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**Impact energy.** Energy required to fracture a part subjected to shock loading as in an impact test. Alternate terms are impact value, impact strength, impact resistance and energy absorption.

**Impact resilience. **Measure of the resilience of rubber obtained by dropping a pendulum hammer against a specimen and measuring rebound. (ASTM D-1054). It is the ratio of 1 minus the cosine angle of rebound, to 1 minus the cosine of the original angle of the pendulum, and is expressed as a %. It is sometimes called % rebound. Impact resilience of metal is an indication of hardness obtained in a rebound test such as the test for scleroscope hardness.

**Impact strength.** Energy required to fracture a specimen subjected to shock loading, as in an impact test. Alternate terms are impact energy, impact value, impact resistance and energy absorption. It is an indication of the toughness of a material.

**Impact test.** A method for determining behavior of material subjected to shock loading in bending, tension or torsion. The quantity usually measured is the energy absorbed in breaking the specimen in a single blow, as in the Charpy impact test, Izod impact test and tension impact test. Impact tests also are performed by subjecting specimens to multiple blows of increasing intensity, as in the drop ball impact test and repeated blow impact test. Impact resilience and scleroscope hardness are determined in nondestructive impact tests.

**Indentation hardness. **Resistance of material to surface penetration by an indentor. See hardness.

**International rubber hardness degrees (IRHD).** Measure of indentation hardness of rubber. For substantially elastic isotropic rubbers IRHD is related to Young’s modulus by the equation: F/M = 0.00017 R0.65 Pl.35 where F is indenting force in kg; M, Young’s modulus in kg/sq cm; R, radius of indentor in cm, and P, penetration in hundredths of mm. IRHD of a rubber is approximately equal to its durometer hardness. A standard test method for determining IRHD is given in ASTM D-1415.

**Izod impact test.** Method for determining behavior of materials subjected to shock loading. Specimen supported as a cantilever beam is struck by a weight at the end of a pendulum. Impact strength is determined from the amount of energy required to fracture specimen. ASTM E-23 describes a standard test procedure, specimens (including notch shapes) and apparatus for Izod impact testing of metals. See also ASTM A-327 (cast iron), ASTM D-256 (plastics) and ASTM D-758 (plastics at sub normal and elevated temperatures). See also Impact test.