Issue 50

G.V. Seretis et alii, Frattura ed Integrità Strutturale, 50 (2019) 517-525; DOI: 10.3221/IGF-ESIS.50.43 519 facturer recommended resin/hardener proportion, which was a 100:50 by weight ratio, and stirred gently using a laboratory mixer for mechanical stirring for a process time of 5 min at 200 rpm. Subsequently, the matrix mixture was coated and hand- rolled on E-glass fabrics in layer sequence under constant stirring [22-25]. For each hand lay-up procedure, four layers of E-glass fabric were employed in [0˚/45˚/-45˚/0˚] T sequence. The GNPs w.t. contents used for UD laminates reinforcement were 5%. The selection of the GNPs content was made based on literature on the content that provides the best perform- ance of the specific nanocomposites [22]. Before the first layer coating, the surface on which the specimens were produced was covered by release paste wax. The hand lay-up procedure applied is well-known and has been presented in explosive view mode in other published works [22,23]. To achieve a 40±1% by volume epoxy proportion in all specimens, both the fabric and the matrix mixture used for coating were weighed before each hand lay-up process and after solidification. Curing cycle All specimens left in ambient temperature for 6 hours before the curing conditions of the Taguchi design of experiments were applied. Therefore, the complete curing cycle applied is presented in Fig.2, where parameter a , T 1 and h 1 represent the heating rate [°C/min], the temperature of the first curing step [°C] and the duration of the first curing step [h], res- pectively. The selected values for each parameter under study (i.e. the design of experiment levels) can be found in Table 2. Figure 2 : The curing cycle applied with the parameters of the Taguchi design of experiments noted. Control factor Level 1 2 3 4 5 A: Heating Rate [ o C/min] 1 2 3 4 5 B: Temperature [ o C] 50 80 100 120 140 C: Time [h] 2 4 6 8 10 Table 2 : Design of Experiments (DOE) factors and their levels. The curing temperature ( T cure ) can be either higher or lower of the glass transition temperature ( T g ) [3-5,22-25]. When T cure > T g , the reaction proceeds rapidly at a rate driven by chemical kinetics. When T cure = T g , vitrification takes place (i.e., material solidifies). Finally, when T cure < T g , the reaction rate decelerates and becomes diffusion-controlled. To include all these mechanisms in the Taguchi design of experiments, apart from the T g temperature, two different temperatures both under and over T g were selected as presented in Table 2. The dimensions of each specimen which underwent 3-point bending tests were 93.6 × 12.7 ×1.1 mm, according to the ASTM D790-03 test method. The specimens which underwent tensile test had a total size of 102 × 6 × 1.1 mm in accord- ance with ASTM D3039/3039M. All specimens were cut at their testing dimensions using a Struers Discotom-2 along with a 40A25 cut-off wheel. To evaluate if tabs were needed on the holding regions of the specimens, the theoretical tab limits were marked on the specimens, as indicated from the above ASTM standard method [22,23]. Since the failure occurred into the theoretical control region no tabs are recommended by the ASTM standard used. For each experiment number (run number) of the Taguchi design of experiments, five specimens were prepared and under- went each test (five specimens for each tensile and five for each flexural test). Experimental set-up and tests An Instron 4482 test machine of 100 kN capacity was used for the both tensile and 3-point bending tests. In accordance with the ASTM standard methods D790-03 and D3039/3039M, all tests were performed in a standard laboratory atmo-