Mechanics of material : Chalk?

Question

If a chalk is twisted how is shear and normal stresses distrubeted over the cross section? Please explain the whole mechanisum of it in terms of stresses on a twisted chalk and how does it break? Is a chalk a ductile material or a brittle material if neither what type of material is it? Please put a useful link where i can find more about stress distribution in a chalk , Torsion in a chalk?

Answer 1

For torsion, shear forces are distributed along a direction that is in the plane of the applied twist. Normal stress are distributed along a surface 45° to that.

Since brittle materials tend to fail in tensile first, if you twist a stick of chalk until it breaks, you will see the cleave or the break-line forms at 45° to the plane of the applied torque. This is typical behavior for brittle materials.

Chalk is brittle. Brittle is defined as having no "transition" where strain increases significantly with respect to additional applied stress (there is no "yield strength").

If you take something not brittle (a Tootsie Roll, for example) and apply the same torque to failure, you will see that it cleaves in the plane of the torque - completely differently than the chalk did. This type of failure is typical for ductile materials.

This type of information about the behavior of materials (parting lines of failures) is particularly valuable in forensics - determining how and why a failure happened.

Answer 2

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