brittle fracture under compression/impact 2

[electricpete]

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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[electricpete]

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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[gr2vessels]

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

 

[desertfox]

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

 

[CoryPad]

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.

 

[desertfox]

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

 

[Compositepro]

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.

 

[BobM3]

I would think a compressive force would produce a tensile hoop stress.

 

[desertfox]

Hi electricpete

I found a couple of sites that might be of interest:-

http://www.dentalmaterials.nl/wear/basics/hertz/hertz.htm

http://www.matsceng.ohio-state.edu/...adture/padturepapers/PadtureHertzianBook1.pdf

regards

desertfox