INCOMPLETE FUSION dard pendulum-type impact machine in breaking a standard specimen in one blow. IMPACT STRENGTH (1) The ability of a material to withstand shock loading. (2) The work done in fracturing, under shock load- ing, a specified test specimen in a specified manner. IMPEDANCE The total passive opposition offered to the flow of electric current. It consists of a combination of resistance, inductive reactance, and capacitive reactance. Note: impedance is usually a function of frequency. IMPERFECT THREAD See Incomplete thread. IMPINGEMENT ATTACK A form of localized corrosion-erosion caused by turbulence or impinging flow. IMPULSE A surge of electrical energy, usually of short duration, of a nonrepetitive nature. INACTIVE TIME The period of time when an item is available, but it is neither needed nor operated for its intended use. INCH-POUND SPECIFICATION Inch-pound specifications have requirements given in rounded, rational, inch-pound units, usually as a result of being originally developed in inch-pound. The magnitudes are meaningful and practical. Inch-pound specifica- tions should include those with rounded rational, inch-pound units only. Note: There have been instances in which magnitudes expressed in metric units as a result of mathematical conversion from rounded rational, inch-pound units are given first (preferred units) with the rounded, rational, inch-pound units given in paren- thesis or in a nonpreferred position. These specifications are inch- pound documents. Inch-pound specifications are developed for items to interface or operate with other inch-pound items. INCH-POUND UNITS The customary system formerly and cur- rently used in the United States (foot, inch, pound, BTU, horse- power, degree Fahrenheit, etc.). INCLUSION (1) A term used to denote the presence, within the body of the glass, of extraneous or foreign material. (2) Inclusions are particles of foreign material such as sand or slag that are embed- ded in the cast metal. (3) A foreign particle, metallic or nonmetallic, in a conductive layer, plating, or base material. INCOMPLETE FUSION A discontinuity caused by localized temperatures that are insufficient to allow fusion of the molten weld metal and nonmolten weld or base metals. 145