Shear and yarn pull-out grip for testing flexible sheets by universal load machines

Highlights

• New testing device for shear (hysteresis) and yarn-pull-out tests of fabrics.

• A mechanic pretension system generates and maintains a quasi-constant preload.

• The grip can be mounted to any tensile machine, no need for auxiliary electronics.

• The transversal preload and the clamping distance can be adjusted in a wide range.

• Various textiles and flexible sheets can be tested rapidly and precisely.

Abstract

The purpose of this research was to develop a novel, multifunctional apparatus that makes possible to carry out two common tests of woven fabrics and flexible sheet-like materials, namely the shear and the yarn-pull out test. We designed an apparatus that can be mounted on a universal load machine and makes possible to test the materials rapidly and precisely.

In this paper we introduce the apparatus and the related simple shear and yarn pull-out test methods, as well as the accuracy and reproducibility of the test results obtained. We carried out cyclic shear and yarn pull-out tests on plain and panama weave materials. We found that the relative deviations of the common shear (G, 2HG, 2HG5) and yarn pull-out parameters were around 5–9% in most cases that confirms the repeatability of the test method. With our method, one can carry out these tests without an expensive, dedicated test device.

Introduction

At human environmental textiles, at flexible sheets & composites (e.g. canopies), and at composite reinforcements it is necessary to know their various mechanical properties in order to be able to design and create complex structures [[1], [2], [3], [4]].

In the former cases the textile or the flexible sheet has to suit to the shape of the body or adequate draping properties are required for the appropriate optical performance. At composite reinforcements, the textile has to conform the shape of the mold. In all these cases, besides the good handling possibilities, appropriate shear properties, flexibility and the role of the friction between the fiber bundles or yarns have high importance (and moreover these things also influence one another) [[5], [6], [7]].

The method for investigating the shear properties of textiles and other flexible sheets has to be simple and rapid to fit the current requirements of the industry. There are widely-known methods making available to determine the shear properties, but these are either complicated, or the stress state, generated by the test setup, is not ideal [8,9].

In case of woven textiles maybe the simplest method is when the specimen in bias direction (the weft and warp yarns are located in ±45° directions to the axis of pulling) is tested by a tensile tester [8]. As the endings of the yarns are not constrained in the shear zone, therefore the shear state is suitable, but complex. The problem is with the determination of the exact location of the pure-shearing zone and as the shear deformation is calculated from the strain of the whole sample it is hard to obtain precise results. The method can be further improved by image processing as Domskiené and Strazdiené [10] and Al-Gaadi and Halász [11] demonstrated. This method is more precise but maybe too complex for industrial purposes and besides, it does not make possible to carry out cyclic tests that is a significant disadvantage.

The other widespread method, when the textile or the canopy sample is fixed to a picture frame that has knuckle-joints in the corners. The two opposite corners of the frame are displaced by the load machine thus the square becomes a rhombus the way the specimen is sheared. This method makes possible to make cyclic tests. The method is simple but not precise because the stress state is not ideal near the clamping bars hence the yarns are bended [[10], [11], [12]]. Orawattanasrikul [13] elaborated a special method to avoid the bending of the yarns during tests. The edges of the textile is not clamped, but pinned into needles. These needles make possible to transfer the shear forces, still makes possible the rotation of the yarns, therefore their bending can be neglected.

The Kawabata's Evaluation System for Fabrics (KES-FB) [14] can also be used for determining the shear properties of flexible sheets and textiles. At this method two parallel 200 mm long sides of a rectangular textile sample are clamped and one of them is moved to shear the specimen (Fig. 1/a) by an F_t [N] force. The initially 50 mm distance of the parallel clamps (X₀) gradually changes (X(t)) as a constant transversal pretension is applied during the test. The system is precise and the experiment can be carried out rapidly. The pretension (f_p [N/m]) is generated by a rotating drum and the requisite displacement is calculated from torque measurements that requires additional electronic control devices. As the device only operates in the ±8° shear angle range, therefore the bending of the yarns along the two edges can be neglected.

The totally automated, computer controlled KES-FB device does a cyclic shear test. During the test the shear motion changes direction as the shear angle reaches +8° or −8°. The schematic of the measurement and a typical specific shear force – shear angle diagram can be seen in Fig. 1. Based on the diagram, the so-called Kawabata parameters can be determined: the hysteresis of shear force at shear angle of 0.5° (2HG [N/m]), the hysteresis of shear force at shear angle of 5° (2HG5 [N/m]), the shear rigidity, calculated from the mean slope of the curve in the region between a shear angle of 0.5° and 5° (G [N/(m°)], respectively).

The yarn pull-out test is also a widely researched topic. Various measuring devices were made to do these kinds of tests, but all of them are one of a kind. Currently, the commonly accepted yarn pull-out measurement device is unknown.

