Issue 50

G.V. Seretis et alii, Frattura ed Integrità Strutturale, 50 (2019) 517-525; DOI: 10.3221/IGF-ESIS.50.43 520 sphere (23±1˚C and 50±5% relative humidity). Test conditioning was kept constant for 6 hours before each test. To meet the test method’s span-to-depth specification, the support span was set at 52 mm for the flexural tests. The recommended from the ASTM methods test speed of 2 mm/min was applied on both tensile and 3-point bending tests. Taguchi design of experiments Taguchi’s method uses a special design of orthogonal arrays to study the entire process parameter space with just a small number of experiments. Using Taguchi’s method the number of experiments to evaluate the influence of control parameters on certain quality properties or characteristics is considerably reduced as compared to a full factorial approach [23,24]. There are three ways of transformation to calculate the loss function, depending on the desired characteristic of the measured value. The characteristic of the desired value can either be the-lower-the-better, the-higher-the-better or the- nominal-the better. The loss function of the ‘‘the-higher-the-better’’ quality characteristic ( y k ), which was used for this study, with m as the mean of the target quality parameter is calculated as shown in Eq.(1) where L ij is the loss function of the i th performance characteristic in the j th experiment [23,24]. ௜௝ ൌ ଵ ௡ ∑ ଵ ௬ ೔ೕ మ ௡௞ୀଵ (1) In the Taguchi method, the S / N ratio n ij for the i th performance characteristic in the j th experiment, which can be calculated using Eq.(2), is used to determine the deviation of the performance characteristic from the desired [23,24]: ௜௝ ൌ െ10log ሺ ௜௝ ሻ (2) Regardless of the categ ry of the performance characteristic, a larger S / N ratio indicates a performance of a better quality. Therefore, the optimal level of the process parameters is the level with the highest S / N ratio. The selection of control factors is the most important step in a design of experiments. It is known that heating rate ( a ), temperature ( T 1 ), time ( t 1 ) are three factors which affect the mechanical behavior of an epoxy matrix and, consequently, of a laminated composite [23,24]. Therefore, it is expected that these three factors also affect the mechanical behavior in the case of epoxy matrix nanocomposites. In this work, the impact of these three factors on tensile and flexural strength of GNPs/glass fabric/epoxy laminated nanocomposites is studied using an L 25 orthogonal array design. The selected levels of the three control factors are presented in Table 2. A NALYSIS OF VARIANCE ( ANOVA ) nalysis of variance (ANOVA) is a statistical tool which examines the hypothesis that the means of two or more populations are equal and, subsequently, it evaluates the significance of one or more factors, by comparing the response variable means at the different factor levels. The significant factors for both tensile and flexural per- formance were temperature and time in this study, at 95% confidence level (see Tables 3 and 4). Specifically, the tensile performance is mostly affected by curing time (31.18%). On the other hand, the flexural performance is mostly affected by curing temperature (32.67%) and secondarily by curing time (18.32%). The main effects plot for the main effect terms in ultimate tensile strength (UTS) and flexural strength for factors a, T 1 , and t 1 are shown in Figs.3 and 4, respectively. Ad- ditional tests were conducted to confirm the results of the above analysis (see Tables 5 and 6). The error achieved was lower than 1% in both cases. The optimal values can be predicted using Eq.(3) [23,24]. ௢௣௧ ൌ ௠ ൅ ∑ ሺ ௜ െ ௠ ሻ ௤ ௜ୀଵ (3) where: n m is the total mean of the response (UTS and flexural strength, respectively) and characteristic under consideration; n i is the mean values at the optimum level and q is the number of control factors that significantly affects curing process of composite. Results and discussion The main effects plot for the main effect terms in UTS for factors a , T 1 , h 1 are shown in Fig.3. From the main effect plots, it has been observed that the UTS of the composite significantly increase for temperature increase from 50 °C to 80 °C. Tem- perature increase up to 120°C does not cause any change in the tensile performance, while further increase again increases A