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Tuesday, July 12, 2016

Tensile Testing Basics

Tensile breaking strength has always been considered the most fundamental strength test run in the Paper Industry. It is more complicated than it appears since it involves tensile, shear and flexural forces acting on the fibers and bonds in the sheet.

In a tensile measurement, a paper specimen of uniform width and length is clamped between two jaws.  The jaws are then separated to exert a tensile stress on the specimen, and the instrument indicates the tensile force when the specimen ruptures. Most modern instruments also indicate the stretch and tensile energy absorption (TEA). TEA is the area under the stress-strain curve. It is proportional to the energy that the paper can absorb up to the breaking point.

In the Paper Industry, vertical and horizontal tensile testers are most common. However, there is the pendulum-type tester as well. Sample width and length are important variables in the tensile test. Longer specimens give a greater chance for weak points to exist, provide initiation points for rupture, and result in lower tensile strengths. TAPPI Test Method T 494 specifies that the test specimen be 25 +/- 1 mm wide and long enough to be clamped in the jaws when they are 180 mm apart. Also, the specimen must have sides parallel within 0.1 mm. 

Tensile breaking strengths are reported as the force per unit width required to rupture the specimen. This is often reported as lb/in or 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. Tensile strength can vary dramatically from machine direction to cross machine direction.

Tensile Specimen Cutter
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 any paper that undergoes a web converting operation where it is subjected to tensile stresses during startup and while being pulled through the process. Printing papers are a primary example of this.

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 long fiber content of the furnish, and increasing the basis weight will all usually lead to improved tensile strength.

Do you see any change in the use of tensile tests and measurements?

Are there applications where a pendulum-type tester is preferred over a vertical or horizontal tensile tester? Why?

Is there a good way to estimate tensile with other tests?

Common Standards:
ISO 1924-2 Paper and board - Determination of tensile properties - Part 2: Constant rate of elongation method (20 mm/min) 
ISO 1924-3 Paper and board - Determination of tensile properties - Part 3: Constant rate of elongation method (100 mm/min) 

TAPPI T494 Tensile properties of paper and paperboard (using constant rate of elongation apparatus) 

NOTE: Tissue and wet tensile are discussed under a different blog post.

Reference: "Properties of Paper: An Introduction" - Scott, W.E & Trosset, S. p. 56-58 (1989)