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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residual stress to reduce thickness. The residual stress of core Al influences the anodized alumina layer properties by altering the pore nucleation and growth in terms of pore size variation and pore density. However, the pore density is primarily affected by the residual stresses; tensile nature residual stresses increase the pore density (Liao and Chung, 2013). The observed differences in residual stress between the two types of specimens can have significant implications on their fatigue behaviour. Residual stress is known to influence the initiation and propagation of fatigue cracks in materials, and these contrasting stress states can affect the fatigue performance of CAA and CMAS-AA differently. However, the stress values are minimal, and the significance of such small values may be neglected from an engineering point of view since the COV of different batches of aluminium alloy is as high as 10%, see MIL-HDBK 5J, 2003. 5.4. Fatigue Test The fatigue behaviour of CAA and CMAS-AA samples was investigated as part of the design data development, and the results are presented in Table 2 and illustrated in Fig. 5 as S-N curves. The data from both types of samples were used to fit the Basquin ’s power law equation between Maximum Stress (σ Max ) and the number of cycles to failure (N f ) as in equation (1) : σ Max = A (N f ) B (1) Where A and B are constants in the power law equation. The fit parameters are tabulated below:

Table 2. Design values from S-N curves

Fatigue Exponent

Fatigue Strength Coefficient

S.No

Condition

MPa

-

1

CAA 2024-T3 sheet specimen, 1.6 mm thickness

3838.5

-0.227

2

CMAS sheet specimens, 1 mm thick

1805.1

-0.156

Despite having similar static properties, the fatigue life of Cladded Al alloy (CAA) specimens was marginally higher at higher stress levels, while Chemically milled, anodized, and sealed (CMAS) samples exhibited better fatigue life at lower stress levels. Fig. 5 illustrates the mean S-N curves for both types of materials, along with a 95% confidence interval, showing some overlap in a significant portion of the confidence interval. This suggests that, in some instances, replacing cladded specimens with thickness reduced by chemical milling followed by anodized material might offer similar structural integrity from the fatigue perspective. Comparing these S-N curves with electropolished (unprotected, mirror-finished bare 2024-T3) specimen data obtained from the MIL handbook (MIL HDBK-5J, 2003), the fatigue life of corrosion-protected CAA and CMAS samples is observed to be 3-5 times lower. Although CAA and CMAS-AA showed similar fatigue life, further investigation is required to understand the reasons for the above reduction. Surface roughness, residual stress, size and distribution of pores etc. may be considered as influencing parameters. Differences in fatigue behaviour can be attributed to various factors, as highlighted in the existing literature. For CMAS samples, the brittleness of the anodized layer and surface irregularities beneath it have been identified as contributors to decreased fatigue life (Shahzad et al., 2010). Furthermore, the choice of sealing method during anodization has been noted to reduce fatigue resilience further (Wragg et al., 2022; Shimpo et al., 2003). Another significant influencer of fatigue behaviour is the average surface roughness. CMAS specimens exhibit significant