FED-STD-228A
2.6 Median. If the numerical values for a given property are arranged in an ascending or descending
order, the median is obtained as follows:
2.6.1 If the number of values is odd, the median is the value in the middle of the series.
2.6.2 If the number of values is even, the median is the arithmetic average of the two middle values.
3. SPECIMEN
3.1 Types.
3.1.1 Two shapes of specimens may be used for tensile strength, elongation, tensile stress, and set of
rubber or thermoplastic insulation. These are the dumbbell and straight specimens. A dumbbell
specimen, as described in method 3021, shall be used whenever it is practicable to prepare it from the
piece (varnished cambric insulation excepted) to be tested. Straight specimens in the form of tubes shall
be used for rubber and thermoplastic insulation and sheaths when it is not practicable to prepare a
dumbbell. A straight specimen 1 inch wide or the full width of the tape if less than 1 inch wide shall be
used for determining the tensile strength of varnished cambric insulation.
3.1.2 Specimens for use in the determination of tearing strength are shown on figure 3111.
3.2 Direction. Specimens shall be taken in the length direction of the wire or cable for rubber or
thermoplastic insulation and sheaths and in the length direction of the tape for varnished cambric
insulation.
4. PROCEDURE
4.1 Unless otherwise specified in the detail specification, tensile strength, elongation, and tensile stress
of rubber or thermoplastic insulation and sheaths shall be determined on the same specimen and during
a single test.
4.2 If five specimens are required for determining any one of the above properties, 4.1, these five
specimens shall be used for determining all of these properties.
4.3 If the piece from which the specimen is to be prepared is too thick or has an uneven surface that may
interfere with the determination of thickness, the piece or portion of the piece shall be buffed as described
in method 3011. The specimen shall be permitted to rest at least 30 minutes between buffing and testing.
Specimens for tension tests shall be buffed in the strip form before cutting with a die.
5. RESULTS
5.1 Unless otherwise specified in the detail specification, three specimens from each inspection unit shall
be tested for tensile strength, elongation, tensile stress, and set of rubber and thermoplastic insulation
and sheath except that under the following conditions five specimens shall be tested:
5.1.1 If one or more values do not meet the specified requirements when tested for compliance with
specifications.
5.1.2 If referee tests are being made.
5.2 The characteristic of the insulation or sheath of the inspection unit shall be the median of the results
obtained from the specimens tested.
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