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The ASTM E208
drop weight test is used primarily to determine the
Nil-ductility transition temperature or NDT of ferritic steels of 5/8 in
thickness and over. This particular drop weight test was initially developed by
the Navel Research Laboratory in 1952 and published as Department of the Navy
document NAVSHIPS-250-634-3. Current specifications are available from ASTM at:
ASTM E208-06 Standard Test Method for Conducting Drop-Weight Test to Determine
Nil-Ductility Transition Temperature of Ferritic Steels.
Nil-ductility transition temperature
is the temperature at which the fracture mode of the steel
changes from ductile to brittle. Above the NDT a piece of steel typically will
stretch or deform 20% - 40% before fracture when loaded to its ultimate tensile
strength. However, below the NDT the same piece of steel will fracture in a
brittle manner like glass when only loaded to yield strength (about half of the
ultimate strength). Once it begins to crack in this manner the crack will
continue to propagate at the speed of sound. It will only stop when it runs out
of steel, the load is released, or until the crack is interrupted by steel that
is behaving in a ductile manner due to different qualities in the steel or
different temperatures present.
The drop-weight test
consists of beam specimens prepared according to ASTM E208 specifications to
initiate a material crack in a selected area of their tensile surfaces at the
start of the test. During the test a series of specimens is subjected to a
single impact load at a progression of selected temperatures to determine the
maximum temperature at which a specimen breaks. The impact load is delivered by
a guided, free-falling weight with an energy of 250 to 1200 ft-lb (340 to 1630
J) according to the yield strength of the steel to be tested. A stop is employed
to prevent deflection of more than a few tenths of an inch.
After the specimen is prepared
properly for the crack
initiation, and conditioned to the proper temperature, the test begins.
The initial test is conducted at a temperature estimated to be near the NDT
temperature. The remaining specimens are then tested at a progression of
temperature intervals to determine the break and no-break performance
temperatures within 10°F or (5°C). The lowest temperature no-break test is then
run again to confirm the results.
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