AS NZS 1462.29:2019 pdf free.Methods of test for plastic pipes and fittings Method 29: Plastics piping and ducting systems一Determination of the long-term hydrostatic strength of thermoplastics materials in pipe form by extrapolation.
5.1.3 Detection of a knee and validation of data and model
Use the procedure given in Annex B to detect the presence of any knee.
After detecting a knee at any particular temperature, split the data set into two groups, one belonging to the first branch (data type A), the other belonging to the second branch (data type B).
Fit the multiple linear regression as described in Annex A independently, using all first-branch (type A) data points for all temperatures and all second-branch (type B) data points for all temperatures.
When studying the data for the occurrence of a knee, attention should be paid to the occurrence of a degradative failure. Such data (usually characterized by a nearly stress-independent line and visually recognizable) should not be considered for the calculation, but should only be used for determination of the extrapolation time (see 5.1.5).
If the automatic knee detection does obviously not correspond with the visual examination of the diagram, then the data points type A and type B in the region of the predicted knee can be manually reclassified to better align the knee point position with the data. In this case all data points at higher stresses than the stress level of the reclassified transition from type A to type B data points shall be declared type A and all data points at lower stresses shall be declared type B. The extrapolation shall be performed again without automatic knee detection. It is recommended in this case that more data points beyond the time of the predicted knee are obtained.
Obtain the maximum test time tmax, by averaging the logarithms of the five longest times to failure, which are not necessarily at the same stress level, but at the same temperature. Test pieces that have not yet failed may be considered as data points for this purpose. All those points shall belong to the population with which all calculations are performed.
If the data at the maximum test temperature are not used for determination of the regression model, these data are only used for the determination of the maximum test time tmax and consequently for the extrapolation time te. Such choices of calculation shall be justified and reported. Extrapolation is not permitted above the temperature range of the regression model.
The data obtained may be used for predicting the strength down to 20 °C below the lowest test temperature, provided that there is no change of state of the material, e.g. glass transition.
NOTE It is recommended that test data be generated at the lowest predicted temperature to demonstrate perform nce.
Examples of the application of the extrapolation time factor are presented in Figures 1 to 3. Figure 2 represents the case that a knee has been detected only at the highest temperature. Figure 3 refers specifically to the case that a knee has been detected at higher temperatures. Values of the extrapolation time factor ke are assigned in 5.2 and 5.3.
NOTE In cases like Figure 2, tmax is positioned at the time of the knee point.AS NZS 1462.29 pdf download.