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A-10 samples and the results compared to the BFF test results under Time-temperature-superpositioning (TTS) has been used the same conditions. for decades and it is the basis for the validation procedures for polyethylene pipe materials in ASTM D2837 and Plastics Pipe Institute (PPI) Technical Report TR-3. TTS is used to project A.4 Service-Lifetime the long-term hydrostatic strength of pressure pipe. Estimation Tests Basically, increasing the temperature of a process like creep, A.4.1 Stepped Isothermal Method (SIM) stress relaxation, or slow crack growth is equivalent to perform- For Long-Term Creep Modulus ing the test at longer and longer times. The higher the tem- and Strain (ASTM D6992) perature, the longer the accelerated time. In the case of traditional TTS, tests are performed at various The Stepped Isothermal Method (SIM) is a special form of elevated temperatures on different samples and the results Time-Temperature-Superpositioning (TTS) that has been used shifted to a lower target temperature. Because of the sample- to extrapolate short-term creep results (24 h) into long-term to-sample variability, the result of TTS can be uncertain and estimates of creep behavior (50, 100 years). It was originally requires tests on many test specimens. developed in these laboratories on polyester (PET) geogrids SIM is a form of TTS in which behavior at multiple used for reinforcement applications. The application of SIM temperatures is observed on a single test specimen, which to PET has been verified and validated by several other labo- reduces the uncertainty of the behavior due to sample-to- ratories comparing the SIM results to conventional creep sample variability. tests performed at room temperature. It has also been used An example SIM test for HDPE was performed under the by others on other PET fibers, Kevlar, and Polyethylene following conditions: Naphthanate (PEN). It has also been used by TRI to examine PP buried struc- tures and most recently on HDPE resins used for corrugated Sample: Type I Dumbbell. drainage pipe. Only preliminary validation tests have been Strain Measurement: Extensometer. performed on PP, but the results are favorable. Initial Temperature: 20C. The main difference between PET and HDPE is their respec- Temperature Steps: 7C (20, 27, 34, 41, 48, 55, 62, 69, 76) tive temperature dependencies at temperatures below 80C. Stress: 1000 psi. HDPE's properties change at a higher rate with temperature Dwell Time: 10,000 seconds (2.78 h). than PET's properties. In fact, the low-temperature depend- ency of PET strength was the main reason SIM was developed. The raw, unshifted data are shown in Figure A-13. The sample-to-sample variability could be as large as the dif- There are nine temperature steps shown on the plot, so the ference in creep rates at two different temperatures. A com- highest temperature was 76C. Notice that the sample yielded parison for the two materials is shown in Figure A-12 below. catastrophically during the early part of the 76C step. 1.2 PET 1 y = -0.003x + 1.069 0.8 Relative Property 0.6 0.4 PE 0.2 y = -0.0117x + 1.2639 0 10 20 30 40 50 60 70 80 90 Temperature (C) Figure A-12. Temperature dependence of PET and PE.