A comparison is made of the crack resistance (R-curve), fracture toughness, and instability behavior of candidate damage-tolerant aluminum-lithium (Al-Li) alloys, 2091 and 8090, and the widely used conventional 2024-T3 alloy. The 2091 alloy was in three heat treatment conditions, T8X, TX, and TY, all artificially aged. The 8090 alloy was in the T81 condition. The crack resistances and fracture toughnesses of 2091-T8X and 8091-T81 were similar to those of 2024-T3, but at a 50 MPa lower strength level. The crack resistances and fracture toughnesses of 2091-TX and 2091-TY were much inferior.
In all cases, stable (slow) crack growth was ductile, but unstable crack growth in 2091-TX and 2091-TY was 100% intergranular and macroscopically brittle. Unstable crack growth in 2091-T8X was 50% intergranular and macroscopically ductile. Fractographic analysis indicated the 2091-TX and 2091-TY alloys to be sensitive to dynamic effects, such that the dynamic fracture toughness could be significantly lower than the quasi-static fracture toughness. This may also be true of 2091-T8X. In view of these results, fail-safe crack arrest tests should be included in any evaluation of damage-tolerant Al-Li sheet alloys for aircraft structures.