## 2 State-of-the-Art Methodology
### P-Diagram of T-Peel Test  
Parameter diagram or P-diagram is constructed to understand all parameters involved in a system as shown in Figure 5. The function of Al-CPP T-peel test is to measure adhesion peel strength in flexible film. Thus, the response or output of T-peel test is peel strength, which is measured in Newton (N). The input of T-peel test is known as signal factor. In the ideal function, the energy transformation occurs for three different specimen widths that are 5 mm, 10 mm, and 15 mm. Signal factor, in this study, is specimen width that is a controllable variable to actualize the intention (variation in peel strength) to achieve robust condition regardless of various width condition [26]. Figure 4: Peel strength increased proportionally to specimen width. Figure 3: T-peel test apparatus schematic diagram. In P-diagram, robustness is optimized by evaluating the control factors and their levels. The noise factor condition is varied accordingly to minimize variation that influences the response. Signal-to-noise ratio (SNR) with dynamic response (Equation (1) is used in this study due to the signal factor existence. A dynamic signal-to-noise ratio (SNR) has been used in this study where the specimen widths of 5 mm, 10 mm, and 15 mm as the signal factor levels are used to measure the peel strength linearity. \(S/N\) ratio,  
\[\eta=10\ \log\ (1/r)\ [(S_{\beta}-V_{\mathrm{e}})/V_{\mathrm{N}}] \tag{1}\]  
where \(S_{\beta}=\) variation caused by the linear effect, \(V_{\mathrm{e}}\) and \(V_{\mathrm{N}}=\) error variance (error variance/DOF), \(r=\) total number of measurements under signal (\(r\) is also the effective divisor due to level changes of signal factor). Noise factor is an uncontrolled factor during normal production or use but is controlled during the experiment. There are three types of noise: outer noise, which is caused by environmental conditions; inner noise, which is caused by the deterioration of elements or materials in the product; and between-product noise, which is caused by piece-to-piece variations between products. Noise factors are likely to produce variability in the response. For noise factor, historical data have proven that the peel angle would vary during exchanging the peel angle setting and during peeling process [2; 28; 29]. Peel angle deviation will affect the peel strength; thus, peel angle is considered as a source of variability. Two noise factors are considered in this study: peel angle deviation \(\pm 2^{\circ}\) and tensile weight \(w\). Peel angle is adjusted to three levels: \(60^{\circ}\), \(90^{\circ}\), and \(120^{\circ}\). Peel angle \(\pm 2^{\circ}\) is a noise factor because it is possible to have an inaccurate reading if the peel angle is changed by the angle adjuster by two degrees due to natural movement, as shown in Figure 6. The noise in peel angle is defined as deviation \(\pm\) 2\({}^{\circ}\) for each level. Noise 1 is the higher level (N1 = +2\({}^{\circ}\) and maximum peel strength value) and Noise 2 is the lower level (N2 = -2\({}^{\circ}\) and minimum peel strength value). The plot for noise factor is shown in Figure 7. After completing the noise strategy, the selection of a control factor is done. The objective of this T-peel test is to satisfy the industry requirement of getting the minimum variation for flexible film. Thus, control factors are selected that may affect variability in the response, and possibly the mean of the response. The controllable factors or inner array are chosen based on testing and design conditions that can affect the variance. The controllable factor selection is based on the historical data of experiment results, preliminary tests, theories and available knowledge, and expert's opinion. The controllable factors are considered from testing condition and design of apparatus condition. Figure 5: P-diagram for flexible film. The control factor from testing condition includes tensile weight that is used to keep the specimen in T-shape, peel angle, peel speed, and peeling curve region.