Geotextile Narrow Strip Tensile Test: Standards, Methods, and Applications

The geotextile narrow strip tensile test is a fundamental laboratory method for determining the tensile strength and elongation properties of geotextiles and geotextile-related products. By loading narrow strip specimens cut in both the machine and cross-machine directions, manufacturers, contractors, and accredited third-party laboratories can verify product quality and demonstrate compliance with standards such as ASTM D5035, ISO 10319, and GB/T 15788. This article explains the test principle, specimen requirements, testing procedures, reported parameters, and typical engineering applications.

Technical Overview

The narrow strip tensile test is one of the most widely performed mechanical tests on geotextiles. In this test, a strip of geotextile is gripped across its full width by the clamps of a constant-rate-of-extension (CRE) tensile testing machine and loaded in uniaxial tension until rupture. The test yields the maximum tensile force, the elongation at maximum force, and the complete force-extension curve, which together characterize the short-term load-carrying capacity and deformation behavior of the material.

The narrow strip method traditionally uses specimens 25 mm or 50 mm wide. Because such narrow specimens are influenced by edge effects, unrestricted lateral contraction (Poisson effect), and grip-induced stress concentrations, international standardization bodies later developed the wide-width tensile test, which uses 200 mm wide specimens and is documented in ISO 10319, ASTM D4595, and GB/T 15788. Despite this development, the narrow strip test remains extremely common in routine quality control, incoming material inspection, and factory acceptance testing, because it is fast, requires little material, and can be performed on standard textile testing equipment.

• ASTM D5035 describes the strip method for textile fabrics, which is commonly applied to geotextile strip specimens.
• ISO 10319 and ASTM D4595 govern the wide-width variant and are generally preferred for design-critical data.
• GB/T 15788 is the Chinese national standard for the wide-width tensile test, while GB/T 3923.1 covers the strip method for textiles.
• Certain national product specifications and project technical requirements continue to specify the narrow strip method for batch release and acceptance testing.

It is important to recognize that narrow strip and wide-width results are not directly interchangeable. For a given material, the two methods can produce different values of strength and elongation, particularly for nonwoven geotextiles, and any comparison or conversion should be based on documented correlation or the governing specification.

Testing Samples

Woven Geotextiles

Woven geotextiles, including slit-film and multifilament fabrics, are tested in both the machine direction (warp) and cross-machine direction (weft). Because the yarn structure is directional, the two directions typically exhibit distinctly different tensile properties, and both must be evaluated. Ravel strip preparation, in which outer yarns are removed to obtain an exact specimen width, is often applied to woven fabrics to eliminate cut-yarn effects at the edges.

Nonwoven Geotextiles

Needle-punched, spunbond, and thermally bonded nonwoven geotextiles can also be evaluated by the narrow strip method. Due to their random fiber orientation and pronounced necking behavior, nonwovens generally show higher scatter in results, which is why testing a larger number of specimens and reporting the coefficient of variation is recommended.

Composite and Other Geotextile-Related Products

Composite geotextiles, such as nonwoven-woven laminates and coated fabrics, may be tested using the narrow strip approach provided the product behaves as a continuous sheet. Specimens containing factory or field seams can be included when seam strength is of interest. For geogrids and other integrally ribbed products, dedicated rib or wide-width geogrid methods are used instead, as the strip concept does not apply.

Testing Items

The narrow strip tensile test generates a force-extension curve from which the following parameters are typically determined and reported:

• Maximum tensile force (ultimate tensile strength), expressed in N or converted to strength per unit width such as N/50 mm or kN/m.
• Elongation at maximum force, expressed as a percentage of the initial gauge length.
• Elongation at break, where the force-elongation curve permits a distinct rupture point.
• Tensile force at specified strain levels, for example 2%, 5%, or 10% extension, when required by specification.
• Initial tensile modulus or secant modulus calculated over a defined strain interval.
• Full force-extension (stress-strain) curve shape, including yield and post-peak behavior where applicable.
• Seam or joint efficiency when specimens incorporate a stitched, welded, or bonded seam.
• Statistical evaluation of replicate results, including mean value, standard deviation, and coefficient of variation.

