1
Transient Testing
• two types: creep & stress-relaxation; dimensional stability of a material
• creep: elongation (strain) vs. time after applying a constant load (stress), s0
• stress-relaxation: stress vs. time at a fixed elongation (strain), e0
1
* creep test (at a constant load (stress); measures strain change with time)
; ε increases with time logarithmically; Figure 4-23C
creep modulus,
l
0F=F
0; σ=σ
o0
)
0) (
( l
l t
t l
e
) ) (
(
0t t E
ce
s
) (
1 e t
solids n
for Hookea (4.61)
- -
0
-
0
e
s e D
fluids Newtonian
for (4.62) -
-
0
-
0
0
dt t
t
s s e
Figure 4-23B
Figure 4-23A
ε=ε(t)
3 elastic
response dominant viscous response dominant
3
shifting by time-temperature superposition principle
T>Tg
T<Tg ( ) ---(4.60) )
( ) 1
(
s
0e
t tt E D
c
c
(11.7) )
(
) log (
) (
) log (
2 1
g g T
g C T T
T T a C
T T
(aT< 1)
(aT > 1)
5
F=F(t) ε=ε
0* stress relaxation test (at a constant strain; measures stress change with time)
- σ decreases with time exponentially
- relaxation modulus,
( ) - - - (4.63) )
(
e
0s
t t
E
; σ=σ(t)
Figure 4-25
shifting by time-temperature superposition principle
T<Tg
T>Tg
( ) - - - (4.63)
) (
e
0s
t
t
E
(aT< 1) (aT > 1)
7
Impact Testing
• measures energy expended up to failure under conditions of rapid loading
• Izod & Charpy tests (Figure on next page)
• falling ball or dart test
• estimated from the area under stress-strain curve in high-speed tensile tests
• typical values of notched-izod impact strength; Table 4-15
• amorphous polymers with bulky substituents and nonlinear backbones; brittle
• unoriented crystalline; increases brittleness below Tg
• low-temperarure secondary relaxation; ductile; high impact strength
• impact strength increases by dispersing small particles (<0.1 mm) within the polymer matrix; ex) HIPS, ABS, rubber modified epoxy
7 (PVC)
(PS)
Izod & Charpy impact tester:
9 9
HIPS
Fatigue Testing
• determine the number of cycles (fatigue life) of applied strain at a given stress before failure, Figure 4-26
• endurance limit: maximum stress for which failure will not occur regardless of how many cycles the stress applied
• fatigue life: decreases with increasing frequency of oscillation with decreasing temperature
• important in evaluating materials when frequent periodic stress loading is encountered; ex) a plastic hinge joint, shoes
11 11
4.5 Solid-State Characterization Methods
4.5.1 Microscopy
• Polymer surface morphology can be characterized at high magnification and resolution by traditional scanning electron microscopy (SEM) and
transmission electron microscopy (TEM).
• During the 1980s, scanning tunneling microscopy (STM) and atomic force microscopy (AFM) were introduced.
• AFM offered superior resolution with few of the specialized sample
preparations such as microtoming, etching, staining, or gold coating that are required for SEM and TEM studies.
• AFM and STM employ a very sharp tip to probe and map the morphology of the surface as illustrated in Figure 4-27A.
• Tip–surface interactions can occur by van der Waals, electrical, or magnetic forces that are measured by deflection of the cantilever.
13 13
4.5.2 Scattering Methods
• As covered in Section 4.2.4, wide-angle X-ray scattering (WAXS) can be used to determine the fractional crystallinity of semicrystalline polymers and to determine the Bragg spacing.
• Polymers can be characterized by small-angle neutron, X-ray, and light scattering.
• Light scattering is useful when the characteristic dimension (D) of the scatterer is greater than l/4p where l is the wavelength of light.
• Neutron scattering can provide information about both the structure and dynamics of a polymer system.
