Browsing by Author "Liaw, PK"

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    Development of crystallographic-orientation-dependent internal strains around a fatigue-crack tip during overloading and underloading
    (Elsevier Science Inc, 2013-05-01) Lee, SY; Huang, EW; Wu, W; Liaw, PK; Paradowska, AM
    In-situ neutron diffraction was employed to directly measure the crystallographic-orientation-dependent (i.e. hkl) internal strains as a function of distance from the crack tip on the pre-cracked Hastelloy C-2000 compact-tension specimen. Both in-plane (IP) and through-thickness (TT) strain evolutions for various grain orientations were examined during tensile overloading and compressive underloading cycles. After overloading, underloading and their combination loadings were applied and unloaded, the significantly different {hkl} residual strain profiles were obtained in the vicinity of the crack tip. The load responses of the {200} grain orientation in both the IP and TT directions were more significant than those of any other orientations. It is suggested that the different orientation-dependent strain distributions around the crack tip are caused by the combined effects of elastic and plastic anisotropy of each {hkl} reflection upon loading and the subsequent development of residual stresses generated near the crack tip during unloading as a result of the plastic deformation. © 2013, Elsevier Ltd.
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    Twinning-detwinning behavior during fatigue-crack propagation in a wrought magnesium alloy AZ31B
    (Elsevier Science SA, 2012-10-30) Wu, W; Lee, SY; Paradowska, AM; Gao, Y; Liaw, PK
    In-situ neutron diffraction was used to investigate the twinning and detwinning behavior during fatigue-crack propagation on a wrought magnesium alloy, AZ31B, compact-tension specimen, where fatigue loading was applied parallel to the plate normal. Reversible twinning and detwinning characteristics were observed as a function of the distance from the crack tip. While twinning was activated above a certain critical stress value, detwinning occurred immediately upon unloading. It is thought that the development of compressive residual stresses generated around the crack tip during unloading is responsible for the detwinning behavior. Neutron bulk texture measurements were performed at several locations away from the crack tip (i.e. locations behind, right in front of, and far away from the crack tip) to quantitatively examine the volume fraction of {10.2}< 10.1 > extension twins in the vicinity of the crack tip. The texture analyses demonstrated that the texture in the fatigue-wake region did not change significantly, compared to that in the undeformed region far away from the crack tip, and approximately 11% of the residual twins were left behind the crack tip. The current results reveal that the reversible twinning and detwinning are the dominant deformation mechanisms for the studied material subjected to cyclic loading, and only a small amount of residual twins remain after the crack propagation. The spatial distribution of twinning/detwinning transitions correlates well with our previous predictions of the stress fields in the vicinity of a fatigue crack tip. © 2012, Elsevier Ltd.