Measuring elastoplastic strain loops in the near crack-tip region using a Stereo Microscope DIC system

•Strain measurements using a stereo microscope DIC system.•Strain loops or Δε measured inside the near crack-tip region during the cyclic loading.•Experimental evidence to investigate the actual fatigue crack growth driving force. In this work, DIC analysis is used to assess the cyclic plastic deformation response, Δε, at the near crack-tip region of propagating fatigue cracks and to identify the effects of other phenomena that happen during crack propagation, such as crack closure and partial crack opening on the measured elastoplastic strain loops. The DIC measurements use a stereo microscope and a very refined speckle pattern on the specimen surface. Fatigue tests were performed in constant ΔK and Kmax conditions on aluminum and steel specimens. Such measurements can be used to investigate the actual fatigue crack growth driving force.