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1. Effects of Silk Humidity Exposure on Silk/Resin Affinity for Silk Reinforced Composites

   https://doi.org/10.5281/zenodo.580785  

   Ava L. Robson, Lauren D. (July 2021, American journal of advanced research)

   Thanks to its comparable specific mechanical properties to glass fibers, silk is a natural fiber that can be used as an eco-friendly alternative to synthetic reinforcing fibers in composite materials. Compared to natural fibers, especially plant fibers, silk enjoys higher mechanical performances, lower density, and higher elongation even at low temperatures, silk also exhibits other attractive qualities like flame resistance and being naturally continuous. However, silk is known to be prone to moisture absorption from surrounding humid environments. Moisture absorption may alter the silk/resin dynamics during composite manufacturing, and later lead to prem-ature degradation in the composite thermomechanical properties. This study investigates the effect of humidity on silk/resin wettability using two different resins (one epoxy and one vinyl ester) and three different silk architectures. Silk fibers are first exposed to different relative humidity environments. Subsequently, the affinity of the conditioned silk to a set of resins is assessed through measurements of silk/resin contact angle over time. Different silk/resin systems were observed to have contrasting responses to humidity exposure. While some silk/resin systems, such as Ahimsa/epoxy, did not show any change after humidity exposure. Other combinations showed tremendous susceptibility of silk/resin affinity to prior exposure of silk to humidity. For instance, although starting at virtually the same initial hydrophobic contact angle of ~123 degrees, Habotai silk/epoxy samples had contrasting wetting times. While the dried Habotai silk reached full wetting after around 5 minutes, the silk samples exposed to humidity took around 1 hour to reach full im-pregnation. These findings demonstrate the importance of humidity exposure control in silk reinforced composites. Keywords: Natural-Fiber Composites, Contact Angle, Silk, Wettability, Humidity. 

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2. Nano Z-threading Strategy Enhanced Multifunctional Carbon Fiber Composites

   Hsiao, Kuang-Ting (November 2021, Prof. Joseph Koo WebSymposium on Polymer Nanocomposites, Advanced Materials WebCongress 2021, 16-18 November 2021.)

   Carbon fiber reinforced polymer (CFRP) composites have been increasingly used to replace metal parts in many industries such as aerospace, marine, automotive, and sporting goods. The CFRP parts compared with their metallic counter parts have the advantages of lightweight, significantly higher tensile strength, stiffer, and corrosion resistant. On the other hand, compared with many metal parts, the CFRP parts have many well-known disadvantages including the lower toughness, lower through-thickness tensile and shear strengths, lower thermal conductivity, lower electrical conductivity, and lower operating temperature. These disadvantages have made the conversion from metal parts into CFRP parts challenging and costly to design, manufacture, and maintain. The use of nanoparticles in polymer has been studied in the recent two decades. Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) have been dispersed in various thermoset and thermoplastic polymers and improved the mechanical, electrical, and thermal properties; however, the properties were not comparable to CFRP. Later, researchers tried to infuse CNTs or CNFs into either carbon fiber preforms [1] or glass fiber preforms [2] for improving the mechanical properties. But the results were marginal and with great uncertainty due to the challenges of nanoparticle dispersion, filtering, and alignment while being infused through the fiber preform. The glass fiber preform experiments ended with relatively more consistent improvement than the carbon fiber preform experiments since that the glass fiber preform has significantly larger pores than the carbon fiber preform' s. The small pore size presented a great challenge for infusing millions of unaligned long CNTs or CNFs through the carbon fiber preform without being filtered. To infuse long CNFs or CNTs through the carbon fiber preform and achieve reliable improvements, especially for 55% or higher carbon fiber volume fraction with increasingly tighter pores, an innovative plan for the processing and nano-reinforcing strategy is necessary. The z-threading strategy [3, 4, 5] has been reported to have consistent experimental successes in achieving the statistically meaningful improvement in multifunctional properties. The manufacturing steps of the CNF z-threaded CFRP (ZT-CFRP) are: (1) disperse the CNFs in a resin, (2) use a strong electrical field to align the CNFs in either the B-stage epoxy film or a CNF/resin impregnated sponge layer, whereas the CNFs are aligned in the through-thickness direction of the film or sponge layer. (3) place the resin film or sponge layer on a preheated dry carbon fiber fabric and press the resin film into the hot carbon fabric and allow the resin flow to carry the well-aligned CNFs to thread through the pores in the carbon fabric. (4) cool down the resin saturated and CNF z-threaded carbon fiber fabric to obtain the ZT-CFRP prepreg. (5) use the ZT-CFRP prepreg to make the ZT-CFRP laminate. Compared with the traditional CFRP, the ZT-CFRP laminates were reported of having improvement in the Mode-I delamination toughness, interlaminar shear strength, longitudinal compressive strength, through-thickness electrical conductivity, through-thickness thermal conductivity, and can reach the carbon fiber volume fraction of 55-80%. It is an effective approach to achieve a multifunctional CFRP for potentially expanding CFRP's applications. 

