Changes in Aging Mechanism

Because every aging mechanism is associated with a specific activation energy, there are three different activation factors. For ail aging mechanisms, the curves are parallel for different temperatures only the distance between curves changes. The distance of the parallel shift is dependent on the activation factor and thus on activation energy [104]. 

While the activation factor represents a measure for the activation energy of the aging mechanism, the slope of the service life curve is a measure of the rate at which degradation progresses in the component. This rate may increase or decrease with a constant activation factor. In Fig. 1.50, this rate is constant for each aging mechanism, but increases with a change in activation factor [104]. 

Consequently, the slope of the curves can be subject to change over time despite of a constant activation factor [104]. 

If the assumption is valid that the service life curves are straight lines, the time shift of the switchover point is  

The characteristic component value at the switchover point at this temperature is  

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Attention to changes in activation energy due to changes in aging mechanisms,... 

If the temperature-time curves are parallel for different aging states, the activation factor can he considered constant over time. Otherwise, a change in aging mechanism is involved [104]. 

Figure 11.9 Correlation between transverse relaxation times (T ) and bulk average crosslink density of a bimodal silicone network, both filled and unfilled, exposed to gamma radiation. In general, over moderate ranges of changes in crosslink density a linear correlation is usually found in a similar family of silicones. Over large ranges of doses (see break between lOMR and 25 MR), non-linearities can be observed, most likely due to changes in aging mechanisms.

As an aside, we note that the FDEMS sensor input information can also be used to detect the onset of phase separation in toughened thermoset systems and to monitor cure in thin film coatings and adhesive bond lines. It is particularly important that the FDEMS sensor is also very sensitive to changes in the mechanical properties of the resin due to degradation. As such, it can be used for accelerated aging studies and as a dosimeter to monitoring the composite part during use to determine the knockdown in the required performance properties with time. 

Daly CH, Odland GF. Age-related changes in the mechanical properties of human skin. J Invest Dermatol 1979 73 83-87. 

McGahon BM, Murray CA, Horrobin DF, Lynch MA. Age-related changes in oxidative mechanisms and LTP are reversed by dietary manipulation. Neurobiol Aging 1999 20 643-653. 

Lanteri CJ, Sly PD. Changes in respiratory mechanics with age. J Appl Physiol 1993 74 369-378. 

Fig. 3.1-52 Changes in the mechanical properties and electrical conductivity of electrical grade E-AlMgSi ( 6101) alloy wire during artificial ageing at 160 °C solution treated at 525 °C, water quenched, naturally aged for 14 days, 95% cold-worked and then artificially aged [1.14]...

In order to estimate the resistance of the vulcanizates to UV aging, their mechanical properties and cross-link density after aging were determined and compared with the values obtained for the composites before UV aging. An aging factor (S) was calculated as the numerical change in the mechanical properties of the samples after and before aging (equation 15.3.)... 

Extensive dynamic viscoelastic measurements on polyvinyl chloride gels with well-defined tacticities and thermal histories have revealed changes in aging which affect the plateau zone as some crystallites melt and others are formed. The appearance of secondary loss mechanisms is attributed to the formation of entan-gelments during the aging process. 

Shogenov, V. N., Kharaev, A. M, Guchinov, V. A. (1988). The Polyarylatesulfone Mechanical Properties Change in Aging Process. In Polycondensation Processes and Polymers. Ed. Mikitaev A.K. Nal chik, KBSU, 14-21. 

Overall, the accelerated ageing study did not reveal any significant changes in the mechanical properties of the adhesives even when unrealistic levels of water are present within the AS 1160 resin. The resin is considered to be fairly resilient to thermal ageing, and the shelf life could be extended beyond the current manufacturer recommended 6-year limit, and storage in low humidity conditions is likely to promote shelf life. 

Luff, A.R., 1998. Age-associated changes in the innervation of muscle fibres and changes in the mechanical properties of motor units. Arm. NY Acad Sci. 854, 92-101. 

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