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 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 Geocell 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.