ORNL-1771 - Oak Ridge National Laboratory

AMP QUARTERLY PROGRESS REPORT
studied include sodium silicate, silica, silicon
nitride, and boric oxide.
The fluoride-to-sodium intermediate heat ex-
changer, which failed in a life test after 1680 hr of
cyclic service in the temperature range 1080 to
15OO0F, was examined, It is probable that the
failures in the tube-to-header welds were caused
by unequal thermal expansion, which caused stress
concentration at the roots of the tube-to-header
welds. These stresses, combined with expansion
and contraction of the tubes, would tend to propa-
gate cracks through the walls. These failures
emphasize the extreme desirability of using back-
brazing as a means of rniriirnizing the notch effect
in tu be-to-header welds.
STRESS-RUPTURE TESTS OF INCONEL
R. B. Oliver
D. A. Douglas
J. H. DeVan
J. W. Woods
Metallurgy Division
The tube-burst test for obtaining information on
the stress-rupture properties of lnconel has been
studied intensively. The stress pattern introduced
into the specimen in this test simulates to some
extent the stress pattern that will be present in
circulating-fuel reactors. The test consists of
stressing a closed-end tube with internal gas pres-
sure. In tests of this type reported previously,' it
WQS observed that lnconel specimens stressed in
this manner showed less ductility and much shorter
rupture life than the uniaxially stressed specimens,
and therefore an intensive study of the multiaxial
stress system was initiated. W. Jordan of the
Mechanics Department of the University of Alabama
is investigating this problem.
Part of the investigation consists of a study of
the theory of stresses in cylindrical pressure ves-
sels, with particular attention to the variations in
stresses calculated by the thin-wall formula vs the
stresses determined by the niore exact Lame' theory.
The stresses under discussion are those in the
walls only, with no consideration given to the end
closure shape, except that the ends are assumed to
be completely closed. The three principal stresses
at a given point are the radial stress (G), the tan-
gential (hoop) stress (ut), and the longitudinal
(axial) stress (uj. The theory of elasticity yields
'R, €3. Oliver, B. A. Douglas, and J. W. Woods, ANP
Qrtnr. Prog. Rep. ]me 10, 1954, ORNL-1729, p 89.
112
the following equations:
(1 ) =
2 2
PIT2 - P OTO
2 2
to - r .
2 2
Tire ( p, *.. p )
2 2
r0 - r . '
where r is the radial distance to the point at which
the stress is desired, rz and ro are the internal and
external radii, and pi and p, are the internal and
external pressures. For the special case of the
cylinder subjected to internal pressure only (p, =O),
the equations reduce to:
(4) u . = p . u - 0 ,
YZ 70
u . = u
UZ
r. 2
a* = PI ___ I
r2 - r2
0 1
where the additional subscripts i and o an the
stress terms indicate stresses at the inner and
outer surfaces, To simplify these equations for
application to cylinders with thin walls, it is fre-
quently assumed that the tangential stress does
not vary across the wall of the vessel. This simpli-
fication results in the following equations for in-
ternal pressures only:
(7)
where the letter s is used to denote stress and I
is the thickness of the wall (t = ro - r.). A simpli-
fied expression for radial stress in thin-walled
cylinders is not commonly used.