The influence of compression velocity on strength and structure for gellan gels

The influence of the velocity of compression on the strength and the fracture profile in compression of cylindrical gellan gels was investigated in the concentration range from 0.81 to 2.5%. The fracture load and compression were obtained by analyzing the load-compression curves taken at various compression velocities. The fracture load F-b initially decreases, and then increases with decreasing compression velocity. The same tendency was observed for the compression velocity dependence of the fracture compression L-b The compression velocity at the minimum that determined from F-b-velocity curve, v(F), was slower than that determined from L-b-velocity curve, vc. In addition, it is found that the gellan gel was compressible to 90% without any macroscopic fracture when the compression velocity is smaller than a certain value V-unbroken Visual inspections indicated that the gel fractured at: an angle of almost 45 degrees from its parallel surface at the compression velocities above vc. On the other hand, several vertical breaks of 3-5mm around a cylindrical surface were observed in the velocity range from v(F) to V-unbroken Two types of fracture simultaneously occurred in the velocity range from v(C) to v(F). Below V-unbroken a gel became a sheet, in which most water flowed out of the gel without changing the radius of cylinder. The characteristic strain rate g, that estimated from V,,broken was interpreted in terms of the collective diffusion of the polymer network. (C) 2001 Elsevier Science Ltd. All rights reserved.