Global Constraint-Insensitive Fracture in SiC Particulate-Reinforced AA 2009 BRIAN P. SOMERDAY and RICHARD P. GANGLOFF Experimental results prove this hypothesis: The effective plastic fracture strain (~) for a discontinuously reinforced metal matrix composite (MMC) is insensitive to globally imposed triaxial tensile stress, because the elastic reinforcement independently produces high local ma- trix constraint during plastic deformation. The value ef is measured for SiC particulate- reinforced AA 2009-T6, with cylindrical circumferentially notched (global constraint ratio, trm/~, of 1.0) and smooth tensile specimens (o-m/6"of 0.3), as a function of temperature. The MMC fractures by microvoid-based processes associated with the SiC at all temperatures. The ratio (r) of smooth to notched specimen ef equals unity from 25 ~ to 200 ~ and is less than values of 2 or higher typical of monolithic A1 alloys at similar global constraint. This result establishes that global constraint does not degrade MMC fracture resistance because of the unique effect of local matrix constraint. The ratio r for the MMC increases from near 1 at 200 ~ to a maximum of 3.3 at 250 ~ indicating a loss of local matrix constraint, possibly due to reduced matrix-particle load transfer or microvoid nucleation at low strains. Uncertainties hinder precise definition of e~ vs global trm/O. Global constraint-insensitive fracture for the MMC suggests that plane strain fracture toughness differences between the composite and un- reinforced matrix are not well defined by smooth specimen ductilities. Further, MMC fracture toughness should be insensitive to global constraint and associated cracked specimen dimensions. I. INTRODUCTION THE intrinsic tensile fracture resistance of a discon- tinuously reinforced (DR) metal matrix composite (MMC) is generally inferior compared to that of the un- reinforced matrix, t~l Average tensile ductility, and more precisely the local strain and normal stress required to initiate microscopic damage, is reduced due to the tri- axial tensile stress which evolves in the composite ma- trix adjacent to particles or whiskers, t2-61 Residual triaxial stress forms in the matrix of DR MMCs during rapid cooling from processing or heat-treatment temper- atures because of the difference in thermal expansion coefficients between the matrix and reinforcement, t71 Additional local triaxiality develops during plastic de- formation due to the constraint of matrix plastic flow by the high stiffness elastic reinforcement, t3-5,8,91 Locally high MMC matrix constraint was demon- strated by finite element modeling (FEM) of smooth uni- axial tensile specimens under imposed uniform nominal strains t3-5,91 and was suggested based on smooth tensile experiments subjected to superimposed hydrostatic con- fining pressure, t~~ In the latter work, reduction of area (RA) at fracture for both 2XXX/SiC/15p-UA and -OA* rose from 10 to 50 pct, whereas RA values for both 6061/AI203/15p-UA and -OA increased from 10 to 80 pct, with the application of 300 MPa pressure. It was concluded that superimposed pressure suppressed both globally generated constraint from specimen neck- ing and local matrix triaxial tensile stress; MMC tensile BRIAN P. SOMERDAY, Graduate Research Associate, and RICHARD P. GANGLOFF, Professor, are with the Department of Materials Science and Engineering, School of Engineering and Applied Science, University of Virginia, Charlottesville, VA 22903. Manuscript submitted September 13, 1993. ductility was accordingly improved because both ele- ments of constraint exacerbate microvoid nucleation, growth, and coalescence (MNG).** *This MMC designation follows Aluminum Association nomenclature.tn~ **The global triaxial tensile stresses at fracture, which evolved dur- ing necking, were calculated for 6061/AI2OJ 15p-XX and monolithic aluminum alloy (AA) 6061-XX specimens tested under 150 MPa ap- plied pressure. Monolithic AA 6061-XX fractured after the neck- induced global triaxial tensile stress exceeded the imposed pressure; that is, the net triaxial stress was greater than zero. However, a net negative global triaxial stress was calculated for 6061/A1203/15p-XX at fracture, suggesting that local matrix triaxial tensile stress was superimposed on that created by global constraint, tm Triaxial tensile stress degrades the intrinsic fracture resistance of materials that microscopically fail via MNG. The MNG models predict that the initiation and growth stages of this dilatant fracture mode are enhanced by triaxial tensile s t r e s s , t13"14'151 Experiments conducted on cylindrical circumferentially notched tensile speci- mens of monolithic A1 alloys and steels demonstrate that material ductility decreases and asymptotically ap- proaches a limiting value with increasing magnitude of triaxial tensile stress, t16-231 Geometric constraint, due to cross-sectional area and plastic-to-elastic deformation gradients in the notch, produces global triaxial tensile stress. 115,171 Typical plots of effective plastic strain to failure, ~, vs the global mean stress-to-effective stress ratio, o-,,/6-, are reproduced in Figure 1 for two mono- lithic AI alloys, tl6AT,2zl Additionally, for monolithic AA 8009, increasing global ~rm/6"from 0.3 to about 1.4 re- suited in a fivefold reduction in ~.t231 Several studiest3-Sm calculated the magnitude of local matrix triaxial tensile stress in strained smooth uniaxial tensile specimens of whisker- and particulate-reinforced A1 MMCs using FEM. This work did not indicate METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 25A, JULY 1994--1471