INFLUENCE OF STRESS/STRAIN CONCENTRATION AND MEAN STRESS ON THE LOW-CYCLE FATIGUE BEHAVIOR OF THREE STRUCTURAL STEELS AT ROOM TEMPERATURE. Page: 8 of 44
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GEAP-5726
ABSTRACT
The influence of stress/strain concen-
tration and mean stress on the low-cycle
fatigue behavior (100 to 50, 000 cycles-to-
failure) at room temperature was determined:
for three low-strength steels: low carbon
steel, 2-1/4 Cr-1 Mo alloy steel, and
Type-304 stainless steel.
Circumferentially grooved' cylinders
with a uniaxial theoretical elastic stress con-
centration factor of K ; 3 were tested under
zero nominal mean stress and a nominal
mean stress equivalent to the stress amplitude.
In each case, load control was used. High
elongation foil strain gages of 1/64-in. and
1/32-in. gage length measured 'cyclic plastic
strains in the root of the notch in the axial
and circumferential directions. The smooth-
.bar, completely-reversed, strain-controlled,
low-cycle fatigue behavior was also determined.
The influence of a nominal mean stress
on the deformation behavior in the root of the
notch is shown. At a given nominal stress
range, the strain range in the root of the
notch is considerably less for the. zero-to-
tension loading than for the completely re-
versed loading. At low nominal stresses,no appreciable change in notch- root' strain
range with cycling can be observed. At
high nominal stresses, which lead to total
notch-root strain ranges above -1. 4%, a de-
crease in the notch-root strain range is ob-
*: served with cycling. This observation
corresponds to the cyclic strain-hardening
behavior of the smooth-bar specimens.
A fatigue strength reduction factor
was determined for crack initiation, which
is defined by the appearance of a 0. 005 to
0.015-in. -long crack in the root of the notch.
This factor depends on the mean stress,
the applied stress range, and the material,
and can be considerably higher than the the-
oretical elastic stress concentration factor.
Crack initiation consumes only a
small percentage of the specimen life; the
rest of the life is consumed by crack growth.
It is suggested that design against low-cycle
fatigue should use specific analysis tools for
each stage of the failure process. The
fatigue strength reduction factor can be used
to determine crack initiation; crack growth
may be calculated by an extension of Fracture
Mechanics concepts.SECTION I
INTRODUCTIONThe treatment of discontinuities in
highly-stressed structural members is
always a challenge in design. Creep and
fatigue, as well as brittle, fracture-is en-
hanced by the presence of stress raisers;
consequently special methods must be devised
for the design against fracture under a given
type of loading. In some types of loadings,
the influence of stress raisers on the frac-
ture behavior is not yet completely under-
stood. One such area is low-cycle fatigue;
i. e. , fatigue that occurs under cyclic plastic
deformation on a microscopic scale.
It would be desirable to approach this
problem from a fundampental viewpoint.
Such an approach would, first of all, have to
concentrate on the development of constitu-tive material laws to represent cyclic strain
hardening (softening) which is a characteristic
phenomenon of the low-cycle fatigue deforma-
tion behavior. Stress analysis, with the aid
of this constitutive law, would then be able
to give information on the magnitude of the
stresses and strains in the root of the notch
at any instant of time. The answer to the
question of how many cycles at a given notch-
root.strain can be endured before cracking
occurs would have to come from an appropri-
ate strength hypothesis, which has yet to be
developed for low-cycle fatigue. (1)
An engineering approach consists 'of
devising .a number of experiments to investi-
gate the influence of important parameters.
Empirical factors can be derived immediately
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Krempl, E. INFLUENCE OF STRESS/STRAIN CONCENTRATION AND MEAN STRESS ON THE LOW-CYCLE FATIGUE BEHAVIOR OF THREE STRUCTURAL STEELS AT ROOM TEMPERATURE., report, January 1, 1968; United States. (https://digital.library.unt.edu/ark:/67531/metadc1035525/m1/8/: accessed April 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.