Textiles, yarn, thread, fabrics, wool, cotton, and other animal and plant-derived fibers are commonly tested to ensure product quality by assessing the performance of materials and making sure they are acceptable towards proper end-use. This blog post will cover certain mechanical testing methods to test fabric and go over the testing standards as well as the recommended equipment.
Measurement Methods of the Mechanical Properties of Fabrics
Tensile testing is the most commonly applied test method for analyzing the mechanical properties of fabric materials. Although the direction of applied force is always in tension, there are a variety of tensile test methods available for capturing the most relevant data for final product usage.
The strip test is a tensile test in which the full width of the test specimen is gripped in the tensile grip jaws of a universal testing machine. During this test, tensile force is applied on the fabric specimen until it ruptures. Mechanical properties to analyze include the force at rupture and the elongation (expressed as percentage) at maximum force and/or at rupture.
Testing standards that can be followed to run strip tests include:
A grab test is a tensile test in which the center part of the specimen width is gripped in the tensile grip jaws. Due to the way the sample is gripped, edge effects, which may cause inaccurate data for fabrics, are eliminated. Similar to the strip test method, tensile force is applied to the fabric specimen until rupture and maximum force is recorded. Fabric specimens most often tested using the grab method are woven and non-woven textile fabrics.
Testing standards outlining grab test instructions include:
Note that the maximum force recorded after running a strip method and the grab method may not be the same due to the effective width of the fabric being tested.
Tear strength is the capacity of a material to withstand the tearing force required to propagate a tear after its initiation.
Tongue tear fabric testing method on an ADMET eXpert 7600
The tongue tear method is often used to measure the tear force and the tear strength of a fabric specimen. This method, outlined by ASTM D2261, requires a specific sample preparation where the rectangular fabric specimen is cut to form two tongues on each end which are then placed on upper and lower grips. Once tensile force is applied, the fabric specimen will tear along the mid-line between the tongues. Fabrics to be tested using the tongue tear method include those made from acetate, acrylic, cotton, flax, nylon, olefin, polyester, rayon, silk, and wool.
Trapezoidal tear is another tear strength test that uses a specimen prepared as an isosceles trapezoid with a small cut on one side. When testing starts and force is applied, the trapezoid tear produces tension along a reasonably defined course such that the tear propagates across the width of the specimen. This test method applies to most fabrics such as woven fabrics, air bag fabrics, blankets, napped, knitted, layered, and pile fabrics. Refer to ASTM D5587 for more information on running fabric strength testing following the trapezoidal tear method.
Trapezoidal tear testing on an eXpert 7600
Seam strength is the strength of the connecting seams constituting the fabric. The grab test method and the strip test method can be used to measure the seam strength of fabric specimen. ASTM D4884 is the test method recommended for measuring the seam strength of geotextiles. ASTM D751 includes specific sections on seam strength testing and adhesion coating testing, described below.
Adhesion Coating Testing
Adhesion coating testing is applicable to fabrics with an adhesive coating compound applied, forming a chemical bond between the adhesive and the fabric material. The bond strength that is created between the coating compound and the fabric material can be measured running adhesion coating tests. If the adhesion is not strong enough, seam strength will decrease. If adhesion is too strong, problems may arise as tear strength will be affected. Standards outlining the minimum required criteria for specific fabric constructions can be used to ensure both the seam and tear strength are acceptable.
ASTM D751 is the most common testing standard for testing the bond strength of coated fabrics. Prior to placing the specimen in the grip jaws, this test requires a minimum of 3 inches of the adhesive layer to be separated from the substrate. The fabric specimen is then mounted on the upper grip and the peeled layer is placed between the jaws of the lower grip.
Puncture testing of fabric specimens determines the strength of a material by measuring the force required to penetrate the specimen. Contact with sharp edged objects in a real-world scenario is simulated by the use of puncture fixtures. Puncture fixtures are often used in the tensile direction but may also be used in the compressive direction. In order to calculate the specimen puncture resistance, the specimen is first stretched and placed on the ring clamp mechanism of the puncture fixture. Load is then applied by the puncture probe until specimen rupture.
Testing standards such as ASTM D751, ASTM D3787, and ASTM D4833 often specify the diameter of the puncture probe that applies force to the specimen being tested. Puncture fixtures are offered with sharp puncture probes with small diameters as well as with large diameters for ball burst testing.
Shear testing ensures accurate data is collected to analyze the draping, flexibility and handling of textiles which show a wide range of complex deformations, such as woven fabrics. Depending on the fabric material, textile fabrics may show anisotropic behaviors and have different strength values in different directions, affecting the bending and tensile properties in various directions.
The ±45 degree off-axis tension test is recommended for shear characterization of woven fabric composites. Prior to applying load, the test specimen is prepared with strain gauges, one perpendicular and one parallel to the specimen length, and another 45ﹾ off-axis to determine the off-axis modulus of Elasticity, the off-axis Poisson’s ratio, and the shear coupling ratio.
The three major methods for shear testing are shown below:
Recommended Equipment for Fabric Strength Testing
A universal testing machine with constant-rate-of-extension equipped with a stationary bottom grip and with a top grip mounted on the moving crosshead is recommended for fabric tensile testing. The machine should be able to move at a constant speed throughout the test. ADMET recommends the eXpert 7600 single column testing machines for lower force applications and the eXpert 2600 dual column testing machines for high-force capacity applications. All ADMET machines are closed-loop servo control systems, ensuring the speed is kept constant during testing. In addition, they are equipped with the eP2 controller and GaugeSafe software or the MTESTQuattro controller and software, which provide the automation and accuracy required for the standardized tests listed in this post.
Pneumatic vise and manual vise grips are recommended for tensile testing of fabrics. Testing standards often recommend grips with specific jaw material and dimensions.
ADMET offers puncture fixtures conforming to specific standards that will come with the recommended diameter and ring clamp dimensions. In addition, puncture fixtures can be customized for customer-specific applications.
Adding an extensometer to the universal testing machine is often the standard for fabric testing in order to measure the most accurate elongation data. Long travel extensometers such as the EX-700CT and the EX-1200CT are recommended for high-stretch materials while the standard EX-3542 can be used for measuring elongation of other fabric specimen.
Specimen cutting dies may be required in the preparation stage of the specimens. Cutting dies are made following the specific sample dimensions delineated in testing standards.