Assessment of Fatigue Crack Initiation Behaviour in Ti-6Al-4V Alloy Using a Coarsened Surrogate Model

The microstructure and fatigue crack initiation process of Ti-6Al-4V alloys were measured using a multimodal technique combining synchrotron X-ray microtomography and electron backscatter diffraction (EBSD) serial sectioning techniques. Various microstructural design variables were generated to describe the shape, size and crystallographic information of the polycrystalline microstructure that is the fatigue crack initiation point. The microstructural information was coarsened based on the similarity between the design variables and their correlation with fatigue crack initiation. An objective function describing the resistance to fatigue crack initiation was also established. By combining these variables, the relationship between the microstructural information and fatigue crack initiation resistance was described by a metamodel in the form of a multidimensional response surface using a support vector machine. A limited number of design variables with a high correlation with transgranular and intergranular fatigue cracking were identified, and the optimum or weakest microstructural patterns for fatigue crack initiation were quantitatively represented. This approach is expected to allow much more efficient microstructure control to enhance the fatigue crack initiation resistance than has previously been possible with the conventional surface-based approach.