Fatigue Crack Paths 2003

Loading axis

250 μ m

Hole

Average crack growth direction

Figure 5. A typical crack of type 3 showing deviations, slip bands, and straigtht

propagations. Specimen 3, crack A, 56000 cycles.

I N F L U E N COEFT H EL O C AMLI C R O S T R U C TOUNRCER A CPKA T H

Zhai and al. [3] have shown in an Al-Li alloy that a key parameter for the local crack

path when a grain boundary is encountered is the twist angle α of the crack planes

before and after the boundary (see Fig. 6). In order to check this mechanism for the

2024 alloy, Schmid factor, twist angles (α) and disorientations angles at grain

boundaries (θ) have been measured from the E B S Ddata. Since our alloy present a

microstructure with pancake/disc grain shapes due to rolling, we have assume (like Zhai

and al.) that the crack meets grain boundaries parallel to the (L,S) plane as described in

Fig. 6.

S

Crack path

L=loading

T= average crack

axis

Specimen

growth direction

surface

θ

in

α

Crackplane grai 2

2

Grain

Crack plane 1 in

grain 1

boundary

Figure 6. Definition of the twist angle α as the misorientation of the crack planes when

a grain boudary is encountered.