The Stepped Isothermal Method (SIM) was developed to predict the accelerated creep of polymers for a lifespan of 50-100 years (ASTM D6992). SIM measures the cumulative plastic distortion of the polymer product by subjecting it to a constant load at different temperatures for a specified time. The SIM method enables the calculation of the allowed strength for long-term design.
In the SIM test the End of Life is not based on the cell wall or seam rupture, but rather a permanent plastic deformation exceeding 10%. Polyolefin (PE, PP) tends to fail unexpectedly at strains above 10% caused by “crazing” in the polymer chemical structure, leading to a loss of confinement and failure. Accelerated creep of the Neoloy® Tough-Cells sample is ~9x lower than the HDPE sample as shown in the test below.
|MECHANICAL PROPERTIES – TENSILE STRENGTH|
|Strength at Yield||24||kN/m||ISO 10319|
|Resistance to Permanent Deformation (Creep) Reduction Factor||< 1.50||-||ASTM D-6992 (SIM)|
|(1) Standard ISO 10319 test modified to achieve more accurate results by using more representative test sample size; strip is cut adjacent to 2 seams and clamped so distance between clamps is 1/2 of cell height; Test direction is perpendicular to seams.|
|Test sample measured at strain rate 20%/min, 23°C;|
This technique “accelerates” the creep by increasing the temperature in steps and then shifts the data (using time-temperature superpositioning principles) to longer time periods. The SIM technique allows for more than 10,000 hours of creep to be simulated in less than 24 hours.