SHIELDING DUE TO ALIGNED MICROCRACKS IN ANISOTROPIC MEDIA

This work presents a continuum damage analysis of the effect of aligne d microcracks on a dominant crack in an otherwise anisotropic medium. This geometry is particularly relevant for composite media with an ali gned reinforcement phase, where aligned cracking can change the magnit ude and principal axes of anisotropy. Provided that the density of ali gned microcracks reaches a stable saturation value near the crack tip, the near-tip stress intensity factor, K-I(s), may be related to the r emote value, K-I(o) in terms of the anisotropic properties of the unda maged and saturated media. Results show that of all types of damage, t hat which increases compliance in a direction normal to the crack plan e typically reduces K-I(s)/K-I(o) near the crack tip most significantl y, while comparable increases in compliance parallel to the crack grow th direction produce a relatively small change. Reduction in K-I(s)/K- I(o) due to increased shear compliance may be 80% of that due to incre ased compliance normal to the crack plane. For damage consisting of al igned microcracks, the most effective reduction in K-I(s)/K-I(o) occur s when the microcracks are oriented parallel, rather than perpendicula r to the main crack. This conclusion holds even for alumina/aluminum a nd graphite/epoxy systems that display large anisotropy. Tn those case s, optimal reduction in K-I(s)/K-I(o) occurs when in addition to paral lel macro and microcracks, the stiffer direction is perpendicular to t he crack surfaces. This case corresponds, for example, to macrocrack e xtension perpendicular to aligned fibers or layers in composite materi als, with pinned microcracking through fiber or layer cross sections.