Chemistry Reference
In-Depth Information
4.12
Toughness and Brittleness
Many polymers that yield and exhibit tough, ductile behavior under the conditions
of normal tensile tests prove to be brittle when impacted. This is particularly true
when the sample contains notches or other stress concentrators. Fracture behavior
is characterized by a variety of empirical tests. None of these can be expected to
correlate very closely with service performance, because it is very difficult to ana-
lyze stress and deformation behavior of complex real articles under the variety of
loads that may be encountered in practice. Impact tests aim to rate the fracture
resistance of materials by measuring the energy required to break specimens with
standard dimensions. The values obtained relate to the experimental conditions
and the geometry and history of the specimen. A single figure for impact strength
is of limited value in itself but such data can be useful for predicting serviceabil-
ity of materials for different applications if they are obtained, say, from a series
of impact tests at various temperatures and sample shapes and are combined with
experience of the performance of similar materials and part shapes under related
service conditions.
Impact tests are often used to locate the brittle-ductile temperature or brittle-
ness temperature. This parameter is generally defined as the temperature at which
half the specimens tested fail in a given test. Because of the nature of most of
these impact tests, this approximates the temperature range in which yielding pro-
cesses begin to absorb substantial portions of the applied energy. As the test tem-
perature is increased through the brittle-to-tough transition region, the measured
impact energies increase substantially and the specimens exhibit more evidence
of having flowed before fracturing.
Ductile
brittle transitions are more accurately located by variable temperature
tests than by altering impact speed in an experiment at a fixed temperature. This
is because a linear fall in temperature is equivalent to a logarithmic increase in
straining rate. The ductile
brittle transition concept can be clarified by sketches
such as that in
Fig. 4.27 [21]
. In the brittle region, the impact resistance of a
material is related to its LEFM properties, as described above. In the mixed mode
failure region, fracture resistance is proportional to the size of the yield zone that
develops at a crack tip during impact. If the yield zone (also sometimes called a
plastic zone) is small, fracture tends to be brittle and can be described by LEFM
concepts. If yielding takes place on a large scale, then the material will absorb
considerable energy before fracturing and its behavior will be described as tough.
The relative importance of the yield zone can be estimated, for a given prod-
uct, by comparing its yield stress and fracture toughness. The parameter proposed
for this purpose is [
K
Ic
/
σ
y
]
2
, where
σ
y
is the yield stress
[22]
. This ratio has units
of length and has been suggested to be proportional to the size of the yield zone.
Higher values indicate tougher materials. In the third region of
Fig. 4.27
, impact
resistance is determined by the capacity of the product to absorb energy by local-
ized necking and related mechanisms, after yielding.
