G N. Morscher, J. D. Cawley/Journal of the European Ceramic Society 22(2002)2777-2787 198 One important factor for a fiber strength determina 0.215t tion is the actual strength of the fibers in the composite after processing. The fiber strength of both the Hn and E0.7 SYL fibers at room temperature found by Yun and DiCarlo was 2800 MPa. 6 However, some strength degradation may occur due to composite processing Curtin et al. have established a composite ultimate strength failure criterion based on global load sharing assumptions 2(m+1)=/m+1 for 28> (13) (m+2) Fig 3. Depth of oxidation into the specimen from the surface(face)of the composite versus rupture time for -815C rupture of BN inter- Eq(13)is for matrix crack saturation where pe is the phase, MI SiC composites. The n widths were approximately 2 crack density and pe would be the crack spacing. The m.4, I5 The arrow for the Hi-Nicalon data point was for a specimen room temperature ultimate strengths of all of the com- that did not have through-thickness cracks, i.e. the data point indi- cates that oxygen ingress was at least that deep, three plies, into the posites modeled in this study are known. Therefore, the specimen. 4 The closed symbols indicate 815C experiments where the ultimate strength of the fibers in the composites could specimens failed in the hot zone. The open symbols indicate specimens be estimated by solving for o. from Eqs.(13)and ( that were tested at 960 oC which had failed outside of the hot zone region at a lower temperature estimated to be -870oC 14 m(m+2)R/m+2 study are indicated in Fig 4. This data was best fit, re 2(m+1)Lom+ plotted on a stresss-time plot and re-fitted to fit the common form Eq. 12 is based on the room temperature ultimate (12) fiber strength, Oo(Rn, of 2800 MPa. Assuming the flaw growth mechanism that causes time-dependent fiber where Cr is the coefficient that best fits the fiber rupture strength-degradation rate at intermediate temperatures data and n is the rupture exponent; both are dependent were the same as for single fiber tests and depends on on the fiber type. This then becomes the time-dependent starting flaw size, the time-dependent fiber strength of reference stress for Eq.(9) fibers in the composite can be estimated from Eq . (12) Sylramic 8 Nicalon Hi-Nicalon Yun and Di Carlo [16] 800c1000C1200c 100h100h100h 0.1 500010000150002000025000300003500040000 Larson Miller Parameter, q=T[log t+ 22],(K, hr ig. 4. Rupture strength in a Larson-Miller format for different SiC fibers from Yun and DiCarlo. 6study are indicated in Fig. 4. This data was best fit, re￾plotted on a stresss-time plot and re-fitted to fit the common form:
0ðt;TÞ ¼ Cf t 1=n ð12Þ where Cf is the coefficient that best fits the fiber rupture data and n is the rupture exponent; both are dependent on the fiber type. This then becomes the time-dependent reference stress for Eq. (9). One important factor for a fiber strength determina￾tion is the actual strength of the fibers in the composite after processing. The fiber strength of both the HN and SYL fibers at room temperature found by Yun and DiCarlo was 2800 MPa.16 However, some strength degradation may occur due to composite processing. Curtin et al.19 have established a composite ultimate￾strength failure criterion based on global load sharing assumptions:
ult ¼
c 2ð Þ m þ 1 m mð Þ þ 2   1 mþ1 m þ 1 m þ 2
; for 2>1 c ð13Þ Eq. (13) is for matrix crack saturation where c is the crack density and c 1 would be the crack spacing. The room temperature ultimate strengths of all of the com￾posites modeled in this study are known. Therefore, the ultimate strength of the fibers in the composites could be estimated by solving for so from Eqs. (13) and (4):
oðcompositeÞ ¼ m mð Þ þ 2 2ð Þ m þ 1 R Lo m þ 2 m þ 1
ult
mþ1 " #1 m ð14Þ Eq. 12 is based on the room temperature ultimate fiber strength, so(RT), of 2800 MPa. Assuming the flaw growth mechanism that causes time-dependent fiber strength-degradation rate at intermediate temperatures were the same as for single fiber tests and depends on starting flaw size, the time-dependent fiber strength of fibers in the composite can be estimated from Eq. (12): Fig. 4. Rupture strength in a Larson–Miller format for different SiC fibers from Yun and DiCarlo.16 Fig. 3. Depth of oxidation into the specimen from the surface (face) of the composite versus rupture time for 815
C rupture of BN inter￾phase, MI SiC composites. The specimen widths were approximately 2 mm.14,15 The arrow for the Hi-Nicalon data point was for a specimen that did not have through-thickness cracks, i.e. the data point indi￾cates that oxygen ingress was at least that deep, three plies, into the specimen.14 The closed symbols indicate 815
C experiments where the specimens failed in the hot zone. The open symbols indicate specimens that were tested at 960
C which had failed outside of the hot zone region at a lower temperature estimated to be 870
C.14 G.N. Morscher, J.D. Cawley / Journal of the European Ceramic Society 22 (2002) 2777–2787 2781