Volume 22, Issue 1 (January 1994)
On the Uncertainties Involved in Quantifying the Fracture Toughness of Al-Li-X Alloys
The fracture behavior of an extruded Al-2.6Li-0.09Zr alloy in a near peak-aged and over-aged condition was found to be primarily intergranular with formation of non-Mode I, crack-divider delaminations at the crack tip especially in the over-aged temper. This complex, irregular fracture mode results in behavior that cannot be accurately measured using the conventional J- and K-based toughness parameters and testing methods. Other observed fracture characteristics that force reassessment of the traditional testing methodologies include crack length variations, mixed mode fracture from crack deflection, and apparent plasticity. The observed crack length variations through the specimen thickness led to 4 to 8% uncertainty in the K-curve for the specimens used. Furthermore, crack deflection corrections lead to an average 4 to 7% and maximum 15 to 18% apparent toughness increase for the material. The alloy exhibited nonlinear load-displacement behavior during fracture that could be corrected by using a plastic zone size modification. The analyses outlined provide insight into the roles the various mechanisms play in the fracture process of this alloy. However, the conventional toughness measures were inadequate in treating the observed fracture behavior.