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Steel in Compression, Tension
Name: John
Status: other
Grade: other
Location: MI
Country: USA
Date: Fall 2010
Question:
I have heard that most materials are stronger in
compression than in tension. Is steel stronger in pure compression
loading or pure tension loading. Is there a way to explain this at a
cellular level or looking at a single grain?
Replies:
Here is a response from Dr. Saurin Majumdar.
Typically, steels fail by necking during tensile tests, i.e., by instability
at peak load beyond which the load (not true stress) decreases and the plastic
strain concentrates at the necked region (plastic strain is no longer uniformly
distributed in the gauge section). The ultimate fracture occurs at the necked
region by coalescence of voids which are initiated by high triaxial hydrostatic
tensile stress in the neck.
The typical deformation in compression test is quite different. There is no
necking or formation of voids and, if the specimen is not designed right, it may
fail by buckling. If buckling is prevented, sometimes the specimen develop axial
cracks (not transverse cracks) due to "barreling" of the specimen which is caused
by frictional resistance to lateral deformation of the specimen at the supports.
David Kupperman
John,
As you already know metals tend to be stronger under tension than under compression.
A coin can be flattened with gentle taps of a hammer, but converting that same force
into tension will not always result in elongation.
If you try to imagine the forces at an atomic or crystal level, you would imagine
that under compression, the grains are allowed to slip past each other, or individual
atoms take up defect locations by sliding into defect slots. Essentially the force on,
say, the z-axis, is transmitted into transverse directions, x and y. While this is
also possible under tension, in order to elongate, the crystals must actually reorganize
themselves and flow along the line of force, the force is applied on the z-axis and
the particles must move along the z-axis (mostly). Some metals can do this (copper,
gold) while others not so easily. Steel, because it is a mixture of iron and some other
atom (carbon, chiefly) comes out in a particular crystal lattice that does not allow
good flow along the line fo force.
Greg (Roberto Gregorius)
Canisius College
"Steel" is not a single substance. There are many (probably hundreds)
alloys that bear the name "steel". Each of these numerous alloys has
different mechanical properties. I do not know of a single simple test to
predict the mechanical properties.
Vince Calder
Typically, steels fail by necking during tensile tests, i.e., by
instability at peak load beyond which the load (not true stress)
decreases and the plastic strain concentrates at the necked region
(plastic strain is no longer uniformly distributed in the gauge
section). The ultimate fracture occurs at the necked region by
coalescence of voids which are initiated by high triaxial
hydrostatic tensile stress in the neck.
The typical deformation in compression test is quite
different. There is no necking or formation of voids and, if the
specimen is not designed right, it may fail by buckling. If
buckling is prevented, sometimes the specimen develop axial cracks
(not transverse cracks) due to "barreling" of the specimen which is
caused by frictional resistance to lateral deformation of the
specimen at the supports.
Dr. Saurin Majumdar (jThorpe)
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