Fatigue crack growth rates and striation spacings have been examined for a high-strength material, Ti-6Al-4V, and a low-strength material, 304 steel, ranging from the linear elastic fracture mechanics (LEFM) regime to the elastic-plastic fracture mechanics (EPFM) regime. The limitation of LEFM fatigue crack growth law was discussed. Departure from the LEFM regime to the EPFM regime was found to result in acceleration of crack growth rate as well as retardation of resharpening of crack tip. The results suggest that the EPFM approach to fatigue crack growth may not be extended by the use of the J-integral concept, so that unloading effects are not included.