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3. Development of a Recyclable Flax Fiber Reinforced Polymer Composite

   https://doi.org/10.5281/zenodo.8092082  

   Das, S; Doshi, S; Millan, E; Mendez, D; Luckenbill, D; Tatar, J (June 2023, Zenodo)

   This study compared the mechanical properties of a recyclable flax fiber reinforced polymer composite (FFRP) with a covalent adaptable network (CAN) matrix to an FFRP composite with a conventional (unrecyclable) epoxy resin matrix. The results indicated that composites fabricated via vacuum-assisted resin transfer molding (VARTM) exhibited up to 19% higher tensile modulus and strength compared to those fabricated via hand layup, attributed to reduced air void content and more uniform fiber alignment. Microscopy evidence supported by mechanical property tests revealed superior adhesion of the CAN matrix to flax fibers compared to conventional epoxy resin. Additionally, a solvent-based method was demonstrated for separating fibers from the CAN matrix, facilitating reuse or upcycling. 

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4. Experimental evaluation on the synergistic effect of PC / ABS / DGEBA blend in fracture toughness enhancement of CFRP composites at cryogenic temperatures

   https://doi.org/10.1002/pc.27504  

   Jayarajan, Aravind; Krishnan_Kutty_Elsy_Bai, Reby Roy (September 2023, Polymer Composites)

   Abstract The current study confirms that modified carbon fiber reinforced polymer (CFRP) composites have higher fracture toughness than unmodified CFRP composites achieved by exploiting the synergistic effect of a polycarbonate (PC)/acrylonitrile butadiene styrene (ABS) blend in toughening the diglycidyl ether of bisphenol A (DGEBA) epoxy resin. The CFRP composite specimens are tested at near cryogenic temperatures using TMA, DMA, and microcrack analysis to determine the best‐suited concentration of ABS in the PC/ABS blend. TMA and DMA results, as well as microcrack analysis at cryogenic temperatures (CT), confirm that the blend 90/10 is effective in reducing the brittle nature of DGEBA resin and increasing bond strength, resulting in the fracture toughness enhancement of CFRP specimens at CT. Further investigation of 90/10 modified CFRP (90/10 m‐CFRP) and unmodified CFRP specimens using Mode II fracture using ENF test and SEM analysis reveal significant reduction in brittle characteristics of matrix with increase in elongation at failure and fracture surface morphologies confirm nano web‐like structures bridging the CF layers, proving to improve fiber/matrix bond strength. This study concludes the effectiveness of hybrid PC/ABS blend in synergistically‐modifying DGEBA resin for improved fracture toughness of CFRP laminates across a wide temperature range (−150°C to 150°C). 

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5. Implementation of Numerical Models to Evaluate Intrinsic Residual Stresses in 3D Woven Composites by Blind Hole Drilling Tests

   https://doi.org/10.12783/asc35/34925  

   Vasylevskyi, Kostiantyn; Tsukrov, Igor; Buntrock, Hilary; Gross, Todd; Drach, Borys (September 2020, Proceedings of the 35th ASC Technical Conference, Hoboken, NJ, USA, 2020)

   null (Ed.)

   3D woven carbon/epoxy composites are often produced using resin transfer molding technique which includes epoxy curing at elevated temperatures. The process may lead to accumulation of the intrinsic residual stresses during cooling of the material caused by the mismatch between carbon and epoxy coefficients of thermal expansion. This paper deals with implementation of mesoscale finite element models to evaluate intrinsic residual stresses in 3D woven composites. The stresses are determined by correlation of the surface displacements observed after drilling 1-mm diameter blind holes with the corresponding predictions of the models. We investigated how a numerical representation of the composite plate surface affects the correlation between the experimental measurements and numerical predictions and how it influences the evaluation of the process-induced residual stresses. It has been shown for ply-to-ply woven composites with different pick spacing that the absence of the resin layer leads to more accurate interpretation of the experimental measurements. The prediction of the average residual stress in the matrix phase of the composite was found to be sensitive to the surface representation accuracy, however, the residual stress magnitude and distribution was not affected fundamentally. 

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