Debond Crack Propagation and Branching under Varying Shear Stresses
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DEBOND CRACK PROPAGATION AND BRANCHING UNDER VARYING SHEAR STRESSES C. M. KENNEFICK, R. M. DICKERSON AND P. O. DICKERSON NASA Lewis Research Center, 21000 Brookpark Road, Cleveland, OH 44135 ABSTRACT Silicon carbide fibers in a Ti24AI+ 1lNb matrix were debonded under varying combinations of shear and compression in off axis loading in a pushout test. Evidence of debond crack branching was recorded for etched specimens using scanning electron microscopy and the interface microstructure was examined by transmission electron microscopy. The regions of high shear stress and multiple weak interfaces promoting crack branching were analyzed in terms of an energy balance for brittle fracture. INTRODUCTION The deflection of cracks at fiber-matrix interfaces in both intermetallic and ceramic matrix composites with subsequent fiber pullout has been shown to be an important fracture toughening mechanism. Interfacial chemical reaction layers from composite fabrication and also the use of fiber coatings to tailor the bond strength or morphology at the interfaces opens the possibility of not one but multiple debonding cracks. Such multiple cracking may increase crack deflection, but it may at the same time increase the number of asperities at a fiber-matrix interface, thereby increasing the stress required for frictional pullout. This paper examines experimental evidence for crack branching in an intermetallic matrix composite and analyzes the energy release rate for crack branching for a main crack under a combination of compressive and shear loading. Theories on the cause of crack branching have focussed on critical crack tip velocities and stress intensity factors, and on the interaction of elastic waves with running cracks. Studies have so far been mainly on homogenous materials and have included a review of fast fracture [1], the interaction of stress waves with cracks [2, 3], and analysis and experimental results of crack branching in polycarbonate [4], in glass [5, 6], and at inclusions inside a matrix [7]. In this study, crack branching during debonding along a fiber-matrix interface in a pushout test is examined. The energy expended in overcoming friction both from residual compressive stresses on the fibers and from dragging along fiber coating fragments during debonding are compared with the overall strain energy release to clarify which frictional mechanism may retard branching. EXPERIMENTAL A three-ply composite of silicon carbide SSC-6 fibers in a Ti24AI + 11Nb matrix, made by a wire arc spray process [8], was successively sliced to produce pushout specimens whose fibers made an orientation angle a from 0 to 40 degrees with respect to the loading axis in a pushout test (Figure 1). The fibers comprised 44 volume percent of the composite, had a diameter of 142 Am, and had an average interedge spacing of 30.3 ± 0.5 Am within each row. Each specimen was 401 ± 3 to 468 ± 8gm thick and the groove in the sample holder in Figure 1 was an average of 366 ± 6 Am wide. Mat. Res. Soc. Symp. Proc. Vol. 288. 01993 Materials Resear
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