TAPPI T 494 Tensile Testing of Paper

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Fundamental behavior of paper in tensile loading in key to understanding the quality of paper. The stress strain plot of paper is a model of the stress-strain tensile behavior of machine-made paper. The graph shown represents the behavior of two strips cut from the same sheet of paper in different directions: machine direction (MD--the direction in which the paper moves during manufacture) and cross-machine direction (CD--the direction at right angles to the machine direction). The stress-strain curves shown provide a fundamental engineering description of the mechanical behavior of paper strips when subjected to tensile stresses as they are pulled to failure by special paper clamps attached at their ends. The initial straight section of the stress-strain curve, which is the elastic deformation of paper, has a slope that defines the property known as Young's modulus.

Along with the thickness of the paper, Young's modulus is a useful concept in engineering design because it is critical in determining stiffness. Tensile breaking strength of paper determined to be the maximum value of stress that occurs at failure. Elongation at rupture is the measure of the maximum percentage of stretch, or the maximum strain, that the paper can achieve before failure.

The differences in the properties of paper in these two directions arise from the tendency of the fibers to align preferentially in the machine direction and from the stretching of the paper web in this direction during manufacture. Another fundamental measure of the tensile strength properties of paper is represented by the area under the stress-strain curve. This area represents the work expended to cause rupture of the tensile strip of paper; this work is called the tensile energy absorption (TEA). Even though the breaking strength of paper in the machine direction is greater than in the cross-machine direction, it may require more work to break a tensile strip in the cross-machine direction because its greater stretch percentage makes the TEA greater in the cross-machine direction than in the machine direction. In certain instances, TEA can prove to be a significant property relating to durability. Tensile energy absorption is a measure of the ability of paper to absorb energy under variable loading conditions and can be used to gauge the durability of papers chat are subjected to repetitive straining.

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