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Glossary
Plastic Deformation.
Deformation that remains after the load causing it is re-
moved. It is the permanent part of the deformation be-
yond the elastic limit of a material. It also is called
plastic strain and plastic flow.
Plasticity.
Tendency of a material to remain deformed, after reduc-
tion of the deforming stress, to a value equal to or less
than its yield strength.
Plasticity Number.
Index of the compressibility of rubber at elevated tem-
peratures. Equal to 100 times the height of a standard
specimen, after a 3 to 10 minute compression by a 5 kg
load (ASTM D-926).
Poisson's Ratio.
Ratio of lateral strain to axial strain in an axial loaded
specimen. It is the constant that relates modulus of rigid-
ity to Young's modulus in the equation:
E = 2G(r+1)
where E is Young's modulus; G, modulus of rigidity;
and r, Poisson's ratio. The formula is valid only within
the elastic limit of a material. A method for determining
Poisson's ratio is given in ASTM E-132.
Proof Stress.
Stress that will cause a specified permanent deformation.
B-23
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