  Fatigue failure driven by stress gradients associated to casting defects was
studied in two cast nickel-based superalloys. The experimental campaign
revealed complex damage phenomena linked to spongeous shrinkages, characterized
by their intricate arrangement of defects in the material medium, forming
defect clusters. Multiple cracks were observed to initiate from defect volumes,
coalescing with neighboring void surfaces along crystallographic planes.
Defects were characterized using X-ray computed tomography, and image-based
finite element (FE) models were constructed as digital representations of each
experimental sample explicitly containing all real casting defects. Numerical
simulations of these FE models under the same conditions as the experiments
revealed that tortuous defects contain small ligaments where very high local
stresses develop. These ligaments initiate early cracks, but due to the limited
stressed volumes, these cracks do not drive the life of the material. A
thorough comparison of simulations with experiments led to the development of
an original method to define stressed volumes and address small ligaments.
Finally, a novel energy-based non-local model was proposed, using two
parameters to predict the fatigue lives of samples containing casting defects
at the mesoscopic scale. The model was validated against samples with varying
porosity levels, sizes, and geometries, accurately predicting fatigue lives
within a factor of 3 compared to experimental results. This new approach
generalizes the application of non-local methods to real casting defects by
considering their shape and stressed volumes to estimate fatigue properties.
