✍ Scribed by Vibhor Chaswal; G. Sasikala; S.K. Ray; S.L. Mannan; Baldev Raj
No coin nor oath required. For personal study only.
Room temperature fatigue crack growth (FCG) testing was conducted after thermal aging for 500 and 5000 h at 823 K and for 500 h at 923 K, on 9%Cr-1%Mo steel normalized at 1323 K and tempered at 1053 K. Though crack growth behavior in Paris regime is not much affected by aging, stress intensity factor range for threshold ( K th ) is found to increase monotonically. Acoustic emission signal analysis indicates identical crack propagation mechanisms in both threshold and Paris regimes. But SEM fractography reveals micro-voiding in the Paris regime only. Also, theoretical analysis of plastic zone size and inter-particle spacing (λ m ) predicts microcleavage in the threshold regime. This indicates FCG occurs as a two-step mechanism, but each regime has a different dominant step viz. initial micro-void nucleation and growth at precipitates, followed by eventual micro-cleavage of inter-particle ligaments. Aging induces solid solution dilution, precipitate coarsening and increases λ m . In threshold regime, due to a tougher matrix, and a smaller cyclic crack increment (␦a N ) than λ m , an increase in K th values is observed. Conversely, Paris regime ␦a N is greater than λ m , making it rather insensitive to aging.
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The fatigue crack propagation behaviour of SAE 1010 steel sheet specimens containing elliptical holes under uniaxial loading conditions at room temperature is investigated. The experimental data is analysed in terms of variability of C and n. Also results are presented for the crack growth rate and