Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 64
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: 20°C.
The main difference between PET and HDPE is their respec- Temperature Steps: 7°C (20, 27, 34, 41, 48, 55, 62, 69, 76)
tive temperature dependencies at temperatures below 80°C. 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 76°C. Notice that the sample yielded
parison for the two materials is shown in Figure A-12 below. catastrophically during the early part of the 76°C 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.
