Fracture toughness and fatigue crack growth rate (FCGR) of 7075T73510 alloy have been studied. The difference in properties between (T-L) and (L-T) direction were analyzed.The results of plane stress fracture toughness indicate that fracture toughness isorientation dependence as shown in Table 1. Fracture toughness in L-T direction is 10 MN m-3/2 higher than that in T-L direction. Typical fracture toughness fracture features are shown in Fig. l. Impurities have less cracks and dimples based on impurities are large and deep, and more shear planes are existed in L-T direction. Dimples in transverse fracture surface are shallow, and there are striplike impurities with small spaceing.FCGR results of 7075T73510 alloy in T-L and L-T direction are shown in Fig. 2. It shows that FCGR is sensitive to stress ratio. When stress intensity (AK) is lower than 10 MPa' m1/2, FCGR in two directions makes little difference, while EKe 10 MPa' m1/2,crack growth rate in T-L direction is faster than that in L-T direction. With the increasing of stress ratio, FCGR in L-T direction and in T-L direction tended to identity. Examination of FCGR fractures shows that fracture features are much difference with different stress ratio at the same stress intensity as shown in Fig.
3. FCGR obtained both in L-T and T-L direction at stress ratio of 0.6 is two time larger than that obtained at stress ratio of 0.1 when UK is almost the same. Fracture features at both directions have no obviousdifference when stress ratio and stress intensity factor (≤10 MPa' m1/2') are fixed.The main difference of fracture toughness in two directions is due to the different volumefraction of cracked impurities. Impurities have little influence on FCGR at a low stess intensity (AK). At a high AK, the increasing of stress concentration and the stringing of largeimpurities cause that FCGR in T-L direction is higher than that in L-T direction. With stressratio increasing, both matrix and impurities take effects, so FCGR in the two directions aregradually identical. (Correspondent; JIAO Chengge, Institute of Aeronautical Materials, P O Box 81(2) Bejing 100095)