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Peel resistance. Torque required to separate an adhesive and adherent in the climbing drum peel test. (ASTM D-1781). It is a measure of bond strength.

Peel strength. Measure of the strength of an adhesive bond. It is the average load per unit width of bond line required to part bonded materials where the angle of separation is 180 deg and separation rate is 6 in./min. (ASTM D-903).

Penetration. Depth to which the striker of a rebound pendulum penetrates a rubber specimen under conditions set in ASTM D-1054. It is an index of the dynamic stiffness or complex modulus of rubber specimens.

Permanent set. Extent to which a material is permanently deformed by a specified load. Usually expressed as % and calculated by dividing the difference in dimensions in the direction of loading before loading and after the load is removed by the original dimension and multiplying by 100.

Pfund hardness number (PHN). Measure of the indentation hardness of coatings. Hemispherical quartz or sapphire indentor is used. (ASTM D1474).

Photoelasticity. Method for observing stress distribution in a part through the use of a transparent model.

Pin test. Alternate term for expansion test.

Plastic deformation. Deformation that remains after the load causing it is removed. It is the permanent part of the deformation beyond the elastic limit of a material. It also is called plastic strain and plastic flow.

Plasticity. Tendency of a material to remain deformed after reduction 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 temperatures. Equal to 100 times the height of a standard specimen after 3 to 10 min compression by a 5 kg load. (ASTM D-926).

Plastometer test. Method for determining ability of rubber to be compressed at an elevated temperature and to recover at room temperature. Results are reported as plasticity number and recovery. Test differs from compressibility and recovery test in that the latter measures behavior of material subjected to short-time loading at room temperature.

Poisson’s ratio. Ratio of lateral strain to axial strain in axial loaded specimen. It is the constant that relates modulus of rigidity to Young’s modules in the equation: E = 2G (r + 1) where E is Young’s modulus; G, modulus or 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-1321.

Proof stress. Stress that will cause a specified permanent deformation.

Proportional limit. Highest stress at which stress is directly proportional to strain. It is the highest stress at which the curve in a stress-strain diagram is a straight line.Proportional limit is equal to elastic limit for many metals.

Pusey and Jones indentation. Measure of indentation hardness of rubber. It is the depth of penetration of a ball indentor under a load applied by a plastometer. (ASTM D-531).