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When you have no convenient access to a tensile tester, and if your web is at least 1 mil thick, the age-old cantilevered beam deflection method may be just what you are looking for. This test is not recommended as a replacement for a good laboratory tensile tester, but it will usually give results to within ± 20% of the value measured with a tensile tester. The test requires:

• web samples about 6 to 15 in. long
• a flat, level surface (preferably near a wall)
• a flat weight (such as a book)
• a good ruler or, even better, a sheet of accurately ruled graph or quadrille paper
• knowledge of the sample's thickness and density.

The diagram shows the basic elements of the test setup and the two key geometric measurements: the deflection at the beam's end, y-end and the cantilevered length, L.

Equations for calculating the deflection at the end of a cantilevered beam or sheet under uniform loading are available from most mechanical and civil engineering handbooks. In this test, the web's own weight provides the uniform loading. The deflection equation for the end of a cantilevered beam or sheet is expressed in terms used in coating and web processes.

y_end=3ÞL⁴/2EZ²

where Þ is the density in lb/in³, L is the extended or cantilevered length in inches, E is the tensile modulus in lb/in², and Z is the web thickness in inches. If we rearrange the deflection equation, we get a simple equation for the tensile modulus in terms of parameters we know or can easily measure.

E=3ÞL⁴/2y_endZ²

A key limit on this test is that the cantilevered length of the sample should be at least five times longer than the deflection at the end of the sample. This is because the beam equations are based on beams with very small deflections and large deflections can introduce significant error.

A table, lab bench, or cabinet next to a wall is a really convenient flat surface because we can attach a piece of graph paper on the wall with which we can easily measure the deflection. Just make sure that a well marked line on the graph paper extends directly out from the top of the flat surface. A typical setup is shown in the second diagram.

Cut a sample at least an inch wide to avoid twisting—wider samples are fine. Place the sample on the flat surface and clamp it with a weight such as a book. If the deflection is not large enough to conveniently measure, extend more film over the edge of the flat surface. If it is too much—more than 1/5 the extended length, for example—reduce the extended (cantilevered) length.

To cancel the effects of curl, be sure to always do two measurements—face up and face down—with exactly the same cantilevered length, and average the deflections.

What if we don't know the thickness? Measure it with a micrometer or caliper. If the thickness is a bit low for the range of the instrument, measure a batch of several sheets and divide the result by the number of sheets.

What if we don't know the density? Weigh a carefully cut and measured sheet on a balance. If no balance is available, use a postal scale. Weigh as many sheets as necessary to get a significant reading and divide by the number of sheets. The volume is the product of the number of sheets and the thickness, width, and length of one sheet.

Above all, remember, this test is a quick and dirty field test and not a substitute for a really precise laboratory tensile test.

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Based in Ft. Mill, S.C., the Assn. of Industrial Metallizers, Coaters & Laminators provides a broad range of services to the converting industry. If you have inquiries concerning products, equipment or material sourcing, converting capabilities or technical issues, the group's "Ask AIMCAL" service is available at its Web site: www.aimcal.org

For more information, contact AIMCAL at 803/802-7820, fax: 803/802-7821.