Microtexture, Asperities, and Crack Deflection in Al-Li 2090 T8E41

Abstract

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.

New direct observations of crack closure processes in Al–Li 2090 T8E41

“…The work of Haase et al . (1998Haase et al . ( , 1999 has, in fact, demonstrated directly that this is the case in Al-Li 2090 T8E41.…”

Internal strains and stresses measured in cortical bone via high-energy X-ray diffraction

“…The much smaller strength of the 22.2 texture (ratio of maximum to minimum azimuthal intensity equaled 1.3) compared to the 00.2/00.4 texture (ratio of 9) is expected for an imperfect (broadened) fiber texture. Using methods outlined elsewhere (Cullity and Stock, 2001;Haase et al, 1999), diffracted beam directions for the maxima and half-maxima points of 00.2 and 22.2 were plotted on a stereographic projection, i.e., at the correct relative azimuthal positions and correct 2h (data not shown). The longitudinal bone axis was plotted at the Wulff netÕs north pole, and the entrance and exit beams corresponded to directions at or near, respectively, the center of the net.…”

MicroCT (microtomography) quantification of microstructure related to macroscopic behaviour: Part 1 – Fatigue crack closure measured in situ in AA 2090 compact tension samples

“…4 are typical of fatigue cracks grown in the centre of plates of AA 2090 T8E41. [3][4][5][6][7][8][9][10][11][12][13][14][15]27,28 The smaller asperities visible in Fig. 4 at top and middle coalesced to form the long mixed mode surface in Fig.…”