PSI - Issue 60

Chinnam Sivateja et al. / Procedia Structural Integrity 60 (2024) 245–255 Sivateja et al. / Structural Integrity Procedia 00 (2023) 000 – 000

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surface pores in the aluminium oxide layer compared to Cladding, resulting in higher stress concentrations due to increased roughness. This disparity may accelerate crack initiation in CMAS samples (Schneider and Fürbeth, 2022; Arola and Williams, 2002). The interplay of various factors affecting fatigue behaviour at different stress levels is crucial. These factors include: 1. Plane stress fracture toughness (K c ): Reduction in thickness leads to higher toughness and extended fatigue life. 2. Surface Roughness: Surface roughness proportionally affects cyclic life. 3. Residual Stress State: Residual stress (Particularly tensile stress) significantly contributes to early crack initiation and may be in the case of CMAS specimens. At lower stress levels, CMAS may benefit from a higher toughness due to its reduced thickness, resulting in longer fatigue life due to higher propagation lives. Conversely, the reduced fatigue life of CMAS-AA versus CAA at higher stress levels arises from the impact of surface roughness and the dominant role of tensile residual stress in initiating early cracks. This can be attributed to earlier lead crack formation originating from multiple cracks initiated at surface notches in CMAS, contributing to their reduced life span.

Fig. 5. S-N plot of chemically milled and bare samples In conclusion, the fatigue behaviour of CAA and CMAS samples shows similarities within their respective ranges. Despite having a similar mean life, the differences in fatigue life at different stress levels warrant further investigation and study. 5.5. Fractographic studies Fractographic studies on the failed samples were conducted using a Scanning Electron Microscope (SEM). The fatigue fracture surfaces exhibited a standard fracture morphology characterized by a thumbnail-type crack, as shown in Fig. 6, commonly observed in various engineering metallic materials. To determine the plane stress fracture toughness (K c ), the crack sizes measured from these fractographs were analyzed using the methods of Newman and Raju (Newman and Raju, 1981). The computed results are tabulated in Table 3. The Variation of the Stress Intensity Factor (SIF) on the crack front at the failure of CAA and CMAS tested at σ max of 350 MPa is depicted in Fig. 7. Results indicated that the CAA samples exhibited a Kc value of 23.9