Pretension and heating are common processes in the production of flexible geogrids. The pretension and heating temperature effects on the tensile and creep behavior of two types of high strength polyester yarns were investigated. Two types of polyester yarns provided from different local manufactures were used. The unit weights of the test yarns are 2013 (Type A) and 1511 (Type B) dynes, respectively. The ultimate tensile strength of the test yarns are 8.8 g/dyne and 8.9 g/dyne, respectively. 20%, 30%, 50% UTS pretension forces and 150℃, 170℃ and 180℃ heating temperatures were used to simulate the manufacturing process. A series of tensile tests (ASTM D2256) and conventional longterm (ASTM D5262) creep tests were performed to evaluate the tensile strength, elongation at rupture and long-term creep strain of the test polyester yarns. Scanning Electronics Microscope (SEM) technique was also used to visual evaluation the surface structure of the yarns before and after the simulated manufacturing process and creep tests.
The test results indicated that the ultimate tensile strength of the polyester yarns decreases bilinearly as the test temperature is increased. The bi-linear tensile strength decrease rate is about -0.37 N and -0.74 N per degree of Celsius, respectively for Type A polyester yarn. The elongation at break for the tested yarns varied from 10.57% to 12.49% for test temperature varying from 20℃ to 80℃, respectively. The tensile strength and elongation at break at around 70℃ test condition was inconsistent with the results from other test conditions. It is believed that the glass formation phenomenon might have some effects on the engineering behavior of the tested yarns at a temperature around 70℃.
Drying the polymer coating by heating would induce polyester yarn shrinkage during the geogrid production process. A minimum of 10% UTS pretension load is required to prevent shrinkage in the test polyester yarns. The single strand tensile test results for the processed polyester yarns indicated that the elongation at break was reduced by 1.5% to 2.0%.
Linear creep strain curves on a semi-log scale diagram were observed from a series of 1000-hour conventional long-term creep tests for polyester yarns processed at different temperatures. The curve slope decreased as the pretension load and heating temperature were increased.
The creep strain rates decrease very rapidly at the initial stage and reach a plateau stage after 50 hours loading during the tests. Secondary creep behavior was observed for the control and processed 1000-hour creep tests. The creep strain rate decreases as the pretension load and heating temperature were increased. Hair cracks were observed from SEM pictures for the samples obtained from a simulated manufacturing process after 1000-hours conventional creep testing. Similar test results were observed for both tested polyester yarns.