Effect of Cyclic Movement

Turning now to the effect of cyclic movement (0 %, 8 %, 15 %, and 25 % strain) on modulus change, the block plots of raw data and averaged data focusing on the movement effects of over 24 distinct robustness factor settings are shown in Figs. 11(a) and 11(b). It is evident that all of the 20 robustness factor settings show that 25 % cyclic movement results in the greatest decrease in modulus. The probability of this happening randomly according to the binomial distribution is virtually zero (<1 x 10~4 %). Thus, the overall pattern of modulus decrease due to 25 % movement is unlikely to stem from random occurrence. Indeed, as shown in Fig. 9(c), the data for 25 % movement are always located to the bottom left of the plots. Further, the height of each block in Figs. 11(a) and 11(b) is large, signifying the prevalence of local effects of movement on modulus decrease. Such consistencies in terms of the local arrangement of 25 % movement within each bar and the large block heights over all settings of robustness factors show the deleterious effect of cyclic movement on modulus reduction. Interestingly, cyclic movements at 8 % strain and 15 % strain do not seem to affect the modu­lus. In fact, the modulus ratios at these values are similar to those in the tests without cyclic movement. This observation suggests that there might be a threshold value of cyclic strain below which the modulus is not affected.

To date, the incorporation of cyclic movement during exposure has not been widely used in routine outdoor testing of sealants, despite the fact that a

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FIG. 10—Block plot of (a) modulus ratio and (b) mean modulus ratio targeting RH across 14 distinct combinations of temperature, cyclic movement, and UV. Fairly con­sistently large block heights and inconsistent local RH level arrangements over all set­tings of robustness factors suggest that RH is a less important factor.

number of studies [31-35] have shown that accelerated aging under cyclic movement simulates the effects of in-service environments more closely. For the sealants studied here, the incorporation of cyclic movement in the durability tests not only will enable the use of an appropriate combination of environmen­tal factors that closely simulates in-service conditions, but also yields an even greater acceleration factor to reduce test times for commercial sealants with a usual target service life of 20 years. It should be noted, however, that in some types of sealants the effect of cyclic movement does not greatly accelerate degra­dation, or it even has a negligible effect. For example, in a study by Enomoto et al. [36], mechanical cycling during outdoor exposure had no effect on the rate of degradation of two-part polyurethane and two-part silicone modified polyether (general purpose) sealants, and only a small effect on one-part polyur­ethane sealants.