October 2004 Shear Strength as a Function of Test Rate 1915 1000 1000 3.3x10-mm/s 3.3x10-mm/s 乙 L Interlaminar 2345 050100150200250300 Time, t(s) Time, t(s) cal examples of force-versus-time curves of SiC /BSAS composite tested in interlaminar and in-plane shear at I loC in air at (a) fast test rate of3.3 mm/s and (b) slow test rate of 3.3x 104mm/s Typical fracture surfaces of DNS test specimens tested at fast decreasing test rate for advanced monolithic ceramics is known as (3. x 10 mm/s)and slow (3.3 X 10 mm/s)test rates are a slow crack growth ("dynamic fatigue")phenomenon, commonly hown in Fig. 6. The mode of shear failure was well typified from expressed by the empirical power-law relation 2 fracture surfaces as delamination of fibers from matrix-rich re- eOns, implying that the fiber-matrix interfacial architecture is the v= a(K,/Kl) st influencing characteristic controlling shear properties of the SiC/BSAS composite. The of a viscous phase was where v, Kr, and Ke are crack velocity, stress intensity factor, and bvious at the low test rate, indicating that more enhanced slow fracture toughness under Mode I loading respectively. In Mode I crack growth occurred at the low test rate. The residual glassy a and n are called slow crack growth (SCG) parameters. Based on hase might have been a major cause of slow crack growth in the this power-law relation, the tensile strength (o can be derived as SiC /BSAS composite under shear. a function of applied tes\i-s or stress rate(o) with some mathematical manipulations IV. Discussion o,= Do"lh The strength degradation in shear with decreasing test rate where D is another SCG parameter associated with inert strength, exhibited by the SiC,BSAS composite in this work is very similar n, and crack geometry. Equation (3)can be expressed in a more to that in tension shown not only by CFCCs such as SiC /CAS. convenient form by taking logarithms of both sides SiC /MAS, SiC/SiC, C//SiC, SiC,/BSAS (2-D).. but also by advanced monolithic ceramics such as silicon nitrides log o log D carbides, and aluminas. The strength degradation in tensio 010582501070a B212X0325018hi22 Fig. 6. Typical examples of fracture surfaces of SiC /BSAS composite tested in interlaminar shear at 1 100C in air at (a) fast test rate of 3. x 10 mm/s and (b) slow test rate of 3.3x 10 mm/s. Shear direction is indicated with arrows.October 2004 1000 800 - 600 <i> 2 400 o 200 0 3.3x10 mm/s Shear Slreni^ih as a Function of Test Rate 1000 1915 In-Plane Interlaminar 800 ii 400 o L J . 200 0 3 - .3x10"* mm/s In-Plane ^^- ^ iterlaminar 2 3 4 5 Time, t (s) 50 100 150 200 250 300 Time, t (s) (a) (b) Fig. 5. Typical examples of force-versus-time curves of SiC|^BSAS composite tested in interiaminar and in-plane shear at 1101.1 "C in air at la) fast test rate of 3.3 X 10"^ mm/s and (b) slow tesi rate of 3.3 X lO'"'* mm/s. Typical fracture surfaces of DNS test specimetis tested at fast (3.3 X 10"' tntn/s) and slow (3.3 X 10"^ mm/s) test rates are shown in Fig. 6. The mode of shear failure was well typified from fracture surfaces as delamination of fibers from matrix-rich re￾giotis, implying that the fiber-matrix interfacial architecture is the most inlluencing characteristic controlling shear properties of the SiC/BSAS composite. The presence of a viscous phase was obvious at the low test rate, indicating that more enhanced slow crack growth occurred at the low test rate. The residual glassy phase might have been a major cause of slow crack growth in the SiC,/BSAS composite under shear. IV. Discussion The strength degradation in shear with decreasing test rate exhibited by the SiC,/BSAS composite in this work is very similar to that in tension shown not only by CFCCs such as SiC,/CAS. SiC,/MAS. SiC,/SIC. C,/SiC. SiC/BSAS (2-D)''-^-'" but also by advanced monolithic ceramics such as silicon nilrldes, silicon carbides, and aluminas.'' The strength degradation in tension with decreasing test rate for advanced monolithic ceramics is known as a slow crack growth ("dynamic fatigue") phenomenon, commonly expressed by the empirical power-law relation'" (2) where w K^. and /T,^, are crack velocity, stress intensity factor, and fracture toughness under Mode I loading, respectively. In Mode 1, a and n are called slow crack growth (SCG) parameters. Based on this power-law relation, the tensile strength ((T,) can be derived as a function of applied test rate or stress rate (<T) with some mathematical manipulations''"'^^ (I, = D[CT]'""*" (3) where D is another SCG parameter associated with inert strength, n. and crack geometry. Equation (3) can be expressed in a more eonvenient form by taking logarithms of both sides l o g CTf - 1 n+ 1 log d- + log D (4) (a) (b) Fig. 6. Typical examples of fracture surfaces of SiC|/BS AS composite tested in interlaminar shear at and (b) slow test rate of 3.3 X 10"'' mm/s. Shear directiun is indicated with arrows. IOO"C in air at (a) fast test rale of 3.3 X 10 ' mm/s