Effect of deformation rate on the stress-strain curves of gelatin gels

We studied the large deformation theology of gelatin gels as a function of shear rate (shear deformation) and compression speed (compressive deformation). We fitted experimental stress-strain curves using the phenomenological BST equation and extracted: moduli, breaking stretch and stress, and a parameter that characterizes the non-linear elastic behaviour at large deformations (the elasticity parameter n). The results could be explained in terms of a mechanism in which rupture of the cross-links is not instantaneous. However, the maximum in the degree of non-linearity of the stress-strain curve (i.e. in n) as a function of deformation rate is more difficult to explain, especially the decrease at higher rates. Apparently, some kind of selection mechanism makes the elastic response of the gel more linear at high deformation rates. We propose that this is due to a combination of the fact that the shortest cross-links break earlier than the longer cross-links at these high deformation rates and the fact that the network bonds are non-linear elastic bonds.