Materials for engineering, 3rd Edition - (Malestrom)

170
Materials for engineering
Crazes are a form of highly localized yielding and often start from scratches
on the specimen surface. The growth of a craze occurs by the extension of its
tip into uncrazed material. They are regions of cavitated material, bridged by
drawn and oriented polymer ‘fibrils’ of approximately 20 µm diameter, as
indicated in the sketch of Fig. 5.7. The craze thickens by lengthening of the
fibrils. The yielding is thus localized into these crack-shaped regions, being
constrained by the surrounding undeformed solid, giving rise to the presence
of local hydrostatic tensile stresses. The voids are interconnected, being
typically 10–20 nm is size, lying between the oriented fibrils, so that the
density of the craze is only about half of that associated with the uncrazed
matrix. Crazing thus results in an overall increase in volume of a polymer
which yields in this way.
The criterion for the nucleation of a craze in a three-dimensional stress
state is of the form:
σ
max
Ω1
( T )
– σ min ≥Λ( T ) +
σ
H
where σ max and σ min are maximum and minimum principal stresses,
respectively, Λ(T) and Ω 1 (T) are material constants, which depend on the
temperature, and σ H is the hydrostatic stress.
Crazes grow slowly at first and absorb considerable amounts of energy,
although they contribute towards an overall weakening of the material. They
eventually turn into true cracks which propagate rapidly.
Brittle fracture
Brittle fracture is often observed in plastics and, in common with other
Craze
5.7 Crazing in a linear polymer: strong strands bridge the microcrack.