Experimentation has been carried out for the Taguchi's DOE as per ASTM standard testing procedure using CT specimens. The load-displacement data has been recorded and plotted, as shown in Fig. 3. For each experiment conducted, using the obtained values of PQ, a, W, the provisional fracture toughness (KQ)of the composite has been determined using the available standard empirical equation [11]and are listed in Table 2.
From Fig. 3, it is observed that, except for Taguchi's process parameters 6, i.e., 6wt% of reinforcements, a/W ratio = 0.50 and specimen thickness = 10mm, all shows similar trends as available in the literature [10–12]. The nature of the load vs COD plot shows the ductile fracture of the AA7075-SiC/Al2O3 hybrid composites.
Table 2
Taguchi's DOE and their fracture toughness values
Sl.
No.
|
Composition
Wt %
|
a/W Ratio
|
Specimen Thickness
mm
|
Fracture Load (PQ) kN
|
KQ
MPa√m
|
1
|
3
|
0.45
|
10
|
4.242
|
17.69
|
2
|
3
|
0.47
|
12
|
4.630
|
17.04
|
3
|
3
|
0.50
|
15
|
5.008
|
16.12
|
4
|
6
|
0.45
|
12
|
5.289
|
18.38
|
5
|
6
|
0.47
|
15
|
5.880
|
17.31
|
6
|
6
|
0.50
|
10
|
3.409
|
16.46
|
7
|
9
|
0.45
|
15
|
6.516
|
18.11
|
8
|
9
|
0.47
|
10
|
4.003
|
17.67
|
9
|
9
|
0.50
|
12
|
4.007
|
16.13
|
From Table 2, it is seen that the load-carrying capacity of the AA7075-SiC/Al2O3 hybrid composites is high for the higher specimen thickness. In contrast, the fracture toughness is high for the lower a/W ratios. Figure 4 shows the effect of Taguchi's DOE parameters on the fracture toughness of AA7075-SiC/Al2O3 hybrid composites. The graph shows that as the a/W ratio increases, the fracture toughness of the said hybrid composites decreases. Therefore, the a/W ratio increases the crack length in the specimen increases that in turn reduces the load-carrying capacity and fracture toughness of the material. From the plot, it is also observed that, up to 12wt% of reinforcements, the fracture toughness increases and further decreases for the higher wt% reinforcements. Thus, it might be because a higher percentage of reinforcements may cause the particle grouping (clustering) in the matrix. Thus it is observed to be a decrement in the fracture toughness of the said hybrid composite.
From Fig. 4, it is seen that thickness of the specimen has little impact on the fracture toughness. As the thickness increases from 10mm to 12mm, the fracture toughness of the material reduces, and further increment in the thickness of the specimen does not influence the fracture toughness of the said composite. This geometry condition of the fracture toughness of the specimen is said to be the plane strain fracture toughness (KIc). Thus the critical fracture toughness (KQ) obtained is considered the plane strain fracture toughness [19] after the thickness of the specimen reaches 12mm. Thus the optimized parameters from Taguchi's DOE are 6wt% of reinforcements, a/W ratio = 0.45 and specimen thickness = 12mm.
Thus to analyze the effect of each parameter on the fracture toughness, it is required to use the ANOVA [20]. The input function for the analysis of variance is experimental fracture toughness values. For the obtained KIc values and Taguchi's DOE, analysis of variance has been carried out, and the result of the analysis is given in Table 3.
Table 3
Analysis of variance for fracture toughness
Source
|
DF
|
Adj SS
|
Adj MS
|
F-Value
|
P-Value
|
% Contribution
|
Composition
|
2
|
0.319
|
0.160
|
01.77
|
0.362
|
05.72
|
a/W ratio
|
2
|
5.060
|
2.530
|
28.02
|
0.034
|
90.74
|
Thickness
|
2
|
0.017
|
0.008
|
00.09
|
0.915
|
00.30
|
Error
|
2
|
0.181
|
0.090
|
|
|
03.24
|
Total
|
8
|
5.577
|
|
|
|
100.00
|
From Table 3, it is observed that the a / W ratio used will have a larger influence on the fracture toughness of said composite (90.7%), whereas the composition and thickness of the specimen have little impact. The increment in the a/W ratio means the increment in the crack length in the geometry of the specimens. Thus larger crack length reduces the load-carrying capacity, in turn, decreases the fracture toughness.
Figure 5 shows the fractography images taken from the scanning electron microscope (SEM) hybrid composites for the compositions 3, 6 and 9wt% of reinforcements. From the graphs, the cracks and voids present in all the compositions of the hybrid composites. The addition of reinforcements from 3wt% to 6wt% increases the barricades to crack growth and propagation, thus increasing fracture toughness. However, the larger sized voids present in the 9wt% of reinforcements may arise due to the clustering [21] of the reinforced particles, which causes uneven distribution of reinforcements in the matrix. Hence, the matrix alone will not take the applied load in those locations and let voids grow bigger. The presence of these voids, on loading condition, becomes cracks that propagate and lead to the microstructure's failure. Increased size of the cracks in the microstructure of the 9wt% of reinforcements causes the reduction in crack propagation resistance, thus decreases the fracture toughness of the material. Also, the fractured surface of the said composites is dull and fibrous, which specify the ductile fracture.