Properties Flashcards
(21 cards)
Stress
Force per unit area
Stress straine test conditions
Typically metals at room temp. Tension, compression, torsion and shear
Strain
Elongation per unit area
Young’s modulus
Defines stiffness of a material
Poissons ratio
Negative ratio of transverse to axial strain
Typical poisson ratio
Isotropic- 0.25. Max (no vol change)- 0.5
Plastic deformation
When material deformed beyond a point, stress no longer proportional to strain and permanent deformation occurs. Due to breaking of bonds with original atom neighbours then reforming with new neighbors. For crystalline solids, it is caused by a process called slip, which involves the motion of dislocation. For noncrystaline, occurs by a viscous flow mechanism.
What is tensile strength
Stress at the maximum on the stress strain curve- the max stress that can be sustained by a structure in tension. At this max, neck is formed, where deformation is confined at.
Ductility
Amount of plastic deformation that has been sustained at structure
Stress stain curve for tensile vs brittle solids
How is ductility expressed
Percentage elongation or percentage reduction in area
Toughness
The ability of a material to absorb energy up to fracture- area under stress strain curve.
How is toughness assessed
For dynamic (high strain rate) lading condition and when a notch is present, notch toughness is assessed using an impact test. For static (low strain) it may be assed from a tensile stress strain test.
How is necking caused
Caused by localised deformation in the tensile specimen. Leads to observed reduction in engineering stress.
True stress
Load divided by the instantaneous cross sectional area over which deformation is occurring
Hardness
Resistance to localised plastic deformation.
How is hardness usually measured
With an indentation test which involves plastically deforming the material by a known force and measuring the deformation. Two types of indentation test: brinell and vickers. BH uses spherical indenter. VH uses diamond shaped indenter.
Rule of thumb for hardness of street
Proportional to tensile strength according to : TS = 3.45 x BH
How to find yield strength
0.2 % offset
