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Increased tensile strength of carbon nanotube (CNT)-grafted carbon fiber (CF) composites has been reported, but the mechanism of this increase is not yet clear. In this study, CNT-grafted CF unidirectional (UD) and woven composites were fabricated using a low-temperature chemical vapor deposition (CVD) and resin transfer molding. Two types of CNTs (short and thin, long and thick) were successfully grown and grafted to CFs without degrading the CFs in the preforms. The CNT-grafted CFs exhibited increased interfacial shear strength (IFSS) similarly regardless of the CNT type. Interestingly, however, long and thick CNT-grafted CF UD and woven composites exhibited significant increases in tensile strength (about 20% and 30%), suggesting other mechanisms besides increased IFSS. The splitting crack initiation under the mixed mode condition was quantitatively characterized for the CNT-grafted CF UD composites, demonstrating that long and thick CNTs delayed the splitting crack initiation. Delayed fiber splitting and increased IFSS were concluded to be the main sources of increased tensile strength of CNT-grafted CF composites. [ABSTRACT FROM AUTHOR]

This study reports on the prediction of the delamination of steel-polymer composites. To characterize the adhesive properties from peeling tests, a simple model was derived based on peeling mechanics, in particular considering plastically dissipated energy of polymer film due to bending. Peeling tests were performed at two angles so that the adhesive properties were determined in both normal and tangential directions. Then, peel strength at arbitrary angle was predicted and compared with experiments, showing that the peel strength at only two angles and peeling mechanics considering the bending of polymer film can accurately describe the adhesive properties of steel-polymer interface. Finally, finite element simulations based on the cohesive zone model were performed to predict the delamination between steel-polymer sandwich composites, demonstrating the validity of cohesive zone model equipped with necessary properties determined using peeling tests and the developed model in this study. [ABSTRACT FROM AUTHOR]

Synergistic effects of carbon fiber-reinforced plastic (CFRP) and steel hybrid laminate composites were investigated systematically by static and dynamic tensile testing. Various hybrid laminate composites were prepared by varying the adhesion between the CFRP and steel, the layup sequence, and the volume fraction of the CRFP. The fracture strain of the CFRP within the hybrid laminate composites increased, e.g., from 1.94% for pure CFRPs to 2.21% for the steel/CFRP/steel case. Finite element analysis and fractography established that transverse compressive stress was the main source of the improvement. Different Poisson's ratios led to transverse compressive stress on the CFRP layer after yielding of the steel during the tensile test. This stress delayed the fiber-splitting behavior of the CFRP, leading to further deformation and thus increasing the breaking strain and tensile strength of the CFRP within the hybrid laminate composites. Our results provide insight into the design of ductile composites using CFRP and steel through optimization of the laminate structure. [ABSTRACT FROM AUTHOR]

The tensile strength of unidirectional (UD) carbon fiber composites was predicted considering the interfacial shear strength (IFSS). First, the effect of the IFSS on the load transfer to the surrounding fibers when a fiber was broken was analyzed using finite element method and then the stress concentration factor (SCF) of each surrounding fiber was determined. The multiple fracture number was calculated using the SCF and a statistical approach that can calculate the probability of fiber breakage propagation due to the stress concentration when the broken fibers exist in neighbor. Finally, the tensile strength of UD carbon fiber composites was predicted using the multiple fracture number and was compared with experimental results, demonstrating the validity of the current method. An optimal IFSS that can ensure the maximum tensile strength of UD carbon fiber composites is discussed based on the calculation results. [ABSTRACT FROM AUTHOR]

This report describes a new twist-spinning process for the manufacture of piezoelectric yarn without the need for additional poling processes. Poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) nanofibers were first electrospun and organized into a web structure. Using rotational and translational motors, the nanofibers of the web were pulled and stretched and, finally, twisted into piezoelectric yarns. The crystallinity, β phase ratio, and mechanical and piezoelectric properties of these twist-spun piezoelectric yarns were characterized to assess the effects of the twist-spinning operation on their microstructure and performance. Twist-spun piezoelectric yarns that had undergone high degrees of stretching exhibited enhancements in both crystallinity and β phase ratio by 83% and 12%, respectively. In contrast, high stretching reduced the tensile strength and modulus of the yarns due to small surface angles. The twist-spun piezoelectric yarn with the highest β phase ratio and lowest modulus attained in this study yielded a piezoelectric potential and piezoelectric voltage constant of 500 mV and 0.412 mVm/N, respectively. Unlabelled Image • Poling-free spinning process was designed for piezoelectric nanofiber yarns. • PVDF-TrFE nanofibers were enhanced in both crystallinity (by 83%) and β phase ratio (by 12%) after yarn spinning process. • Piezoelectric yarn showed high mechanical (tensile strength of ~25 MPa) and piezoelectric (potential of 500 mV and g 33 of 0.40) properties. [ABSTRACT FROM AUTHOR]