AbstractThe cyclic plastic work of fatigue crack propagation,Qpw, and the effective surface energy,U, were measured for an annealed 1008 steel in humid air and dry nitrogen using calorimetryQpwwas not significantly affected by the humid air environment suggesting that the hydrogen generated by the oxidation of iron at the crack tip does not strongly affect the cyclic properties for this materialUwas identical and constant for the two environments for crack growth rates greater than ∼ 100 A/cycle. Below da/dN∼ 100 A/cycleUdecreased rapidly with decreasing δKin the humid air environment It was concluded from these two observations that the main effect of hydrogen on crack growth in this alloy was to lower the atomic surface energy, γ, thereby lowering the local fracture s