Creep crack growth and cavitation damage in a 12% CrMoV steel

Creep crack growth of a 12% CrMoV steel has been investigated at 650°C in SENT specimens with and without side-grooves approaching plane strain and plane stress conditions respectively. Experiments have been conducted under different loading conditions including constant $\text{C}{}^{\mathrm{\ast }}$, the J-integral rate parameter. Transitions in crack growth behaviour are observed early in the crack growth and later when the crack becomes long with respect to the specimen width. The early transition is dominated by the initial development of cavitation damage ahead of the crack tip. Once this transition is complete, the correlation between $\text{C}{}^{\mathrm{\ast }}$ and the crack growth rate becomes valid. This correlation is independent of loading conditions, it coincides with theoretical expectations and it is valid until the second transition where the extent of damage has a dominant influence on the stress field ahead of the crack tip. A trend has been observed that crack growth is slower in smooth-sided specimens as compared to the crack growth in side-grooved specimens for the same value of $\text{C}{}^{\mathrm{\ast }}$. Metallographic investigations indicate that crack growth occurs by the formation and growth of cavities and microcracks ahead of the crack tip. A comparison of the damage in smooth-sided and side-grooved specimens shows that creep damage is much more extensive under plane stress conditions than under plane strain conditions for the same value of $\text{C}{}^{\mathrm{\ast }}$. This difference is due to the multiaxial stress field, produced by the side-grooves, which influences cavitation and microcrack formation in the material. It also accounts for the smaller crack growth rates in smooth-sided specimens.