Grab and Strip Tensile Strengths for Woven Fabrics: An Experimental Verification

In the present study we have attempted to verify a theoretical model of grab versus strip strengths of fabrics [Textile Research Journal, 2003, 73(2), 165–171] through experimental investigation. Five different fabrics were included in this study, and three related parameters were investigated including the modulus ratio of the specimen (shear versus tensile moduli), the specimen gauge length and the ungripped specimen width. The ratios of grab versus strip tensile strengths were computed from the experimental results and compared with the values predicted from the theoretical model. The predicted results are compatible with the measured values, suggesting that the theoretical model is capable of estimating the tensile strength of a grab specimen. Tensile strength is one of the most important mechanical properties for woven fabrics. To quantify the tensile strength of a piece of fabric, two testing methods are often used, namely the grab test and the strip test. Each testing method has its own advantages and drawbacks. Specimens in the grab test are easier to prepare, and the testing condition is closer to the load application on a fabric in practical use. However, the results of the grab test may not be as accurate and interpretable as those of the strip test, but the preparation of unraveled strip specimens usually takes up time [1, 2, 3]. Both testing methods have been standardized as the ASTM standard D5034-95 for the grab test and D5035-95 for the strip test, respectively. Given the wide application of both testing methods, it is desirable to establish the relationship between these two methods from both theoretical and experimental viewpoints. A few studies have been reported work towards establishing the relationship between the grab and strip tests. These early investigations attempted to explore the relationship from empirical approaches [2, 5]. However, the breaking mechanisms and physical implications involved cannot be obtained from those studies. Recently, Pan [3] conducted a theoretical investigation to relate the grab and strip tensile strengths of a fabric. In his model a grab specimen is basically divided into two portions, the gripped part held by the machine grips, acting essentially like a strip specimen, and the ungripped parts on each side of a grip, as shown by Figure 1a. A herringbone deformation mode as seen in Figure 1b was adopted for the ungripped portions caused by the tensile load during the test. According to continuum mechanics, the shear forces within the herringbone elements contribute to the generation of tensile stress in the ungripped portion. With the assumptions of roughly linear mechanical behavior of a fabric specimen as well as the negligence of the Poisson effect, the tensile stress in the ungripped portions can be expressed in terms of gauge length, ungripped specimen width, machine clamp width, and tensile and shear modulus of the specimen, among other variables. The overall tensile strength for a grab specimen can thus be calculated as the combination of the contributions from both ungripped and gripped parts. In other words, the tensile strength of the ungripped portions obviously determines the difference between the grab and the strip tensile strengths for a fabric specimen. In the present study we attempted to verify the predictions of Pan’s model [3] through experimental investigation. According to the theoretical predictions, we first studied the influence of the tensile and shear moduli by measuring five different fabrics with the sample size determined from the aforementioned two ASTM standards. Then, we chose one fabric type and investigated the effects of the specimen gauge length and the ungripped specimen width, respectively. The ratios of the grab versus strip tensile strengths were computed from the experimental results and finally compared with the values predicted from Pan’s model [3]. To whom correspondence should be addressed: tel.: 1-530-7526232???, fax: 1-530-752-7584???; e-mail: [email protected] Current affiliation: Division of Kinesiology, University of Michigan, 401 Washtenaw Ave, Ann Arbor, MI 48109, U.S.A. NOVEMBER 2005 789 Textile Res. J. 75(11), 789–796 (2005) DOI: 10.1177/0040517505057525 © 2005 SAGE Publications www.sagepublications.com Predictions from the Theoretical Model Figure 1a shows the schematic diagram of a grab specimen with the length of 2w and the width of 2h. The machine clamps holding the specimen are 2b wide and a long. The overall tensile strength for the specimen is contributed from both the gripped portion held by the machine clamps and the ungripped portions including both top and bottom parts. The herringbone deformation is assumed for the ungripped portions of the specimen subjected to an external tensile load. The overall tensile force from both the top and bottom ungripped portions of the specimen was derived in detail by Pan in [3]