The yarn pull-out test can be used to determine the interaction between the yarns of woven fabrics. During the test, the force needed to overcome the mostly frictional connection between the yarn to be pulled out and the crossing yarns is measured. The interaction between the yarns has a fundamental role on the material properties. It influences the shearing, bending, wrinkling, etc. properties, moreover the impact strength and the energy absorbing capacity of woven fabrics, hence it is very important to know this interaction, especially for modelling their mechanical behavior [15,16].

The principle of the yarn pull-out test can be seen in Fig. 2, while Fig. 3 shows a typical result of a single measurement. During the test, the rectangular woven fabric specimen has to be clamped on both sides, then a single yarn is pulled out along the centerline in parallel with the clamps. It is important that the upper end of the yarn is clamped and pulled, the other end of the yarn has to be able to move free. At the first, static stage of the yarn pull-out test (Fig. 3) due to the increasing pull-out force, the yarn being pulled-out straightens out and stretches, which leads to shear deformation on both sides of the yarn. The specimen itself deforms leading to a displacement (d_t). The second, kinetic stage starts when the pull-out force reaches the peak value and overcomes the static friction force among the yarns. At this moment the yarn starts passing through the crossing yarns, leading to the increasing displacement of the yarn (d_y). The pull-out force (F_po) is gradually decreasing with periodic waves and tending to zero as the yarn passes through more and more transversal yarns. The magnitude of F_po also influences the vertical deformation of the specimen itself (d_t) that also decreases during this stage of the test.

In the case of another, modified test setup the free end of the pulled yarn overhangs the bottom edge of the fabric sample [18,19]. Fig. 4 shows a typical pull-out curve when the overhang was 50 mm. Due to this modification the kinetic phase of the yarn pull-out divides in two more phases. In the first region of the kinetic phase the yarn to be pulled out passes through the same number of crossing yarns, hence the end of the yarn does not reach the bottom edge of the fabric sample. In this phase the mean value of the periodic tensile force after a quick decrease sets at an approximately constant value. When the end of the yarn to be pulled out reaches the bottom edge of the fabric sample, the second region of the kinetic phase starts. This time the pulled yarn passes through fewer and fewer crossing yarns, hence the mean value of the periodic tensile force decreases and tends asymptotically to zero.

Several papers have been published on the yarn pull-out test and its results [11,[17], [18], [19], [20], [21], [22], [23]]. Using the capstan equation some [11,16] tried to determine the coefficient of friction among yarns from the yarn pull-out test results. However, the accuracy of the applied methods has not been proved yet. Knowing the maximal yarn pull-out force and the tensile strength of the yarn, Pan and Yoon [22] determined the critical yarn length. The critical yarn length is similar to the critical fiber length widely used in composite mechanics [24]. The critical yarn length equals the half of the critical fiber length and it means the smallest length of the embedded yarn that breaks due to the load instead of slipping out of the fabric.

Yarn pull-out test is not a standardized test, therefore it does not include a standard measuring apparatus. Researchers usually build different clamps for this purpose, but these are typically only U-profile static clamps that are only suitable for yarn pull-out tests. Hence, many times they try to deduce the shear properties of fabrics from yarn pull-out tests [25,26].

To determine the shear properties of woven fabrics, we have previously described some well-known and widely used methods, including the Kawabata shear test. This measurement can be only carried out with the KES-FB1-A Tensile and Shear Tester, which is an expensive, stand-alone device for testing the tensile and shear properties of fabrics and from time to time it requires maintenance and calibration.

In our study, the aim was to design and construct an apparatus that makes possible to combine the simplicity of the above-mentioned test methods and the precision of the KES-FB system in order to determine the shear properties of textiles, textile composites and other flexible sheets. A grip that makes possible to maintain transversal pretension of the textile during the tests akin to the KES-FB shearing system was designed. Moreover, the same apparatus can be used for yarn pull-out tests of woven textiles. The pretension is applied by clearly mechanical ways during the tests leading to a simple, easy-to-build, symmetrical construction. There is no need for a complete device dedicated to only shear or yarn pull-out experiments as the grip can be mounted on almost any kind of universal load machine. With the apparatus the tests can be carried out rapidly and the evaluation of the results can be done with the aid of the software of the load machine. In this paper we demonstrate the applicability of this measurements and the accuracy of the results in case of both yarn pull-out and shear tests through some examples of technical materials.

In the current paper we describe the construction of the new apparatus we designed, as well as the shear and yarn-pull out tests we made. Shear tests (with the same principle of the KES-FB system) were made on a glass fabric, a polyester fabric and a canopy material, yarn-pull out test were made on another glass fabric and on a viscose fabric, moreover we introduced new parameters for the yarn-pull out test for the detailed evaluation of the results.