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Earlier, crack opening as a function of position within an optically opaque notched tensile sample (NT-3) of Al-Li 2090 was reported for four loads by Guvenilir and co-workers; these measurements were made with high-resolution X-ray computed tomography. This paper reports the same type of results for a second notched tensile sample (NT-4): the authors believe this to be the second sample whose crack closure was quantified as a function of position. The crack within NT-4 was found to be more planar than, and to close differently to, that in NT-3. In NT-4, the crack 'zipped' shut from the tip; this is in contrast to the earlier results on the asperitydominated crack in NT-3, where contact occurred primarily behind the open crack tip. During unloading of NT-4, the crack faces came into contact above the closure load defined by the intersection of tangent lines to the upper and lower portions of the load-deflection curve; this is similar to previous observations in sample Estimates of da/dN as a function of ∆K for samples NT-3 and NT-4 agree with others' measurements, which suggests that the closure observations of this paper also apply to compact tension samples of Al-Li 2090.

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Roughness-induced closure is held to be responsible for very low fatigue crack growth rates observed in certain plate orientations of Al-Li 2090 T8E41, and the geometry of asperities producing this closure correlates with macrotexture. Little work, however, has focused on the role of individual grain orientations (microtexture) or of average orientation within small groups of adjacent grains (mesotexture) on the crack's path through a sample, i.e., on whether the variation in grains' orientations determines where the crack will deflect. This paper reports synchrotron X-ray microbeam diffraction mapping of the three-dimensional microtexture in samples of Al-Li 2090. Groups of adjacent pancake-shaped grains are found to have very similar orientations, producing nearly single-crystal regions approaching thicknesses of 0.3 mm along the sample's S (short-transverse) direction. These near-single-crystal volumes produce large asperities with surfaces having substantial Mode III character, asperities which appear over the range of stress intensity ranges observed (∼5 to ∼25 MPa√m). Results of these experiments suggest not only that this type of mesotexture plays an important role in determining fatigue crack path in compact tension samples of Al-Li 2090 but also that specific orientations of the groups of grains lead to large crack deflections.