Comparison of picture frame and Bias-Extension tests for the ...

Fibers and Polymers 2013, Vol.14, No.2, 338-344

DOI 10.1007/s12221-013-0338-6

Comparison of Picture Frame and Bias-extension Tests for the Characterization of Shear Behaviour in Natural Fibre Woven Fabrics I. Taha*, Y. Abdin1, and S. Ebeid Faculty of Engineering, Ain Shams University, Cairo, Egypt Mechanical Engineering Department, The British University in Egypt, Cairo, Egypt (Received March 2, 2012; Revised July 21, 2012; Accepted July 28, 2012)

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Abstract: The investigation of the shear behaviour of technical natural fibres is vital for the insurance of aesthetics and performance of light weight, high strength, and eco-friendly composites. In this study, Egyptian jute fibre plain weave fabrics of various areal densities were investigated to describe their shear behaviour in terms of shear forces, shear angles and shear lock angles, using the Bias-Extension and the Picture Frame test methods. Results show that both methods are valid for natural fibres and produce comparable results. Whereas the Bias-Extension test presents a fast and simple test procedure, the analysis of the results is more complex due to the interaction of non-shear components. On the other hand, the Picture Frame test proves to be time consuming and in need of a more complex test rig, but results in pure shear deformations throughout the sample. Keywords: Fabric textiles, Polymer-matrix composites, Textile composites, Shear behaviour, Draping behaviour

and hence a relatively low in-plane shear resistance [8]. Although this deformation mechanism can accommodate large fibre rotations, at some point, the rotating tows contact each other restricting further shear and leading to subsequent lock up (lock angle) [6,9-11]. The shear resistance increases rapidly when the lock angle is reached, and the assumption of the negligible shear resistance (easy deformation) is no longer valid [11]. While the shear lock angle separates the low and high stiffness parts of the curve, it is not very precise for many fabrics which depict a progressive increase in stiffness [6]. Although there is a considerable amount of literature analysis [6,10,12-23], of the shear behaviour of fabrics, aiming at developing prototype tests, there is no set standard test method for determining the in-plane shear behaviour of textile reinforcements. Nevertheless, there are three main known techniques used to measure fabric shear compliance and locking angle, namely the Direct Shear Force Measurement (DSFM) method, the Bias-Extension (BE) test method, and

Introduction The use of fibre reinforced composites has gained a considerable amount of recognition within the last decades. The variations in fibre types (glass, carbon, natural, etc.), shape dimensions and orientation have led to the development of a vast basis for selection allowing for the flexible tailoring of end product properties. Since all processing techniques of fibre reinforced polymer composites are related to moulding operations, the deformation behaviour of the textiles over an open mould or within a closed mould is vital for the guarantee of the aesthetics and performance of the end product. Woven fabric is the most common continuous dry textile material form, where the specific weave pattern affects its deformability characteristics, as well as the handleability and structural properties. The deformation behaviour of textiles, known as “drape”, is considered an important mechanical property that affects the functionality of fabrics [1-5]. The definition of drape has been largely discussed in the literature. Recently, draping has been defined as “the extent to which a fabric will deform when it is allowed to hang under its own weight” (BS 5058:1973; British Standard Institute, 1974b) [1,2]. The draping or deformation behaviour of the woven fabrics is greatly associated with the relative motion of yarns, thus primarily depending on the shear behaviour of the fabric, which determines its performance properties when subjected to a wide variety of complex deformations, as well as its conformance to the required shape. Pure shear consists of a trellising action, whereby the tows in the fabric rotate about the cross-over points [6,7]. Textile reinforcements are favoured for draping over complex surfaces as they offer possibility for large rotations between warp and weft yarns

Figure 1. Different methods for measuring the shear properties of textiles: (a) direct shear force measurement, (b) bias-extension test method, and (c) picture frame test method.

*Corresponding author: [email protected] 338

Shear Behaviour in Natural Woven Fabrics

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acceptable description of their shear behavior for subsequent modeling and simulation of the final composite. The urge of this study results from the rising needs for lighter, costefficient eco-friendly reinforcements for polymer composite applications in every-day products, on the one hand, and the missing knowledge on the draping behavior of natural fibres and the adequacy of applying current test methods for this group of materials, on the other hand.

