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brittle fracture under compression/impact 2

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electricpete

Electrical
Joined
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Simple question.

If I tap a spherical marble with a hammer, it may split in two.

Would that be brittle fracture?
If so, is there tensile stress involved?

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Eng-tips forums: The best place on the web for engineering discussions.
 
On second thought, let's put the spherical marble into a vice and increase pressure until it fails.

Same question. Is it a brittle failure? Is there an tensile stress in this problem?

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Eng-tips forums: The best place on the web for engineering discussions.
 
Yes, if you hit the marble with the hammer at -270 deg C, it will be a brittle fracture;- not if you hit it at ambient temperature. Same as with a fish, if you drop it on the floor at -100 deg C, it will shatter instead of splatting on the floor. When you fabricate the pressure vessel, often you need the hammer to work with;- however, don't use the hammer on it when it is operating at -150 deg C. Get it?
cheers,
gr2vessels
 
hi electricpete

Yes it is a brittle fracture and no I don't think there is any tensile stress involved if the marble is put purely under compression.

regards

desertfox
 
Yes it is brittle, if defined as low-energy fracture. Glass has no plastic deformation, so it absorbs little energy during fracture.

Yes, there are tensile stresses. Imagine a small void/pore/crack in the marble (they are there, just maybe not resolvable by unaided eyesight). Even if there is a compressive stress on one axis of the flaw, the perpendicular axis has a tensile stress.

This is an easy thought experiment - there must be tensile stresses, otherwise the marble will not fracture. If you hydrostatically compress the marble (bottom of the ocean, inside a pressure vessel), there is no fracture.
 
Hi Corypad

Yes your right about tensile stresses at the voids.
I looked at it from a point of view of a Mohr stress circle
with only a compressive force acting.
I never considered a void or flaw, out of interest would these tensile stresses be calculatable.

desertfox
 
A void or flaw is not required. A compressive force on a point on the marble creates a complex stress pattern in the marble that includes tensile and shear stress. On a molecular level all failures are tensile falures.(Molecules move apart, cracks open). As Corypad said, with uniform hyrdrostatic compressive loading there is no pressure that will cause a solid marble to fail.
 
I would think a compressive force would produce a tensile hoop stress.
 
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