Quantitative Evaluation of Fracture Toughness-Microstructural Relationships in Alpha-Beta Titanium Alloys

The fracture toughness of two alpha-beta titanium alloys containing an alpha platelet in a transformed beta matrix has been examined in terms of the microstructural parameters controlling the fracture initiation and propagation in the alloys. Equations have been formulated that show that the highest toughness values of both alloys were associated with the finest platelet spacings and the thickest alpha platelets. It is proposed that the fracture initiation process in both alloys is controlled by the distance between the platelets, the fracture toughness of the alloys being dependent on the distance between active centers of void nucleation, i.e., as a function of the alpha platelet thickness and spacing between the platelets. Seven models of ductile fracture relating fracture toughness to mechanical property and microstructural parameters have been compared in their ability to predict the toughness of the alloys after solution treatments, which produce varying platelet thickness and inter-platelet spacings. The principle has been adopted following Rice and Rosengren and Hutchinson (HRR)[1,2] that there must be a 1/x energy singularity at the crack tip, which also prescribes the stress and strain distribution ahead of a crack tip. Any model not incorporating these requirements should be rejected.