redundancy) ranging from 2 to 6. The results show that blockage tolerance is greatly enhanced for panels optimized while considering blockages and for panels with higher nodal degree. For example, the 6-degree O 1 design only has a temperature rise of 7 °C when a single channel is blocked, compared to a 35 °C rise for the 2-degree O 0 design. Thermography experiments on PDMS panels validate the IGFEM solver and the blockage tolerance of optimized panels.

The integration of microchannels within structural composites enables a range of multifunctional responses such as thermal management and self-healing. In this work, we investigate how microchannels affect the crashworthiness of the host material. Corrugated panels are fabricated with aligned microchannels (ca. 400 µm diameter) at different channel spacing (10 mm and 1.2 mm), orientation with respect to the loading direction, and alignment with respect to the surrounding fiber-reinforcement. Specific energy absorbed (SEA) is measured by compression testing of samples with a chamfer edge trigger. SEA was preserved within 10% for all test cases. Flat (non-corrugated) panels are also tested to demonstrate that microchannels can serendipitously trigger stable, energy absorbing failure modes that lead to improved crashworthiness. Non-vascular panels without an edge chamfer fail catastrophically when compressed. In dramatic contrast, vascular panels fail in a stable fashion triggered by crack initiation at the microchannels, yielding ca. 10 times more energy absorption.

The crashworthiness of discontinuous recycled carbon fiber composites was studied. Sinusoidal crush specimens were manufactured of acrylonitrile butadiene styrene, polyphenylene sulfide, and a common structural epoxy reinforced with recycled carbon fibers. Samples were tested at a range of strain rates and temperatures representative of automotive conditions. Crush debris size was quantified using sieving to correlate with energy absorption. The effect of manufacturing flaws such as embedded foreign objects, porosity, and ductile tearing was studied by comparing x-ray computed tomography images with crush test data gathered using digital image correlation. The results show great promise for discontinuous fiber organosheet composites as a novel material for automotive crash structures, exceeding the energy absorption capabilities of many continuous fiber thermosets.

Electric vehicle (EV) batteries require both thermal regulation and crash protection. A novel battery packaging scheme is presented that uses microvascular composite panels with 2D channel networks to accomplish both objectives. Microvascular carbon fiber/epoxy composite panels are fabricated by vacuum assisted resin transfer molding, with the channel network formed by post-cure vaporization of an embedded polylactide channel template. Panel cooling performance is evaluated for parallel, bifurcating, serpentine, and spiral channel designs at different coolant flow rates and channel diameter. The spiral design provides the best thermal performance, but requires high pumping pressure (>100 kPa) at the flow rates needed for adequate cooling (>30 mL min −1 ). The bifurcating design and a network obtained by computational optimization offer much lower pressure with slightly reduced thermal performance. Channel diameter has negligible effect on cooling performance, but strongly affects pumping pressure. Computational fluid dynamics (CFD) simulations are also performed and correlate well with the experimental data. Simulations confirm that microvascular composite panels can cool typical battery cells generating 500 W m −2 heat flux below the target temperature of 40 °C.

This paper explores the development of a long (greater than 10 mm in length) chopped carbon fiber reinforced polyphenylene sulfide (PPS) composite. A novel manufacturing approach that uses compression molding to infuse thin PPS resin films sandwiching non-woven mats of chopped carbon fiber is developed for obtaining a composite organo sheet. The relationship between processing, microstructure and the resulting thermo-mechanical properties of the composite is evaluated. Laminated composite samples are prepared using a compression molding process using custom aluminum molds with simple design for scalability in size. The consolidated panels are subsequently annealed at various temperatures to induce cold crystallization to evaluate its role on mechanical properties. Dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) are used to explore the thermo-mechanical properties of the composite. Annealing increased the thermal resilience of the composite specimens through a crystallization mechanism. Optical microscopy is used to evaluate the microstructure of the material. A tensile modulus of over 25 GPa and a failure strength of over 200 MPa are routinely obtained, showing significant promise for manufacturing lower cost and high throughput discontinuous carbon fiber composites with the potential of utilizing recycled carbon fiber for reinforcement.

In the last 50 years, intensification of agricultural grassland management has led to over 97% of all grassland being ‘improved’ in the UK. Despite grassland being an important habitat for birds in Europe, both in terms of area and preference by several species of conservation concern, the impact of grassland intensification on both breeding and wintering birds is poorly understood. This study compared habitat characteristics and bird diversity, in winter, between improved and unimproved lowland grassland in England comprising 44 unimproved fields (350 ha) and 59 improved fields (290 ha) at 7 paired sites. Key habitat differences were found between grassland types. These included, on improved fields, a denser sward, less surface water, more dung flies and more fields with seeding grass. Species that are dependent on soil invertebrates (particularly earthworms) were found to be more numerous on unimproved grassland, whilst generalist insectivores such as corvids were much more numerous on improved grassland. Grassland improvement may have been one important factor behind the recent increases in numbers of some corvid species in the UK.

Almost a third of the bird species designated as Species of European Conservation Concern exploit agricultural grasslands, yet few studies have focused on their use as foraging habitats for birds. This study investigated the influence of variation in sward structure, grassland management and landscape variables on the use of 77 grass fields by 14 field-feeding bird species wintering on lowland mixed farmland in southern England. Multiple logistic regression was used to model the proportion of bird-count visits in which each species was encountered as a function of the recorded habitat variables for each field. Variation in sward height and density were associated with frequency of occurrence for 12 bird species and larger areas of bare earth and occurrence of winter grazing by stock animals were correlated with greater frequency of occurrence by 11 bird species. Two rapidly declining species, skylark (Alauda arvensis) and yellowhammer (Emberiza citrinella), were recorded more frequently on fields with higher numbers of seeding grasses. We suggest that mosaics of fields managed as short-term leys and permanent pastures with low-intensity cattle grazing over the autumn and winter would provide the combination of heterogeneous sward structure, areas of bare earth and presence of some seeding plants necessary to maximise the range of bird species able to use a given area of agriculturally improved grassland throughout the winter.