Crack closure and slip irreversibility in the fatigue crack growth of 316L stainless steel were investigated using digital image correlation (DIC). A polycrystalline and a oriented specimen were tested to study mode I fatigue crack growth, while oriented specimens were used to study mixed mode (I and II) fatigue crack growth. DIC was used to determine the crack growth rates and crack tip displacement fields during the fatigue loading cycles. During this study, a least-squares regression algorithm using the vertical and horizontal displacements was developed, and the stress intensity factors, KI and KII, and the T-stress for the isotropic and anisotropic cases were determined. To further scrutinize the crack closure in both modes I and II, two-point digital extensometers were placed along the crack flanks. Crack opening displacement profiles were used to identify the loads at which each mode opened. The crack closure levels were compared between the polycrystalline and the two oriented specimens. Plastic zones associated with the fatigue crack growth were determined for all specimens using an anisotropic yield criterion. The strains in the plastic zone obtained from digital image correlation were investigated in detail and a dependence on the crystallography and load ratio R was observed. Slip irreversibility in front of the crack tip was measured and showed an increasing trend with increasing crack length; this information is critical for describing the crack growth behavior.
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