# Failure Detection within Composite Materials in System Engineering Applications
## 2 Composite Structures  
Composite material is formed by combining two or more distinctly different materials. The characteristics of this combination of components are superior to that of the separate materials; it is frequently desirable for composites to have lighter and mechanically stronger properties, though requirements may necessitate improved thermal or electrical insulation; as such, the composite would be designed for the specific applications. All the constituent materials of the composites have distinct characteristics. The matrix (or binder) and the reinforcement are generally the two elements that make up composites. To make a composite, at least one of each element is necessary. The reinforcing material considered in the case study presented in this article for continuous CF laminate is CF, which may be tweaked to modify the desirable traits of the resultant composite. The epoxy resin matrix protects the reinforcing material from chemical and environmental influences while also maintaining its structural integrity. Reinforcement is held in place by resin in order to properly transfer force [28]. Polyepoxides commonly known as epoxy is class of reactive prepolymers and polymers which comprise epoxide groups. A thermosetting polymer which during the curing phase turns from a liquid to a solid. There are many properties of epoxy resin in different product variations that offer desired parameters such as viscosity, curing time duration, adhesion and varying degrees of hardness when fully cured. Epoxies are probably the most versatile group of adhesives, and this is demonstrated by its enormous range of applications from small jeweillery to aviation. The US fighter aircrafts F-14 and F-15 dating back approximately 40 years used epoxy for the skins of the empennages, originally used in secondary structures, the benefits were soon realised and become more common in primary structures such as the wings and fuselage. The F-15 used around 2% composite material whereas the modern F22 now utilises 24% composite material. More impressively, the European Eurofighter exterior skin of CF in which 40% of its structural weight is made up of composite material [29]. The epoxy resin market is forecast to reach a staggering 12 Billion USD by 2027 [30]. Following is a comparison between epoxy resin and polyester resin to demonstrate its superiority:  
**Bond strength:** As compared to polyester resin (3.44 MPa), epoxy has a bonding strength of up to 13.7 MPa. In comparison, epoxy resin and polyester resin can resist up to 13.7 MPa and 3.4 MPa respectively. Polyester resin does not sure well as a thin film and so is less than ideal as an adhesive. In terms of bonding, epoxy will not bond to polyethylene, polypropylene, ethylene propylene diene monomer known as EPDM synthetic rubber, Teflon or Tedlar. **Resistance:** Chemicals and environmental conditions do not affect the durability of epoxy resin. It is also resistant to cracking and peeling. Furthermore, due to its great moisture resistance, epoxy resin is well suitable for water submerged applications. Whereas, moisture resistance is limited in polyester resin, which is also prone to fracture. There are however marine grade versions of polyester resins available. **Durability:** Compared to polyester resins, epoxies have a couple of year shelf life, with little potency loss, provided that the resin and hardener have not been mixed or exposed to contamination. However, polyester resins are significantly become more brittle over the course of time. **Cost:** As a result of high strength of epoxy resin and the extreme requirements of formulation, they can be more expensive than conventional adhesives and this frequently offers a legitimate cause to choose other adhesives over epoxy resins. Epoxy resin's higher cost of on low-cost products, for instance jewellery, makes it harder to justify its use, thus adhesives with lesser requirements are regarded more acceptable.