Special Issue on “Progress of Fiber-Reinforced Composites: Design and Applications”

1. Introduction

Fiber-reinforced composite (FRC) materials are widely used in advanced structures and are often used to replace traditional materials such as metal components, especially those used in corrosive environments. They have become essential materials for maintaining and strengthening existing infrastructure due to the fact that they combine low weight and density with high strength, corrosion resistance, and high durability, providing many benefits in performance and durability. Modified fiber-based composites exhibit better mechanical properties, impact resistance, wear resistance, and fire resistance. Therefore, the FRC materials have reached a significant level of applications ranging from aerospace, aviation, and automotive systems to industrial, civil engineering, military, biomedical, marine facilities, and renewable energy.

In order to update the field of design and development of composites with the use of organic or inorganic fibers, a Special Issue entitled “Progress of Fiber-Reinforced Composites: Design and Applications” has been introduced. This editorial manuscript gathers and reviews the collection of twelve article contributions, with authors from Europe, Asia and America accepted for publication in the aforementioned Special Issue of Applied Sciences.

2. Fiber-Reinforced Plastic Composites

In recent years, the uses of composite fiber-reinforced plastic (CFRP) for several manufacturing structures have increased. Seven articles have been published in this Special Issue related to the synthesis and application of CFRP. The work of Troschitz and his team [1] reports the procedure of embedding metal weld inserts as an interface in glass-fiber-reinforced polypropylene sheet thermoplastic composites via compression molding in high quality and without fiber damage. The composite component was joined with steel sheets using conventional spot-welding guns. Numeric welding simulations were used for the determination of welding process parameters. Using to this innovative technology, high-quality joints were obtained.

A very interesting concept is developed by Yanlei Wang and his team [2] based on an thickness-dependent accelerated ageing method for tensile strength retention and water absorption of basalt fiber-reinforced polymer (BFRP) laminates. The evaluation of the tensile properties, water absorption, and degradation mechanism of BFRP laminates was conducted after their immersion on either alkaline solution or deionized water. The BFRP laminates were synthesized by a wet-layup method, and the influence of thickness on their tensile properties and water absorption were investigated under hygrothermal environment together with degradation mechanism. The obtained results revealed that the tensile properties and the water absorption of BFRP laminates were affected by specimen thickness.

Vito and his team [3] performed studies focuses on the physical structure of a 5G smart light pole and its multidisciplinary design process. Experiments were conducted for the determination of the design drivers of a composite 5G smart pole and the connecting design between signal penetration, finite element modeling, and computational fluid dynamics for thermal analysis. The results indicate the significant effects of thermal loading on the material selection.

In the work of Peng et al. [4], the slip load and slip factor of the glass-fiber-reinforced plastic (GFRP) GFRP–GFRP slip-critical connections were investigated. The impact on the long-term effects of the creep property in composite elements under the pressure of high-strength bolts was also evaluated via pre-tension force relaxation tests. It was proved that a high-efficiency fastener connection can be obtained using stainless steel cover plates with a grit-blasting surface treatment, while the effects of the creep property are negligible.

In the work of Riccio et al. [5] the influence of the material fracture toughness on the capability of a composite fuselage barrel to tolerate an impact on a rigid surface was investigated. The effects of intralaminar fracture energy variations on the impact deformation of the barrel were evaluated comparing the numerical results in terms of displacements and damage evolution for the three analyzed material configurations. According to the obtained results, a relevant influence of the in-plane toughness on the global dynamic response of the fuselage barrel was observed.

In another study, Sellitto et al. [6] reported the investigation of tensile behavior of a hybrid metallic–composite stiffened panel. A numerical–experimental investigation into the mechanical behavior of a fastened omega-reinforced composite fiber-reinforced plastic (CFRP) panel joined with a Z-reinforced aluminum plate was discussed. It was proved that the investigated hybrid panel does not experience any damage in composite sub-components up to a tensile load of 300 kN, while, at this loading level, some metallic sub-components experience extensive plastic deformation. Moreover, it was revealed that the omega stringer joints are able to arrest the skin-stringer debonding growth avoiding a drastic reduction of the load-carrying capability of the panel.

