In situ near‐tip normal strain evolution of a growing fatigue crack

Abstract

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

A parametric study of DIC measurement uncertainties on cracked metals

“…In this study, physical features were used in physical experiments, where the noise level was found to stabilise when the subset size is 50 × 50 pixels 2 or above, a value used in a previous study of full-field near-tip strain mapping. [14] Under loads, large deformations and large gradients as well as discontinuities due to the crack opening present major challenges to image correlation. As shown in Figure 7b, increasing load led to significantly increased SDs, although the trend in strain random errors under loads is similar to that under zero load, that is, higher SDs for smaller subset sizes.…”

“…The error analyses were carried out under three selected loading conditions equivalent to stress intensity factors (SIF) of 0, 20, 30 MPa√m using an Instron servo-hydraulic testing machine. A LaVision DIC system was used for image acquisition, including a 12-bit CCD camera mounted on an optical microscope with a Schneider Kreuznach 50 mm lens and an extension tube of 100 mm [14] (Figure 3). The lighting setup and the aperture position were critical for imaging the etched surface, since the surface reflection of the metal is less bright and shows less contrast than, for example, a speckle pattern on painted white background.…”

“…In physical experiments, painted speckles are sized around 5-10 μm [40] and are considered suitable for a resolution of about 10 μm/pixel, [41,42] though resolutions of 2.5 to 0.7 μm/pixel have been achieved recently. [12][13][14] In order to achieve a resolution of around 0.5 μm/pixel, which is close to the optical microscopy resolution limit of circa 0.2 μm, [43] methods such as exposing material surface texture by etching have been considered. Etching has been used for steel, [44] copper, [45] and at micro-scale under SEM, [46] where sufficient contrast may be achieved from fine-grained microstructures or materials with sub-grain features, such as in martensite or bainite.…”

“…[2,3] Later algorithms for more robust and faster convergence were proposed, [4,5] which enabled this technique to be used for a range of applications, including displacement and strain measurements for fracture or crack growth analysis. [6] DIC was also utilised for the determination of stress intensity factors, [7,8] for fatigue crack growth analysis, [9][10][11] and more recently carried out in situ, [12][13][14] which allows tracking of instantaneous near-tip deformation during crack propagation under cyclic loading.…”

In Situ Measurement of the Strain Field at the Fatigue Crack Tip Based on Sub-Image Stitching and Matching DIC

“…A combination of the DIC technique and a microscopic camera system can provide enough spatial resolution to spatially and temporally measure the crack tip strain field during fatigue crack growth process. These characteristics have made micro-DIC the first choice for fatigue crack growth research in recent years, rather than scanning electron microscopy–digital image correlation (SEM-DIC) or synchrotron radiation X-ray devices that require more complex technology and specialized equipment [ 20 ]. Zhu et al [ 20 ] studied the normal strain near the fatigue propagation crack of a 316L stainless steel compact tension (CT) specimen under the action of tensile cyclic load by micro-DIC.…”