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Monday, April 18, 2016

From the Testing Lab: Tensile

Most tensile testers today are vertical or horizontal and subject the specimen to a constant rate of elongation.

Units of Reported Values
     Tensile breaking strengths are reported as the force per unit width required to rupture the specimen. Some of the common units for reporting tensile breaking strength are lb/in and kN/m. Tensile breaking strength corrected for grammage is called the tensile index.
     A similar quantity, called the breaking length, is also used for reporting grammage-corrected tensile strength. Breaking length is defined as the length of a strip of given paper that will cause it to break under its own weight. It is calculated by dividing the tensile breaking strength by the grammage. The tensile strength may be expressed in kilograms force per meter for this calculation or one can use customary basis weight and tensile units.

Why is Tensile Breaking Strength Important?
     Tensile strength is a direct indication of the durability and potential end use performance of a number of papers that receive direct tensile stresses in use, such as wrapping, bag, gummed, tape, cable wrapping, twisting papers, and printing papers.
     In general, a certain minimum tensile strength is required of an paper that undergoes a web converting operation where it is subjected to tensile stresses while being pulled through the process. Printing papers are a primary example of this.
Tensile property data for several different grades of paper
How is Tensile Strength Increased?
     There are several ways to increase the tensile strength of paper. For example, increasing beating or refining, increasing wet pressing, adding a beater adhesive, increasing the long fiber content of the furnish, and increasing the basis weight will all usually lead to an improvement in tensile strength.

Source: Scott, William E., Trosset, Stanley, Properties of Paper: An Introduction. TAPPI Press, 1989.