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J. Tong et alii, Frattura ed Integrità Strutturale, 25 (2013) 44-49 ; DOI: 10.3221/IGF-ESIS.25.07 44 Special Issue: Characterization of Crack Tip Stress Field Near tip strain evolution under cyclic loading J. Tong, Y.-W. Lu, B. Lin University of Portsmouth, UK Y. H. Tai Rolls-Royce plc, UK J.R. Yates University of Manchester, UK A BSTRACT . The concept of ratchetting strain as a crack driving force in controlling crack growth has previously been explored at Portsmouth using numerical approaches for nickel-based superalloys. In this paper, we report the first experimental observations of the near-tip strain evolution as captured by the Digital Image Correlation (DIC) technique on a compact tension specimen of stainless steel 316L. The evolution of the near-tip strains with loading cycles was studied whilst the crack tip was maintained stationary. The strains were monitored over the selected distances from the crack tip for a given number of cycles under an incremental loading regime. The results show that strain ratchetting does occur with load cycling, and is particularly evident close to the crack tip and under higher loads. A finite element model has been developed to simulate the experiments and the simulation results are compared with the DIC measurements. K EYWORDS . DIC; FE; Ratchetting; Crack tip mechanics; Fatigue crack growth. I NTRODUCTION he concept of ratchetting strain as a crack driving force in controlling crack growth has been investigated utilising a variety of constitutive models, including elastic-plastic, visco-plastic and crystal-plastic formulations, for nickel- based superalloys [1-3]. Crack tip deformation fields were examined for both stationary and growing cracks at room and 650  C using the finite element method. Distinctive strain ratchetting behaviour near the crack tip was identified in all cases, leading to progressive accumulation of tensile strains normal to the crack growth plane. It was hypothesised that this tensile strain may be responsible for material separation leading to crack growth. Most recently the concept has been applied successfully [4] to fatigue crack growth of a nickel alloy in vacuum at a range of temperatures, where the influence of oxidation is removed. Although this latest work appears to be very encouraging, no direct experimental evidence is yet available to support this line of reasoning. In this paper, we present the first series of experimental results on the measurement of the near-tip strains as a function of the number of loading cycles in a compact tension specimen of stainless steel 316L using the DIC method. Finite element analysis has also been carried out on the specimen and the near-tip strain results from the simulation are compared with those obtained experimentally. T