Stress Relaxation Behavior

Apparent moduli versus relaxation time curves for specimens before and after exposure to motion at 25 % maximum strain, 75 % RH, and 30°C and with UV radiation are shown in Fig. 5(a). The curves represent the average of up to four replicates, and the vertical bars indicate one standard deviation. The difference seen in Fig. 5(a) between the two curves is significant. Note that there is no change in the curve shape, implying that the time dependence of the apparent modulus is very similar before and after exposures. However, the magnitude of the apparent modulus decreased by a small amount after exposure. In order to facilitate comparison between different exposure conditions, stress relaxation data are presented as a modulus ratio (F) as a function of the relaxation time [Eq 3, Fig. 5(b)]. In such a graph, no change would be represented by a horizon­tal straight line at F = 1. A horizontal line above or below F = 1 indicates that ex­posure caused a vertical shift in the stress relaxation curve but no change in shape; i. e., the time dependence did not change. Something other than a hori­zontal straight line indicates a change in the time dependence. The experimen­tal uncertainty can be shown as a hashed region on either side of F = 1, so if the points for a given curve fall within this region, there are no changes outside the experimental uncertainty.

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