Extrapolation methods, lifetime prediction

Two different approaches for lifetime prediction are presented. The underlying lifetime limiting processes have been identified in two cases. Mathematical expressions of chemical/physical relevance were used for the lifetime predictions for PE hot-water pipes and cables insulated with plasticized PVC. Accelerated testing, extrapolation and validation of the extrapolation by assessment of the remaining lifetime of objects aged during service conditions for 25 years were successfully applied to cables insulated with chlorosulfonated polyethylene. Polyolefin pipes exposed to chlorinated water showed a very complex deterioration scenario and it was only possible to find a method suitable for predicting the time for the depletion of the stabilizer system. 

Lifetime predictions of polymeric products can be performed in at least two principally different ways. The preferred method is to reveal the underlying chemical and physical changes of the material in the real-life situation. Expected lifetimes are typically 10-100 years, which imply the use of accelerated testing to reveal the kinetics of the deterioration processes. Furthermore, the kinetics has to be expressed in a convenient mathematical language of physical/chemical relevance to permit extrapolation to the real-life conditions. In some instances, even though the basic mechanisms are known, the data available are not sufficient to express the results in equations with reliably determined physical/chemical parameters. In such cases, a semi-empirical approach may be very useful. The other approach, which may be referred to as empirical, uses data obtained by accelerated testing typically at several elevated temperatures and establishes a temperatures trend of the shift factor. The extrapolation to service conditions is based on the actual parameters in the shift function (e.g. the Arrhenius equation) obtained from the accelerated test data. The validity of such extrapolation needs to be checked by independent measurements. One possible method is to test objects that have been in service for many years and to assess their remaining lifetime. 

In the present discussion it is tacitly assumed that the thermal analysis technique identifies the proper life-determining reaction and that the detailed chemistry and physics of the various failure mechanisms is as assumed, in order to allow us to concentrate on the kinetics and the precision of the chosen extrapolation methods. The safest lifetime prediction is necessarily the one with the shortest extrapolation — in other words, the test made under conditions close to those the material experiences in service. 

Lifetime prediction studies on polymeric materials rely heavily on the use of accelerated thermal aging exposures. Most accelerated aging methods first expose the virgin material to various accelerated environments. Then the changes that occur in the material are documented. The goal is to extrapolate the accelerated results obtained in order to predict the material lifetime imder ambient aging conditions (1). 

Such methods include predictions from tests of accelerated aging [7-23] and from weatherability based on empirical formulas (extrapolation, ageing kinetics model derivation) [5, 6], while others exploit new ways for detection of service lifetime [24—26]. Some methods are based on monitoring of cumulative damage [27-30], etc. [31]. 

In practice a method is established which estimates the limits of application by time-temperature-extrapolation of the measured damage processes. So a lifetime prediction is possible by using the time-temperature-shifting-principle (Figure 4). 

This revised approach improved the scatter in both Eqs. (6.100) and (6.102), as seen in the illustrations Fig. 6.116. Various parameters were also suggested for the extrapolation of time-to-rupture with varying success certain ones have been used to predict the in-service lifetime of a component operating at high temperatures. Of these methods, the two most popular ones have been discussed in this section. The reader may turn to the professional literature in order to choose the most appropriate method for a given specific application. 

Thermoanalytical methods, such as TG, where degradation of a material can be measured under conditions that accelerate its rate and the resulting parameters extrapolate to predict a service lifetime could have great commercial importancel" in the construction industry. They could be used not only for planning economic replacement before catastrophic failure occurs or avoiding premature replacement, but also for developing specifications for quality assurance and control tests and formulations. 

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