Rupture lifetime extrapolation

Accelerated testing depends critically on selecting a parameter whose effect on service life is so well understood that long lifetimes at low values of the parameter can be predicted from shorter lifetimes at higher values. The parameter may be the prime cause of degradation, such as in a stress-rupture test where longer lifetimes at lower loads are predicted by extrapolation from short lifetimes at higher loads. It can also be a secondary parameter, such as when temperature is increased to accelerate chemical attack while the principal factor, chemical concentration, is kept constant. This is because there is more confidence in the relation between rate of reaction and temperature than in the relation of rate of reaction to concentration. It is clearly essential that extrapolation rules from the test conditions to those of service are known and have been verified, such that they can be used with confidence. 

Lifetime prediction of plastic pipes is performed by extrapolating from higher loads and lower times to lower loads and higher times, remembering to use only data corresponding to brittle , low energy ruptures to the right of the knee in the curve (see Section 4.9). 

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. 

Sufficient data for HDPE geomembrane materials are not usually available from pipe pressure tests which would enable a direct extrapolation of the stress-rupture curve at 23 °C or 40 °C according to the instmctions of ISO 9080 for small stresses and longer times. The emphasis is on the brittle branch of the stress-rupture curve which results from brittle failures and determines the failure behaviour at small stresses and extremely long service lifetimes. However, one can use the experience accumulated with HDPE materials over many decades (Schulte 1997 Krishnaswamy 2005) and fall back on the extrapolation factors of DIN 16887 or ISO 9080 which were checked by measured stress-rupture data of polyethylene pipes. These factors Ke give the time limits of a permissible extrapolation of the stress-rupture curves in the following sense. Let us assume that hoop stress versus times-to-failure data are measured at several higher test tempera-... 

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