EFFECTS OF PRE-STRAIN ON THE NON-LINEAR VISCOELASTIC PROPERTIES OF A MODEL SEALANT
Christopher C. White, Donald L. Hunston, Kar tean Tan
The non-linear viscoelastic properties of a model sealant were measured using stress relaxation tests conducted at different strain levels, x. Some specimens were measured with no previous strain history while others were given a preconditioning that involved two cycles of stretching the specimen to a maximum strain of m, returning it to 0 % strain, and allowing it to recover. The sample geometry specified in ASTM-C719 was used. Apparent modulus data were calculated from the relaxation curves using rubber elasticity theory. In all cases, modulus data over a three decade range of time could be modeled with a power law relationship characterized by an exponent, n, and a value for modulus at 100 s, E100. In experiments with preconditioning and x < m, the value of n, which represents the time dependence of the response, was independent of both m and x. This means the curves could be superimposed to give a time-strain master curve via simple vertical shifts. For the range of strains that is often of interest for sealants, below 25 %, plots of E100 vs 1/(1+x) were linear for each value of m, and the slopes of the lines were similar; i.e., approximately independent of m. When a specimen was tested without preconditioning or at a strain higher than that used in the preconditioning (x > m), the behavior was more complex. The slopes, n, were no longer independent of x, and plots of E100 vs 1/(1+x) deviate from linearity at the higher strains. Nevertheless, for all conditions tested, the behavior of the specimen depended on the largest strain it had seen in its previous strain history and was relatively independent of its history at strains below this maximum.