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Strain hardening modulus: a measure for ranking time to failure of random polypropylene pipe materials

 

Papers # 2016 Berlin

The resistance to Slow Crack Growth of HDPE is related to the strain hardening modulus as determined from a tensile test at 80 °C. Correlatio ns between traditional methods (e.g. Full Notched Creep Test, Notched Pipe Test) and other accelerated tests (e.g. fatigue testing) show that the same failure mechanism is probed and that the strain hardening modulus can be used to study and rank the expected SCG behavior of HDPE materials. With the huge benefits achieved for the HDPE pipe grades using raw materials, the logical next step is expansion of the benefits of the strain hardening method to other pipe grade materials like PP. The strain hardening method is optimized (e.g testing temperature, testing speed) for r-PP pipe materials. For different PP pipe grade materials strain hardening is determined and ranking of the strain hardening results to time to failure of pipes is discussed. 

Random Poly propylene (r-PP) is widely used in cold and warm water pressure pipe applications. The traditional test method (Internal Pressure Test) requires large volumes of material and long testing times (>1 yr) in assessing this property in order to guarantee lifetimes of 50 years and above. These extremely long testing times are a serious concern in using these methods for determination of time to failure behavior of resins and manufactured pipes as well as in the development of new grades, in batch release testing and quality control. Additionally, material wastage, energy costs and water use in traditional methods are undesirable from a sustainability point of view. In recent years within SABIC, a simple but effective test method – tensile strain hardening modulus assessment – requiring only small amounts of material (grams) and delivering reproducible, reliable and accurate results within only a few hours was developed for HDPE pipe materials. The resistance to Slow Crack Growth of HDPE is correlated to the strain hardening modulus as, determined from a tensile test at 80 °C 2,3. Correlations between traditional methods (e.g. Full Notched Creep Test, Notched Pipe Test) and other accelerated tests (e.g. fatigue testing) show that the same failure mechanism is probed and that the strain hardening modulus can indeed be used to study and rank the expected SCG behavior of HDPE materials. With the huge benefits achieved for the HDPE pipe grades using raw materials, the logical next step is expansion of the benefits of the strain hardening method to other materials like PP. The strain hardening method is optimized (e.g testing temperature, testing speed) for r-PP pipe materials. For different r-PP materials strain hardening is determined,

Related keywords : Strain Hardening, random PolyPropylene, slow crack growth, pipe failure.
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