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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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