Three-Orthogonal-Direction Stress Mapping around a Fatigue-Crack Tip Using Neutron Diffraction (original) (raw)
2012, Metallurgical and Materials Transactions A
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Fatigue Crack-Tip Stress Mapping Using Neutron Diffraction
Fatigue crack growth experiments were carried out on the 304L stainless steel compacttension (CT) specimen under a load control mode. Neutron diffraction was employed to quantitatively measure residual strains/stresses and the evolution of the stress fields in the vicinity of a propagating fatigue-crack tip. Three principal stress components (i.e. crack growth, crack opening, and through-thickness direction stresses) were examined in-situ under loading as a function of distance from the crack tip along the crack-propagation path. The stress/strain fields measured both at the mid-thickness and near the surface of the CT specimen were compared. The results show that much higher compressive residual stress fields are developed in front of the crack tip near the surface than at the mid-thickness. The change of the stresses ahead of the crack tip under loading is more significant at the midthickness than near the surface.
Residual Strain Distribution around a Fatigue-crack Tip Determined by Neutron Diffraction
An analysis of residual stress, one of the contributory factors to the crack tip driving force, is extremely important to probe the fatigue crack growth mechanism and to further develop the life prediction methodology. Since fatigue crack growth is governed by crack-tip plasticity and crack closure in the wake of the crack tip, the investigation of residual stain/stress field in both behind and in front of the crack tip is crucial. In the current work, a 304L stainless steel compact-tension specimen is pre-cracked under constant-amplitude cyclic loading. Neutron diffraction is employed to directly measure the three orthogonal residual strain fields with 1-mm spatial resolution as a function of distance from the crack tip. The mapping results show that the three orthogonal residual-strain distributions around the crack tip depend on the stress multiaxiality, not following a single Poisson relationship to each axis.
2010
The spatially resolved neutron-diffraction residual stress mappings were performed on five compact-tension (CT) specimens subjected to various variable-amplitude fatigue loadings (e.g. overload, underload and their mixed loads) during fatigue crack propagation. Three principal residual-stress components (i.e. longitudinal, transverse and normal stresses) were measured as a function of the distance from the crack tip along the crackpropagation direction. The shape of respective crack tips on the five CT specimens was examined using scanning electron microscope. The results show the distinct residualstress fields near the crack tip and significant changes in the crack-tip geometry for five different loading cases. It is thought that the combined effects of the changes in the residual-stress state and crack-tip geometry seem to be a key factor to account for the observed transient crack-growth phenomena.
In Situ Characterization of the Fatigue Crack-Tip Stress Fields
A neutron diffraction technique was used to directly measure residual stress fields around a fatigue-crack tip at the mid-thickness and near the surface of the compact-tension specimen, where the fatigue crack was grown with a R-ratio of 0.1 under a load control mode. The stress mapping was conducted as a function of distance from the crack tip with 1-mm spatial resolution of neutron beam along the crack-propagation direction. The residual strain fields near the surface were much more significant than those at the mid-thickness. The evolution of the residual stress fields was examined in situ under applied load, in order to investigate the distribution of stresses in the vicinity of the crack tip. The three orthogonal stress components (i.e., crack growth, crack opening, and through thickness) of the specimen under loading were calculated from the directly-measured three orthogonal strain components without any assumptions of a plane strain or plane stress condition. A clear distinction for the evolution of the crack-tip stress field was observed along the direction of the through-thickness of the sample, which might account for the intrinsic differences of a crack-opening process and resultant crack-tip driving force between at the mid-thickness and near the surface.
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