MIL-PRF-3135H
4.5.5 Indentation. Four panels shall be prepared in accordance with 4.4.1. The underlayment system shall be
applied to each panel at a thickness of 6.4 millimeters (0.25 inch). Two panels shall be allowed to cure for 96 hours
at referee conditions. Two panels shall only be allowed to cure for 24 hours at referee conditions. The thickness of
the underlayment shall be measured in three locations not less than 38 millimeters (1.5 inches) from any edge of the
specimen, and not less than 64 millimeters (2.5 inches) from the center of an adjoining measurement. The thickness
of the underlayment at each of these three locations shall be measured with a micrometer dial gage with a 0.1-
kilogram (4-ounce) weight and a flat foot with a 6.4-millimeter (0.25-inch) diameter, to an accuracy of ±0.7937
millimeter (±0.03125 inch). A permanent marker shall be used to mark these three locations. Each panel shall be
mounted on a plate designed to support both the panel and a load of 907.2 kilograms (2,000 pounds) without
allowing the specimen to slide. The specimen mounting plates selected for the indentation tests shall be checked for
flatness before being used. Indentations shall be made on the deck covering underlay material at each of these three
locations by placing a load of 907.2 kilograms (2,000 pounds) over a flat faced, circular indenter for 30 minutes at
each spot. The indenter's flat face shall have an area of 1 square inch and its perimeter shall be rounded to a radius
of 0.39688 millimeter (0.015625 inch). Care shall be taken to ensure that specimen is maintained parallel and level
to the ground throughout the 30-minute indentation period.
4.5.5.1 Initial indentation. Immediately after the weight is lifted from each spot on each panel, the thickness of
the underlayment shall be measured at that spot. The thickness of the underlayment immediately after the weight is
removed shall then be subtracted from the thickness of the underlayment measured initially at that location. The
difference shall then be divided by the original measured thickness (before the weight was applied) and the result
multiplied by 100 percent. The initial indentation percentages for each of the three spots on a given panel shall then
be averaged and the average initial indentation percentage recorded for that panel. Follow the same procedure for
the remaining three panels.
4.5.5.2 Residual indentation. Two hours after the weight is lifted from each spot on each panel, the thickness
of the underlayment shall be measured again at that spot. The thickness of the underlayment two hours after
removal of the weight shall then be subtracted from the thickness of the underlayment measured initially (before the
weight was applied) at that location. This difference shall then be divided by the original measured thickness
(before the weight was applied) and the result multiplied by 100 percent. The residual indentation percentages for
each of the three spots on a given panel shall then be averaged and the average residual indentation percentage
recorded for that panel. Follow the same procedure for the remaining three panels.
4.5.6 Resistance to elevated temperatures. Two plates shall be prepared in accordance with 4.4.1, with the
exception that the plates shall be 150 millimeters by 50 millimeters by 3.18 millimeters (6 inches by 2 inches by
0.125 inch). The underlayment shall be applied to each plate at a thickness of 6.4- millimeters (0.25- inch) thickness
and allowed to cure to service per manufacturer's instructions. The underlayment shall then be scribed with a line
parallel to and approximately 25 millimeters (1 inch) from one of the 51-millimeter (2-inch) edges. The edge closest
to the scribe shall be identified as the "reference edge." The distance between this line and the reference edge of
each steel plate shall be measured to the nearest 0.25 millimeter (0.01 inch). The specimens shall then be suspended
vertically from the end opposite the reference edge in an oven maintained at a constant temperature of 71.1±1.1 °C
(160±2 °F) for 5 hours. Immediately after the 5-hour period, remove the panels from the oven; examine the
underlayment for softening by touch (wear gloves) on the end opposite the "reference edge." The specimens shall
then be allowed to cool to room temperature. When the specimens have cooled to room temperature, the distance
between the "reference edge" and the scribe on each plate shall be measured again. The difference in the distance
between the scribe and the "reference edge" before and after heating shall be calculated for each plate, and the
results averaged and recorded as the underlayment slip.
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