Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 113

i ......... ' ..... Method" ..... '

Constant strain amplitude

(plastic or total strain)

(several specimens)

Incremental step

(one specimen)

Multiple step,

Increasing strain

(one specimen)

Multiple step,

decreasing strain

(one specimen)

' Strain wavefo~_ ~ ~i

.... , r ,,, ,,,

Figure 1.112 Different strain waveforms used to measure a cyclic stress-strain curve

from pressings involving different types of forming strain 169 have also shown that
the increase in strength from the cold work during forming may be lost during
cyclic deformation. It is a safe procedure, therefore, to disregard the increase in
strength due to forming strains for design calculations.

Cold work embrittlement

Any structure must clearly have sufficient strength to survive normal use and
sufficient ductility to withstand any unusual impacts. It has been found, however,
that certain IF steels may be subject to a condition known as secondary cold
work embrittlement (SCWE). This condition may lead to brittle fracture after the
material has previously received severe strains such as those encountered during
deep drawing. The cracking is accompanied by very little plastic deformation
and consequently very little energy absorption and the initial fracture surfaces
have been reported to be mainly intergranular. 17~ There is little evidence that
SCWE has caused any serious problem in service, 172 but nevertheless it has been
the subject of extensive research to show how the condition may be avoided.
The condition has been mainly studied using three types of laboratory test,
but the most commonly used test involves a cylindrical cup. A circular blank
of steel is first deep drawn to form the cup and a truncated conical plunger is
used to open the cup end to determine whether the rim of the cup is brittle. The
plunger may be pushed slowly into the cup as illustrated in Figure 1.113, using a