Thermal stresses from large volumetric expansion during freezing of biomaterials

Thermal stresses were studied in freezing of biomaterials containing signif icant amounts of water An apparent specific heat formulation of the energy equation and a viscoelastic model for the mechanics problem were used to an alyze the transient axi-symmetric freezing of a long cylinder. Viscoelastic properties were measured in an Instron machine. Results show that, before phase change occurs at any location, both radial and circumferential stress es are tensile and keep increasing until phase change begins. The maximum p rincipal tensile stress during phase change increases with a decrease in bo undary temperature (faster cooling). This is consistent with experimentally observed fractures at a lower boundary temperature. Large volumetric expan sion during water to ice transformation was shown to be the primary contrib utor to large stress development. For very rapid freezing, relaxation may n ot be significant, and an elastic model may be sufficient.