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National Advisory Committee for Aeronautics, Technical Notes - Influence of Tensile Strength and Ductility on Strengths of Rotating Disks in Presence of Material and Fabrication Defects of Several Types

The significance of tensile strength and ductility in the pre-
sence of defects and the strength-reducing effects of several types
of defect were investigated for some rotating disks. The types of
defect investigated included laminar-type irregularities, eutectic
melting, and shrink porosity.

Strengths of disks containing irregularities increased with
increasing tensile strength of similar material free from irregulari-
ties independently of ductility at ductilities in excess of 14.9-percent
elongation. The best compromise between tensile strength and ductility
in material containing shrink porosity occurred at 6.8-percent
elongation.

Disks having laminar-type irregularities at ductilities above
14.9-percent elongation exhibited no loss in strength due to the
irregularities. At 0.0-percent elongation, the same irregularities
produced a loss in strength of 42 percent. A material possessing
eutectic melting suffered a 25-percent loss in strength at 2.8- and
at 14.0-percent elongation. The loss in strength due to shrink
porosity varied from an average of 26 percent at 57.5—percent elonga-
tion to 58 percent at 2.4—percent elongation.

A significant question to be considered in the specification of
chemical composition and mechanical and thermal treatment of rotor
disks is the compromise between tensile strength and ductility. High
ductility is usually considered desirable for overcoming stress
concentrations associated with rotor design or defects that sometimes
result from the fabrication procedure. The attainment of high due-
tility can, however, usually be achieved only by sacrifice in tensile
strength. Information on the optimum combination of tensile strength
and ductility for disks in which various types of stress concentrations
are expected would therefore be very useful.

An investigation of the relative influence of tensile strength
and ductility on the burst strength of rotating disks with and with-
out central holes is described in reference 1. The materials studied
in that investigation were free of defects so that the only stress
concentrations involved were those associated with disk geometry,
such as central holes. Under these conditions, at ductilities higher
than approximately 5 percent elongation, the primary variable affecting
the bursting strength of a disk was found to be the tensile strength of
the disk material.