crack nucleation

[sara_87]

In Solid Mechanics, what is 'crack nucleation'??
and how does this relate to fatigue and creep loading?
thanks

[Astronuc]

In Solid Mechanics, what is 'crack nucleation'??
and how does this relate to fatigue and creep loading?
thanks Crack nucleation would be the initial process whereby sufficient quntity of dislocations would accumulate to allow a separation of crystal planes.

[sara_87]

why do the dislocations happen in the first place, i mean what causes it.
what do you mean by crystal planes?

[Astronuc]

why do the dislocations happen in the first place, i mean what causes it.
what do you mean by crystal planes? The crystal planes are the planes of atoms in the grains (assuming polycrystalline solid) of metal (alloy) which form a regularly ordered array of atoms. Examples of crystal (lattice) structure are simple cubic, body-centered cubic, face centered-cubic, hexagonal (close-packed), triclinic, monoclinic, orthorhombic, rhombohedral, and tetragonal.

Dislocations are imperfections in the crystal planes such that atoms of one plane are slightly mismatched in relation to the adjacent plane(s). There are point dislocations, lines of dislocations and screw type dislocations. These happen because metals are not pure, atoms of different elements (in alloys or compounds) have different sizes, and in polycrystalline material, the grains do not align perfectly and thus have mis-matched surfaces that can generated dislocations when the material is stress. Cold working produces dislocations which more from the grain boundaries to the interior of crystals, and at high levels of cold work, dislocations can form channels such that when the material is annealed, new grain boundaries form.

[spacester]

"i mean what causes it."

In a general Engineering context, my answer would be "stress concentrations": an applied load that is within a material's stress limits until a geometrical discontinuity is encountered.

One classic case is a sharp inside corner. Just a little bit of radius on that corner makes a big difference. A larger radius removes the stress concentration by allowing the principal stress to be gradually reoriented.