PSI - Issue 41

Kushal Mishra et al. / Procedia Structural Integrity 41 (2022) 248–253 Mishra & Singh / Structural Integrity Procedia 00 (2022) 000 – 000

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decrease. However, at the highest strain rate of 0.2 s -1 , there is a slight increase in tensile ductility, and the UTS increases to the highest value obtained over the entire strain rate range.

Table 1. Tensile and compressive properties of highly cross-linked epoxy at different st rain rates

Compression test

Tensile test

Yield strength under compression in MPa 94.5 ± 0.9 104. 6 ± 2.4 100. 5 ± 10

Maximum Post Yield softening stress in MPa 93.8 ± 0.6 103.2 ± 3 98.4 ± 9.4 118. 7 ± 3

Strain rate

Elastic Modulus in GPa 1.67 ± 0.08 2.03 ± 0.02 2.04 ± 0.08 2.20 ± 0.01

Fracture stress in MPa

% post-yield softening

Fracture strain

0.0003

0.7 ± 0.3 1.4 ± 0.6

65.8 ± 4.3 67.3 ± 0.9 57.8 ± 2.8 78.1 ± 3.2

0.06 ± 0.008 0.04 ± 0.0003 0.033 ± 0.001 0.058 ± 0.01

0.005

0.02

2 ± 0.9

0.2

121.7 ± 2

2.4 ± 1.1

Fig. 2 shows the fracture surface of tensile tested specimens which are tested at strain rates as listed in Table 1. All the specimens failed in a brittle manner with no necking and the fracture surfaces were flat. Also, for all the specimens, the fracture was init iated at one of the four corners, and a clear fracture init iation region was observed. Higher magnification images of the fracture in itiation region are presented in Fig. 2(b,d,f,h). This reg ion contains features very similar to the crazes. Crack in itiation regions consisting of crazes are expected to be present at the corner as a re latively higher stress concentration is expected at corners. At low strain rates, the crazes are p resent uniformly in the crack in itiation region. As the strain rate increases, the distribution becomes sparse, and the fracture surface appears more uneven locally. At low strain rates, the polymer chains would have enough time to reptate, and disentanglement crazing is dominant, which results in slightly higher tensile strains (Deb lieck et a l., 2011; Kramer and Berger, 1990). As strain rate increases , chain disentanglements become difficu lt, and crazing happens through the scission of covalent bonds, which require higher energy (Deblieck et a l., 2011; Han et a l., 1998; Kramer and Berger, 1990). Following the crack in itiation region is the catastrophic crack p ropagation region which encompasses the balance area of the fracture surface.

Fig.1 (a) Tensile test st ress- strain curvecarried out at different strain rates. The st rain rates are listed in Table 1. (b) Variat ion of ‘tensile st ress at failure’ and ‘tensile strain at failure’with ‘tensile st rain rate’.