Near crack tip strain fields in a stationary and a growing fatigue crack have been studied in situ using the Digital Image Correlation (DIC) technique in a compact tension specimen of stainless steel 316 L under tensiontension cyclic loading. The evolution of near-tip strains normal to the crack plane was monitored at selected locations ahead of the crack tip in consecutive cycles during fatigue crack growth experiments. A stationary crack was examined first to provide a baseline reference whilst the evolution of the strain field ahead of a growing crack was monitored in situ at peak loads during fatigue cycling. The results show that strain accumulation with loading cycle occurred at all tracked locations in both cases, and it is particularly evident close to the crack tip. Moreover, a higher strain accumulation rate was found near the growing crack tip than that near the stationary crack tip. The results on near-tip strain evolution were collected for the first time in situ during the fatigue crack growth experiments, which may hopefully inform a physical-based modelling strategy of fatigue crack growth.

A B S T R A C T Normal strains near a growing fatigue crack have been studied in situ using the Digital Image Correlation technique in a compact tension specimen of stainless steel 316L under tensiontension cyclic loading. An error analysis of the measured displacements and strains has been carried out, and the results show that the precisions of displacements and strains in the direction perpendicular to the crack plane and ahead of the crack are better than those parallel to the crack plane and in the wake of the crack. The errors in the measured displacements are in the range of 0.06 to 0.1 μm and between 0.2% and 0.4% for the measured strains, the latter are well below the critical strain at the onset of crack growth about 8%. Strain ratchetting was found ahead of the growing fatigue crack tip, albeit very close to the crack tip. a = crack length K = stress intensity factor ΔK = range of stress intensity factor P max = maximum load P min = minimum load R = load ratio V y = normal displacement value V y = average normal displacement value W = width of Compact Tension specimen x 0 = horizontal coordinate of the crack tip (in pixel) y 0 = vertical coordinate of the crack tip (in pixel) ε yy = normal strain Δε yy = range of normal strain