PSI - Issue 14

M.K. Singh et al. / Procedia Structural Integrity 14 (2019) 475–481 M. K. Singh, R. Kitey / Structural Integrity Procedia 00 (2018) 000 – 000

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Table 1 Elastic and fracture properties of milled-fiber composites.

Filler size (inch)

Flexural modulus E f (GPa)

Flexural strength σ fu (MPa)

Fracture Toughness K IC (MPa m )

Neat epoxy

4.18 ± 0.15 4.41 ± 0.11 5.15 ± 0.11 5.48 ± 0.44

145 ± 15 125 ± 22 120 ± 12 87 ± 30

0.67

1/32

0.99 ± 0.14 2.51 ± 0.53 2.49 ± 0.80

1/8 1/4

Fig.3. Flexural stress vs. flexural strain curves of milled-fiber composites obtained by conducting flexural experiments

4.2 Fracture analysis of composites Fracture behavior of composites is analyzed next by plotting load vs. crosshead displacement data obtained from the fracture tests (see, Fig. 4). Nearly linear curve until fracture initiation is observed for all cases. This is followed by an instantaneous drop in the load for the neat epoxy and 1/32 ” milled-fiber specimens. On the contrary continually increasing crosshead displacement subsequent to fracture initiation is observed for the remaining two filler cases. Post fracture area under the curv e, indicating crack growth resistance, suggests that 1/8” and 1/4” fillers provide significant resistance to crack propagation. This is attributed to the crack bridging provided by the slender fillers. Due to large filler lengths it is suggested that the fiber fracture along with fiber pull-out are the dominant toughening mechanisms in 1/8” and 1/4” reinforcement cases which is evident from micrograph of fractured surface (see, fig. 5 ). On the contrary in 1/32” milled -fiber reinforcement fiber bridging is followed by fiber pull-out due to the shorter length of fillers. It is evident from the fracture toughness values of the composites tabulated in Table 1 that the two larger fillers, 1/8” and 1/4” , significantly contributed towards toughening the epoxy matrix. The K IC values of the two composites are measured to be more than 3.5 times when compared to the neat epoxy counterpart. On the contrary 1/32” fillers enhance the fracture toughness of neat epoxy by ~ 1.5 times.