Material properties of semi-sweet biscuits for finite element modelling of biscuit cracking

Material properties required for the modelling of biscuit cracking (known in trade as checking) have been experimentally determined for semi-sweet ('rich tea') biscuits. Biscuits were prepared with several different fat concentrations, and studied at different moisture contents. Moisture diffusion coefficients were determined from moisture uptake plots obtained when biscuits were subjected to step changes in humidity. At relatively low moisture contents the diffusion coefficient increased with increasing moisture. However, this increase levelled off beyond a moisture content of about 6%, independent of fat content. Hygroscopic expansion coefficients were determined from the slope of strain vs. moisture content data. The expansion of the biscuit with moisture uptake was measured using digital speckle pattern interferometry (DSPI) following the application of step changes in humidity. Mechanical properties such as fracture stress, fracture strain and bending modulus were measured using 3-point bending. All showed considerable dependence on moisture content over a range of 4-12%. The knowledge of these material properties was used as an input to a semi-coupled diffusion stress finite element analysis (FEA). There was a good agreement between experimental results and finite element predictions for equilibrated biscuits subjected to changes in external humidity.