Biomedical Engineering Reference
In-Depth Information
(c)
(b)
(a)
Figure 5.7
A schematic representation of the fracture
surface of PEEK 450G showing how (a) the major crack
begins, with inclusions/microflaws in the sample; (b)
the major crack propagates and cracks form at the
inclusions; (c) parabolic curves are generated by the
intersection of the major crack front and these growing
fractures; and (d) fast fracture occurs. Courtesy of Ref.
[29]
.
yielding followed by cavitation, then void coales-
cence, flaw formation, and crack growth; zone II
showed no yielding, there was simply cavitation
followed by crack growth through crazes; and zone
III was simply fast fracture (
Figs 5.13 and 5.14
). We
also observed that increasing
k
t
increased the
proportion of the fracture surface that was occupied
by zone III. It is believed that the changes in the
micromechanism are due to the triaxial stress state
experienced by the notched specimens and the
dependence of yielding on the VM stress and of
Figure 5.8
Scanning electron micrograph of a fracture
surface from a PEEK 450G CT specimen monotonically
loaded to fracture. Courtesy of Ref.
[31]
.
cavitation on hydrostatic stress. In zone I, the VM
stress is greater than the yield stress, and the hydro-
static stress is greater than the cavitation stress;
therefore, both yielding and cavitation occur; in zone
II, the VM stress is below the yield stress, but the
hydrostatic is greater than the cavitation stress, so
