Representing crack growth in additively manufactured Ti-6Al-4V

The aerospace industry is now seriously considering the use of Additive Manufacturing (AM) both in new aircraft design and also for aircraft sustainment. However, as explained in MIL-STD 1530 certification requires the operational life of the airframe to be determined by a damage tolerance analysis. In addition, MIL-STD-1530 reinforces this requirement and that and the role of testing is merely to validate or correct the analysis. This means that if AM Ti-6Al-4V produced parts are to be used as load carrying members it is important that the crack growth (da/dN) versus stress intensity change (Delta K) curves be determined and, if possible, a valid mathematical representation is determined. Within the aerospace industry, the NASGRO computer program is the most widely used to compute crack growth and hence to determine operational life. In this context, the present paper demonstrates that the associated crack growth curves can be represented reasonably well by the Hartman-Schijve variant of the NASGRO crack growth equation regardless of the specific AM process studied, the power level used and the build direction. This is achieved by applying the Hartman-Schijve equation to data produced for a wide variety of AM processes. It is also shown that the variability in the various da/dN versus Delta K curves is captured reasonably well by proper changes in the threshold and the effective fracture toughness terms. A consequence of the comparative analysis of the data indicates that if AM-produced parts are to be used on operational aircraft a focus of future research should be on minimizing the size of the nucleating material discontinuities. This includes small naturally occurring cracks that arise due to the stress concentrations associated with the surface roughness as well as material discontinuities in the bulk of the material.