Analysis of stress and failure for food products during a simultaneous heat and mass transfer process

Myoung-Ho Kim, Purdue University

Abstract

The purpose of this study was to develop a numerical solution procedure for predicting the stress state and failure in a food product during a simultaneous heat and mass transfer process (SHMT). A cracker was chosen as a model food product. Several material properties relevant to the stress and checking problem were also measured experimentally. The cracker was modeled as a homogeneous axisymmetric short cylinder. The temperature and moisture distribution within a cracker during cooling process were described by a set of heat and mass transport equations. These equations were numerically solved by a finite element software package to provide the temperature and moisture content gradient data during cooling process. Seven different cooling practices--ambient, forced, and tempering were studied to examine how the process variables affect the stress and checking development. The thermo-hydro viscoelastic boundary value problem was first mathematically described to calculate the stresses due to temperature and moisture gradient. From this mathematical formulation, finite element equations were derived for stress prediction using the principle of virtual work. Compressive stress developed on the surface and tensile stress was set up in the region near the center of a cracker during cooling. Somewhere between the center and surface, the stress changed from tensile to compressive. Except near its edges, a cracker was free of axial and shear stresses. The change in axial moisture gradient was found a major factor for the development of thermo-hydro stresses. The magnitude of stresses developed was roughly proportional to the amount of axial moisture gradient change during the cooling process. Checking was predicted only for the forced ambient cooling practice. The critical location for checking initiation was predicted to be the edges. The delayed checking which is frequently cited in the baking industry can not be predicted with the model developed in the present study. Inclusion of the crack propagation mechanism would improve the predictability of the existing stress and failure model. To prevent checking, it was suggested the initial moisture gradient be equilibrated as much as possible while the cracker is maintained warm.

Degree

Ph.D.

Advisors

Okos, Purdue University.

Subject Area

Agricultural engineering|Food science

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