The Science Behind Fabric Stiffness Testing

By Brian Shiels, Senior Development Engineer, PBI Performance Products, Inc.

Stiffness test

As you’ve probably noticed, there is a recent trend towards lightweight systems for turnout gear. Along with that trend, many fabric suppliers are shifting their focus towards the stiffness, or conversely the flexibility, of their fabrics being offered as part of these new lightweight systems. Anyone can easily perceive if a fabric is extremely stiff or flexible. But there’s a long-standing scientific test method that is used to determine, objectively, even the smallest differences in fabric flexibility. This article will explain how labs are using that test method to assign a standard quantitative value for stiffness to the new fabrics coming onto the market.

ASTM International is widely recognized as one of the leading standards developing organizations in the world. NFPA 1971 currently references 40 different ASTM standards; that’s 10 more references than all other standards developing organizations combined. For that reason, many labs have turned to ASTM standards when looking for a new way to evaluate a fabric property.

To evaluate fabric stiffness, most labs perform ASTM D4032 Standard Test Method for Stiffness of Fabric by the Circular Bend Procedure, which was originally developed in 1981 by ASTM Committee D13 on Textiles. Although the test equipment has evolved over the past 35 years, the principle of the test remains unchanged: to determine the amount of force that is required to push a fabric sample through a round hole. Naturally, a stiffer fabric requires more force to be pushed through the hole, while a more flexible fabric requires less force.

Logistically, performing the test is quite simple. Because the sample is forced through a round hole, this test simultaneously averages the stiffness of fabrics in all directions. As such, and unlike many fabric test methods, this test does not have to be duplicated in both the length and width directions of the fabric. Additionally, this test is applicable to all types of fabrics: wovens, knits, and nonwovens. So, it can be used to evaluate all aspects of firefighter protective clothing: knit hoods, outer shells, moisture barriers, thermal liners, and even all composite layers together.

Stiffness tester schematicThe apparatus used for performing the stiffness test is quite simple as well. Testing is conducted on a 4-inch-square steel platform, with a 1.5-inch-diameter hole, or orifice, in the center. The edge of the orifice is beveled to a 45-degree angle to prevent snagging during the test. Centered above the orifice is a 1-inch-diameter plunger, which is used to force the fabric sample through the orifice. The size of the plunger leaves ¼-inch clearance around all sides. The plunger is mounted to a force-measurement gauge, which records the maximum force applied during the test. Older test equipment might have a manually actuated plunger and an analog (dial-type) force-measurement gauge, while more modern equipment is supplied with pneumatic actuators and a digital force-measurement gauge for better precision. [Figure 1: Schematic of stiffness tester from ASTM D4032*]

To ensure consistent fabric properties day after day, all test specimens are first conditioned to approximately 70 degrees Fahrenheit and 65% relative humidity. Once the fabrics are adequately conditioned, test specimens are cut to 8 inches by 4 inches. To ensure the results are representative of the fabric in question, five individual specimens are cut from different locations on a roll of fabric. Next, the specimens are folded in half, resulting in a 4-inch-square folded test specimen. For consistency in testing fabrics that have a different face and back, samples are always folded so the technical face of the fabric is on the outside.

When a specimen has been prepared, it is placed on the orifice platform and is ready for testing. At this point, the plunger is activated and begins pushing the sample down through the orifice. Once the plunger has forced the fabric through the specimen, the maximum force required to do so is displayed on the force-measurement gauge. In the case of modern digital models, this force can be measured to the nearest 0.001 pounds. Once the force is recorded, the gauge is reset and these steps are repeated until all five specimens are tested. These five results are then averaged and reported as fabric stiffness.

Although it’s important to note that this is not an NFPA requirement, ASTM D4032 is the optimal test to compare fabric stiffness. Comparing the average stiffness of fabrics can help to quantify differences in their perceived comfort. A higher maximum force requirement in this test indicates a stiffer fabric. Stiffer fabrics translate to stiffer garments, which require more force to move around while performing normal firefighting activities. If a garment requires more force to move about freely, the body will naturally expend more energy to perform those same activities. More energy expense leads to quicker exhaustion and greater risk of heat stress. While there is a trend towards lighter-weight fabrics, attention should also be given to the flexibility of these fabrics.

As with all the tests performed on turnout gear, only so much can be replicated in a laboratory. The real test is wearing the garment and assessing the fabric flexibility in the field.

BRIAN SHIELS is the senior development engineer for PBI Performance Products, Inc. Shiels also sits on a variety of technical committees in National Fire Protection Association, ASTM International, and American Association of Textile Chemists and Colorists. He has worked for more than 10 years in testing and development of protective clothing and equipment. He has a BS in chemistry and an MS in textile chemistry.

*Figure 1 is reprinted, with permission, from ASTM D4032-08(2012), Standard Test Method for Stiffness of Fabric by the Circular Bend Procedure, ASTM International, West Conshohocken, PA, 2012, www.astm.org