Page 75 - Engineering Plastics Handbook

Properties 49

where G = shear modulus, MPa (psi)
τ= shear stress, MPa (psi)
γ= shear strain, cm/cm (in/in) %

γ= D
H

where D = displacement of cubical specimen, cm (in)
H = height of cubical volume, cm (in)

V
γ* = shear strain rate =
H
where V = velocity of displacement caused by applied force
H = height of cubical specimen

Sinusoidal time-varying (STV) flow demonstrates differences between
elastic and viscous properties, and it demonstrates viscoelastic behav-
ior in more complex time-varying flow [14].
With a beam equation, shear modulus is expressed as [7]

/
FA FA
/
G = =
M/L tanθ
where F = shear load, N (lb)
2 2
A = cross-sectional area of applied shear load, cm (in )
M = shear elongation, cm (in)
L = unit length of beam, cm (in)
θ= shear angle (strain), rad (deg)
Shear force is applied tangentially to the surface of a part. Shear
stress—the tangential force divided by the applied force area—can be
tensile, flexural, and compressive.

Creep
Creep and stress relaxation are the principal causes of failure due to defor-
mation (strain) for long-term service applications, and engineering plas-
tics usually are designed for long-term service. Aplastic gear or mechanical
fastener such as a nut and bolt, nail, or screw that produces creep and
stress relaxation can malfunction. The result can be catastrophic failure
of the entire product system. Aplastic gear produces dynamic creep, which
is caused by a fluctuating applied load or fluctuating temperature. A
stationary mechanical fastener produces static creep, which is caused by
a constant applied load and typically at constant temperature. At the
macromolecular level, creep is the result of a delay in response to load
by the macromolecules during gradually increasing strain. When the
load is removed, strain gradually reduces (stress relaxation). The two