Page 77 - Engineering Plastics Handbook

Properties 51

1. Initial application of load, instantaneous initial strain, elastic and
plastic strains, along the y axis at virtually 0 min.
2. Primary phase, with elastic and plastic strain combined; creep rate
(slope of curve) decreases rapidly.
3. Secondary phase, which shows further decrease in creep rate
(decrease in total strain increase per minute). The secondary phase
shows a shallow, virtually linear slope.
4. Tertiary phase, when strain rate increases again, leading to fracture,
product failure.
ASTM D2990 cautions that this creep curve “is an idealized curve.”
Presentation of total strain (y axis) and time (x axis) as instantaneous,
primary, and secondary stages depends on the time scale (x axis). Not
all polymeric materials have a secondary phase; and the tertiary creep
phase occurs at high stresses for ductile materials [1].

Clear Engineering Plastics
Transparent (clear) engineering thermoplastics are used increasingly for
medical, automotive, and architectural products; windows, skylights,
panels; compact disks, fiber-optics sleeves; lenses, reflectors, and light
transmission pipes. ASTM D1003 (ISO 14782/13468), “Haze and Luminous
Transmittance of Transparent Plastics (Hazemeter or Spectrometer),”
describes optical properties of thermoplastics.
Light pipes are used to transmit light from a light source to an outlet
interfaced with a clear medium, usually air or water [9]. Light pipes are
typically clear, solid, curved or bent plastic rods. The objective with light
pipe design is to transmit the maximum amount of light. The distance x
between the light pipe and the incident light is based on the design. One
way to reduce the number of reflections and light path length is to design
a convex entrance. Curved light pipes are designed with a maximum angle
of bend—radius of curvature—to avoid or minimize light loss. The maxi-
®
mum radius of curvature for Lexan polycarbonate, e.g., is 51°. At angles
below the maximum radius of curvature, there is light loss; and there is
a minimum radius of curvature. To calculate the minimum radius of cur-
vature [9], use
R n + n
min = p a
t 2( n − n )
p a
where R min = minimum radius of curvature, deg
t = diameter or thickness of light pipe, mm (in)
n = refractive index of light pipe = 1.585 for Lexan
p
polycarbonate
= 1.00 for refractive index of air
n a