A novel application of speckle interferometry for the measurement of strain distributions in semi-sweet biscuits

The spontaneous formation of cracks in biscuits following baking, also known as checking, is an issue that manufacturers would like to be able to predict and avoid. Unfortunately the mechanisms driving this phenomenon are not well understood. Speckle interferometry was used to study moisture-induced in-plane strain development in biscuits. This sensitive and non-contacting technique for measuring surface displacements has two major advantages over more commonly used methods; firstly, strains can be detected at a far higher sensitivity (down to 2 × 10−6) than previously accessible and secondly the method is a whole-field technique, enabling observation of the development of strain distributions during moisture migration. For biscuits exposed to step changes in humidity, initial strain rates of up to 10−5 min−1 were measured, which decreased as the moisture content approached equilibrium, leading to an accumulated strain of ∼10−2 after 48 h. Under these conditions, a homogeneous, uniform strain distribution was observed. The data were used to calculate the hygroscopic expansion coefficient, which was linearly related to moisture content and provides the necessary constitutive link between strain and biscuit moisture content needed to model biscuit checking.