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National Advisory Committee for Aeronautics, Technical Notes - A Critical Review of Notch Sensitivity in Stress Rupture Tests

The English and German literature on notch stress—rupture test—
ing was reviewed and information obtained on: (1) the effect of
notching on the rupture strength in general and, in particular, the
influence of the unnotch ductility on the notch sensitivity; (2) the
effect of chemical composition and heat treatment on the notch stress—
rupture characteristics of low=alloy heat—resisting steels; (5) the
mechanism of stress—rupture embrittlement and notch sensitivity in
these lowealloy steels; and (4) a comparison of the influence of
notch geometry on the notch strength in stress-rupture tests and in
conventional tensile tests.

In a stress-rupture test, the strength at a given time for a
notched specimen may be either greater or smaller than the corres—
ponding strength of an unnotched bar. Whether the strengthening or
weakening effect predominated appeared to be related to the unnotched
ductility. For the alloys considered, the notch strength was greater
than the unnotch strength when the ductility exceeded approximately
10 percent. Below an unnotch ductility of approximately 5 percent,
the strength of the notched bar fell below that of the unnotched
specimen and the trend of notch rupture strength with time closely
paralleled that of the unnotch ductility.

Lowealloy Cr-Mo steels containing from 0.9 to 1.5 percent nickel
were extremely notch sensitive in rupture tests at 952° F (500° 0)
when either quenched and tempered or normalized and stress relieved.
Elimination of the nickel appeared to reduce the notch sensitivity
greatly. Annealing or sphereodizing Cr—Ni—Mb steels greatly reduced
the strength at short times to rupture. Such structures, however,
are not notch sensitive and when compared with the normalized or the
quenched-and—tempered steels may possess superior notch rupture
strength at very long times to rupture.

An analysis of the data for notch rupture tests on lowealloy
steels indicated that a precipitation reaction may be responsible for
the rupture embrittlement and consequent notch sensitivity of the
Cr-Ni-Mo steels.