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A rapid fatigue characterization method using full-field temporal surface temperature measurements has been used to study the effect of microstructural modification in unidirectional carbon fiber reinforced plastics (UD- CFRP) via electrically aligned Z-threaded carbon nanofibers (CNF). 1 wt% CNF were aligned in the Z-direction via electric means using a patented roll-to-roll process, enabling ZT-CNF-CFRP prepreg production. Three conf igurations were tested under fatigue: ZT-CNF-UD-CFRP (ZTE), UD-CFRPs with Unaligned CNF, and UD-CFRPs without CNF (Control). Mean surface temperatures measured via passive infrared thermography (IRT) was used to estimate the fatigue limit for these materials using a staircase loading method. Further, harmonic analysis of the obtained temporal full-field temperature data was used to monitor the damage evolution. Finally, the fatigue limit was also determined using the residual threshold method based on the second harmonic signal. Fatigue limits obtained for the three configurations via the bi-linear method were 62.36 ± 0.42 % σ 64.7 ± 1.83 % σ uts for Unaligned and 49.29 ± 2.47 % σ uts uts for ZTE, for Control. While the presence of 1 wt% CNF improves the fatigue limit; the effect of Z-threading could not be accurately quantified since the Z-threading manufacturing process was found to increase the matrix content of the composite. CNF Z-threads increased thermal conductivity, enabling better in situ damage monitoring. Different failure modes were found and discussed to understand the roles of CNF in the fatigue behavior of UD-CFRP laminates. 

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. 

Previous studies have shown that carbon nanofiber (CNF) z-threaded carbon fiber-reinforced polymer (ZT-CFRP) laminates exhibit improved mechanical performance in comparison to traditional carbon fiber-reinforced polymer (CFRP) laminates when exposed to extreme elevated temperatures. Z-threaded reinforcement is a technique for strengthening the through-thickness of a laminate by introducing perpendicularly aligned carbon nanomaterial to be threading into the continuous fiber array. Improved performance has already been observed in properties such as interlaminar shear strength (ILSS) without extreme heat exposure, but there has also been evidence that z-thread inclusion may mitigate strength loss due to thermal degradation of the matrix. This study examined how ILSS was diminished in both CFRP and ZT-CFRP samples with matrix degradation caused by extreme temperature exposure. Test samples were heated to 350 ˚C for 10 minutes and then allowed to return to room temperature. SBS testing in accordance with ASTM D2344 was conducted on both untreated and heat-treated samples for comparison. All samples were at room temperature during testing. It was found that ZT-CFRP samples (with 0.5wt% CNF concentration the matrix) exhibited higher ILSS with and without heat treatment over the traditional CFRP samples with and without heat treatment by +33.96% and +25.12%, respectively. ZT-CFRP ILSS was found to decrease by 10.584 MPa (-14.56%) after the extreme heat treatment, while CFRP ILSS decreased by only 4.627 MPa (-8.53%). Microscopic image analysis was also performed to provide insight into how the CNF z-threads may have provided a mechanism for retaining ILSS performance even with matrix thermal degradation. 

Previously reported vertical UL-94 testing results showed that carbon nanofiber z-threaded carbon fiber-reinforced polymer (ZT-CFRP) laminates have significantly improved flame resistance capabilities compared to traditional carbon fiber-reinforced (CFRP) laminates. Shorter flame self-extinguishing times and no flame propagation were reported. These characteristics provided evidence that ZT-CFRP’s unique microstructure, combined with its inherent strengthened mechanical, thermal, and electrical properties, has the potential to have more favorable high-temperature applications than traditional CFRP. This study examined the interlaminar shear strength (ILSS) enhancement of ZT-CFRP laminates, in comparison to traditional CFRP, when exposed to gradually increased temperatures. This was accomplished through the use of a furnace and an in-house constructed three-point bending apparatus capable of supplying static loading to determine the temperature at which failure occurred. The apparatus was loaded with a specimen and then placed inside the furnace where the temperature was allowed to increase based upon a consistent heating schedule. It was observed that ZT-CFRP samples had an approximately 30 ˚C improvement in temperature handling capabilities while exposed to an interlaminar shear load when compared to CFRP samples. Microscopic image analysis was also performed to observe how CNF z-threads contributed to the improved performance observed for ZT-CFRP at extreme elevated temperatures. 

In-plane shear strength is an important issue for the structural integrity of carbon fiber reinforced polymers (CFRP). In this study [± 45°]4s in-plane shear test was performed for both Z-threaded CFRP (ZT-CFRP) and traditional CFRP. A newly developed proprietary Bisphenol-F based epoxy blend was used in this study. A significant improvement of +24% in the in-plane shear strength for ZT-CFRP was observed. There was a notable difference found in the failure modes between control CFRP and ZT-CFRP samples. Intralaminar and interlaminar delamination modes were noticed in control CFRP samples spreading to all plies whereas the ZT-CFRP samples experienced a confined failure in the interlaminar region. Digital image correlation (DIC) showed more uniform stress distribution and higher strain in ZT-CFRP, which suggested ZT-CFRP was stronger and tougher under the in-plane shear testing. Microscopic analysis of failure mode indicated that the z-threaded CNFs act as effective nano-structural reinforcement between and inside the laminas to keep the interlaminar/intralaminar bonding stronger.