Measuring Geomembrane Tensile Strength – 1” or Dumbbell?
Abstract: Specifying a tensile strength or % elongation for any given application is necessary to avoid any possible failure in the geomembrane. There are many factors to consider including: testing equipment, thickness, load cell, type of grips, and sample geometry. Although ASTM D7176 currently requires a 1” tensile strip, they will be considering a dumbbell-shaped sample as this provides more consistent results. This blog explores current industry standards and methodologies in testing and reporting unsupported geomembrane strength and elongation, while providing recommendations for improvements.
Tensile properties are used to estimate if a material is suitable for a given application. Tensile strength and percent elongation for any given application is necessary to understand and avoid any geomembrane failure.
ASTM D7176 and FGI 1115 lists, among the many specifications, the desired tensile strength required for each thickness, which ranges from 10 mils to 60 mils. Tensile strength is a good measure of a material’s toughness. For example, a 30-mil specimen has a minimum tensile strength of 73 lbs./inch while a 60 mil. is specified at 137 lbs./inch. As the thickness increases so will the amount of force needed to pull it apart and destroy it.
There are different types of testing equipment, but all are designed to run the same tests. Typically, the most often used tensile testing machine in the lab is from the Instron Corporation, whereas in the field, a common testing machine used is manufactured by Demtech. The sample is separated at a given speed until the membrane breaks. The load is measured intermittently until failure. Tensile strength is reported either in psi (pounds/square inch) or lbs./in (pounds/inch width). ASTM D 7176 results are reported in pounds per inch.
According to ASTM D7176, “Certified properties are tested based on a quantity of material produced. Certified properties are tested once per lot, or once every 18,000 kg of material (40,000 lb.), whichever is more frequent.” Five samples are to be selected for tensile strength both in the machine and cross machine direction. Manufacturers may take more test samples during production. Each sample is die cut to be 1” x 8” in accordance to ASTM D882, known as the 1” strip tensile.
ASTM D882 specifies the type of grip and set up which is critical in measuring tensile strength and elongation. For unreinforced products, flat grips are better than aggressive grips. When using flat grips, rubber pads work best for thin gauge samples to ensure the sample will not slip out during testing. Aggressive serrated grips have a tendency to puncture the membrane which may weaken the sample and cause premature failure at the grips. This can be emphasized with higher pressure of the grips such as when using manual tightening.
Sample geometry is another aspect of this standard that is often overlooked. ASTM D7176 and FGI 1115 specs both require the sample to be a 1-inch strip in accordance with ASTM D882. There has been some discussion about changing this method over the last couple of years due to inconsistent result from different labs. EPT found in a recent study of a 30-mil membrane with a specification of 73 lbs./in, the results had a variation of over 50% due to the type of grips used and pressure applied.
To address this issue, a dumbbell geometry was adopted by ASTM D412 or D638 for unreinforced products. As seen in the diagram below, the sample resembles a dumbbell, such that the width of the sample in the middle is significantly less than the grip area, which are clamped down and stretched. The middle section sees approximately 95% of the force and that section will stretch until it breaks. This method takes out the variable due to the grips and pressure applied.
Tensile measurements are just one piece of the puzzle when reviewing a product to be used in a given application. Although, further work could be done to evaluate different polymers and thicknesses, the variation in results would be lower using dumbbell shaped samples rather than 1 inch strips. The results given by the proposed new standard should represent an accurate value of tensile strength. Therefore, if the results do not meet the current specification, it may be the equipment and/ or sample geometry rather than the material.