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ISSUE No.12 - NOVEMBER 2000
Tensile Properties of Polyethylene Geomembranes
by George Yazdani, MSc., P.E.
Tensile properties of polyethylene geomembranes are sensitive to the test specimen geometry and strain rate. The pre-dominant test method used by the polyethylene geomembrane industry to measure the tensile properties of smooth and textured HDPE and LLDPE geomembranes is ASTM D 638 " Tensile Properties of Plastics". A second test method is ASTM D 882 "Tensile Properties of Thin Plastic Sheeting", which covers thin polyethylene sheeting of less than 1.0 mm (40 mil) in thickness. Both the ASTM D 638 and the ASTM D 882 are index tests designed to produce tensile properties for manufacturing quality control and third party quality assurance. A third test, ASTM D 4885 "Determining Performance Strength of Geomembranes by the Wide Width Strip Tensile Method", is designed to determine the per-formance strength of geomembranes. This article will present tensile properties of 1.5 mm (60 mil) smooth and textured HDPE liners based on the three test methods to demonstrate the influence of specimen geometry and strain rate on these properties.
ASTM D 638
This is the industry standard test method for polyethylene geomembranes. A test was performed using Type IV specimens (Figure 1) on each of the 1.5 mm (60 mil) smooth and textured HDPE geomembranes. The strain rate was 50 mm/minute (2 in/minute). The percent elongations were calculated based on the gage lengths of 33 mm (1.3 in - the length of the narrow section) for yield elongation and 50 mm (2 in) for break elongation.
FIGURE 1. Type IV Specimen
| Dimensions (see drawings) | mm | | | |  |
|
| W | — Width of narrow section | 6 | | (0.25) |
| L | — Length of narrow section | 33 | (1.30) |
| WO | — Width over-all, min | 19 | (0.75) |
| LO | — Length over-all, min | 115 | (4.5) |
| D | — Distance between grips | 64 | (2.5) |
| R | — Radius of fillet | 14 | (0.56) |
| RO | — Outer radius | 25 | (1.00) |
ASTM D 882
This test method is for tensile properties of thin plastic sheeting, but is not commonly used in the polyethylene geomembrane industry. The tests performed on the 1.5 mm (60 mil) smooth and textured HDPE liner used specimens with a uniform width of 25 mm (1 in). The strain rate was 50 mm/minute (2 in/minute). The only difference between this test and the ASTM D 638 was the shape of the specimens (i.e. rectangular verses "dog bone")
ASTM D 4885
This is a performance test intended as a design aid used to determine the ability of a geomembrane to sustain the stresses and strains under design conditions. The tests performed on the 1.5 mm (60 mil) smooth and textured HDPE liner used specimens with a uniform width of 200 mm (8 in). The strain rate was 1.0 mm/minute (0.04 in/minute). The tests were terminated at 525% elongation, which was the maximum extension limit of the test device. None of the specimens failed by rupture during the tests. The differences between this test and the previous tests are the specimen width and the strain rate. All three tests were performed under the same environmental conditions. Test data are presented in Table 1 and 2.
Discussion
Smooth Liner: The index test data from ASTM D 638 and ASTM D 882 methods are in Table 1 and Figure 2 and 3. The data indicate that the "dog bone" specimens and the 25 mm (1 inch) wide strips have similar yield properties. The tensile strength at break is nearly the same as the yield strength for the 25 mm (1 in) wide specimens, but the break strength of the "dog bone" specimens is much higher than its yield strength. As shown in Figure 3, the strength of the "dog bone' specimen starts to increase at a faster rate at about 300% elongation. This could be because the material starts to draw strength from the wider section of the "dog bone" once it exceeds 300% elongation. The difference between the break tensile properties of the "dog bone" and the 25 mm (1 in) wide specimens is due to their different geometry.
