## 1 Introduction  
The landing gear system is considered to be one of the most significant systems of an aircraft. An aircraft experiences the highest structural loads at the time of landing and thus the design of the landing gear system is crucial. There are various configurations of landing gear systems depending on the design requirements of the aircraft. Designers in the aviation industry are always focused on saving weight in order to improve aircraft performance, pay-load capacity and fuel efficiency. Developments in the field of fiber-reinforced polymer composite materials have assisted designers in introducing lightweight aircraft components and systems owing to their high strength-to-weight ratio compared to conventional components and systems. Few lightweight aircraft are equipped with fiber-reinforced composite landing gear struts which are generally not retractable and are fixed with the fuselage. However, for long endurance flight requirements of any aircraft, retractable landing gear systems become a primary requirement. Composite materials are now widely used in almost all industries because of their low weight and high strength. Fiber-reinforced composite materials are one of the responses of the emerging applied sciences and technologies in the industry. Composite materials are designed and developed using two or more constituent materials having different chemical and physical properties. All the constituents of the composite material do not merge or dissolve into each other, rather the individual identity of each constituent material is retained. As a result, different phases are developed in the composite material, comprisingcontinuous and discontinuous or discrete phases. The continuous phase is referred to as the'matrix' and the discontinuous or discrete phase is called the'reinforcement'. The reinforcement can comprise long fibers, short fibers or particles. Fiber-reinforced composites in which all the fibers lie in the same direction are called unidirectional (UD) fiber-reinforced composites. A schematic representation of such unidirectional fiber-reinforced composites is shown in Figure 1. The continuous matrix may be constituted of polymer, ceramic, or metal material. The reinforcement or discontinuous phase is much stronger and stiffer than the continuous phase forming the matrix. The reinforcement is primarily responsible for strengthening the matrix. The key role of the matrix is to hold the reinforcement within the composite and to transfer and distribute the applied load across the discontinuous phase (reinforcement) [1]. The fibers may be long fibers or short fibers. Fibers are also used in the form of woven fabrics during the manufacture of composite materials. Further advanced types of fiber-reinforced composites are known as hybrid composites in which there are more than two constituent materials. In some advanced designs of composite materials, hollow or solid metal rods are used as reinforcements in addition to the fibers. The type of fiber, its volume ratio and orientation are important in the determination of the mechanical and physical properties of fiber-reinforced composite materials. The proportion of fiber volume in composite materials plays a basic and vital role in the determination of the strength and other mechanical properties of the composite materials.This is why the proportion of fiber volume is one of the key design parameters. In addition, the manufacturing processes also affects the mechanical behavior of composite materials. The orientation of fibers in a composite material is decided according to the desired mechanical properties for the application. The mechanical properties of the composite materials depend on the distribution, chemical and physical interactions of its fibers and matrix. Experimentation is the simplest and most direct way to determine the mechanical properties of unidirectional fiber-reinforced composites.