February 2021 - Drop Weight Testing
Drop Weight Testing was developed by the Naval Research Laboratory in 1952 and is still used by naval, nuclear, freight, and railcar industries to determine the Nil-Ductility Transition (NDT) Temperature for a variety of ferritic steels.
The NDT is defined as the temperature at which the fracture mode of the material changes from ductile to brittle or the maximum temperature at which a material exhibits brittle fracture behavior. Below the NDT, the probability of ductile fracture is negligible. Materials are susceptible to crack initiation and/or fast crack propagation at lower applied energy or stress levels once they reach brittle fracture mode. In most cases, this transition occurs at a low (colder) temperature. The wreck of the RMS Titanic is a great illustration of the effect of cold temperatures can have on steel. Drop Weight Testing is used, in part, to avoid these sorts of catastrophic failures.
Prior to testing, each specimen is machined to a predetermined size and a weld bead is deposited on one face. The weld bead is then notched to a specified depth in order to produce a crack initiation point. After being cooled/heated to a specified temperature, the specimen is placed at the base of a drop tower. A free-falling weight is then dropped from a prescribed height of the face opposite the weld bead. The specimen is inspected to determine if a crack has been initiated in the start notch, and if so, has it propagated to either edge of the tension surface. If the crack has reached the edge of the specimen, the test temperature is deemed to be below the material's NDT (i.e. the material is brittle).
Common specifications that invoke this procedure include:
ASTM E208, the governing specification for the standard method of conducting drop-weight testing
NAVSHIPS-250-634-3, the document originally published by the Department of the Navy pertaining to drop-weight testing