# Failure Detection within Composite Materials in System Engineering Applications
## 2 Composite Structures
**Safety:** Polyester resin off gassing of volatile organic compounds (VOCs), especially its catalyst methyl ethyl ketone peroxide (MEKP), is high explosive, similar to acetone peroxide, and acts as a strong oxidising agent and is corrosive and hazardous to health. Epoxy resins are associated with contact dermatitis through irritation or sensitisation effects, photodermatitis can also occur. Using the tensile modulus known as Young's modulus (E), which estimates the rigidity of a flexible material; CF composites are classified. It is important to note that the Young's modulus may vary depending on the orientation so can be used to characterize the isotropicity of the material, where isotropic indicates that the material's mechanical characteristics are same in all directions. A high value of the Young's modulus is the characteristics property of stiffer materials. In CF materials, when the force is applied, due to directional fibre nature of CF and high value of Young's Modulus, it possesses anisotropic property. CF is classified into five types: ultra-high modulus (>450 GPa), high modulus (350-450 GPa), intermediate modulus (200-350 GPa), low modulus and high tensile (100 GPa, tensile strength > 3.0 GPa), and super high tensile modulus (modulus > 450 GPa). High-performance composites are created from the layer of the fiber sheet. These sheets have adhered together. The layer from which composite is constructed is known as a ply. Plies are a collection of layers that are piled on top of one another in carbon composite constructions as shown in Figure 1. In order to transfer the load force, each layer is attached to the next layer. The structural stability is greatly compromised if this bond is disrupted; it is also typical for the plies orientation to shift from those directly above and below. This increases strength in a desired direction which is deemed suitable for the application. It is also preferable to have symmetrical stacked ply make up as this reduces the chances of warping during the curing process. It is, however, also common for non-symmetrical layer makeups, although extra skill is required as to ensure the structure is as intended after the curing process [28]. During the production process, composites might develop defects as a due to improper use or poor quality. The most prevalent types of failure includes; deamination of plies, debonding of fibre matrix, cracking of the matrix and fibre breakage [31]. Stress concentrations caused by thermal shrinking create matrix fractures. Debonding can happen when the bonding forces (physical, mechanical, or chemical) that hold the bond together are weakened. Whereas the most common type of failure between the above stated ones is the delamination (as discussed in Section 1). It can manifest itself in a variety of ways such as a fracture inside the resin or adhesive or reinforcement, and de-bonding of the resin from the reinforcement. Process-related material flaws may include voids or areas of high fibre content and/or high matrix content as well as fibre misalignment and/or laminate  
Figure 1: Plies in a stacked composite [0\({}^{\circ}\)/\(\pm\)45\({}^{\circ}\)/90\({}^{\circ}\)] [2]. In the three-point flexural test, the maximum separation distance for Specimens 1-3 was established at 70 mm to prevent observed specimen sliding when the sample spacing was more than this value. This distance was used to test the following specimens.