# Optimization of Carbon Fiber Reinforced Plastic Curing Parameters for Aerospace Application
## 1 Introduction  
Carbon fiber reinforced plastics (CFRPs) have proven to be the most suitable alternative to aluminum alloys for structural applications in the aerospace industry. Due to their superior properties, the use of CFRPs has increased in recent years [1, 2]. Some of the properties suitable for aerospace applications are high strength-to-weight ratio, impact toughness, hardness, dimensional stability, corrosion resistance, fatigue resistance, noise reduction, low radar cross-section, and the capability to be molded into large and complex shapes [3, 4, 5, 6]. Tensile strength, impact toughness, and hardness are some of the desired properties for the skin of an aircraft. Tensile strength is the most fundamental requirement for the design of any structural component. The structure of commercial and military aircraft is semi-monocoque in construction. The entire skin of the fuselage, wings, and empennage is divided into regular geometric units. Each unit of the skin can be considered as a thin platesubjected to aerodynamic pressure. The required strength for most of the aircraft skin can be estimated by Equation (1) [7; 8]. \[\sigma_{max}=\frac{\beta qb^{2}}{t^{2}} \tag{1}\]  
where  
\(\sigma_{max}\) is maximum stress in the skin patch (MPa);  
\(\beta\) is a plate dimensions ratio (dimension less);  
\(q\) is uniformly distributed aerodynamic pressure on the skin of the selected portion (MPa);  
\(b\) is a short dimension of the skin patch (mm);  
\(t\) is the thickness of the skin patch (mm). Besides being dependent on the dimensions, the maximum allowable stress is a function of aerodynamic pressure acting on the skin panel and it must be below the yield strength of the material. Furthermore, a fracture is less likely to occur if the maximum applied stress is below the tensile strength of the skin panel. Adequate impact toughness of composite makes the skin damage tolerant and enables it to sustain normal impact loads such as runway debris [9], bird strikes, gale strikes during normal operation, and tool drops during maintenance [10]. The impact toughness is also correlated with Mode I and II interlaminar crack growth resistance [9; 11] (Section 3.2.2) influencing the structural integrity of the material. The hardness of the aircraft skin helps to resist indentation, scratching, and abrasion. Prepreg polymer matrix composites (PMCs) are generally used for the construction of aircraft skin due to the superior quality of the finished product. It is well established that the mechanical properties of prepre PMCs are strongly dependent on the curing cycle (CC) [12; 13; 14; 15]. The CC can be programmed for desired levels of processing parameters. The effect of various CC parameters such as autoclave pressure [16; 17; 18; 19], vacuum application time, curing temperature, curing time [20; 21; 22; 23; 24; 25], heating/cooling rate [26], and the number of stages for curing [27] has been investigated in the literature. Amongst these, temperature, time, and pressure are the most influential in dictating the mechanical properties of CFRP [20; 23; 24; 25; 28; 29; 30]. Temperature and time are interlinked as there is always an optimal temperature for a selected time to obtain an optimum mechanical property. It is often desirable to know those optimum values of temperature and time [31; 32]. Autoclave pressure has also been found to heavily influence the volume fractions of the voids in the matrix [17; 18]. Increasing the pressure causes a decrease in matrix voids thus increasing mechanical properties. A combination of certain (optimal) values of temperature, time, and autoclave pressure results in optimum values of mechanical properties of composites that have been studied in the literature. For example, [PERSON] et al. investigated the effects of curing parameters on tensile, compressive, and flexural characteristics of CFRP [33]. The authors compared experimental results to find out the optimal curing parameters for the resulting optimized properties.