Materials and Methods Materials Five commercially available jute woven fabrics were supplied by El-Manasra, Egypt. The fabrics are all of the plain weave type and show measured variations in density and weave dimensions as listed in Table 1.

Figure 2. Deformation zones in an idealized bias-extension test sample [6,9,11].

the Picture Frame (PF) test method, as schematically presented in Figure 1. Authors reported the complexity of realizing the test conditions for the DSFM technique in addition to the overlap of both tensile and shear stresses throughout the fabric, which makes further analysis more complex, easily leading to false conclusions. In contrast, the BE technique is widely used since the uniaxial tension of a bias-cut fabric is relatively simple, does not require special equipment and can be carried out on any extensometer. However, sample length-to-width ratio must be greater than 2, in order to obtain a pure shear zone where the restraining effects of the clamps become insignificant [13]. Moreover, there are typically three deformation zones involved in the shear force estimation, as depicted by Figure 2. Therefore, normalization of the obtained forcedeformation curves is necessary to get useful shear characterization curves. The main advantage of the PF method is the uniformly induced shear in all areas of the fabric sample, allowing pure shear force values to be directly obtained. The limitation of the test method, however, is the time consuming preparation of a suitable frame and clamping of the fabric sample onto it, taking care to ensure perfect fibre alignment. A collaborative benchmark effort [24] was initiated in 2004 to compare between the results obtained from both PF and BE tests. Although there were consistencies in the data provided, it was concluded that there were still significant deviations that require further understanding before a standard test procedure could be developed. In this study, the BE and PF tests are conducted on natural fibre woven fabrics and compared to each other, targeting an

Bias-Extension Test Rectangular specimens of 300×75 mm2 were tested on a Lloyd LRXPLUS universal testing machine. Samples providing a gauge length of 200 mm were clamped onto the machine in such a way that the warp and weft directions of the tows are oriented initially at ±45 o to the direction of the applied tensile force as schematically illustrated in Figure 1(b). Testing was performed with a 2.5 kN load cell at a crosshead speed of 50 mm/min up to the termination at first failure signs. A total of five samples was chosen for the test to represent a broad area of each of the supplied fabric rolls. The measured shear force in this method does not only reflect pure shear deformations, giving rise to the necessity of normalizing the resulting force-deformation curves. Picture Frame Test Samples cut to the shape and dimensions illustrated in Figure 3(a) were clamped to a square frame of 145×145 mm2 at an initial tow orientation of 0/90 o to the frame arms. The hinged frame (Figure 3(b)) was further mounted onto the universal testing machine Lloyd LRXPLUS and subjected to tensile force at a crosshead speed of 50 mm/min with a 2.5 kN load cell. The test was terminated at 100 mm displacement. Again, five samples were chosen for the test to represent a broad area of each of the supplied fabric rolls. In contrast to the Bias-Extension test the resulting force-displacement Table 1. Jute woven fabric properties Material type NF_192 NF_241 NF_250 NF_272 NF_616

Fibre diameter (µm) 1311 1368 1129 1333 2553

Yarn count (tex) 340.8 371.6 288.9 311.1 770.4

Areal density (g/m2) 192.12 240.95 250.47 272.38 615.75

Tensile modulus (MPa) 13.4 20.9 25.5 27.5 2.5

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curves directly describe uniform shear behaviour within the fabric, as schematically presented in Figure 1(c). Shear Lock Angle Determination The shear lock angle was determined graphically as the inflection point of the shear compliance curve [12]. This can be identified by fitting linear trends to the first and last five degrees of data points. The interception point represents the shear lock angle.

Results Bias-Extension Test Results Figure 2 shows an idealized test sample under bias extension deformation. When the gauge length is stretched from H to H+d, the sample can be divided into three different regions as follows [6,9,11]: Region A: the warp and weft have free yarn ends resulting in pure shear deformation in this zone, related to the global sample elongation d. In Region B one yarn direction is clamped at one end, the other direction is free at both ends. The global stretching of the specimen

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leads to half the shear deformation (half the shear angle) in this zone compared to Region A. In Region C the warp and weft yarns have clamped ends and there is no deformation in this zone. In this respect, the deformation in Region A can be considered equivalent to the deformation produced by the pure shear of a picture frame [25]. Figure 4(a) presents the resulting force-displacement curves for several NF_241 samples. It can be observed that the test leads to satisfactory reproducibility of the results. For further analysis, the various curves are averaged into one forcedisplacement curve to remove slight deviations, such as to simplify subsequent calculations. Figure 4(a) also shows that beyond a displacement of around 60 mm, the sample starts to slide and slip within the holding clamps. This can be considered a disadvantage of the Bias-Extension test in contrast to the Picture Frame test. This phenomenon was also reported in the literature [26,27]. Figure 4(b) shows a comparison of the force-displacement curves resulting from the Bias-Extension test for the various jute fibres under consideration. It can be observed that the higher the fabric density, the greater the force required

Figure 3. Picture frame (a) sample shape and size, (b) assembly, and (c) resulting shear deformation.

