Chemistry Reference
In-Depth Information
Brittle Region
Tough Region
Mixed Mode
Failure Region
Impact resistance
relates to yield stress,
propensity to cold
draw, etc.
Impact resistance
relates to linear
elastic fracture
toughness
Impact resistance
relates to size of
plastic zone at
crack-up
Temperature
FIGURE 4.27
Ductile
brittle behavior in impact resistance
[21]
. The transition between the zones varies
with the rate of impact and type of test.
Notches act as stress raisers and redistribute the applied stress so as to favor
brittle fracture over plastic flow. Some polymers are much more notch sensitive
than others, but the brittleness temperature depends in general on the test speci-
men width and notch radius. Polymers with low Poisson ratios tend to be notch
sensitive. Comparisons of impact strengths of unnotched and notched specimens
are often used as indicators of the relative danger of service failures with compli-
cated articles made from notch sensitive materials.
Weld lines (also known as knit lines) are a potential source of weakness in
molded and extruded plastic products. These occur when separate polymer melt
flows meet and weld more or less into each other. Knit lines arise from flows
around barriers, as in double or multigating and use of inserts in injection mold-
ing. The primary source of weld lines in extrusion is flow around spiders (multi-
armed devices that hold the extrusion die). The melt temperature and melt
elasticity (which is mentioned in the next section of this chapter) have major
influences on the mechanical properties of weld lines. The tensile and impact
strength of plastics that fail without appreciable yielding may be reduced consid-
erably by double-gated moldings, compared to that of samples without weld lines.
Polystyrene and SAN copolymers are typical of such materials. The effects of
