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Standard: |
TAPPI |
Method: |
TAPPI T 494 |
Title: |
Tensile properties of paper and paperboard
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TAPPI T 494 describes the procedure, using constant-rate-of-elongation equipment, for determining four tensile breaking properties of paper and paperboard: tensile strength, stretch, tensile energy absorption, and tensile stiffness. This procedure is applicable to all types of paper and paperboard within the limitations of the instruments used, whether the instruments perform horizontal or vertical tests or whether they are manually operated or computer controlled. It is also applicable to handsheets, with modifications, as specified in TAPPI T 220 “Physical Testing of Pulp Handsheets.” It does not apply to combined corrugated board.
Tensile strength is indicative of the strength derived from factors such as fiber strength, fiber length, and bonding. It may be used to deduce information about these factors, especially when used as a tensile strength index. For quality control purposes, tensile strength has been used as an indication of the serviceability of many papers which are subjected to a simple and direct tensile stress. Tensile strength can also be used as an indication of the potential resistance to web breaking of papers such as printing papers during printing on a web fed press or other web fed converting operations. When evaluating the tensile strength, the stretch and the tensile energy absorption for these parameters can be of equal or greater importance in predicting the performance of paper, especially when that paper is subjected to an uneven stress such as gummed tape, or a dynamic stress such as when a sack full of granular material is dropped.
Stretch (sometimes evaluated in conjunction with bending resistance) is indicative of the ability of paper to conform to a desired contour, or to survive nonuniform tensile stress. It should be considered important in all papers, but is of particular importance in papers where stress-strain properties are being modified or controlled. This includes creped paper, pleated paper, air-dried paper, and paper that has been made extensible through mechanical compaction. Stretch is evaluated in decorative papers and certain industrial grades such as paper tapes and packaging papers, both as an index of how well the paper will conform to irregular shapes and, along with tensile energy absorption, as an indication of the paper’s performance under conditions of either dynamic or repetitive straining and stressing. Stretch has also been found important in reducing the frequency of breaks on high-speed web fed printing presses such as are used to print newspapers.
Tensile energy absorption is a measure of the ability of a paper to absorb energy (at the strain rate of the test instrument), and indicates the durability of paper when subjected to either a repetitive or dynamic stressing or straining. Tensile energy absorption expresses the “toughness” of the sheet. An example of this is a multi-wall sack that is subject to frequent dropping. In packaging applications such as multi-wall sacks, favorable drop tests and low failure rates have been found to correlate better with tensile energy absorption than with tensile strength. Tensile stiffness tells of the stiffness of the sheet and often gives a better indication of the mechanical response of the sheet to converting forces than does failure criteria.
Basis weight required for calculations.
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