lecture #30 : 3.11 mechanics of materials f03 -...
TRANSCRIPT
LECTURE #30 :3.11 MECHANICS OF
MATERIALS F03INSTRUCTOR : Professor Christine OrtizOFFICE : 13-4022 PHONE : 452-3084WWW : http://web.mit.edu/cortiz/www
• PLASTICITY OF POLYMERS II
SUMMARY : LAST LECTUREI. Molecular Origins of Plasticity in Metals• edge and screw dislocations, 2-D and 3-D movement of dislocations, slip systems,
planes, directions, polycrystalline plastic deformationII. Stress versus Strain Curves for Polymers in Uniaxial Tension• structure of semicrystalline polymers: amorphous+cystalline regions, chain folding,
lamellar, spherulites (macroscopically isotropic)
Semicrystalline polymers (T>Tg):Ι. linear elastic deformationII. homogeneous plastic deformation,
strain hardeningIII. NECK forms and grows unstably, i.e.
inhomogeneous plastic deformationIV. neck stabilizes V. COLD DRAWING, neck increases
in length by extracting polymer fromunnecked region of sample
VI. entire sample is drawnVII. begin stretching of completely drawn
sample, strain hardeningVIII. fracture
σ
ε
σ
ε
highly oriented polymer fiber
amorphous, glassy polymer
(T<Tg)
semicrystallinepolymerT>Tg
elastomer (T>Tg)
σ
ε
σmax
Lo
εf
I.
II.
III.
IV. V.VI.
VII.
•
σY
VIII.
MACROSCOPIC MECHANICAL
BEHAVIOR
Polyethylene :used in milk jugs, toys, pens,
ice traysNylon (polyamide)
Plastic Deformation of Semicrystalline Polymers
crystalline lamellae
amorphous region
Stages and Mechanisms of Plastic Deformation1) elongation of amorphous chains (uncoiling along stress axis)2) rotation/tilting of lamellae crystallites toward tensile axis3) separation of crystallites into block segments (partial melting)4) further stretching of crystallites and orientation of amorphous regions along tensile axis, void formation, stress whitening5)2nd strain hardening regim:recrystallization to a fiber like oriented structure, high stiffness
σ, ε
σ, ε
Comparison of Amorphous and Semicrystalline Polymers
Some Applications of Amorphous Polymers
Plastic Deformation of Amorphous, Glassy Polymers or Networks (T < Tg)
σ
ε
Crazing in Amorphous, Glassy Polymers (T<Tg)
σ,εσ,ε
Crazing in Amorphous, Glassy Polymers (T<Tg)
σ,εσ,ε
entanglementoriented polymerchains
fibril
CRAZE NANO
STRUCTURE
main fibrilcross-tie fibril
craze-bulk interface
craze
bulkbulk
polycarbonate
Crazing in Amorphous, Glassy Polymers (T<Tg)
σ,εσ,ε
Shear DZ’s in Amorphous, Glassy Polymers (T<Tg)
σ,εσ,ε
Macroscopic sample
~ necking
Microscopic structure
CRAZING OR SHEAR DZ’s?
σ,ε
σ,ε
σ,εσ,ε
CONSEQUENCES FOR IMPACT RESISTANCE
bisphenol-A polycarbonate
(crazing)
Plastic Deformation and Fracture of Amorphous Components
Plastic Deformation and Fracture of Amorphous Components
TOUGHENING OF AMORPHOUS POLYMERS
TOUGHENING OF AMORPHOUS POLYMERS
Typical uniaxial tensile stress-strain behavior of polystyrene (PS), medium-impact PS (MIPS), high-impact PS (HIPS), and poly(acrylonitrile-co-styrene-graft-butadiene) ABS.