IMPACT STRENGTH OF CONTINUOUS-CARBON-FIBER-REINFORCED SILICON-NITRIDEMEASURED BY USING THE SPLIT HOPKINSON PRESSURE BAR

Three-point impact bending tests, using the split Hopkinson pressure b ar method, were performed to evaluate the fracture resistance of monol ithic silicon nitride (SN) and carbon-fiber-reinforced silicon nitride (CFRSN) ceramics. By applying ramped incident-stress waves in the spl it Hopkinson pressure bar apparatus, relatively smooth stress-time cur ves could be recorded without using any artificial filtering process. The maximum load in the load-deflection curve of the CFRSN material in creased, in comparison to its static value, when impact testing was ap plied. Such behavior was substantially different from that of the mono lithic SN material, for which the maximum load values from impact and static testing were almost the same. The time dependence of strength i n the CFRSN ceramic was then investigated by using relaxation tests, a nd the impact strength behavior could be explained by these results. A lso, the shear strength was significantly dependent on the deformation rate, whereas the tensile strength was almost independent of it. The experimental results were compared with the numerical predictions of t he stress distribution that were obtained by using finite-element anal ysis.