Doing CANTILEVERS, we consider all of the following EXCEPT:
a) small in all diameters
b) high yield strength
c) minimal contact
d) small occlosogingival length.***
The elastic limit is the stress from which a material stops deforming in an elastic, reversible manner and thus begins to irreversibly deform.
For a fragile material, it is the stress to which the material breaks, in particular because of its internal micro-cracks. The Griffith criterion then makes it possible to estimate this threshold constraint.
For a ductile material, it is the zone in red on the graph opposite, beyond the elastic domain E represented in blue in which the increase of the stress gives a reversible strain to the suppression of this constraint (and often fairly linear depending on this constraint). The deformations under the elastic limit remain permanent, they are plastic deformations. They are usually measured or verified by means of a tensile test.
In the medium of the technique and by abuse of language, one often uses "elastic limit" for yield strength, which is unsuitable because in itself the limit is a quantity; she is not elastic.
Elastic deformation occurs by reversible deformation of the material structure by a change in interatomic distances5. Plastic deformation occurs by displacement of dislocations, which are crystalline defects. The appearance of these movements, occurring at the threshold of the elastic limit, depends on several factors, the main ones being:
the interatomic cohesion forces: the greater the bonds between atoms, the more difficult it is to move them so the higher the elastic limit is;
- the crystalline structure: the shifts (displacements of the dislocations) are made more easily on the atomic planes having a high density; the crystals with the most sliding possibilities are the crystals of cubic structure with centered faces; in fact, the most ductile materials (gold, lead, aluminum, austenite in steels) have this type of structure;
foreign atoms block dislocations (Cottrell cloud, pinning); pure metals are more ductile than alloy metals;
dislocations are blocked by grain boundaries; the more grain boundaries, so the smaller the crystallites, the higher the yield strength;
- the dislocations are blocked between them; the more the material contains dislocations, the higher the elasticity limit (hardening);
the atoms can reorganize under the effect of the thermal agitation (dynamic restoration and recrystallization, mounted dislocations), the speed of deformation thus intervenes;
- in the case of rolled or extruded products, there is a texture (crystallography) and an elongation of the grains in a given direction, therefore an anisotropy of the elastic limit (the size of the crystallites and the orientation of the dense crystallographic planes do not are not the same according to the direction considered); we speak of "fiber" (in the figurative sense), the resistance is more important in the direction of rolling or extrusion than in the transverse directions.
a) small in all diameters
b) high yield strength
c) minimal contact
d) small occlosogingival length.***
The elastic limit is the stress from which a material stops deforming in an elastic, reversible manner and thus begins to irreversibly deform.
For a fragile material, it is the stress to which the material breaks, in particular because of its internal micro-cracks. The Griffith criterion then makes it possible to estimate this threshold constraint.
For a ductile material, it is the zone in red on the graph opposite, beyond the elastic domain E represented in blue in which the increase of the stress gives a reversible strain to the suppression of this constraint (and often fairly linear depending on this constraint). The deformations under the elastic limit remain permanent, they are plastic deformations. They are usually measured or verified by means of a tensile test.
In the medium of the technique and by abuse of language, one often uses "elastic limit" for yield strength, which is unsuitable because in itself the limit is a quantity; she is not elastic.
Elastic deformation occurs by reversible deformation of the material structure by a change in interatomic distances5. Plastic deformation occurs by displacement of dislocations, which are crystalline defects. The appearance of these movements, occurring at the threshold of the elastic limit, depends on several factors, the main ones being:
the interatomic cohesion forces: the greater the bonds between atoms, the more difficult it is to move them so the higher the elastic limit is;
- the crystalline structure: the shifts (displacements of the dislocations) are made more easily on the atomic planes having a high density; the crystals with the most sliding possibilities are the crystals of cubic structure with centered faces; in fact, the most ductile materials (gold, lead, aluminum, austenite in steels) have this type of structure;
foreign atoms block dislocations (Cottrell cloud, pinning); pure metals are more ductile than alloy metals;
dislocations are blocked by grain boundaries; the more grain boundaries, so the smaller the crystallites, the higher the yield strength;
- the dislocations are blocked between them; the more the material contains dislocations, the higher the elasticity limit (hardening);
the atoms can reorganize under the effect of the thermal agitation (dynamic restoration and recrystallization, mounted dislocations), the speed of deformation thus intervenes;
- in the case of rolled or extruded products, there is a texture (crystallography) and an elongation of the grains in a given direction, therefore an anisotropy of the elastic limit (the size of the crystallites and the orientation of the dense crystallographic planes do not are not the same according to the direction considered); we speak of "fiber" (in the figurative sense), the resistance is more important in the direction of rolling or extrusion than in the transverse directions.
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