Measuring and testing – Sheet – woven fabric or fiber
Reexamination Certificate
1999-10-27
2002-12-03
Noori, Max (Department: 2855)
Measuring and testing
Sheet, woven fabric or fiber
C073S826000
Reexamination Certificate
active
06487902
ABSTRACT:
TECHNICAL FIELD
The present invention relates to tensile testing of membrane materials, and more particularly to an apparatus and method for biaxial tensile testing of membrane materials.
RELATED ART
Traditionally, the tensile properties of fabrics are evaluated in uniaxial testing in which force-deformation response of a fabric is measured along one of the major axes (machine or cross). Results of uniaxial tests produce indices of relative tensile behavior but are inadequate for many applications. However, in actual use, the textile fabrics rarely experience true uniaxial forces. In most instances, the forces are imposed simultaneously in more than one direction resulting in responses that are quite different from that under uniaxial force. Since most fabrics or orthotropic membranes possess two principal directions and the forces or deformations in most cases can be resolved into two orthogonal components, it is important to understand their behavior under two dimensional force or deformation. Application of forces or extensions simultaneously along two orthogonal axes is referred to as “biaxial”.
Often, the objective of tensile testing of fabrics is to characterize their nature of failure in terms of breaking elongation and strength. However, for many applications it may be more important to find out the relationships between applied stresses (or strains) and resultant strains (or stresses), e.g.,
&sgr;
x
=f
1
(&egr;
x
,&ggr;
y
,&ggr;
xy
),&sgr;
y
=f
2
(&egr;
x
,&egr;
y
,&ggr;
xy
),&sgr;
xy
=f
3
(&egr;
x
,&egr;
y
,&ggr;
xy
)
where &sgr;
x
and &sgr;
y
are stresses in machine and cross directions, respectively, and &sgr;
xy
is the shear stress. Similarly, &egr;
x
and &egr;
y
are strains in machine and cross directions, and &ggr;
xy
is the shear strain. These constitutive relationships between stress-strain parameters are well known for linear orthotropic materials. However, the stress-strain relationships of textile fabrics are far more complex and non-linear.
The need to measure the constitutive laws of fabric behavior, in addition to studying their nature of failure, has received considerable attention in the literature. A number of studies have been reported in the area of theoretical modeling of fabric deformation under uniaxial as well as biaxial tensile forces. Other researchers have reported various experimental methods of evaluating fabric behavior under biaxial tensile forces.
Multi-directional test devices have been described by Ariano (“Rubber Stretched by Forces in Two Directions Perpendicular to one Another”,
Rubber Chemistry and Technology
, 13, 92-102 (1942)) as early as 1942. He developed a static two-dimensional tester for rubber films. Anderson (“A Method for Obtaining Stress-Strain Relations in Non-Isotropic Flexible Sheet Material Under Two-Dimensional Stress”,
Journal of Scientific Instruments
, 24, 25 (1947)) and Boonstra (“Stress-Strain Properties of Natural Rubber under Biaxial Strain”,
Journal of Applied Physics
, 21, 1098-1104 (1950)) used a pressure cylinder apparatus suitable for impermeable sheet materials. Treloar (“The Swelling of Cross-Linked Amorphous Polymer under Strain”,
Transactions of Faraday Society
, 46, 783-789 (1950)) developed an apparatus for imposing four-directional planar strains in rubber, but without any provision for measuring forces. The two-dimensional force-extension tester reported by Reichard, Woo, and Montgomery (“A Two Dimensional Load-Extension Tester for Woven Fabrics”,
Textile Research Journal
, 23, 424-4248 (1953)) and later used by Woo, Dillon, and Dusenbury (“The Reaction of Formaldehyde with Cellulosic Fibers, Part II: Mechanical Behavior”,
Textile Research Journal
, 26, 761-783 (1956)) is a modified uniaxial tensile tester that was designed to extend a fabric along two perpendicular directions. However, the instrument could measure the force developed in one direction only under moderate levels of strain. Checkland. Bull, and Bakker (“A Two-Dimensional Load-Extension Tester for Fabrics and Film”,
Textile Research Journal
, 28, 399-403 (1958)) reported a two-dimensional force extension tester in which a four-jaw, self-centering lathe chuck was used as the straining device. The sample size was quite small, as was the strain level.
All these devices were equipped with solid clamps that did not allow any deformation along the clamps, a critical requirement, for large or finite deformation. Additionally, none of the above investigators either defined or examined the role of control variables in interpreting their experimental results. Of the multi-directional testers already reviewed, only two were specifically designed for and readily applicable to the laboratory testing of fabrics. In both cases, the clamps were moved at constant rates to apply strains in the sample. However, as Klein (“Stress-Strain Response of Fabrics underTwo-Dimensional Loading, Part I: The FRL Biaxial Tester”,
Textile Research Joumal
, 29, 816-821 (1959)) pointed out, in both cases neither the forces nor the extensions are controlled in the central region of the test specimen. Thus, while general trends in biaxial behavior can be determined from these machines, considerable generality is lost and quantitative comparison of different materials is rather impossible.
Klein described a biaxial tester with solid clamps and proposed a set of design criteria for a biaxial tester based on sound theoretical analysis. He pointed out that in a biaxial testing system, either both forces or both extensions must be specified or controlled while the other should be measured. Klein chose forces as independent variables because of his particular requirements. He then concluded that a biaxial test could be considered completely controlled and reproducible by the control of a single factor, i.e., the ratio of forces or extension in the two directions. Klein designed his instrument to measure force-extension behavior of a cruciform sample (2 in.×2 in.) under different levels of force-ratio.
Freeston et al. (“Mechanics of Elastic Performance of Textile Materials, Part 18. Stress-Strain Response of Fabrics under Two Dimensional Loading”,
Textile Research Journal
, 37, 948-975 (1967)) in 1967 reported a biaxial tester developed by the Air Force Materials Laboratory. The load-frame of the test system described by Freeston et al. was similar to Klein. However, the clamping system of this instrument was designed to rotate freely about an axis perpendicular to the plane of the test specimen to permit shear deformations.
An important factor to be considered in biaxial testing of fabrics or other similar membranes is the distribution of stress and strain in the test specimen. Perfect homogeneity of strain in a test device is almost always unattainable. However, test instruments must be designed to minimize the variations of stress and strain. None of the prior art instruments described thus far allow for homogeneous distribution of stress and strain. In testing a flat specimen of a membrane/fabric, it is necessary to allow the specimen to undergo tensile strain in the direction along each clamp. In the vicinity of the solid clamps, these strains are not allowed to develop. Consequently, these “boundary effects” become particularly severe in cases of small samples.
A number of testers reported in the literature have provided for deformation along each clamp. Two of these, Rivlin et al. (“Large Elastic Deformations of Isotropic Materials, VII: Experiments on the Deformation of Rubber”,
Philosophical Transactions of the Royal Society
, 243, 251-288 (1951)) and McRory et al. (“Experimental Investigation of the Biaxial Load-Extension Properties of Plain, Weft-Knitted Fabrics”,
Textile Research Journal
, 47, 233-239 (1977)) applied stresses with what was essentially point contact, while Sakaguchi et al. (
Journal of the Society of Materials Science
, Japan, 17, 365 (1968)) and Kawabata et al. (“The Finite-Deformation Theory of Plain-Weave Fabrics, Part 1: The Biaxial-Deformation Theory”,
Joumal of The Textile Institute
, 64, 21-46 (
Jenkins & Wilson, P.A.
Noori Max
North Carolina State University
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