**4.4 Failure root cause and prevention**

Failures of components are frequently activated by defects introduced during manufacturing [33]. Due to the prevalence of manufacturing defects, critical components are usually subjected to a thorough inspection to prevent defective parts from entering service, and sometimes this effort is unsuccessful. Several shortcomings are associated with the casting operations, which are potential causes of product failure. For example, it is well known that core or subsurfaces discontinuous, including voids, blowholes, shrinkage cavities, pipes and porosities created during ingot solidification, are identified sources of imperfections during succeeding manufacturing steps or in service as these weak points initiate cracks in the components that eventually lead to their failure [34]. The oxide inclusions in castings could indirectly induce crack initiation by enhancing local stress concentration, promoting cleavage fracture and detrimental to the fracture toughness of the components [12]. Nonmetallic inclusions also create discrepancies such as thermal expansion mismatch, stiffness mismatch, chemical mismatch, and ductility mismatch [22, 35], which ultimately impair the cast products' mechanical properties and service performance. It is therefore imperative to understand the impact of inclusion defects on the structural integrity and the fracture toughness of cast parts about their interaction with the main characteristics of fracture mechanics, including defect crack size, loading and material toughness.

As illustrated in **Figure 14**, nonmetallic inclusions in the rotor's swan neck reduce the disc's adequate thickness. The design thickness in the rotor neck region is 6.62 ± 0.01 mm, but the inclusions defects were positioned at approximately 1.620 ± 0.237 mm away from the rotor inner radial perimeter. During the braking regime, the disc swan neck can be exposed to cyclic compressive or tensile loads (**Figure 14**). However, inclusions in the neck region would reduce the swan neck's design load-bearing capacity, consequently
