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@BOOK{Larour:94600,
author = {Larour, Patrick},
title = {{S}train rate sensitivity of automotive sheet steels :
influence of plastic strain, strain rate, temperature,
microstructure, bake hardening and pre-strain},
address = {Aachen},
reportid = {RWTH-CONV-105791},
year = {2010},
note = {Zsfassung in dt. und engl. Sprache. - Urspr. ersch.: Aachen
: Shaker, 2010; Zugl.: Aachen, Techn. Hochsch., Diss., 2010},
abstract = {This experimental work shows the different parameters
influencing the strain rate sensitivity behaviour of
automotive sheet steel grades in crash conditions. Most
investigations have been performed in the strain rate range
[0,001-200/s] and temperature range [233-373K] with
servohydraulic tensile testing machines. Additional
Split-Hopkinson bar testing results up to 1000/s have also
been included at room temperature. The focus has been laid
on the “apparent” strain rate sensitivity, determined
based on multiple dynamic flow curves at constant strain
rate in a semi-logarithmic (Beta10 values) or logarithmic (m
values) way. It has been shown that the dynamic behaviour in
the investigated strain rate and temperature range is
clearly thermally activated for a wide range of automotive
sheet steels. This means that an increase in strain rate is
nearly equivalent to a decrease in temperature. The strain
rate sensitivity values are dependent on the strain rate
range considered, as well as on the temperature and plastic
deformation range chosen. The strain rate sensitivity
decreases with increasing plastic strain level due to a
gradual exhausting of work hardening potential combined with
adiabatic softening effects. The strain rate sensitivity
increases with decreasing temperature or increasing strain
rate, which is often omitted when considering literature
data. The strain rate sensitivity is also dependent on the
microstructure investigated. The strain rate sensitivity
decreases strongly with increasing strength level,
especially below 400MPa yield strength or 500MPa tensile
strength, and is stabilised at a low level for AHSS and UHSS
steel grades. The strain rate sensitivity decreases for
single phase ferritic mild, HSS and HSLA steel grades,
mainly due to solid solution alloying with Mn, Si and P
elements. Long range mechanisms such as precipitation
hardening, grain refinement or cold work do not influence
the strain rate sensitivity. With increasing second hard
phase content, the strain rate sensitivity decreases due to
the decrease of relative volume content of strain rate
sensitive ferrite in multiphase steels. The TRIP effect
decreases the strain rate sensitivity in low alloy TRIP
steels in comparison to dualphase steels. High alloy TRIP
steels show some negative strain rate sensitivity in the low
strain rate and high strain range. A significant decrease in
the TRIP effect intensity is seen at strain rates above 1/s
at high strain level, which is related to an adiabatic
temperature increase. Uniaxial, plane strain or biaxial
pre-straining up to around 0,10 equivalent strain does not
influence the strain rate sensitivity of sheet steels in
comparison to the as-delivered material. A bake hardening
heat treatment with or without pre-straining does not
influence the strain rate sensitivity significantly neither.
Forming and/or bake hardening does not affect particularly
the subsequent strain rate sensitivity in crash conditions.
Cold work or bake hardening introduces obstacles to
dislocations, which are rather of athermal nature, so that
the strain rate sensitivity is not influenced. For high
alloy TRIP steels, the magnitude of subsequent TRIP effect
is increased with increasing pre-straining level, which
slows down adiabatic stress softening quite effectively.
This experimental work allows some reliable comparisons
between different alloying concepts and helps to identify
the parameters influencing effectively the strain rate
sensitivity, such as strain rate, temperature, plastic
deformation, solid solution alloying, second phase hardening
and TRIP effect. This work delivers additionally a wide
database for strain rate sensitivity values of automotive
sheet steel grades, which, can be referred to for further
experimental and modelling investigations.},
keywords = {Hochfester Stahl (SWD) / Temperatur (SWD) / Plastische
Deformation (SWD) / Mikrostruktur (SWD) / Bake-Hardening
(SWD)},
cin = {522110 / 520000},
ddc = {550},
cid = {$I:(DE-82)522110_20140620$ / $I:(DE-82)520000_20140620$},
typ = {PUB:(DE-HGF)3},
urn = {urn:nbn:de:hbz:82-opus-32713},
url = {https://publications.rwth-aachen.de/record/94600},
}
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