In another study, Hassan et al. [7] investigated the optimal conditions for a banana/epoxy composite in order to synthesize a sandwich structure where carbon/Kevlar twill plies acted as the skins. The structure was examined based on low-velocity impact and compression after impact tests. The results depicted a low peak load and larger damage area in the optimal banana/epoxy structures. The impact damage area increased with increasing impact energy. The optimal banana composite and synthetic fiber systems were proven to offer a similar residual strength and normalized strength when higher impact energies were applied.

3. Fiber-Reinforced Concrete Composites

Concrete and cement-based materials are commonly used in structural members and equipment in civil and industrial infrastructure. Two articles have been published in this Special Issue related to the fabrication, characterization, and application of these composites.

The improvement effect of fiber on the brittle failure of cement-treated subgrade soil was investigated by Wei Wang et al. [8]. A series of triaxial unconsolidated undrained (UU) tests were performed on samples of polypropylene fiber-cement-treated subgrade soil (PCS) in several polypropylene fiber mass contents. The outcome of this study denotes that it is feasible to modify cement subgrade soil with an appropriate amount of polypropylene fiber to mitigate its brittle failure.

The study of Jong-Han Lee et al. [9] examined the effect of steel fibers on the shear failure mode and breakout resistance of anchors embedded in steel fiber-reinforced concrete (SFRC). The relationship between the tensile performance of SFRC beams and the shear resistance of SFRC anchors was also assessed. The obtained results revealed that the calculated shear resistance of anchors in both SFRC and the plain concrete were in good agreement with the measurements. Moreover, the energy absorption capacity depicted a linear increase with that of the SFRC beam.

4. Fiber-Reinforced Carbonaceous Composites

Carbon-fiber-reinforced carbon, or carbon-carbon, is a composite material based on carbon fibers incorporated into a carbonaceous matrix. Three articles have been published in this Special Issue related to the production, characterization and application of these composites. In the work of Kailong Xu et al. [10], 3D four-step braided composites were aged in a cyclic hygrothermal environment. An accelerated hygrothermal aging spectrum for a military aircraft was applied. The microscopic damage morphologies of the composites were examined in order for the damage evolution determination with cyclic hygrothermal aging days to be determined. A split Hopkinson pressure bar was employed to evaluate the dynamic compressive mechanical property along the longitudinal direction of the 3D four-step braided composites at various cyclic hygrothermal aging days.

In the paper of Yanfeng Zhang et al. [11], the mechanical properties and compressive shear failure behavior of bonded–bolted hybrid single-lap joints of C/C composites at high temperature were studied. The interrelationship of the compression shear loading mechanism and the variations in stress distribution between bonded joints and bonded–bolted hybrid joints at high temperature were explored. The progressive damage of hybrid joints and the variations in the ratio of the bolt load to the total load with displacement were obtained. The variations in the ratio of the load shared by the bolt to the total load with displacement were obtained.

Finally, in the study of Kartsonakis et al. [12], the fabrication of a carbon fiber process was investigated, using high-density polyethylene (HDPE) and lignin esterified with either lactic acid or poly(lactic acid). The modified compounds were blended with HDPE thermoplastic polymer to synthesize composite precursors suitable for carbon fiber production. Stabilization and carbonization were performed, and the corresponding carbon fibers were obtained. This bottom-up approach was evaluated as a viable route considering large scale production for the transformation of lignin in a value-added product.

5. Future Strategies

Although the Special Issue has been closed, more in-depth research in the field of fabrication and characterization of fiber-reinforced composites is expected. It can be anticipated that more friendly production methods of fibers and their corresponding composites will be demanded in the future for several applications. In this case, suitable strategies should be ready for consolidation and utilization.