Fatigue Crack Paths 2003

k p r o p a g a o n t i

k p r o p a g a o n t i

C r a c

C r a c

10μm

10μm

(a)ΔJIII/ΔJI = 0.5 (b) ΔJ III /ΔJ I = 2

Figure 7. S E Mmicrographs of mixed-mode fatigue fracture surface.

=2x104 N/m. In displacement-controlled tests of mixed-mode loading, the total J

integral ranges, ΔJ, and the mode ratio of ΔJIII/ΔJI were nearly constant during crack

extension of about 1 m mfrom the pre-crack, and the crack propagation rate was also

nearly constant. In Fig. 6, the mean propagation rate was plotted against the total J

integral range. The relations between da/dN and ΔJ for mode I and III were also shown

in Fig. 6. The crack propagation rate was expressed as a power function of the J-integral

range for pure mode I, and also for pure mode III. The propagation rate is lower under

mode III loading than under mode I at the same ΔJ value. All the data of mixed mode

lie between the relations for mode I and III. Therefore, the relation between da/dN and

ΔJ for mode I gives a conservative estimate for the case of mixed-mode loading. At the

same ΔJ, the crack propagation rate decreases with increasing ratio of ΔJIII/ΔJI.

Fractography

The fatigue fracture surface of all the specimens whose mixed modedata were indicated

in Fig. 7 was macroscopically flat and did not show factory-roof type topography.

Figure 7 presents S E Mmicrographs of fatigue fracture surface made under ΔJIII/ΔJI=

0.5 and 2. Striations can be seen in the micrograph for mixed-mode cases, although

more amount of rubbing marks coexist as the mode III component becomes bigger.

Those striations suggest that the mechanism of mode I crack propagation is operating

even in mixed-mode propagation. On the other hand, no striations were observed on the

fracture surface made under pure mode III. Only rubbing marks were seen extending

perpendicular to the growth direction.

The striation spacing was measured on the fatigue fracture surface made under mode

I and mixed-mode conditions. Figure 8 shows the relation between the striation spacing, s, and the crack propagation rate. In the range of crack propagation rate between

7 1 1 0 − × and

6 4 1 0 − × mm/cycle, the striation spacing is equal to the crack propagation

rate.

C O N C L U S I O N S

(1) The J-integral value under mixed-mode (I+III) loading estimated from the simple

method was in good agreement with that obtained by the energy method for long

cracks.