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1. 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. Themore »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.« less
2. EFFECTS OF CARBON NANOFIBER Z-THREADS ON THE LONGITUDINAL COMPRESSIVE STRENGTH OF UNIDIRECTIONAL CFRP LAMINATES

   Kirmse, Sebastian ; Kim, Keonhyeong ; Ranabhat, Bikash ; Hsiao, Kuang-Ting ( May 2019 , SAMPE 2019 Proceedings, May 20-23, Charlotte, NC.)

   In this study, unidirectional carbon fiber prepregs that contain long carbon nanofiber (CNF) z threads as a through-thickness (z-directional) reinforcement were manufactured. The CNF z threads are long enough to thread through multiple carbon fiber (CF) arrays, which creates a multi-scale CNF/CF/resin-composite. The CNF z-threaded prepregs were manufactured using an electric-field aligned flow-transferring process. It was hypothesized that the CNF z-threads with the zig-zag threading pattern reinforces the interlaminar and intralaminar regions of the CFRP laminate thus improve the compressive strength by reducing the chance of carbon fiber buckling. Compressive testing was performed per modified version of ASTM D695 (i.e., SACMA SRM 1R 94) to evaluate the compressive strength of the CNF z-threaded CFRP (ZT-CFRP) laminates. The samples were manufactured using AS4 carbon fibers, EPON 862/Epikure-W resin and a 1wt% CNF content. ZT-CFRP testing results were compared with unaligned CNF-modified CFRP (UA-CFRP) and unmodified CFRP samples to investigate the impact of the CNF z-threads on the compressive strength. Results showed an increase of ~15% for the compressive strength of ZT CFRPs, whereas the UA-CFRPs experienced a decrease of ~8% when compared to unmodified CFRPs. It was concluded that CNF/carbon fiber interlocking stops and delays crack growth, and helps to stabilizemore »carbon fibers from further buckling.« less
3. Flammability characterization of carbon nanofibers and nanotubes z-threaded CFRP laminates

   Hsiao, Kuang-Ting ; Wu, Hao ; Uddin, Md. N ; Hou, Yanan ; Taylor, William W. ; Koo, Joseph H. ( October 2022 , CAMX Conference Proceedings. Anaheim, CA, October 17-20, 2022.)

   Carbon fiber reinforced polymer (CFRP) composites have been increasingly used in many vehicles such as airplanes, automobiles, and ships due to the advantages of high-strength, high-modulus, lightweight, and corrosion resistance. CFRP structures enhance the vehicle's performance, energy-efficiency, comfort, and safety. However, a common safety concern is how the CFRP materials perform when the vehicle is in fire and if there are enough time to safely evacuate the passengers. The elevated temperature can soften and decompose the polymer matrix, delaminate the CFRP laminate, and burn the CFRP through the contact with oxygen. As a result, the thermal and flammability response of CFRP is important for considering CFRP for vehicle applications; and some specialty high-temperature or flame/smoke/toxicity-proven resins have been investigated for CFRP parts manufacturing due to the needs. In this paper, a novel flame resistant hypothesis of utilizing the unique nano/micro- interlocked fiber reinforcing structure of the long-range carbon nanofiber z-threaded CFRP (ZT-CFRP) composite laminates for improving the flammability performance will be investigated. The carbon nanofibers (CNT) and carbon nanotubes (CNT), which have excellent thermal and mechanical properties, will be dispersed in an epoxy resin and will zig-zag thread through a carbon fiber fabric using an electrical/flow assisted impregnation process tomore »create the unidirectional ZT-CFRP prepregs, respectively, which will be further processed into ZT-CFRP composite laminates. The UL-94 flammability test will be employed to characterize the ZT-CFRP laminates' flammability performance against the control baseline data of the regular CFRP, all without using any flame retardant chemicals. An impressive self-extinguishing flammability characteristic of the CNF based ZT-CFRP samples has been distinctly identified from all the samples. The UL-94 testing results and the effectiveness of using the long-range nanofiber z-threading strategy for enabling the novel nano/microstructure-induced flame resistant and self-extinguishing characteristics will be discussed.« less
4. 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.
5. MECHANICAL BEHAVIOR MODELING OF UNIDIRECTIONAL CARBON FIBER REINFORCED POLYMER COMPOSITES REINFORCED WITH Z-DIRECTIONAL NANOFIBERS

   Fariborz Bayat, Kuang-Ting Hsiao ( May 2018 , International SAMPE Symposium and Exhibition)

   This paper utilizes a periodic unit cell modeling technique combined with finite element analysis (FEA) to predict and understand the mechanical behaviors of a nanotechnology-enhanced carbon fiber reinforced polymers (CFRPs) composite. This research specifically focuses on the study of novel Z-threaded CFRPs (ZT-CFRPs) that are reinforced not only by unidirectional carbon fibers but also with numerous carbon nanofibers (CNFs) threading through the CFRP laminate in the z-direction (i.e., through-thickness direction). The complex multi-scaled orthogonally-structured carbon reinforced polymer composite is modeled starting from a periodic unit cell, which is the smallest periodic building-block representation of the material. The ZT-CFRP unit cell includes three major components, i.e., carbon fibers, polymer matrix, and carbon nanofiber Z-threads. To compare the mechanical behavior of ZT-CFRPs against unmodified, control CFRPs, an additional unit cell without CNF reinforcement was also created and analyzed. The unit cells were then meshed into finite element models and subjected to different loading conditions to predict the interaction among all their components. The elastic moduli of both unit-cells in the z-direction were calculated from the FEA data. By assuming the CNFs have the same mechanical properties of T-300 carbon fiber, the numerical modeling showed that the ZT-CFRPs exhibited a 14% improvement inmore »z-directional elastic modulus due to the inclusion of 1 wt% CNF z-threads, indicating that ZT-CFRPs are stiffer compared to control CFRPs consisting of T-300 carbon fibers and epoxy.« less