191 / 352
187
A cyclic plastic zone forms at the crack tip and the growing crack leaves behind the
plastic wake. If the plastic zone is sufficiently small, that it is embedded within an elastic
singularity region, the conditions at the crack tip are uniquely defined by current
K
value,
and the crack growth rate is characterized by
K
max
and
K
min
(Fig. 17). Fatigue cracks
propagation is often followed by striations on a fracture surface. Striations are small
registered perpendicular to the direction of crack propagation.
Figure: 13. Fractograph of NIOMOL (F- fatigue, SZ- stretch zone, C- cleavage) /12/
0
100
200
300
400
500
600
0
10
20
30
40
Measurment number
w,t and h in um
w
t
h
Figure 14: Typical variation for NIOMOL in the measured
w
,
t
and
h
/12/
Figure 15: A schematic showing the method for determination of
J
szw
using SZW