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1. Preliminary experimental study of using nano-enhanced epoxy adhesive to bond carbon nanofibers z-threaded CFRP laminates

   Warren, Ryan A.; Taylor, William W.; Islam, Mohammad R.; Hsiao, Kuang-Ting (October 2023, CAMX 2023 Technical Proceedings)

   Previous studies have provided evidence that reinforcement of epoxy adhesives with nanostructures such as carbon nanofibers (CNFs) produces higher strength bonded joints between carbon fiber reinforced polymer (CFRP) laminates and shifts bond-line failure modes from the adhesive into the laminate. Despite this, there has been no research dedicated to applying reinforced adhesives to the bonding of nano-reinforced CFRP such as CNF z-threaded carbon fiber reinforced polymer (ZT-CFRP) laminates, which have been proven to exhibit increased interlaminar shear strength, mode-I delamination toughness, and compressive strength over traditional CFRP. This study examined the effectiveness of using CNF reinforced epoxy adhesives for unidirectional ZT-CFRP laminate bonding through single-lap shear tests using the ASTM D5868-01 standard. Unidirectional CFRP laminate samples bonded with both epoxy adhesive and CNF reinforced epoxy adhesive were also tested for comparison. It was found that the average shear strength observed for ZT-CFRP samples bonded with CNF reinforced epoxy adhesive was approximately 44% and 26 % higher than that of CFRP samples bonded with epoxy adhesive and CNF reinforced epoxy adhesive, respectively. Microscopic image analysis was performed to examine the mode of bond failure. The roles of nanomaterials in the fracture mechanism of the adhesives and the composite laminates are also discussed. 

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2. Core–Shell Rubber Nanoparticle-Modified CFRP/Steel Ambient-Cured Adhesive Joints: Curing Kinetics and Mechanical Behavior

   https://doi.org/10.3390/ma17030749  

   Okeola, Abass Abayomi; Hernandez-Limon, Jorge E; Tatar, Jovan (February 2024, Materials)

   Externally bonded wet-layup carbon fiber-reinforced polymer (CFRP) strengthening systems are extensively used in concrete structures but have not found widespread use in deficient steel structures. To address the challenges of the adhesive bonding of wet-layup CFRP to steel substrates, this study investigated the effect of core–shell rubber (CSR) nanoparticles on the curing kinetics, glass transition temperature (Tg) and mechanical properties of ambient-cured epoxy/CSR blends. The effects of silane coupling agent and CSR on the adhesive bond properties of CFRP/steel joints were also investigated. The results indicate that CSR nanoparticles have a mild catalytic effect on the curing kinetics of epoxy under ambient conditions. The effect of CSR on the Tg of epoxy was negligible. Epoxy adhesives modified with 5 to 20%wt. of CSR nanoparticles were characterized with improved ductility over brittle neat epoxy; however, the addition of CSR nanoparticles reduced tensile strength and modulus of the adhesives. An up to 250% increase in the single-lap shear strength of CFRP/steel joints was accomplished in CSR-modified joints over neat epoxy adhesive joints. 

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3. 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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4. Evaluation of the effect of z-threaded carbon nanofibers on the improvement of flexural properties of carbon fiber-reinforced polymer laminates using the 3-point bending test

   Hasan, Obaidul; Warren, Ryan A; Hsiao, Kuang-Ting (August 2025, International Conference on Composite Materials (ICCM))

   As a next generation composite material, carbon fiber reinforced polymer (CFRP) has great potential to be widely used in manufacturing industries due to its outstanding mechanical properties. The high strength to weight ratio, and high stiffness inherent to CFRPs make them a desired material in various kinds of applications. CFRPs frequently experience bending loads while in use for such things as aircraft, automobiles, bridges, etc. Anisotropic behavior and limited in through thickness properties are major concerns which affect the performance of CFRPs. Moreover, in the interlaminar region, traditional CFRPs are often vulnerable to matrix sensitive damage such as compressive failure, delamination, and shear failure due to the absence of enough strength in through thickness direction. The tensile and compressive stress generated by the bending loads can weaken the interlaminar shear properties due to the absence of fibers in through thickness and ultimately can lead to catastrophic failure. This study introduces a novel approach with z-threaded CFRP (ZT-CFRP), which incorporates electrically aligned z-threaded carbon nanofibers (CNFs) as reinforcement. Flexural test using 3-point bending was performed on both control CFRP and ZT-CFRP samples reinforced with 1.0 wt.% carbon nanofiber z-threads. The results showed a 15% improvement in the flexural strength and about 36% linear elastic range increase for the ZT-CFRP laminates compared to the unmodified CFRP laminates, and validated the effectiveness of nanofiber Z-threading strategy in strengthening composite materials against flexural loading. 

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5. IMPROVE THE THROUGH-THICKNESS ELECTRICAL CONDUCTIVITY OF CFRP LAMINATE USING FLOW-ALIGNED CARBON NANOFIBER Z-THREADS

   Bikash Ranabhat, Kuang-Ting Hsiao (May 2018, International SAMPE Symposium and Exhibition)

   Traditional Carbon Fiber Reinforced Plastics (CFRPs), carry high in-plane strength and electrical conductivity but exhibit intrinsic weaknesses in strength, toughness and conductivity in the through-thickness direction (i.e. z-direction). This paper presents a novel approach to align and thread Carbon Nanofibers (CNFs) through the porous medium (Carbon fiber fabric) using an interesting radial-flow alignment method and manufacture a novel CNFs z-threaded CFRP prepreg. This new radial-flow alignment approach is unique and has been found highly effective to z-thread the array of carbon fibers (diameter ~ 7 microns) with numerous long CNFs (length ~ 50-200 microns) under a Scanning Electronic Microscope (SEM) analysis. Experimental tests performed on a cured laminate sample prepared by this novel technique with 1 wt% aligned CNF concentration showed a significant improvement on the z-directional electrical conductivity for direct current (DC). The 1 wt% CNFs z-threaded CFRP was found about 100 times as conductive as the control CFRP; whereas the unaligned 1 wt% CNFs modified CFRP was only about 16 times as conductive as the control sample. 

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