Abstract
Fatigue crack growth tests under Hi-Lo two-step, repeated two-step and programmed loadings were performed on grain-oriented 3% silicon iron in a high-resolution, field emission type scanning electron microscope, using a specially designed servo-hydraulic fatigue loading system, and direct, real time observations were made, partly with aid of image processing technique.
Under Hi-Lo two step loadings, the relationships of crack growth direction and crack tip opening displacement to crack growth rates were found quite different from those of constant amplitude loading test. On the contrary, the relationship between crack tip opening displacement and effective stress intensity range under repeated two-step loading could be considered similar to that of constant amplitude loading. Moreover, the change of crack tip opening displacement was found to correspond well to the range-pair of stress intensity range. From the above mentioned observation results, microscopic and macroscopic controlling parameters of fatigue crack growth were discussed.
