3.2 | Crack initiation and propagation
Figure 8 shows the partially broken blades. And the crack sources on the
fracture surface of the broken turbine blades are observed by the
optical microscope (OM) (Figure
9(a)). The number of crack sources of the PSP treated blades (C2, C5) is
less than that of the untreated (B2, B4) blades. Therefore, it is
determined that the PSP treatment can significantly reduce the number of
crack sources while the vibration stresses are below the threshold
vibration stress. In contrast, the number of crack sources of PSP
treated blade (C9) is more than that of untreated blade (B9) while the
vibration stress is above the threshold value. This indicates that the
number of crack sources cannot be reduced by the PSP treatment while the
vibration stresses are above the threshold vibration stress.
The crack sources of untreated (B2, B9) and PSP treated (C2, C9) blades
are shown in Figure 9 (b)-(e). The cracks of B2, B9 and C9 blades
initiate from the top surface. In contrast, the crack initiation
position of the C2 blade is the matrix near the top surface. Especially
for the C9 blade, the
lamellate structure
can be observed near the crack source on the fracture surface.
Figure 10 shows the fatigue striations of the crack propagation section
on the fracture surface of untreated (B2,B9) and PSP treated (C2,C9)
blades. All the selected observation areas are 100um away from the crack
source at the critical position. According to the measurement results of
fatigue striations marked in Figure 10, the width of fatigue striations
for the PSP treated blade (C2) is smaller than that of the untreated
blade (B2). This indicates that the PSP treatment is able to reduce the
growth rate of the cracks due to the compressive residual stress while
the vibration stresses are below the threshold vibration stress, for
which it is beneficial to improve the fatigue property of superalloy.
However, the crack growth rate of B9 blade is greatly accelerated by the
large vibration stress compared with that of B2 blade, which
demonstrates that CCF damage significantly increases. Especially for the
C9 blade, the fracture surface near the crack source is characterized by
the small dimples accompanied with the lamellar structure.