Page 387 - Biaxial Multiaxial Fatigue and Fracture

The Environment Effect on Fatigue Crack Growth Rates in 7049 Aluminium Alloy at ... 371

Figures 7 (a) and (b) show the dependence of on R for specified constant fatigue crack
growth rates in vacuum for OA and UA A1 7049 alloy, respectively. These figures again
demonstrate the different deformation features of the two microstructures.

32
30
28
26
24
22
Fa
E 18
B ;:
3::

8
6
4
2
-1.0 0.5 R 0.0 1 .o

3

Fig. 7. hKth values for different FCGR at different R-ratios
for (a)OA alloy and (b) UA alloy, in vacuum.

Vasudevan and Sadananda [48,49] have pointed out that two critical threshold values,
namely A&, and K,, are the necessary requirements for a crack to advance. Likewise two
critical values above the threshold regime are necessary to obtain specified constant fatigue
crack growth rates. This is shown in principle in Fig. 1 (a). In order to obtain defined crack
growth rates of (dalcW),, (daldN)~. . .the critical values &,I, &ha.. .and Kmx,l, Kmax,z.. .are
needed. As a result, the values, which have been reconstructed from data of the OA alloy as in