# Recent Advances in Additive Manufacturing of Fibre-Reinforced Materials: A Comprehensive Review
## 1 Introduction  
The need for lightweight materials in the manufacturing industry led to the invention of polymers in the early 20 th century. Inventions such as vinyl, polyester, and phenolic compounds allowed replacing traditional materials, such as wood and metal, with materials of high stiffness-to-weight ratio. The further evolution of these materials brought carbon fibre (CF), alongside epoxy resins, into to the automotive and aerospace industry, starting during the 1970s [1]. Carbon fibres and thermosetting epoxy matrices create strong interfacial bonds; thus, carbon fibre composites provide better corrosion resistance, fatigue, and better overall mechanical properties. As a key factor for the composites, the epoxy matrices have evolved in the last decades, both in the area of time consumption for hardening and for the different harnesses that can be achieved [2; 3; 4; 5; 6]. Therefore, there is a perfect combination of epoxy/fibre for each application. In contrast, thermoplastic matrices do not provide the adhesion that epoxy ones have, but other advantages can make them preferable to work with [7; 8]. Due to high melt viscosity, thermoplastics offer great processability, and consequently can be much easier to process, and are better suited forrapid production of complex geometries, with reduced manufacturing costs, since there is no need for demanding curing time. In advance, the range of thermoplastic materials that can be used is wider and more accessible than that of epoxy resins [6; 9; 10]. This can provide an advantage to additive manufacturing (AM) technology, particularly the composite fibre coextrusion (CFC) additive manufacturing technique that has emerged recently. In the CFC 3D printing method, fibres are impregnated into the printed product and act as reinforcement materials for the object. The impregnation can be achieved either with continuous fibre implants or by filling with a large number of short fibres. The volume percentage of impregnated fibres cannot exceed 40-50% volume of fibres in the matrix, while the typical fraction for common carbon fibre manufacturing techniques can be as high as 67% [11]. Each method has its advantages and disadvantages. Continuous fibre can provide superior mechanical properties but result in higher fibre consumption and, consequently, higher costs. In contrast, short fibre infills improve the mechanical properties compared to simple thermoplastic print but are not as good as continuous fibres. Additionally, short fibres can more easily damage the nozzle by causing clogs, whereas continuous fibres can prevent this [12; 13]. However, short-fibre impregnation is less expensive and is preferable for applications such as transporting electrical currents, which can be useful in creating electrical sensors [14; 15] used in various situations like sports, the automotive industry, or even in the field of 3D-printed fibre-reinforced concrete composite applications. This can be very useful for civil engineering applications [16; 17; 18], where the integration of piezoresistive and piezoelectric sensors within fibre-reinforced AM structures [19; 20] can enable real-time monitoring of structural integrity, while the fibre reinforcement adds strength and durability to printed materials, which is essential in structural applications like bridge components, load-bearing walls, and modular building sections [21; 22; 23; 24]. In this context, the scope of this review is to offer a comprehensive summary of the existing technologies and breakthroughs that have occurred in recent years in the field of AM. This review will provide descriptions of each technology and the materials that can be used. Furthermore, the mechanical behaviour of 3D-printed composites will be presented, with results validated by mechanical tests from the existing literature, including quasi-static, dynamic, and anisotropic testing. Additionally, the review will highlight potential future steps to enhance the current perspective of AM in production faculties.