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Processes 2020, 8, 1147 8 of 16 1 Coefficient of Variation = x 2.7. Analysis of Variance (ANOVA) Σ(x − x)2 n − 1 (1) Three-way analysis of variance ANOVA was used to determine the significance and percentage contribution of each individual parameter and their interaction between each other based on a 90% confidence level. The variability of thickness variation and spring effect results is represented by Log10 of the standard deviations. Logarithms are taken in the case of standard deviations; when randomly distributed, such data will adequately fit a normal distribution, whereas untransformed standard deviation data do not [44]. 3. Results and Discussion 3.1. Thickness Variation of Laminated Composites In terms of thickness variation, the lesser CoV is desirable as it illustrates a lower variation in the thickness across the entire laminated composite. The lesser variation in the laminate thickness ensures uniformity in the composite structure. The results for CoV in the laminate thickness produced on the concave corner of the mold and their variability are shown in Figure 5a,b respectively. The grand means (Y) are mentioned in the center right, F-stat shows the calculated F-statistic values while F-stat90% illustrates the critical F-statistic value at 90% confidence. The grand means of the CoV and variability for concave-cornered laminates are 0.024 and −1.288, respectively. Within the scope of this study, vacuum bagging technique is the only parameter to demonstrate a statistically significant effect on the laminate thickness with an F-stat of 44.51(>F90%). SVB was chosen as a base technique to compare with others since it is a conventional method. SVB exhibited the highest variation in thickness, followed by DVB, while MSVB exhibited the lowest thickness variation. The highest CoV of 0.032 (3.2% deviation) in the SVB is attributed to the bridging effect, in which the laminate does not conform to the mold shape completely, hence the thickness in the concave corner is higher than the rest of the laminates. It is difficult for the complex-shaped laminate (concave in this case) to conform to the shape of laminate with only 1 atm pressure. Although the corner radius is provided in the mold to prevent major corner deviation, the decrease in the reaction pressure on the corner regions causes insufficient pressure application on the concave corner regions, as demonstrated by Arthur Levy and Pascal Hubert [34]. The thickness variation in DVB witnessed a 22.84% improvement when compared to those of SVB. Whereas, MSVB technique witnessed a tremendous improvement of 110.81% when compared to those of SVB technique with a CoV of 0.015. The use of intensifiers on the concave corner increases the pressure distribution conditions to prevent bridging. This finding has also been reported in a previous study [29]. The thickness deviations reported in this study for laminates with concave corner produced using MSVB technique are in good agreement with their findings. Curing cycles and their interaction with bagging techniques were found to be statistically insignificant. This shows that the main factor in controlling the thickness of laminated composites (with concave corners) is attributed to the bagging techniques and consolidation method. The variability in the results of thickness variation in the concave corner illustrates the statistical significance of bagging techniques with an F-stat of 73.71 (>F90%). SVB and DVB techniques exhibited massive variations with standard deviations of 0.073 and 0.058, respectively. The variability in the results is better understood by plotting the average thickness of laminates across the length, as shown in Figure 6. The thickness fluctuates excessively in SVB when compared to those of MSVB and DVB. It is noticed that the variation in the flat flange of the laminate is lower than the inclined flange in all three cases. The highest corner thickening is also observed in SVB and DVB which are averaged at 2.455 mm and 2.420 mm, as shown in Figure 6a,c, respectively. Major corner thickening in these techniques in comparison to the rest of the laminate thickness is offsetting the CoV values. Whereas, MSVB witnesses a significant improvementPDF Image | Effects of Processing Parameters for Vacuum-Bagging
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