The modification of carbon fibers for improving adhesion between fibers and an epoxy resin in composite materials has become the focus of attention. In this work the carbon fiber coating process has been devised in a way preventing the stiffening and clumping of fibers. To improve interactions between coated fibers and a resin in composites, four types of silica coatings with different organic functional groups (3-aminopropyl–coating 1, 3-mercaptopropyl–coating 2, 2-(3,4-epoxycyclohexyl) ethyl–coating 3, methyl–coating 4) were obtained. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to distinguish the changes of a carbon fibers surface after coating deposition. The thickness of the obtained coatings, including the diversity of thickness, was determined by transmission electron microscopy (TEM). The increase in surface free energy (SFE) of modified fibers, including the distinction between the polar and dispersive parts, was examined by wettability measurements using a tensometric test. The developed coating preparation process allowed to cover fibers separately with nanoscale silica layers, which changed their morphology. The introduction of organic functional groups resulted in surface free energy changes, especially an increase in specific polar surface energy components.

Epoxy resin used commonly as a matrix for polymer composite materials has good handling properties, but is too brittle. That is why various modifiers are used to increase the flexibility of products based on epoxy resin. This leads to two issues: how to efficiently increase the toughness of the resin without impacting significantly other properties, as well as how to measure the toughness in composite materials. The work aimed to show how the addition of a reactive rubber modifier will affect the fracture toughness of the obtained laminates during the longitudinal shear test (Mode II fracture). In total, three epoxy-glass laminates with different matrices were made and subjected to the End-Notched Flexure test according to ASTM D7905/D7905M standard: (1) the basic matrix of Epidian 6 resin, (2) Epidian 6 modified with the addition of 10% of Albipox 1000 reactive liquid rubber and (3) Epidian 6 modified with the addition of 10% of Hypro 1300X16 ATBN reactive liquid rubber. Based on the obtained results, it can be seen that the modulus of elasticity for the modified laminates was decreased compared to the laminate of pure epoxy resin (by ~ 25%). However, the addition of reactive rubbers increased the fracture toughness of the modified epoxy-glass laminates in the Mode II longitudinal shear test (G_IIc) by ~ 40–60%. Thus the benefits of modification outweigh the drawbacks if fracture toughness is an important designing consideration in a given application. The applicability of ENF method is successfully tested, but potential drawbacks are indicated—careful control of specimen thickness is necessary.

Although cooling at ambient temperature is widely used and is said to be safe and convenient, faster cooling may have an influence not only on the time of the manufacturing process but also on the mechanical response, especially the residual stress. The study aimed to investigate the influence of the cooling rate after curing on the mechanical response of filament-wound thick-walled carbon fiber reinforced polymer (CFRP) rings. Three cooling rates were taking into consideration: cooling with the oven, at room temperature, and in the water at 20 °C. The splitting method was used to examine the residual strains. In the radial compression test, the mechanical response was investigated between the rings with different cooling regimes. The FEM analysis of the compression test in elastic range was also performed. Both the splitting method and the radial compression test showed no significant difference in the mechanical response of the CFRP rings. The presented results showed that the fast-cooling rate slightly decreases the mechanical performance of the filament-wound rings.

This paper presents the experimental results of composite rebars based on GFRP manufactured by a pultrusion system. The bending and radial compression strength of rods was determined. The elastic modulus of GFRP rebars is significantly lower than for steel rebars, while the static flexural properties are higher. The microstructure of the selected rebars was studied and discussed in light of the obtained results—failure processes such as the delamination and fibers fracture can be observed. The bending fatigue test was performed under a constant load amplitude sinusoidal waveform. All rebars were subjected to fatigue tests under the R = 0.1 condition. As a result, the S-N curve was obtained, and basic fatigue characteristics were determined. The fatigue mechanism of bar failure under bending was further analyzed using SEM microscopy. It is worth noting that the failure and fracture mechanism plays a crucial role as a material quality indicator in the manufacturing process. The main mechanism of failure under static and cyclic loading during the bending test is widely discussed in this paper. The results obtained from fatigue tests encourage further analysis. The diametral compression test reflects the weakest nature of the composite materials based on the interlaminar compressive strength. The proposed methodology allows us to invariantly describe the experimental transversal strength of the composite materials. Considering the expected durability of the structure, the failure mechanism is likely to significantly improve their fatigue behavior under the influence of cyclic bending. The reasonable direction of searching for reinforcements of composite structures should be the improvement of the bearing capacity of the outer layers. In comparison with steel rebars (fatigue tensile test), the obtained results for GFRP are comparable in the HCF regime. It is worth noting that in the near fatigue endurance regime (2–5 × 10⁶ cycles) both rebars exhibit similar behavior.