Page 73 - Engineering Plastics Handbook

Properties 47

2. Stress and strain by steady (linear) shear at different times and tem-
peratures
3. Creep and stress relaxation
4. Parallel superposition
5. Squeeze flow, pull-off tests, and other linear tests
Data analysis [3] includes

1. Long-term property predictions using time-temperature superpositions
2. Linear viscoelastic functions and interconversions
3. MWD data
4. User-defined variables and models
5. Cut-and-paste graphics
6. Custom reports

The company’s dielectric analyzer (DEA) measures capacitive and con-
ductive properties over different frequencies, temperatures, and times to
determine the permittivity ε′, loss factor ε′′, dielectric loss tangent tan δ,
and ionic conductivity. The DEA measures data at eight decades from
0.003 to 300,000 Hz [3]. Decades and octaves quantify frequency changes.
Decades mean that data are measured at frequency intervals that
are 10 times apart. Frequencies at 5 and 50 Hz, 10 and 100 Hz, 25 and
250 Hz are one decade apart; 2000 and 200,000 Hz are two decades apart
(2000 to 20,000 is one decade, 20,000 to 200,000 is the second decade).
Frequency intervals of octaves are 2 times apart, 5 and 10 Hz are one
octave, 1000 and 2000 Hz are one octave [11, 12].

Applied torque
Roark’s formulas for torsional stress from a twisting motion can be
expressed for beam loading conditions [4], e.g., for a power transmission
application. Shear stress developed in a material subjected to a speci-
fied torque in a torsion test is estimated by [5]

S = Tr
J
where S = shear stress developed in a material subjected to torque or
torsional shear stress, MPa (psi)
T = torque, N⋅m (ft⋅lb)
r = distance from axis of twist to outermost fiber of specimen,
cm (in)
4
4
J = polar moment of inertia, cm (in )