Thermomechanics of fracture in freezing of biomaterials

Water has nine percent volumetric expansion when it freezes to ice. Biomaterials which have a high moisture content also undergo a large volume change during freezing. This volumetric expansion can cause thermal stress and lead to fracture, as documented by researchers in the fields of food preservation and biomaterial cryopreservation. Transient thermal stresses during freezing of a biomaterial have been analyzed in the present work using uncoupled equations of heat transfer and viscoelasticity. The equations were solved numerically using the software package ABAQUS. Mechanical properties input to the model were measured using an Instron 4502 machine with an environmental chamber. Gradual freezing of the biomaterial was incorporated using an apparent specific heat formulation of the energy equation. Frozen biomaterials become more brittle at lower temperatures and higher loading rates. Stresses, which are hydrostatic tensile before the phase change, decrease during the phase change and become stable residual stress much later than the temperature distribution becomes uniform. Crack initiation and propagation have been modeled and verified by the crack test. Multistep freezing is suggested as a possible way to reduce crack formation during the freezing of biomaterials.