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Deformation Transitions and their Effects on the Long-term Performance of Polyethylene and its Pressure Pipe
- Author / Creator
- Tan, Na
Ductile-to-brittle (DB) transition is of tremendous importance for lifetime prediction of polyethylene (PE). Majority of current methods for determining critical stress for the DB transition of PE products use notched coupon specimens in order to shorten the test timeframe. However, those work are mainly for characterization of the crack resistance performance, not for determining critical stress level for the DB transition. Therefore, the ultimate goal of this research is to develop a new short-term test, based on notch-free specimens, to predict critical stress for the DB transition of PE for load-carrying applications. The key assumption for the approach used in the research is that below a critical stress level, brittle failure occurs as a result of the limited extent of deformation due to the constraint of deformation in the amorphous phase; above this critical stress level, ductile failure dominates the failure mode due to the activation of collective crystallographic slips which accommodate large plastic strain. Thus, it is expected that critical stress for the DB transition should not exceed the stress level for the onset of global yielding in the crystalline phase. Also, the critical stress contains two components, one is the time-dependent viscous stress and the other the time-independent quasi-static stress. In view that the quasi-static stress should dominate the long-term load-carrying performance of PE, the viscous stress should be excluded from the critical stress that is considered for the long-term applications.
Based on the above assumption, uniaxial creep tests are firstly conducted on three PE plaques differing mainly in mass density, and master time-to-failure curves are constructed through the use of a stress-time-temperature (StT) parametric method. PE with the highest density shows the well-known DB transition; PE with the medium density shows a different transition which was detected through the change in the dependence of failure time on the applied stress, but the failure mode remained ductile. Therefore, this transition is named ductile-ductile (DD) transition. To investigate the correlation between creep deformation and activation energy at the secondary creep stage, Norton Power Law (NPL) and Eyring’s Law are used to analyze the results. The analysis suggests a strong correlation between the DB transition and activation energy at the secondary creep stage, and that the Monkman-Grant relationship is applicable to ductile failure, even with the DD transition. Therefore, rather than the use of creep tests with failure time up to 13 months, short-term creep tests have been investigated, and found to have the potential for predicting the critical stress for the DB transition.
The study also included development of a new short-term test, named multi-relaxation (MR) test which is proven to be capable of detecting two critical quasi-static stresses for change of mechanisms involved in deformation. Six PE plaques with different mass densities were studied and the results show that the 1st critical stroke has very similar values among six PEs of different mass densities. More interestingly, ratio of the quasi-static stress at the 1st critical stroke to the yield stress from the standard tensile test shows little dependence on PE density. Therefore, it is possible to use the popular short-term tensile test to characterize the critical quasi-static component of the applied stress to initiate plastic deformation in the crystalline phase, which is expected to play a significant role on the long-term, load-carrying applications of PE.
Finally, the study has correlated the critical quasi-static stresses from the MR test with the critical stress for the DB transition from the creep test. The results on compression-molded PE plaque show that the critical quasi-static stress for the onset of global yielding in the crystalline phase correlates well with the critical stress for the DB transition. Therefore, such a concept is also transferred to PE pipe. The results suggest that critical quasi-static stress for the onset of plastic deformation in the crystalline phase is close to the hydrostatic design basis (HDB) value based on the long-term hydrostatic strength (LTHS) determined from hydrostatic pressure tests, but the MR test takes less than two weeks to complete, while the LTHS requires more than 1 year to measure. Therefore, the MR test can be used as an alternative method to characterize PE pipe performance, especially for preliminary screening or in-service monitoring of PE pipe performance.
- Graduation date
- Fall 2021
- Type of Item
- Doctor of Philosophy
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