The analysis of the frictional effect on stress - strain data from uniaxial compression of cheese

Uniaxial compression tests were performed on Gruyere and Mozzarella cheeses. It was observed that shorter samples appeared stiffer when no lubrication was used. This dependence on sample height was eliminated when a synthetic grease lubricant with polytetrafluorethylene (PTFE) was used. Therefore, the true stress-strain curves, i.e. free of frictional effects, were determined. Methods for reproducing these curves using data from unlubricated tests were then sought. It was shown that the true stress-strain curves can be determined by testing samples of increasing heights until the difference between consecutive curves is negligible. The curve corresponding to the tallest sample can then be taken to represent the true stress-strain curve. If size or shape limitations do not allow testing of sufficiently tall samples, quadratic extrapolation of the results may be performed. Alternatively, an iterative finite element analysis could be used. The latter is a more accurate but more time consuming method than the extrapolation procedure. In addition it requires that the coefficient of friction, μ, is known. It was shown that the latter can be derived from an analytical scheme. These values of μ were approximately 0.1 for Gruyere and 0.3 for Mozzarella and they were in close agreement with numerical predictions.