A novel experimental micromechanics technique SIEM (Speckle Interferometry with Electron Microscopy) is employed to examine the plastic deformation field at crack tip during fatigue process. A fatigue cracked specimen made of AISI-SAE 1035 carbon steel was tested under a three-point bend load. Both displacement and strain fields were mapped over an area of 50μm by 40μm at the crack tip. Accumulation of the local damage due to fatigue was revealed by showing the evolution of the plastic zone at the crack tip. Crack growth was found in mixed mode at small scale. The variation of plastic crack opening displacement with the number of fatigue cycles in both mode I and mode II were determined. Two mechanism of crack propagation, crack branching and crack-wake ligament bridging, were observed and clearly demonstrated on the deformation map.

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