The effect of constraint-induced normal stress on the failure of notched TiAl components

Titanium aluminides are candidates for replacing nickel superalloys in some aircraft engine components. In uniaxial tension tests, these materials experience plastic strains at failure that place them in-between traditional definitions for ductile and brittle materials. This study considers the appropriate continuum mechanics failure criterion for these materials under multiaxial loading conditions (based on either the maximum equivalent plastic strain or the maximum normal stress). The material tested is a Ti-47.0Al-2.0Cr-1.9Nb alloy having a predominantly lamellar γ/α ₂ microstructure. Cylindrical notched tensile specimens that experience elevated normal stresses in their interiors due to circumferential constraint during plastic deformation have been investigated. Results are presented that quantify reductions in failure loads due to elevated normal stress, compared to those predicted by finite element models using a maximum equivalent plastic strain criterion. To properly interpret the experimental results, the effects of notch strengthening must be included in the model predictions. Model and experimental results suggest that this TiAl alloy has some sensitivity to normal stress and that a combined failure criterion is needed to accurately predict failure under multiaxial loading conditions. A fracture initiation and failure mechanism requiring a combination of normal stress and plastic straining is suggested that is consistent with observed features at fracture initiation sites.