Testing Methods

Specimen Preparation

Samples are taken from representative locations of the roll, avoiding damaged edges and selvage regions. Specimens are cut with their long axes parallel to the machine direction and to the cross-machine direction. Typical narrow strip dimensions are 50 mm in width (25 mm ravel strips are also permitted by some standards) with a gauge length of 100 mm, and an overall length sufficient to provide full grip engagement at both ends. At least five valid specimens per direction are tested, and ten are recommended for nonwovens to account for variability. All specimens are conditioned in a standard laboratory atmosphere, commonly 20 ± 2 °C with 65 ± 5% relative humidity, for a minimum of 24 hours before testing in accordance with the conditioning requirements referenced by ISO, ASTM, or GB textile testing standards.

Test Procedure

• Verify the calibration of the CRE tensile testing machine and load cell before testing.
• Set the initial gauge length, typically 100 mm for narrow strip specimens, and select the constant rate of extension, commonly 20 mm/min (approximately 20% strain per minute) unless the governing standard specifies otherwise.
• Apply a small pretension to remove slack when testing highly extensible nonwovens, as permitted by the standard.
• Mount the specimen centrally in the grips, ensuring the longitudinal axis aligns with the line of force, then clamp securely using wedge or pneumatic grips at least as wide as the specimen.
• Start the test and continuously record the force-extension data until the specimen ruptures.
• Discard and replace any specimen that fails within the jaws or at the grip faces, unless the standard allows such jaw breaks to be reported separately.

Calculation and Reporting

The maximum tensile force is read directly from the force-extension curve. Tensile strength per unit width is calculated by dividing the maximum force by the specimen width; for a 50 mm wide strip, multiplying the maximum force by 20 gives the strength in kN/m. Elongation at maximum force is calculated as the extension at maximum force divided by the initial gauge length, expressed as a percentage. The test report should identify the test standard, specimen dimensions, testing direction, gauge length, extension rate, conditioning details, individual and mean results, coefficient of variation, and any deviations such as jaw breaks.

Applications

The narrow strip tensile test supports a broad range of quality assurance and engineering activities across the geosynthetics industry:

• Factory quality control and batch release testing by geotextile manufacturers.
• Incoming inspection and acceptance testing of delivered rolls on construction sites and in contractor laboratories.
• Verification of mechanical properties for geotextiles used in highway and railway subgrade separation and filtration layers.
• Material screening for reinforced embankments, steep slopes, and retaining structures, in combination with wide-width data.
• Qualification of geotextiles for landfill liners, leachate collection systems, and environmental containment works.
• Evaluation of fabrics for hydraulic applications such as revetments, canal linings, and scour protection.
• Research, product development, and comparative benchmarking of new geosynthetic materials by accredited third-party laboratories.

FAQ

What is the difference between the narrow strip test and the wide-width tensile test?

The narrow strip test uses specimens 25 mm or 50 mm wide, while the wide-width test specified in ISO 10319, ASTM D4595, and GB/T 15788 uses 200 mm wide specimens. The wider specimen reduces edge effects and grip influence and provides data more representative of in-service behavior, which is why wide-width results are generally preferred for design. The narrow strip test remains popular for routine quality control due to its speed, simplicity, and low material consumption, but the two methods do not produce directly interchangeable results.

What are the typical specimen dimensions and how many specimens are required?

A common narrow strip configuration is 50 mm in width with a 100 mm gauge length, although 25 mm ravel strips are permitted by some standards. A minimum of five valid specimens per direction is normally required, and ten specimens per direction are recommended for nonwoven geotextiles because of their higher inherent variability.

How is tensile strength expressed?

The maximum tensile force is divided by the specimen width to obtain strength per unit width. Results may be reported as N/50 mm or converted to kN/m; for a 50 mm wide strip, the maximum force in kN multiplied by 20 yields the tensile strength in kN/m. Elongation is always reported as a percentage of the initial gauge length.

What extension rate is used?

The test is performed on a constant-rate-of-extension machine, typically at 20 mm/min for a 100 mm gauge length, which corresponds to a nominal strain rate of 20% per minute. The exact rate must follow the governing standard or project specification, since strength and elongation results are sensitive to loading rate.

Can narrow strip results be used directly in limit-state design?

Generally not. Design procedures for geosynthetic reinforcement rely on wide-width or in-soil tensile data combined with partial factors or reduction factors covering creep, installation damage, and durability. Narrow strip values are best suited for quality control, acceptance testing, and comparative screening, and any transfer to design parameters should be supported by documented correlation between methods.