The onset of sub-critical crack growth during slow strain rate tensile testing(SSRT) is assessed through a combined experimental and modeling approach.A systematic comparison of the extent of intergranular fracture and expectedhydrogen ingress suggests that hydrogen diffusion alone is insufficient to ex-plain the intergranular fracture depths observed after SSRT experiments ina Ni-Cu superalloy. Simulations of these experiments using a new phase fieldformulation indicate that crack initiation occurs as low as 40% of the timeto failure. The implications of such sub-critical crack growth on the validityand interpretation of SSRT metrics are then explored.