Measuring and testing – Specimen stress or strain – or testing by stress or strain... – Specimen clamp – holder – or support
Reexamination Certificate
1999-06-28
2001-01-23
Noori, Max (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
Specimen clamp, holder, or support
Reexamination Certificate
active
06176142
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to composite coated jaw faces for testing clamps or grips, specifically an improved surface which enables testing at lower clamp pressures and provides higher overall testing accuracy and reproducibility.
TECHNICAL BACKGROUND
The use of constant-rate-of-extension (CRE) tensile testing machines to determine fiber and fabric properties is well known to those skilled in the art. These machines (e.g., those made by the Instron Corporation, Canton, Mass.; MTS Systems Corp., Eden Prairie, Minn.; United Calibration Corp., Huntingdon Beach, Calif.) have clamps or grips which are designed to grasp the ends of the material (fiber, fabric, etc.) to be tested. There are many different types of clamps or grips, and the selection of the one best suited to the testing procedure is described in supplier literature (see for example “Guide to Advanced Materials Testing”, Instron Corp., CAT-8701, pp. 131-140) as well as various ASTM methods (see for example ASTM D2261-96).
The clamps are designed to provide enough compressive force to securely hold the ends of the material or fiber to be tested, so that when the sample is stretched or flexed, for example, the ends do not pull out of the clamps nor are they sheared, but rather remain in place. The clamps on the CRE instruments are generally made of steel and other materials and the opposing faces of the clamps are often coated with a resilient material. This material can be comprised of elastomers such as Neoprene® or Adiprene®, for example. This surface is generally about 0.050 inches thick, which helps prevent the sample from being pinched at the clamp to clamp contact edge. However, these clamps do not prevent slippage and damage of materials made from high strength fibers or the fibers themselves, and additional clamp pressure is necessary when testing these materials, sometimes on the order of 500 psi or greater. This added pressure allows the test sample to be pinched at the clamp to clamp contact edge, and the yield/break point is no longer in the center of the testing zone. Additionally, these clamp faces are scarred by the high-strength materials, and must be recoated after relatively few test runs, thus resulting in additional expense and instrument down time.
There are a wide variety of clamp faces and coating materials described in the art, but none which describe the current composite coating. U.S. Pat. No. 4,194,402 describes the use of wedge-shaped clamp faces made of low-friction material such as Teflon®. U.S. Pat. No. 5,329,820 describes the use of “O”-rings of butadiene acrylonitrile rubber as clamp faces.
SUMMARY OF THE INVENTION
The present invention relates to an improvement in a clamp or grip used in a materials testing machine of the type having a pair of grips or clamps, with grip or clamp faces, for gripping two ends of a test specimen, power means for applying loads to said test specimen through said grips, and control means for controlling said power means, the improvement comprising coating said grip or clamp faces with a material comprised of an aramid-epoxy composite. The aramid-epoxy composite may be further comprised of aluminum oxide fiber or other materials to improve friction characteristics, and the aramid material may be spunlace, woven fabric or pulp.
The invention further relates to a process for coating these faces by:
a) mixing an epoxy resin material with an appropriate catalyst material;
b) spreading the catalyzed epoxy resin material onto a sheet of release film;
c) placing aramid material onto the top of the catalyzed epoxy resin material, thereby allowing the epoxy material to soak through said aramid material so that substantially all air voids are removed, and thereby forming an aramid-epoxy composite;
d) placing said clamp faces down on top of the aramid-epoxy composite, said clamp faces having been roughened to provide good adhesion;
e) allowing the aramid-epoxy material to cure and adhere to the clamp faces;
f) removing the coated clamps from the release film;
g) removing the excess aramid-epoxy material from the edges of the clamp faces; and
h) roughening the coated clamp faces to provide a matte finish.
This invention also relates to a method for using these coated clamps to test generally high-strength materials at substantially lower clamping pressure, for example 65 psi line pressure as compared to a “house pressure” of 90 psi line pressure.
REFERENCES:
patent: 3840255 (1974-10-01), Daghe
patent: 3906619 (1975-09-01), Shaffer
patent: 4012039 (1977-03-01), Yerke
patent: 4194402 (1980-03-01), De Nicola
patent: 5329820 (1994-07-01), McMahon
E. I. Du Pont de Nemours and Company
Noori Max
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