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National Advisory Committee for Aeronautics, Technical Notes - Effects of Design Details on the Fatigue Strength of 355-T6 Sand Cast Aluminum Alloy

Static and fatigue tests were made on 355-T6 sand—cast plate—type
aluminum alloy specimens incorporating holes, bosses, and ribs.

In the static tests the tensile strength of the plain plate—type
specimen agreed within 3 percent with the strength of separately cast
test bars. In terms of the average stress on the minimum net section
of one specimen of each type, all the design details studied reduced
the static strengths of the platertype specimens. The reductions ranged
from 4650 to 9000 psi or 13 to 26 percent.

0f the design details studied, the unreinforced hole was the most
effective stress—raiser for fatigue lives from 10,000 to 25,000,000 cycles.
The unreinforced hole in the sand—cast specimen was not so severe in
reducing the fatigue strength as a similar hole in wrought l7S—Th,
275—116, 53s—T6, or lll-S-T6 plate.

The fractures of both the static and fatigue specimens with a boss
or rib were generally in cross sections close to the edge of the boss
or rib.

The effects of surface conditions, internal porosity, blow holes,
dross inclusions, and so forth on the fatigue strength of aluminumsalloy
sand castings have been investigated at the Aluminum Research Labora—
tories of the Aluminum Company of America using plate-type specimens of
355-T6 alloy. (See reference 1.) -In a comparison of results of tests
of specimens containing the above defects with results of tests of
similar specimens containing no intentional defects, it was found that
the defects studied did not cause losses of strength in proportion to
their supposed importance. The specimens containing excessive porosity
had fatigue strengths within 10 percent of the average for normally
sound specimens.

In addition to the foregoing investigation a similar investigation
has been made of the effects of design details on the fatigue strength
of 355-T6 sand-cast aluminum alloy using the same general type of speci-
men. After consideration of the types of design details encountered in
practice several types were selected for this investigation as follows:
Unreinforced holes, bosses with and without holes, and ribs. The effect
of changing fillet radii on bosses was included.

The object of this investigation was to study the effects of design
details such as open holes and reinforcements upon the static and fatigue
strengths of 355-T6 aluminumealloy sand-cast specimens. The reinforce-
ments include bosses, with and without holes, and ribs. A comparison
will be made of the effects of these design details with the effects of
normal casting defects.