Lezione 6: Plasticity, yielding and ductility Flashcards
What is yield strength?
Yield strength is the stress beyond which a material becomes plastic - deformation is permanent. It’s determined by standard tensile testing procedures.
How to find yield strength for metals
Yield strength σy is defined by a 0.2% offset from the linear elastic region. When strained beyond σy, most metals work harden, causing the rising part of the curve. Maximum stress is defined as the tensile strength σts
How to find yield strength for polymers
σy is identified as the stress at which the stress-strain curve becomes markedly non-linear, typically around a strain value of 1%. The behavior of the polymer beyond the yield point depends on its temperature relative to the materials glass transition temperature Tg
How to find yield strength for glasses and ceramics
Glasses and ceramics have a yield strength; however, it is so large that it is never reached during a tensile test – the material fractures first
The elastic limit σel is defined by the end of the elastic region of the stress-strain curve – this is the value generally used to compare the strength of ceramics with other materials
What is plastic strain?
Plastic strain, εpl, is the permanent strain resulting from plasticity
What is ductility?
Ductility is a measure of how much plastic strain a material can tolerate and is measured by the elongation εf.
εf is dependant on the sample dimensions and is therefore not a material property.
How are hardness tests done?
In a hardness test, a diamond or ball shaped indenter is pressed into the surface of a material. The hardness of the material is determined by its resistance to the indentation. It’s a strength value but does not have units of MPa or psi – units change based on which hardness scale is used.
What is meant by ideal strength?
Ideally, the strength of a material is the force necessary to break inter-atomic bonds
A bond is broken if it is stretched beyond about 10% of its original length (0.1a0). Further calculations that account for the curvature of the force-distance curve predict a ratio of 1/15.
Talk about crystalline imperfections
Defects in metals and ceramics prevent materials from achieving their ideal strength. Among the common defects we find:
- vacancies
- solute atoms on interstitial and substitutional sites
- dislocations
- grain boundaries
Talk about vacancies
A vacancy is a site at which an atom is missing. While vacancies play a role in diffusion, creep and sintering, they do not influence strength.
Talk about solute atoms
- Substitutional solid solutions: dissolved atoms replace those of the host
- Interstitial solid solution: dissolved atoms squeeze into spaces between the host atoms
Dissolved atoms rarely have the same size as the host material, so the surrounding lattice is distorted.
Talk about dislocations
A dislocation is an extra half-plane of atoms in the crystal. Dislocations distort the lattice and make metals soft and ductile.
Talk about grain boundaries
Grain boundaries form when differently oriented crystals meet. The individual crystals are called grains, the meeting surfaces are grain boundaries.
What are edge dislocations and how are they formed?
Edge dislocation is made by cutting, slipping and rejoining bonds across a slip plane. The dislocation line separates the part of the plane that has slipped from the part that has not. It’s called edge dislocation because it is formed by the edge of the extra half-plane
What are screw dislocations and how are they formed?
In a screw dislocation, the upper part of the crystal is displaced parallel to the edge of the cut rather than normal to it. All dislocations are either edge or screw or mixed, meaning they are made up of little steps of edge and screw.
