A new DCPD-based methodology to measure fatigue crack propagation rates of internal cracks in additively manufactured Scalmalloy®

Fatigue crack growth tests performed under ambient conditions typically involve cracks with surfaces exposed to air. However, in real components, cracks often initiate from internal defects and propagate under different environmental conditions compared to the conventional crack growth tests. Previous studies have indicated that, for Ti- and Al- based alloys, internally-initiated cracks exhibit slower growth rates compared to surface cracks. Nevertheless, a robust and direct experimental characterisation of crack propagation rates from internal defects remains limited, mainly due to the challenges associated with the detection of internal crack front with sufficient spatial resolution. This work presents an experimental investigation of internal crack propagation in Scalmalloy® using the direct current potential drop (DCPD) technique. Specimens containing CAD-seeded internal defects of controlled size were produced via laser-based powder bed fusion process and successively subjected to shot peening to suppress surface-initiated failures. The DCPD technique was calibrated through interrupted tests and validated against fracture surface measurements. Crack propagation experiments were also carried out for determining crack growth rates of surface defects which were used to benchmark the data obtained from the internal defects. The proposed methodology provides a practical and versatile framework for the characterisation of internal crack growth behaviour. Moreover, the crack propagation curves measured for the present Scalmalloy® have important implications for fatigue life prediction, enabling a more accurate and less conservative assessment of internal defects within a damage-tolerant design approach.