Integration of steel fibers (SF) in carbon fiber (CF) reinforced polymer composites (CFRPC) allows improvement of electrical conductivity while maintaining excellent mechanical properties, since SF also contribute to the load-carrying capacity. Due to their high ductility, also energy absorption and structural integrity can be improved. Within this study, a preforming process for hybrid carbon/SF preforms based on dry fiber placement (DFP) is developed and validated. The investigations cover the production of bindered SF rovings, the production of hybrid preforms via DFP of spread and nonspread SF rovings on CF noncrimp fabrics (CF-NCF) as well as the production of hybrid laminates via vacuum-assisted resin infusion (VARI). The laminate quality was evaluated by microscopic images and mechanical tensile testing. A higher SF volume content within the SF areas and more homogeneous SF layers in the preform (fewer matrix-rich zones) were achieved by processing nonspread SF rovings. The more homogeneous SF layers within the samples with nonspread SF rovings compared to spread SF rovings led to higher stiffness and strength of the specimens for tension loadings and therefore to best results.
Carbon-fiber-reinforced polymers (CFRPs) are the standard lightweight composite material for structural applications in aviation. The addition of metallic fibers to CFRPs to form metal/carbon-fiber hybrid composites (MCFRPs) has been shown to improve the elastic and plastic properties and to enable a non-destructive method for structural health monitoring over the material’s service life. In this paper, the results from the fatigue experiments on these hybrid composites at −55, 25 and 120 °C are discussed. Multidirectional CFRP and MCFRP laminates, fabricated using the autoclave method, were tested and compared under different fatigue loading conditions, while being simultaneously monitored for temperature and electrical resistance. Magnetic phase measurements were additionally carried out for the chosen metastable austenitic steel fibers in the MCFRPs. The results show that the improved ductility of the hybrid composite due to the presence of the steel fibers leads to better performance under fatigue loads and a less-brittle failure behavior. Based on the chemical composition of the metastable austenitic steel fibers, a temperature and plastic deformation-dependent phase transformation was observed, which could potentially lead to a method for non-destructive structural health monitoring of the hybrid composite over its service life.
The integration of ductile continuous steel fibers into thermoset carbon fiber reinforced polymer (CFRP) enables significant enhancements of its damage tolerance and crashworthiness. Due to their high strain at failure, the embedded steel fibers provide alternative load paths after failure of the brittle carbon fibers. The resulting post damage performance of the hybrid composite depends on the proportions of the different types of reinforcing fibers, their individual properties, the laminate architecture, and particularly on the steel fiber resin adhesion. So far, common constitutive laws for fiber reinforced composites have very limited suitability for the complex interrelation between the nonlinear plastic behavior of steel fibers and the elastic behavior of the carbon fibers. For this reason, a novel analytical method is developed to predict the failure performance of fiber hybrid composites. In principle, the analytical approach is based on a structural dynamic analysis of the fracture gap formation during failure and thus unloading of the brittle carbon fibers. The present paper covers the derivation of this analytical approach and its exemplary application to a steel and carbon fiber reinforced hybrid composite. A final comparison with an experimentally obtained stressstrain curve validates the theoretical model introduced.
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