Relaxation Constant in the Folding of Thin Viscoelastic Sheets

If one folds a thin viscoelastic sheet under an applied force, a line of plastic deformation is formed that shapes the sheet into an angle. This study determines parameters that define this angle and shows that, no matter how much load one applies, it is impossible to make angles less than a certain minimum angle in a definite time. Moreover, it is shown that regardless of whether the sheet is released freely afterward or kept under load, a logarithmic relaxation process follows the first deformation. The slope of this logarithm is the same in both conditions and depends neither on the applied force nor on the thickness of the sheet, which indicates that it is directly a probe of the molecular mobility of the material. This intrinsic relaxation constant is measured at approximately 0.1 and 6 for Mylar and paper sheets, respectively. It is also suggested that the observed minimum angle of folding can be defined as a characteristic index for the plasticity of different materials.