Table 1. Tensile Properties of Poly-Flex 1.5 mm (60 mil) Smooth HDPE Geomembrane
| Tensile Properties |
ASTM D 638 - Type IV
"Dog Bone" @ 2 ipm |
ASTM D 882
1" Wide @ 2 ipm |
ASTM D 4885
8" Wide @ 0.04 ipm |
|
| Strength at Yield. lb/in |
156 |
159 |
119 |
| Elongation at Yield, % |
19 |
21 |
21 |
| Strength at 525% Elongation, lb/in |
190 |
140 |
112 |
| Strength at Break, lb/in |
281 |
155 |
NA* |
| Elongation at Break, % |
794 |
938 |
>525* |
Values are for machine direction.
* Due to limitation of the test device, the tests were terminated at 525% elongation with no failure.
FIGURE 2.
Tensile Strength Properties of
1.5 mm (60 mil) Smooth HDPE Liner
FIGURE 3.
Tensile Properties of
1.5 mm (60 mil) Smooth HDPE Liner
Textured Liner: The index test data presented in Table 2 and Figure 4 and 5 indicate that the "dog bone" and the 25 mm (1 inch) wide specimens have similar yield properties. The tensile strength at break is nearly the same as the yield strength for the 25 mm (1 in) wide specimens. The break strength of the "dog bone" specimens is higher than the break strength of the 25 mm (1 in) wide specimens, but its break elongation is much lower. Similar to the smooth sheet, the strength of the "dog bone' specimen begins to increase at a faster rate at about 300% elongation (Figure 5). Again, the difference between the break tensile properties in the two tests is due to the geometry of the specimens.
Table 2. Tensile Properties of Poly-Flex 1.5 mm (60 mil) Textured HDPE Geomembrane
| Tensile Properties |
ASTM D 638 - Type IV
"Dog Bone" @ 2 ipm |
ASTM D 882
1" Wide @ 2 ipm |
ASTM D 4885
8" Wide @ 0.04 ipm |
|
| Strength at Yield. lb/in |
142 |
147 |
113 |
| Elongation at Yield, % |
16 |
19 |
21 |
| Strength at 525% Elongation, lb/in |
168 |
126 |
99 |
| Strength at Break, lb/in |
182 |
150 |
NA* |
| Elongation at Break, % |
537 |
961 |
>525* |
Values are for machine direction.
* Due to limitation of the test device, the tests were terminated at 525% elongation with no failure.
FIGURE 4.
Tensile Strength Properties of
1.5 mm (60 mil) Textured HDPE Liner
FIGURE 5.
Tensile Properties of
1.5 mm (60 mil) Textured HDPE Liner
Test presented in Table 1 and 2 and Figure 6 and 7 for the ASTM D 4885 test method show similar patterns between yield and break tensile properties for both the smooth and textured sheets. The wide width tensile strengths were noticeably lower than the corresponding ones from the index tests. This is true for both the smooth and the textured sheet. This is attributed to the specimen geometry and the very low strain rate. A higher strain rate produces higher yield strength and lower elongation, particularly in textured geomembranes.
FIGURE 6. Tensile Strengths of 1.5 mm (60 mil) HDPE Smooth Liner
FIGURE 7. Tensile Strengths of 1.5 mm (60 mil) HDPE Textured Liner
Conclusion
This experiment indicates that the tensile properties of polyethylene geomembrane are sensitive to specimen geometry and strain rate during testing. Both ASTM D 638 and ASTM D 882 are index test methods intended for routine acceptance testing of geomembrane by the liner manufacturers and the third party quality assurance laboratories. The shape of the specimen in the index tests significantly influences the tensile properties at break for HDPE liner. The data obtained from these tests are not intended and should not be used for design purposes.
As a performance test, the ASTM D 4885 method is not intended for routine acceptance testing of geomembranes, but to establish performance characteristics of geomembrane materials. The design of a HDPE geomembrane should be based on its tensile yield and never on tensile break properties.
The intent of this article was not to show an exact comparison of test data from different test methods, but to show general patterns for the tensile characteristics of HDPE geomembranes based on different specimen geometry and strain rates.
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