In addition, there is secured by the use of solid
dies, a strong, clean fillet under the head, the point where strength
is most needed.
Commencing with a countersunk head as the strongest form of head, the
greater the fillet permissible under the head of a rivet, or bolt, the
greater the strength and the decrease in liability to fracture, as a
fillet is the life of the rivet.
If rivets are made of iron, the material should be strong, tough, and
ductile, of a tensile strength not exceeding 54,000 pounds per square
inch, and giving an elongation in _eight inches_ of not less than
twenty-five per cent. The rivet iron should be as ductile as the best
boiler plate when cold. Iron rivets should be annealed and the iron in
the bar should be sufficiently ductile to be bent cold to a right
angle without fracture. When heated it should be capable of being
flattened out to one-third its diameter without crack or flaw.
[Illustration: FIG. 15. Solid Die Rivet.]
[Illustration: FIG. 16. Open Die Rivet.]
If rivets are made of steel they must be low in carbon, otherwise they
will harden by chilling when the hot rivets are placed in the cold
plates. Therefore, the steel must be particularly a low grade or mild
steel.
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