last class more mechanical testing fracture toughness - pre-cracked specimen creep - constant load,...
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Last class
More mechanical testing
Fracture toughness - pre-cracked specimen
Creep - constant load, elevated temperature
Fatigue - constant load, S/N curves, endurance limit
Last class
More mechanical testing
Fracture toughness - pre-cracked specimen
Creep - constant load, elevated temperature
Fatigue - constant load, S/N curves, endurance limit
Fracture surfaces
Brittle vs. ductile
Crack origins, chevron markings
Beachmarks
Fracture surfaces
Brittle vs. ductile
Crack origins, chevron markings
Beachmarks
Today
Atom arrangements in metals
Dislocations
Today
Atom arrangements in metals
Dislocations
Crystal structures of materials - where are the atoms?
Polymers - mostly not crystallinestructure extremely complicated
Ceramic materials - complicatedmany different types of atom
arrangements
Crystal structures of materials - where are the atoms?
Polymers - mostly not crystallinestructure extremely complicated
Ceramic materials - complicatedmany different types of atom
arrangements
Metals - really simple - most have one of three typesMetals - really simple - most have one of three types
Crystal structures of pure metalsCrystal structures of pure metals
Most pure metals exhibit one of three types
1. cubic close packing (ccp or A1)2. hexagonal close packing (hcp or A3)3. A2 (almost universally referred to by
the confusing notation 'bcc')
Most pure metals exhibit one of three types
1. cubic close packing (ccp or A1)2. hexagonal close packing (hcp or A3)3. A2 (almost universally referred to by
the confusing notation 'bcc')
Crystal structures of pure metals
cubic close-packed (ccp)
close-packed plane of atoms
Crystal structures of pure metals
cubic close-packed (ccp)
close-packed plane of atoms
Crystal structures of pure metals
cubic close-packed (ccp)
ABCABC layer sequence close-packed plane of atoms
Crystal structures of pure metals
cubic close-packed (ccp)
ABCABC layer sequence close-packed plane of atoms
AB
C
A
AB
C
Crystal structures of pure metals
hexagonal close-packed (hcp)
Crystal structures of pure metals
hexagonal close-packed (hcp)
Zn, Cd, Co, Ti, Zr……Zn, Cd, Co, Ti, Zr……
close-packed plane of atoms
hexagonhexagon
Crystal structures of pure metals
hexagonal close-packed (hcp)
ABAB layer sequence close-packed plane of atoms
Crystal structures of pure metals
hexagonal close-packed (hcp)
ABAB layer sequence close-packed plane of atoms
AB
AB
AB
Fe, V, Cr, Mo, W, Ta……Fe, V, Cr, Mo, W, Ta……
Crystal structures of pure metals
A2 structure - so-called "bcc" metal structure
almost close-packed atom planes
Crystal structures of pure metals
A2 structure - so-called "bcc" metal structure
almost close-packed atom planes
some empty space some empty space
Dislocations
And now…the rest of the story (on plastic deformation)!
Dislocations
And now…the rest of the story (on plastic deformation)!
Mechanism of plastic deformation connected with existence of defects in atom arrangement known as dislocations
Mechanism of plastic deformation connected with existence of defects in atom arrangement known as dislocations
Ideally, atom arrangement within a crystal repeats perfectly Ideally, atom arrangement within a crystal repeats perfectly
Mistakes (defects) in repetition occur in reality Mistakes (defects) in repetition occur in reality
Dislocations
Situation is this:
strength of a material w/ no dislocations is
Dislocations
Situation is this:
strength of a material w/ no dislocations is
20-100 times greater than ordinary materials 20-100 times greater than ordinary materials
Think of edge dislocation as extra plane of atoms partially inserted into crystal
Think of edge dislocation as extra plane of atoms partially inserted into crystal
Dislocations
These things are real!
We can see them! (in an electron microscope)
Dislocations
These things are real!
We can see them! (in an electron microscope)
Dislocation line
0.00001 cm
Dislocations
Move under a shear stress
Dislocations
Move under a shear stress
Dislocations
Move under a shear stress
Dislocations
Move under a shear stress
Dislocations
Move under a shear stress
Dislocations
Move under a shear stress
Dislocations
Move under a shear stress
Dislocations
Move under a shear stress
Dislocations
Move under a shear stress
Dislocations
Move under a shear stress
Dislocations
Move under a shear stress
Dislocations
Move under a shear stress
Dislocations
Permanent change in shape results
Dislocations
Permanent change in shape results
Dislocation has disappearedDislocation has disappeared
Dislocations move along a slip planeDislocations move along a slip plane
Dislocations
Watch a real one move!
Dislocations
Watch a real one move!
QuickTime™ and aCinepak decompressor
are needed to see this picture.
Dislocations
If hundreds of thousands of dislocations move through material, microscopic steps produced in the surface as below
Dislocations
If hundreds of thousands of dislocations move through material, microscopic steps produced in the surface as below
Dislocations - Initial overviewDislocations - Initial overview
Material w/ NO dislocations is very strongMaterial w/ NO dislocations is very strong
Dislocations weaken a materialDislocations weaken a material
But it cannot be deformed plasticallyBut it cannot be deformed plastically
But dislocations make plastic deformation possibleBut dislocations make plastic deformation possible