Near-tip strain evolution under cyclic loading: In situ experimental observation and numerical modelling

Abstract

International audienceThe 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 quantitative experimental evidence of near-tip strain ratchetting with cycles, as captured in situ by digital image correlation (DIC) technique on a compact tension specimen of stainless steel 316L, using both Stereo and SEM systems. The evolution of the near-tip strains with loading cycles was monitored whilst the crack tip was kept stationary. The strains normal to the crack plane were examined over selected distances from 6 to 57 lm to the crack tip for a number of cycles. The results show that strain ratchetting occurs with loading cycles, and is particularly evident close to the crack tip and under higher loads. 3D finite element models have also been developed to simulate the experiments and the results from the simulation are compared with those from the DIC measurements. This is the first time that near-tip strain ratchetting has been captured in situ at the peak loads during cyclic loading

In Situ Experimental Study of Near-Tip Strain Evolution of Fatigue Cracks

“…3. Although scatters are evident at all locations, a trend of increasing strain with cycle is also apparent, demonstrated by the trend lines for Points 1-5, consistent with the observation by Tong et al [13]. During Test 1, the strain values registered at Point 1 are 3.5 to 4.5 times higher than the strain values of Point 5.…”

“…Measuring near-tip strains has been carried out by a number of researchers, notably by Li and Orlecky [25], Peralta et al [33] and Tong et al [13], amongst others. Measurements were carried out in SEM utilizing Bgridsd eposited onto the specimen surfaces to obtain the desired texture for DIC measurements.…”

“…This criterion has been successfully applied to rationalise fatigue crack growth in nickel-based superalloys, although direct experimental validation was not attempted until very recently, when the first experimental evidence of near-tip strain ratchetting was reported from in situ experiments performed using both stereo and scanning electron microscope (SEM) Digital Image Correlation (DIC) systems [13].…”

“…Most recently, we [13] reported an in situ and quantitative experimental evidence of near-tip strain accumulation with cycles, using both DIC coupled with optical microscopes and a grid technique with SEM imaging. The experiments were carried out on CT specimens of stainless steel 316 L under a series of step-wised loading blocks with increasing constant amplitude, whilst the crack tip was maintained stationary.…”

Application of digital image correlation for estimation of fatigue crack closure level

“…According to the proposed formulation, it is important to measure the size of the plastic zone ahead of the crack tip and construct a crack propagation theory based on the plastic zone ahead of the crack tip. The data related to the estimation of the strain field behind and ahead of the crack tip can be found in [12][13][14][15].…”