PSI - Issue 28

Shirsha Bose et al. / Procedia Structural Integrity 28 (2020) 843–849 S.Bose et. al/ Structural Integrity Procedia 00 (2019) 000–000

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3.2. Deformation Behaviour The deformation process was recorded for specimens of collagen films until failure for both Col D (Fig. 4a) and Col A (Fig. 4b). With the increase in tensile deformation for Col D , the crack initiation was accompanied by an unstable (catastrophic) brittle facture. Generally, for brittle materials, the crack propagation might be affected by extrinsic toughening mechanisms, such as crack deflection, crack bridging or micro-cracking (Nalla et al., 2003). Col D exhibited crack path trajectories dominated by sudden, unstable brittle failure with very little plastic deformation (Fig. 4a). Tough collagenous tissues such as bone (Nalla et al. , 2005) and dentin (Nalla et al., 2003) were also known to demonstrate similar brittle failure. The stress-strain curves (Fig. 3a) and the crack-propagation behaviour (Fig. 4a) obtained for Col D suggested that collagen films in-air did not exhibit plastic deformation. Thus, the fracture toughness in this case may be estimated using linear-elastic fracture-mechanics parameter K IC – the critical mode-I stress-intensity factor.

Fig. 4a. Deformation of collagen film in-air conditions.

Collagen films subjected to in-aqua (Col A ) test environment depicted a contrasting deformational behaviour (Fig. 4b). The increase in the tensile deformation led to slow crack-tip opening, followed by crack blunting and rapid tear propagation with extensive plastic deformation. The initial slow crack-tip opening might be due to the straightening of collagen fibrillar bundles, while the rapid blunting process occurred when the crack-tip stress surpassed the interfacial cohesive strength (Yang et al. , 2015). In-aqua condition led to ductile failure of collagen films, suggesting that the deformation mechanism may be comparable to that of soft collagenous tissues (Yang et al. , 2015; Pissarenko et al. , 2020). Extensive plastic deformation was observed for Col A , resulted in stress-strain curves (Fig. 3b) and the crack propagation behaviour (Fig. 4b). Hence, for such behaviour, the fracture toughness may be quantified using elastic plastic fracture-mechanics (EPFM) parameter J C , i.e. the critical J-integral value.