Vol.25, No.3, 2025, pp. 413–418
https://doi.org/10.69644/ivk-2025-03-0413 

Fractography investigations may unveil the pathway behind the failure by examining the fracture surfaces, and it is a subject that requires careful consideration, particularly in aircraft structures. Fractography can identify crack initiation sites, the direction of crack growth, any associated defect in the microstructure, the environmental effect on fracture, and the type of stress within the material. Proper investigations are needed to identify the origin of failure, the direction of crack propagation, and the related fracture mechanism. This is achieved by assessing the marks on the fracture surface under the microscope. Most of the failures are the result of multiple influences and are rarely single in metallic alloys. Fatigue striations are the most prevalent indicator on the fractured surface of ductile metal alloys in the Paris region of the crack growth curve. In this study, failure analysis is performed after performing crack growth tests for different microstructures in AISI 4340 steels, especially in the near threshold region, and root causes of failure are compared for each case. Crack propagation and morphological changes up to fracture have been observed in AISI 4340 steel. Under SEM and stereomicroscope examination, each stage of crack propagation is detailed with failure analysis.

Keywords: • failure analysis • fatigue • heat treatment • steel • threshold