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Outinen, Kaitila and Mäkeläinen (2001)
reported an experimental study on the mechanical properties of
cold-rolled, hot dip zinc coated structural steel S350GD+Z (Z35)
manufactured in accordance with EN 10147. Test pieces were cut
out from a steel sheet with nominal thickness of 2mm, longitudinally
to rolling direction. Both transient-state and steady-state tests
were conducted. Figure
1 shows the stress-strain curves of Grade S350GD+Z steel
at elevated temperature, plotted from the transient-state test
data with a heating rate of 20°C/min.
Figure
2 shows the reduction factors for yield strength at different
strain levels based on the test results. The 0.2% proof strength
was determined by the 0.2% proof stress method, whereas the
1.0% and 2.0% yield strengths were determined directly at the
respective total strains. The design curves corresponding to
respective strain levels in accordance with prEN1993-1-2 and
BS5950-8 are also included for comparison.
The evaluation of the test data shows that the
reduction factor for yield strength of light gauge steel is highly
dependent on the strain level where the yield strength is determined.
A conservative approach is to take the reduction factor of the
0.2% proof stress as the design strength, similar to what has
been adopted in prEN1993-1-2. Figure 36 shows that the design
curve of prEN1993-1-2 follows closely with the test values of
0.2% proof stress, with slightly higher values.
Under less conservative conditions, the reduction
factors for yield strength corresponding to higher strain levels
can be applied, which are allowed in BS5950-8. The comparison
in Figure 36 shows that the design values at 2.0% strain level
match the test data quite well for the temperature range from
400–600°C.
Lee, Mahendran and Makelainen (2003) reported
an experimental study on the mechanical properties of light gauge
cold-formed steel. Three steel grades of G50, G500 and G300 (minimum
yield strength of 550, 500 & 300 N/mm2, respectively) were
investigated upon the variable thickness from 0.4 to 1.2mm. In
this study, only the steady-state tests were conducted from 100 – 800°C
at intervals of 100°C.
Figures
3, Figure
4, Figure
5, and Figure
6 show the test results of the yield strength at 0.2% (offset),
0.5%, 1.5% and 2.0% strain, respectively. The 0.2% proof strength
was determined by the 0.2% proof stress method, whereas the
0.5 to 2.0% yield strengths were determined directly at the
respective total strains. The design curves corresponding to
respective strain levels in accordance with prEN1993-1-2 and
BS5950-8 are also included for comparison.
According to the test results, Lee et al. (2003)
reported a better strength reduction factor for higher steel
grades (G500 and G550) than the lower steel grade (G300) in the
temperature range of 400 – 750°C, and the thickness
of materials had negligible influence on yield strength.
Compared to the design curve of prEN1993-1-2,
the test data are always at the high side although the design
curve consistently follows the general trends of test data. The
possible factors include:
- The conservative approach adopted by prEN1993-1-2 to define
0.2% proof stresses as the design values.
- The higher yield strength obtained from the steady-state
test methods conducted.
The design curves of BS5950-8 do not correspond
well with the test data. At lower strain levels of 0.2% and 0.5%,
the design curves always tend to be conservative. At higher strain
levels of 1.5% and 2.0%, the design curves tend to be slightly
unconservative, below 400°C, and become conservative above
400°C.
Figure
7 summaries the test data on elastic modulus, showing that
lower steel grade of G300 retained higher elastic modulus than
steel grades of G500 and G550 at elevated temperatures. PrEN1993-1-2
specifies the design curve of elastic modulus for light gauge
steel based on hot-rolled steel which can be seen to be slightly
unconservative compared to the test data.
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