An experimental investigation of the subcycle fatigue crack growth mechanisms at different scales is proposed in this paper for aluminum alloy and steel using in-situ mechanical testing and imaging analysis. At lower resolution, digital images are taken with optical microscopy at the micrometer scale. Digital image correlation is used to get the strain distribution in front of the crack tip to analyze the plastic deformation behavior. At higher resolution, images are taken with scanning electron microscopy at the nanometer scale. Automatic image tracking is used to obtain detailed crack tip deformation and crack growth. The epxrimental study is performed for two types of metallic materials, i.e., aluminum 7075-T6 and ANSI 4340 steel. Crack closure is observed in allumnium materials, but not in steels. Their plastic zone variation shows very different behavior under constant amplitude loadings. The underlying mechanism is discussed. Finally, the potential application of the proposed investigation method to other materials and to other types of mechanical damage is discussed.