Effect of viscosity of an oil environment on fatigue crack growth rate in AISI 316 stainless steel

The effect of viscosity in the range of 10–101500 cS on fatigue crack growth rate, FCGR, (ΔK = 11 − 25 MPa √m) in 316 stainless steel tested in silicone oil was investigated and compared with behavior in air. The four orders of magnitude variation in viscosity had no effect on FCGR, nor on the crack opening load Pop. The FCGR and Pop were smaller in the oils than in air for ΔK < ~ 18 MPa √m, but no difference occurred at larger ΔK values. Microscopy observations revealed more zig-zags in the crack path for the air environment compared to the oils; also, cleavage facets on the fracture surface and debris along the crack path occurred in air but not in the oils. It is concluded that the observed difference between FCGR in the oils compared to air results mainly from shielding by the oil of the crack tip against chemical reactions.