Biaxial deformation of dough using the bubble inflation technique. I. Experimental

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The bubble inflation test has been used to determine the equi-biaxial stress-strain curve of flour/water dough. This was achieved by undertaking experimental measurements of strain, wall thickness and radius of curvature at the top of the bubble as well as applied pressure. It was observed that the bubble was spherical initially but changed to an elliptical shape at large strains. The analysis derived by Bloksma (1957) was also used to calculate stress and strain at the top of the inflated bubble from gauge pressure and bubble volume data. It was found that the analysis led to accurate bubble heights at moderate strains only, a consequence of the non-spherical bubble shape. In addition, at the top of the bubble, the analytical strain was larger and the thickness was much smaller than the experimental values. The bubble wall thickness distribution was more uniform than the analytical predictions. The discrepancies in bubble height, radius of curvature, strain and thickness had a major effect in the analytical stress-strain curve, as both stress and strain were overestimated, the stress being in error by a factor of four at large strains. Therefore, it is concluded that when the bubble inflation test is used, it is necessary that experimental readings of strain, thickness and radius of curvature as well as pressure should be made to ensure accurate stress-strain curves.