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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. 

Low fiber-direction compressive strength is a well-recognized weakness of carbon fiber-reinforced polymer (CFRP) composites. When a CFRP is produced using 3D printing, the compressive strength is further degraded. To solve this issue, in this paper, a novel magnetic compaction force-assisted additive manufacturing (MCFA-AM) method is used to print CFRP laminates reinforced with carbon nanofiber (CNF) z-threads (i.e., ZT-CFRP). MCFA-AM utilizes a magnetic force to simultaneously levitate, deposit, and compact fast-curing CFRP prepregs in free space and quickly solidifies the CFRP laminate part without any mold nor supporting substrate plate; it effectively reduces the voids. The longitudinal compressive test was performed on five different sample types. ZT-CFRP/MCFA-AM samples were printed under two different magnetic compaction rolling pressures, i.e., 0.5 bar and 0.78 bar. Compared with the longitudinal compressive strength of a typical CFRP manufactured by the traditional out-of-autoclave–vacuum-bag-only (OOA-VBO) molding process at the steady-state pressure of 0.82 bar, the ZT-CFRP/MCFA-AM samples showed either comparable results (by −1.00% difference) or enhanced results (+7.42% improvement) by using 0.5 bar or 0.78 bar magnetic rolling pressures, respectively. 

The matrix sensitive weaknesses of Carbon Fiber Reinforced Polymer (CFRP) laminates are usually magnified by mainstream additive manufacturing (AM) methods due to the AM-process-induced voids and defects. In this paper, a novel Magnetic Compaction Force Assisted-Additive Manufacturing (MCFA-AM) method is used to print Carbon Nanofibers (CNF) Z-threaded CFRP (i.e., ZT-CFRP) composite laminates. The MCFA-AM method utilizes a magnetic force to simultaneously support, deposit, and compact Continuous Carbon Fiber Reinforced Polymer (C-CFRP) composites in free space and quickly solidifies the CFRP part without any mold; it effectively reduces the voids. Past research proved that the zig-zag threading pattern of the CNF z-threads reinforces the interlaminar and intralaminar regions in the ZT-CFRP laminates manufactured by the traditional Out of Autoclave-Vacuum Bag Only (OOA-VBO) method, and enhances the matrix sensitive mechanical, thermal, and electrical properties. In this study, the longitudinal compressive test (ASTM D695, i.e., SACMA SRM 1R-94) was performed on the MCFA-AM printed ZT-CFRP laminate. The results were compared with unaligned CNF-modified CFRP (UA-CFRP), control CFRP, and no-pressure CFRP samples’ data to investigate the impact of the CNF z-threads and MCFA-AM process on the CFRP’s performance. The 0.5-bar MCFA-AM printed ZT-CFRP showed comparable longitudinal compressive strength with the 1-bar OOA-VBO cured CFRP. 

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. 

Three-dimensional (3D) printing with continuous carbon-fiber-reinforced polymer (C-CFRP) composites is under increasing development, as it offers more versatility than traditional molding processes, such as the out-of-autoclave-vacuum bag only (OOA-VBO) process. However, due to the layer-by-layer deposition of materials, voids can form between the layers and weaken some of the parts’ properties, such as the interlaminar shear strength (ILSS). In this paper, a novel mold-less magnetic compaction force-assisted additive manufacturing (MCFA-AM) method was used to print carbon nanofiber (CNF) z-threaded CFRP (ZT-CFRP) laminates with significantly improved ILSS and reduced void content compared to traditional C-CFRP laminates, which are printed using a no-pressure 3D-printing process (similar to the fused-deposition-modeling process). The radial flow alignment (RFA) and resin-blending techniques were utilized to manufacture a printing-compatible fast-curing ZT-CFRP prepreg tape to act as the feedstock for a MCFA-AM printhead, which was mounted on a robotic arm. In terms of the ILSS, the MCFA-AM method coupled with ZT-CFRP nanomaterial technology significantly outperformed the C-CFRP made with both the traditional no-pressure 3D-printing process and the OOA-VBO molding process. Furthermore, the mold-less MCFA-AM process more than doubled the production speed of the OOA-VBO molding process. This demonstrates that through the integration of new nanomaterials and 3D-printing techniques, a paradigm shift in C-CFRP manufacturing with significantly better performance, versatility, agility, efficiency, and lower cost is achievable.