## 1 Introduction  
The phrase \"composite material\" implies that two or more materials with different properties have been macroscopically mixed to become a useful material. These composites have different properties than those of the individual components of the mixture. In a composite material, each component retains its original properties in a mixed state; therefore, the properties are generally present in a non-uniform state from a microscopic perspective. These composite materials can generally be divided into five categories: fiber, particulate, laminar, flake, and skeletal composites. Among them, fiber composites are the most widely used and developed materials. The most recently used fiber-reinforcing agent is carbon fiber (CF), which is mixed with various matrix materials [1; 2; 3; 4; 5; 6; 7; 8]. However, these carbon fibers are expensive and have disadvantages. Harmful substances can be generated in the process of mixing these fibers to make a composite material [9; 10; 11]. To overcome these shortcomings, various alternative fibers have recently been used, such as glass fibers [12; 13; 14; 15; 16; 17; 18]. Glass-fiber-reinforced plastic (FRP) is a composite material made of glass fibers that has been reinforced with synthetic resin as a matrix material. Glass-fiber-reinforced composite materials have a low modulus of elasticity and fewer disadvantages than carbon fiber in terms of strength. Various materials have been developed and used as fibers in order to overcome the disadvantages of fiber-reinforcing agents. Among the most recently used materials, ultra-high molecular weight polyethylene (UHMWPE) is significant. UHMWPE is a type of polyethylene (PE) and is also called high modulus PE (HMPE). PE polymers combine in a long chain with a high molecular weight structure. According to these long polymer properties, UHMWPE does not break even when an external force is applied, and it has the ability to disperse the external force in the direction of the chain. In addition, it has excellent wear and corrosion resistance. Therefore, many studies are being conducted to replace it with composite materials in which steel is used as an additive [19; 20; 21; 22; 23]. Since its invention in the 1950s, UHMWPE has been widely used in ships (ropes and sails for yachts), automobile parts, and medical materials (artificial joints); fabrics made from UHMWPE are also used as materials for military and police body armor, outdoor backpacks, and bags. However, UHMWPE has low heat resistance. In general, UHMWPE deforms at approximately 80-100 \({}^{\circ}\)C and melts at approximately 130 \({}^{\circ}\)C. To overcome these shortcomings, an appropriate matrix must be used to compose and utilize a composite material. Polypropylene (PP) is the most commonly used matrix that compensates for the shortcomings of UHMWPE. PP is formed by bonding methyl groups (CH\({}_{3}\)) to the carbon atoms of a PE molecular chain. Generally, PP films are more transparent and slightly harder than PE films. Consequently, PP is the most commonly used substance for automobile parts as well as interior and exterior materials. It has a low specific gravity among plastics of 0.9, hence it is widely used to manufacture lightweight vehicular parts. A composite material that is manufactured and processed by mixing PP and UHMWPE is lightweight and high-strength. In addition, owing to the nature of the composite material, the bonding force at the surface where the two different materials are bonded is important. The creation of composite materials by mixing materials of the same polymer series is expected to exhibit excellent effects. Composite materials produced through this fiber reinforcement mechanism can be classified by fiber length into short-, long-, and continuous-fiber composites. As long-fiber-reinforced composites can be molded using a simple thermoforming process, they have advantages in terms of their productivity and the formability of complex shapes compared to traditional continuous-fiber-reinforced composites.