Rate of strain hardening. Rate of change of true stress as a function of true strain in a material undergoing plastic deformation. An alternate term is modulus of strain hardening.

Recovery. Index of a material's ability to recover from deformation in the compressibility and recovery test (ASTM F-36), the deformation under load test (ASTM D-621) and the plastometer test (ASTM D-926). In the compressibility and recovery test it usually is reported with compressibility and given as %. It is calculated by dividing the difference between recovered thickness and thickness under load by the difference between original thickness and thickness under load. In the deformation under load test it indicates the extent to which a nonrigid plastic recovers from prolonged compressive deformation at elevated temperature. It is given as % and is calculated by dividing the difference between height recovered 11/2 hr after load is removed and height after three hr of loading by the change in height under load. In the plastometer test it indicates the extent to which an elastomer recovers from compressive loading at elevated temperature. It is equal to plasticity number minus recovered height. See also Rockwell recovery.

Recovery test. Method for measuring compressibility and recovery of gasket and seal materials. (ASTM F 36).

Reduction of area. Measure of the ductility of metals obtained in a tension test. It is the difference between original cross section area of a specimen and the area of its smallest cross section after testing. It is usually expressed as % decrease in original cross section. The smallest cross section can be measured at or after fracture. For metals it usually is measured after fracture and for plastics and elastomers it is measured at fracture.

Relative modulus. Ratio of the modulus of a rubber at a given temperature to its modulus at 73 F. It is determined in the Gehman torsional test.

Relaxation. Rate of reduction of stress in a material due to creep. An alternate term is stress relaxation.

Repeated bent test. Method for determining ductility of relatively ductile metals such as silicon steel sheet and strip. Specimens about 1 in. wide and 6 in. long are held in jaws and bent 90 deg. Then they are bent back and forth through 180 deg. Results are reported as number of bends (including the original 90 deg bend) required to cause failure. (ASTM A-344).

Repeated blow impact test. Method for judging impact properties of cast iron. Hammer is dropped on specimen from increasing heights until fracture occurs. Results are reported as height of the last drop prior to fracture (repeated blow impact value). (ASTM A-327).

Residual elongation. Measure of ductility of plastics. It is the elongation of a plastic specimen measured 1 min after rupture in a tension test.

Resilience. Measure of recoverable elastic energy in a deformed material. It is the amount of energy released when a load is removed from a specimen. It is equal to deformation energy minus electric hysteresis.

Rockwell hardness number (RHN). Index of indentation hardness measured by a steel ball or diamond cone indentor. RHN is given in various scales (B, C, R, etc.) depending on indentor and scales used. ASTM E-18 details a standard method for determining RHN for metals and gives a table of scale symbols. ASTM D-785 gives standard method for measuring RHN of plastics; ASTM B-294 covers cemented carbides and ASTM A-370, steel products.

Rockwell penetration. Measure of indentation hardness of rubber. It is the resistance to penetration by a specified indentor under specified load applied with a Rockwell hardness tester. (ASTM D-530). It usually is reported with Rockwell recovery.

Rockwell recovery. Extent to which hard rubber recovers from indentation. It is obtained in same test as Rockwell penetration (ASTM D-530) and is reported with it.

Rockwell superficial hardness. Measure of surface hardness of thin strip or finished parts on which large test marks cannot be tolerated or shapes that would collapse under normal Rockwell hardness test loads. (ASTM E-18).

Ross flexing machine test. Method for measuring crack growth resistance of rubber by repeatedly bending pierced specimen and measuring growth of crack that develops. (ASTM D-1052).

Rupture resistance. Indication of ability of rubber to withstand tensile loading. It is the load required to rupture a rubber specimen under conditions set out in ASTM D-530.

Rupture strength. Nominal stress developed in a material at rupture. It is not necessarily equal to ultimate strength. And, since necking is not taken into account in determining rupture strength, it seldom indicates true stress at rupture.