Figure 4. Force-displacement curves resulting from bias-extension tests for (a) various NF_241 samples and (b) various types of jute fibres.



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Figure 6. Picture frame test (a) at the beginning and (b) at the end of the test.

Figure 5. Normalized shear forces versus shear angles resulting from bias-extension and picture frame tests.

accomplishing deformation. This is comprehensible with respect to the higher freedom given by the low density fabrics to the individual yarns to move past each other, thus allowing greater deformations at lower values of axial force. Simple kinematic analysis [17] as defined by γ = 90 – 2θ –1 =90 – 2cos (L0 + d/ 2L0) of the bias extension sample (Figure 2) provides the shear angle γ in Zone A, as a function of fabric size, and the cross-head displacement d, where Lo = H-W, and H and W are the original height and width of the specimen, respectively. In a subsequent step, a normalization of the BiasExtension axial force results is necessary to describe the pure shear deformation behaviour in the active zone A. Assuming uniform shear angles in zone A, normalization is achieved according to Cao [28], using the relationship Fsg(γ) = H γ 1 --------------------------------- ⎛----- – 1⎞ × F × ⎛cos--γ- – sin ---⎞ – W × Fsh⎛ --γ-⎞ cos --γ⎝ 2 ⎝ 2⎠ 2 2⎠ (2H – 3W)cosγ ⎝W ⎠

where F is the torque exerted on the machine load cell and Fsh the measured shear force. Figure 5 shows the shear force curves obtained from normalization of the bias extension results. The figure shows that the normalization stage did not have any influence on the general trends observed in the force-displacement curves of Figure 4(b). Picture Frame Test Results The Picture Frame test was conducted on the various natural fibre fabrics as can be depicted in Figure 6, showing the beginning and end of test. The measured force-displacement curves are further presented in Figure 7(a). These show that despite difficulties in sample fixation into the frame and the resulting human error leading to variations in initial clamping forces, it is possible to achieve reproducible results. Again, for further calculations and analysis these curves were merged into one average force-displacement diagram, as presented in Figure 7(b). It becomes clear that the force needed to accomplish shear deformation in the fabric is small at the beginning of the test and witnesses a continuous rapid increase at larger displacements, as also observed by Hamila and Boisse [6]. Closer examination of the resulting curve allows its division into

Figure 7. Force-displacement curves (a) resulting from picture frame tests for various NF_241 samples and (b) merged into one average characteristic curve.

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two distinct zones [6,9]. The first region describes the beginning of the test, where deformation is mainly governed by the relative rotation of the neighbouring yarns and is limited by the friction between warp and weft yarns, resulting in forces that are almost negligible. At larger displacements, the second stage describes a different behaviour, where the fabric warp and weft threads begin to pack tightly until the geometry of the weaving does not allow any further rotations, thus resulting in a steep increase in force and fabric stiffness. This phenomenon is often referred to as “shear locking”. Assuming that the shear deformation across the sample is homogeneous and that the shear angle of the fabric is the same as that exhibited at the frame hinges, the axial forcedisplacement curves presented in Figure 7 can be converted into shear force versus shear angle applying [11] cosθ = 2Lframe + d/2Lframe and Fsh = F/2cosθ , where θ is the frame angle and Fsh the resulting shear force. The shearing angle γ is commonly assigned a value of zero at the initial stage when weft and warp yarns are perpendicular to each other [28]. Thus, the shear angle γ was calculated from the geometry of the picture frame depicted in Figure 3(c) as given by γ = 90 – 2θ . Figure 5 shows the relationship of the shear force and shear angle calculated for all fabrics under investigation. All fabric types exhibit very close values of Fsh at the beginning of deformation. However, with further deformation, the values start to largely deviate. The low density natural fibre woven provides the least shear resistance. A gradual increase in shear force can be observed with increasing areal density of the fabric. Using these shear compliance curves, the shear lock angle can be graphically determined as indicated in Figure 8, as suggested by Scouter [12], for the case of the NF_241 fabric.

