Abstract
The cyclic diametral strain and stress response of macroscopically
untextured (nominally isotropic) bainitic Cr-Mo-V steels has been studied.The
total axial strain amplitudes were controlled and chosen so that a range of
ratios of plastic and elastic elongations were used extending from 0.04 to 5.
The trend of the cyclic diametral strain was sometimes found to drastically
deviate from the commonly used Poisson's ratio when the ratio of plastic and
elastic elongation was around 3 for the uncycled material.The unusual initial
increase in cyclic contraction ratio for these conditions was attributed to
strain concentration and the decrease to strain decent ration.A condition for
these unpredictable macroscopic effects seems to be that the effective
strengthening structure should be sufficiently unstable during the cyclic
strain applied.At room temperature fatigue slip bands of high local density
and number are created in these conditions.Such conditions were found to be
satisfied at ambient temperature if the carbides were spaced widely enough -or
more generally- if the temperature of testing was elevated.The basic reason
for the unexpected combination of softening and decreasing diametral strain
range seems to be gradual latent hardening in the slip band bundles which
carry the cyclic plasticity.At elevated temperature applied strains larger
than ~0.3 % give rise to an increasing mechanically activated dynamic recovery
which operates despite a dense carbide dispersion.The amount of recovery and
simultaneous dislocation annihilation increase and act to lower the flow
strength with rising strain.Their extent depend on the strain rate.An apparent
maximum in dynamic recovery was observed as a minimum in cyclic yield
strength at the same strain for which the pronounced unpredictable diametral
strain was observed.Similar diametral strain effects in monotonic tension
tests on different materials reported in the published literature indicate
that the effects are most probably related to the particular dominant mode of
slip at strain levels for which the ratio of plastic and elastic strain
(ea~eE) is around three.Slip is then dominantly planar.Careful shape control
of the specimen gauge section is necessary for reproducible diametral strain
because of the unstable nature of the material in the actual conditions of the
cyclic straining which produce the unexpected behaviour.
Original language | English |
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Qualification | Doctor Degree |
Awarding Institution |
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Award date | 24 Apr 1987 |
Place of Publication | Espoo |
Publisher | |
Print ISBNs | 951-38-2855-7 |
Publication status | Published - 1987 |
MoE publication type | G4 Doctoral dissertation (monograph) |
Keywords
- low alloy steels
- stresses
- strains
- deformation
- cyclic loads