CREEP-FATIGUE CRACK GROWTH AND FRACTOGRAPHY OF METALLIC MATERIALS IN AIR AND VACUUM AT ELEVATED TEMPERATURES

Creep-fatigue crack growth behaviour of a Type 304 stainless steel, a quenched 21/4Cr-1Mo steel, Hastelloy X, a Ti-6242 alloy, and a low carbon steel under different reversed loading patterns (P-P, C-P and C-C) were investigated in air and a vacuum environment. The results are discussed in the light of fracture mechanics and fractography. Crack growth rates for all of the materials tested were successfully correlated in terms of the cyclic J integral range (Δ J ) irrespective of the loading patterns. In the low growth rate region, where fatigue fracture was predominant, crack growth rates of all the materials were about the same for the same value of Δ J. On the other hand, growth rates were somewhat different, depending on the creep ductility of the material in the region of high growth rate, where creep fracture was predominant. Materials with lower ductility exhibited higher growth rates for the same Δ J values. Differences were insignificant between the crack growth rates in air and vacuum and were consistent with the small differences observed in the fracture surface morphology in the two environments.