Figure 8. Graphical determination of the shear lock angle α=γlock for the NF_241 fabric resulting from the picture frame test.

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Figure 9. Shear lock angle α for the various fabrics under investigation resulting from the picture frame test.

Figure 9 further illustrates the resulting shear lock angles for the other fabric types under investigation. It can be observed that the shear lock angle values are inversely correlated to the trends indicated by the shear force in Figure 5, where generally fabrics showing low shear resistance have high lock angles, indicating the occurrence of wrinkling at higher deformations. Thus, the lower the textile density the easier the shear deformation and the more postponed yarn interlocking occurs, which is comprehensible in terms of the greater flexibility of movement and rotation for the individual yarns.

Discussion The force-displacement curves resulting from the BiasExtension and the Picture Frame test, as presented in Figures 4(a) and 7, respectively for the case of the NF_241 fabric, show very similar orders of trends, indicating the validity of both methods. Furthermore, the shear behaviour of the investigated fabrics, as described by both techniques, is presented vis-à-vis in Figure 5. Whereas the Bias-Extension test makes use of a simple rectangular shaped specimen were the weft and warp tows are oriented in ±45 o to the loading direction, the Picture Frame test requires a special shaped sample (Figure 3(a)) that is further to be pinned into a frame. The design and clamps of the frame must be carefully accomplished in order to avoid excessive forces at the pinned fabric ends. Hence, the complexity of the Picture frame test lies in the development of the frame and the time consuming pinning of each specimen. The current study shows that with careful implementation it is possible to obtain reproducible and descriptive test curves. The theoretical equation for the determination of shear force components and shear angle


was proven to be in agreement with actual angle measurements between the displaced tows. These measurements were conducted on still pictures taken at certain intervals of the shear test. On the other hand, complexity of the Bias-Extension test lies in its analysis. When subjecting the woven specimen to shear deformations 3 zones of zero-shear, mixed tensionshear, and pure-shear deformations. Therefore, a normalization of the resulting axial forces becomes necessary, in order to eliminate all non-shear factors. The use of related kinematic equations resulted in shear behaviour of good agreement with those obtained from the Picture Frame test, as depicted in Figure 5. However, it was found that the Picture Frame shear trends are higher than those of the Bias-Extension, which was also reported in literature [22,29,30]. This observation is interpreted based on the setup of the PF test where fibre yarns are rigidly clamped on the frame. It is assumed that taking the geometry and kinematics of the picture frame test into account, resulting tension on fibre yarns would remain small during the test. However, Launay et al. [29] reported that this aspect is questionable, especially that the tensile stiffness of fabrics is high. Thus a small tensile strain of the yarns would lead to significant strain energies on the woven fabric. From Figure 5 it can be further observed that the higher the tensile strength of the fabric, the higher the deviation between PF and normalized BE graphs. This again validates the argument of the existence of tensile forces acting on the PF sample during the test.

Conclusion The shear behaviour of variously dense natural Jute fibre woven fabrics is described by axial force-displacement curves using the Bias-Extension and more complex Picture Frame test methods. An important outcome of this work is the adaptability of both test methods for natural fibre examination, although such tests have been developed on the basis of synthetic fibres having different surface textures. Comparable results are obtained by the use of both methods. However, test simplicity and validation of the normalization equation for natural fibres suggest the use of Bias-Extension method. Due to the effect of extreme clamping conditions occurring in the frame rig causing stretching and tension of the fibre yarns, shear force measurement is partially affected by the tensile behaviour of the samples and does not reflect the pure shear resistance of fabrics. The method suggested by Scout [12] for the determination of the shear lock angles in woven textiles needs to be further studied. This method is based on the graphical detection of the inflection point in the shear compliance curves, which is as much vulnerable to human error as the other common method based on the visual identification of the onset of fabric wrinkling during the test procedure.


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The contact angle, however, remains an important property for the designer and composite manufacturer, due to the complexity of fabric behaviour close to the extreme states when draped over 3D structures. The current study confirms previous researches in that the resistance to shear increases with increasing areal density of the fabrics due to the relative restriction of the tows to motion.

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