Thermal fracture in a biomaterial during rapid freezing

Thermal stresses leading to fracture during rapid freezing of a biomaterial are studied using potato tissues as an example. The thermal model includes the gradual freezing of the tissue, that is, the presence of a mushy zone. Water in the biomaterial expands almost 9% during freezing and develops tr ansient stresses when the material is frozen from all sides. A viscoelastic model for the tissue is used with mechanical properties changing continuou sly during freezing. A comparison of stresses developed during freezing, wi th experimentally measured failure strength, is used as a criterion for ini tiation of a crack. The strain energy release rate G is calculated using th e J integral. Experimental evidence of a catastrophic fracture during very rapid freezing (boundary temperature of -200 degrees C) suggests that the G value is likely to exceed the critical rate (G,) at this temperature but n ot so for slow freezing at a boundary temperature of -40 degrees C, for whi ch no experimental fractures are observed. A multistep freezing process con sisting of initial slow freezing followed by fast freezing to reach the fin al temperature of -200 degrees C reduces the strain energy release rate, an d experiments show that catastrophic failure can be avoided with this proto col.