The procedure for estimating long-term hydrostatic strength or pressure-strength is essentially an extrapolation with respect to time of a stress-time or pressure-time regression line based on data obtained in accordance with Test Method D 1598
Note 38212;Test temperatures should preferably be selected from the following: 40°C; 50°C; 60°C; 80°C; 100°C. It is strongly recommended that data also be generated at 23°C for comparative purposes.
The hydrostatic or pressure design basis is determined by considering the following items and evaluating them in accordance with 5.4.
Long-term hydrostatic strength or hydrostatic pressure-strength at 100 000 h,
Long-term hydrostatic strength or hydrostatic pressure-strength at 50 years, and
Stress that will give 5 % expansion at 100 000 h.
The intent is to make allowance for the basic stress-strain characteristics of the material, as they relate to time.
Results obtained at one temperature cannot, with any certainty, be used to estimate values for other temperatures. Therefore, it is essential that hydrostatic or pressure design bases be determined for each specific kind and type of plastic compound and each temperature. Estimates of long-term strengths of materials can be made for a specific temperature provided that calculated values, based on experimental data, are available for temperatures both above and below the temperature of interest.
Hydrostatic design stresses are obtained by multiplying the hydrostatic design basis values by a service (design) factor.
Pressure ratings for pipe may be calculated from the hydrostatic design stress (HDS) value for the specific material used to make the pipe, and its dimensions using the equations in 3.1.11.
4.5.1 Pressure ratings for multilayer pipe may be calculated by multiplying the pressure design basis (PDB) by the appropriate design factor (DF).
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 (LTHSP). The HDB is a material property and is obtained by evaluating stress rupture data derived from testing pipe made from the subject material. The PDB is a product specific property that reflects not only the properties of the material(s) from which the product is made, but also the influence on product strength by product design, geometry, and dimensions and by the specific method of manufacture. The PDB is obtained by evaluating pressure rupture data. The LTHS is determined by analyzing stress versus time-to-rupture (that is, stress-rupture) test data that cover a testing period of not less than 10 000 h and that are derived from sustained pressure testing of pipe made from the subject material. The data are analyzed by linear regression to yield a best-fit log-stress versus log time-to-fail straight-line equation. Using this equation, the material''s mean strength at the 100 000-h intercept (LTHS) is determined by extrapolation. The resultant ......
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