RESULTS The R-curves of the composites reinforced with the large whiskers (TWS-400) with various Vw were experimentally obtained as shown with closed circles in Fig.1. Each R-curve behavior typically comprised two stages divided at a bend in the curve. At the first stage of R-curve, KIc( α) steeply increased from 4±0.2MPa√m without a visible crack extension. The initiation of R-curve was, however, sensitively detected through the onset of nonlinear load-displacement relation in the bend test of SENB specimens. At the second stage beyond the bend, KIc( α) gradually increased as the crack macroscopically extended. The value of KIc( α ) at the bend increased with Vw. In addition, the plateau of R-curve increased with Vw. The R-curves simulated for various values of Vw were shown as the solid lines in Fig.1. Good agreements between observed and simulated R-curves were confirmed, while the fitting is relatively insufficient for the composites having lower Vw-values. In addition, the bend was successfully reproduced in our R-curve simulation. The R-curve behavior of another composite reinforced with the small whiskers (TWS-100) of 20vol.% referred to as Compo-sw was shown with the closed squares in Fig.2 along with that of a composite with the large whiskers (TWS-400) of 20vol.% referred to as Compo-lw (closed circles) for comparison. A significant deference in their R-curves was seen: At the second stage of R-curve behavior, KIc( α) of Compo-lw showed a steeper increase and reached a higher plateau than that of Compo-sw, though the differences in KIc( α) at the initiation and at the bend in each R-curve are insignificant. The R-curves simulated for both the composites were shown as the solid lines in Fig.2. Good agreements between observed and simulated R-curves were demonstrated not only for Compo-lw but also for Compo-sw. 0 100 200 300 400 500 600 4 5 6 7 8 Crack extension length /μ m Critical stress intensity factor /MPa√m 20vol.% 5vol.% 10vol.% 15vol.% Whisker content 0 100 200 300 400 500 600 4 5 6 7 8 Crack extension length /μ m Critical stress intensity factor /MPa√m TWS400(Radius=0.65μ m, Length=40μ m) TWS100(Radius=0.2μ m, Length=30μ m) Figure 1: R-curves of an alumina matrix composite reinforced with silicon carbide whiskers (TWS400) for various whisker volume fractions. Closed circles are experimental and solid lines are simulated R-curves. Figure 2: R-curves of the composite reinforced with whiskers of small (TWS100) and large (TWS400) sizes. The former and the latter are denoted with closed squares and circles, respectively. Solid lines are R-curves simulated for each composite.
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