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1.1 This test method describes two essentially equivalent procedures: one for obtaining a long-term hydrostatic strength category based on stress, referred to herein as the hydrostatic design basis (HDB); and the other for obtaining a long-term hydrostatic strength category based on pressure, referred to herein as the pressure design basis (PDB). The HDB is based on the material''s long-term hydrostatic strength (LTHS),and the PDB is based on the product''s long-term hydrostatic pressure-strength (LTHS
1.2 Unless the experimentally obtained data approximate a straight line, when calculated using log-log coordinates, it is not possible to assign an HDB/PDB to the material. Data that exhibit high scatter or a "knee" (a downward shift, resulting in a subsequently steeper stress-rupture slope than indicated by the earlier data) but which meet the requirements of this test method tend to give a lower forecast of LTHS/LTHSP. In the case of data that exhibit excessive scatter or a pronounced "knee," the lower confidence limit requirements of this test method are not met and the data are classified as unsuitable for analysis.
1.3 A fundamental premise of this test method is that when the experimental data define a straight-line relationship in accordance with this test method''s requirements, this straight line may be assumed to continue beyond the experimental period, through at least 100 000 h (the time intercept at which the material''s LTHS/LTHSP is determined). In the case of polyethylene piping materials, this test method includes a supplemental requirement for the "validating" of this assumption. No such validation requirements are included for other materials (see Note 1). Therefore, in all these other cases, it is up to the user of this test method to determine based on outside information whether this test method is satisfactory for the forecasting of a material''s LTHS/LTHS P for each particular combination of internal/external environments and temperature.
Note 18212;Extensive long-term data that have been o......
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