Laird and Smith [1] proposed a plastic sliding-off mechanism for the stage II fatigue crack growth via striation formation. In their view, the fatigue crack extension results solely from the different character of the deformation at the crack tip during loading and unloading. In particular, the crack tip blunts during the loading stage and folds into a double notch during the unloading stage, resulting in striation formation. In order to verify Laird's plastic blunting mechanism for crack propagation via striation formation for ductile fcc single crystals, a FEM calculation was performed for a rectangular plate with an initially sharp crack under plane strain conditions. The plate was subjected to a fully reversed tension-to-pressure cyclic load perpendicular to the crack plane (Mode 1). No initial radius for the crack tip was assumed. The actual shape of the crack tip followed from an initially sharp crack by repeated re-meshing. To model the constitutive behavior typical for ductile fcc single crystals, a geometrically nonlinear form of Cailletaud's model based on the multiplicative elastic-plastic decomposition of the deformation gradient was implemented into the FE program ABAQUS. Only octahedral slip systems were considered. The crack propagation simulations were carried out for cracks with crack plane (001) for two different crack growth orientations [110] and [100]. The external cyclic load was imposed in the (001) direction. Using repeated re-meshing for severely distorted elements at the advancing crack tip, deformation patterns in the sense of Laird's mechanism for fatigue-crack propagation with striation formation were obtained for [110] crack growth direction. The simulation for [100] crack growth direction with the same stress level as for [110] direction also yielded crack extension by the progressive large deformations, but without striations formation.
INTRODUCTIONWhile the statement that a fatigue crack advances by an increment ∆a in each cycle, except at very low growth rates in the near-threshold region, is generally accepted, there is a lack of agreement as to which mechanism controls the crack growth process. Two different mechanisms have been proposed to describe fatigue crack extension. One is based on a plastic sliding-off at the tip of the advancing crack, while the other is in terms of damage accumulation at the crack tip. The slidingoff mechanism associates crack extension with blunting of the crack tip during loading followed by re-sharpening upon unloading. This approach, which is usually used to explain fatigue crack growth via a mechanism of striation formation, was fist proposed by Laird and Smith [1] for smooth blunting, and later by Neumann [2] and Pelloux [3] for blunting via alternating shear. Fatigue striations are ripples on the fracture surface, and are often observed as a result of subcritical crack growth in ductile materials during cyclic loading. For cyclic loads in the Paris regime of fatigue crack advance, it has been found that the spacing between adjacent...
The purpose of this work is the application of continuum thermodynamics to the extension of standard crystal plasticity to account for the effects of the development of geometrically necessary dislocations (GNDs) on the material behavior. To this end, following Nye, Kondo, and many others, local deformation incompatibility in the material is adopted as a measure of the density of GNDs. Their development results in additional energy being stored in the material, resulting in additional kinematic-like hardening effects. The current approach generalized previous ones in that the thermodynamic formulation is based on the notion of generalized energy flux. A detailed comparison of the current approach and its results with previous such approaches and their results is given.
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