# Optimization of Carbon Fiber Reinforced Plastic Curing Parameters for Aerospace Application
## 3 Results and Discussions
### Multi-Response Optimization of CFRP Mechanical Properties  
The outcome of experimental results shows that maximum tensile strength and impact toughness was achieved at two different CCs. To solve such issues of multiple responses, the desirability function was introduced by [PERSON] [41]. [PERSON] and [PERSON] [39] further modified the concept later and suggested using a composite desirability function to solve multiple response optimization problems [10; 40; 54; 55; 56]. According to this approach, each of the properties was converted into an individual desirability value \"\(d_{i}\)\" which varied over a range of 0 to 1, (0 \(\leq d_{i}\leq\)1). The value of \(d_{i}\) increased with the desirability of the corresponding property. For the maximization of three properties (tensile strength, impact toughness, and hardness), Equation (5) was used to calculate the desirability of an individual property.  
\[d_{i}=\begin{cases}0;&y_{i}<L\\ \left[\frac{(y_{i}-L)}{(T-L)}\right]^{w_{i}};&L\leq y_{i}\leq T\\ 1;&y_{i}>T\end{cases} \tag{5}\]  
where  
\(d_{i}\) is the desirability of an individual response,  
\(y_{i}\) is the actual property value,  
\(L\) is the lower bound of the property,  
\(T\) is the target property value,  
\(w_{i}\) is the weight of the desirability function.  
The individual desirability di was then combined by using the geometric mean, as expressed in Equation (6):  
\[D=(\prod_{i=1}^{n}d_{i})^{\frac{1}{n}} \tag{6}\]  
where  
\(D\) is the composite desirability,  
\(n\) is the number of responses.  
The results are summarized in Table 7. The highest desirability values obtained for \(TS\) are at CC8 (140 \({}^{\circ}\)C, 120 min, 7 bar), i.e., 1, 0.941, and 0.964, with the maximum \(TS\) of 1420 MPa. For \(IT\), the highest desirability values are at CC4 (140 \({}^{\circ}\)C, 120 min, 3 bar), i.e., 1, 0.947, and 0.977, with the maximum \(IT\) of 17.4 J. For hardness, optimal CC was found to  
Figure 11: Surface plots of hardness (_HD_) at a constant pressure of (**a**) 3 bar; (**b**) 7 bar. Hardness was observed to increase with temperature, time, and pressure. The optimum hardness was observed at CC8 (140 \({}^{\circ}\)C, 120 min, and 7 bar), represented by a black dot.  
be the same as for \(TS\), with desirability values of 1, 0.894, and 0.750. The maximum \(HD\) value obtained at optimal CCS is 57.6 HRB. Optimization of \(TS\) and \(IT\) at different CCs is consistent with the findings of [PERSON] and [PERSON] [49, 50].  
To overcome the issue of two different optimal CC, and simultaneous optimization of mechanical properties, the concept of composite desirability was applied using Equation (6). The results are tabulated in Table 7.  
There were two CCs at which maximum properties were achieved, i.e., CC4, where maximum \(IT\) was achieved, and CC8, where maximum \(TS\) and \(HD\) were achieved. Hence, the composite desirability function at these two CCs was compared. The largest composite desirability values (i.e., 0.271, 0.222, and 0.288 (avg = 0.260)) were obtained at CC8, with \(TS\) of 1420 MPa, \(IT\) of 16.5 J, and \(HD\) of 57.6 HRB. The summary of the composite desirability function is appended in Table 8.