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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. 

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.