ASTM D6693, formally titled Standard Test Method for Determining Tensile Properties of Nonreinforced Polyethylene and Nonreinforced Flexible Polypropylene Geomembranes, is one of the most frequently referenced test standards in the geosynthetics industry. Every HDPE, LLDPE, and fPP geomembrane specification sheet includes tensile property values determined by this method. Yet many engineers and facility owners who review these spec sheets do not fully understand what the test measures, how the results relate to field performance, or what constitutes an acceptable result.
This guide explains the D6693 test method in practical terms, describes what each reported property means, and provides guidance on evaluating tensile test results for specification compliance and fitness for purpose.
What ASTM D6693 Measures
ASTM D6693 is a uniaxial tensile test. A dumbbell-shaped specimen is cut from the geomembrane sheet, clamped in the jaws of a tensile testing machine, and pulled at a constant rate until it breaks. During the test, the machine continuously records the applied force and the elongation of the specimen. From this data, four primary properties are calculated: tensile strength at yield, elongation at yield, tensile strength at break, and elongation at break.
The specimen geometry is specified as a Type IV dumbbell per ASTM D638, with a narrow section width of 6 millimeters and a gauge length of 33 millimeters. The crosshead speed (rate of jaw separation) is 50 millimeters per minute. These parameters are critical because tensile properties of polymers are rate-dependent -- testing at a different speed would produce different results. All specimens must be conditioned at 73 plus or minus 4 degrees Fahrenheit for at least 24 hours before testing.
Understanding the Stress-Strain Curve
The tensile test produces a stress-strain curve that is characteristic of the polymer type. For HDPE geomembranes, the curve has a distinctive shape: an initial steep linear region (elastic deformation), a peak at the yield point, a drop in stress as the material begins to neck, a long plateau region where the material draws and extends at relatively constant stress, and finally a rise and ultimate failure at the break point.
The yield point represents the transition from elastic to plastic deformation. Below the yield stress, the material will return to its original shape when the load is removed. Above the yield stress, permanent deformation occurs. In field applications, geomembranes should ideally operate below their yield stress to avoid permanent deformation, though the large elongation capacity beyond yield provides a significant safety margin against catastrophic failure.
The Four Key Properties
Tensile strength at yield is the maximum stress the material can sustain before permanent deformation begins. For a standard 60-mil HDPE geomembrane conforming to GRI-GM13, the minimum specification value is 126 pounds per inch of width (or 21 kN/m). This property is most relevant to design engineers because it represents the upper limit of the material's elastic range.
Elongation at yield is the percentage increase in gauge length at the yield point. For HDPE, this is typically 12-13%. This relatively low value reflects the stiff, semicrystalline nature of HDPE. It means that HDPE liner panels can accommodate only about 12% strain before they begin to deform permanently -- an important consideration when designing for thermal expansion, settlement, and subgrade irregularities.
Tensile strength at break is the stress at the point of specimen rupture. For 60-mil HDPE per GRI-GM13, the minimum is 84 pounds per inch of width. This value is lower than the yield strength because the material has necked down to a thinner cross-section during the drawing process. The break strength divided by the original cross-sectional area gives the engineering stress at break, which is always less than the yield stress for HDPE.
Elongation at break is the total percentage increase in gauge length at failure. For HDPE, the minimum specification value is 700%. This extraordinary elongation capacity is one of the most important properties of HDPE geomembranes -- it means the material can stretch to 8 times its original length before rupturing. In the field, this ductility allows HDPE liners to accommodate significant subgrade movement, differential settlement, and localized stress concentrations without tearing.
Machine Direction vs Cross-Machine Direction
Geomembrane sheets are manufactured by extrusion, and the manufacturing process imparts a degree of molecular orientation in the machine direction (the direction of extrusion). ASTM D6693 requires that specimens be tested in both the machine direction (MD) and the cross-machine direction (XMD or TD). Tensile properties may differ between the two directions, particularly elongation at break.
GRI-GM13 specifies minimum values that must be met in both directions. Some manufacturers report only the lower value, while others report both MD and XMD results separately. When reviewing test data, verify which direction is being reported. A material that meets specification in one direction but not the other does not conform.
What Abnormal Results Indicate
- Low elongation at break (below 500%) may indicate oxidative degradation, excessive crystallinity, or contamination during manufacturing. This is a red flag that warrants further investigation, as reduced ductility significantly increases the risk of stress cracking in the field.
- Yield strength significantly above specification may indicate a high-density resin that trades ductility for stiffness. Verify that elongation values are also meeting specification -- very stiff material can be difficult to conform to irregular subgrades.
- Break strength approaching or exceeding yield strength suggests the material is not necking properly during the test, which may indicate excessive cross-linking or an unusual resin formulation.
- High variability between specimens (coefficient of variation above 10%) may indicate inconsistent sheet thickness, poor resin mixing, or manufacturing process control issues.
- Brittle failure (no yield point, low elongation) is a serious defect. HDPE should always exhibit ductile failure with a clear yield point and extensive drawing. Brittle failure suggests degradation, contamination, or fundamentally unsuitable material.
Field Relevance of Laboratory Tensile Data
It is important to understand that ASTM D6693 is a quality control test, not a design test. The test is performed at a constant strain rate on a small specimen under uniaxial tension at room temperature. Field conditions involve multiaxial stress states, variable temperatures, long-term sustained loads, chemical exposure, and UV degradation. The tensile properties measured by D6693 do not directly predict how the material will perform under these complex field conditions.
However, the D6693 results serve as a reliable indicator of material quality and consistency. A material that meets GRI-GM13 tensile specifications has demonstrated that its resin quality, manufacturing process, and thickness are within acceptable parameters. Changes in tensile properties over time -- measured by testing retained specimens or samples extracted from installed liners -- can indicate whether the material is degrading in service.
Specification Standards That Reference D6693
Several industry specifications use ASTM D6693 as the test method for tensile property requirements. The most widely referenced are the Geosynthetic Research Institute (GRI) test methods and specifications.
- GRI-GM13: Specification for HDPE smooth and textured geomembranes. This is the most commonly specified standard for HDPE liners in containment applications.
- GRI-GM17: Specification for LLDPE smooth and textured geomembranes. LLDPE has lower yield strength but higher elongation than HDPE.
- GRI-GM18: Specification for fPP smooth and textured geomembranes. Flexible polypropylene has no distinct yield point, so only break properties are reported.
- State-specific regulations often reference GRI specifications directly or impose equivalent tensile property requirements.
Testing Frequency and CQA Requirements
During manufacturing, geomembrane producers perform D6693 testing on every roll or at specified intervals (typically every 20,000 to 40,000 square feet of production). The results are reported on the manufacturer's quality control certificate that accompanies each shipment. During installation, Construction Quality Assurance (CQA) programs may require additional confirmatory testing by an independent laboratory, particularly if the material has been stored for an extended period or exposed to UV light before installation.
For critical containment applications, EFI USA recommends reviewing the manufacturer's QC data for every roll delivered to the site, and performing independent D6693 testing on a minimum of one sample per 100,000 square feet of installed liner. This independent verification provides an additional level of confidence that the installed material meets specification.
Understanding ASTM D6693 and the tensile properties it measures is fundamental to specifying, purchasing, and accepting geomembrane materials. If you have questions about interpreting test data for your project, EFI USA's technical team can review your specifications and test results to ensure the material is appropriate for your application.
