HENAGER et al. SUBCRITICAL CRACK GROWTH: PART I 3729 Table 1. Physical properties of the materials tested(variability given as 95% confidence intervals) Composite designation CG-C Hi-C Fiber architecture 2D plain weave fiber mats 2D plain weave fiber mats thermal cvi at-1050°C othermal cvi at-1050° eramic-grade nical Fiber vol ameter(um) 14±2.3 ol. fractio Composite coating CVD SIC CVD SIC Modulus (GPa) 122±21 172±23 Fracture toughness, Ko(MPa m")5 nterface sliding stress(MPay 8.4+6.6 72.1±33.5 aWe do not have access to all the processing details for these vendor-supplied materials. However, the processing of these CVI-SiC/SiC The modulus is calculated from the measured from the linear portion of loading during flexural testing of SENB specimens at elevated temperatur ture compliance. "Calculated using peak load as described in ASTM E399 mEasured by fiber push-in test at room temperature as described in [43] mats.t Before matrix infiltration, a 1-um-thick carbon 3. RESULTS interphase layer was deposited on the fibers by chemi- 3. 1. Displacement-time cures each bend-bar were coated with a 2-um-thick layer The displacement-time curve for a representative of silicon carbide, deposited by CVD, to provide oxi- CG-C specimen tested at a constant load of 630 N at dation resistance and to protect the surfaces from 1373 K for 8x10 s in argon is a nonlinear curve( Fig damage. These materials will be abbreviated as"Hi- 1), suggestive of a transient creep curve, but indicate the fiber type and the interphase com- accompanied by subcritical, time-dependent crack position(Table 1) growth in a multiply-cracked damage zone(see The materials studied previously were fabricated in below) that extends from the notch root much as a a similar manner. t The Nicalon-CG fibers were also mode I crack. The data shown in Fig. I are the midp- oated with either a I-um-thick carbon interphase int displacement of the notched bar as a function of layer, a 0. 4-um-thick carbon/boron nitride(C/BN) time, t, and are compared to a functional fit of the interphase layer, or a I-Hm-thick multi-layer form nterphase layer consisting of carbon/boron carbide/boron nitride (C/B,C/BN), with the carbon f= alt exp(-bIDIe layer next to the fiber. These materials will be abbreviated"CG-C. The time-independent mechan- ical properties of the CG-C materials in argon were corresponds to a Sherby-Dorn creep equation entical, within experimental uncertainties, in a,b, and c are independent fitting parameters pendent of the interphase chemistry and thickness Composite properties and identification details are listed in Table 0.004 We tested Hi-C specimens at 1373, 1423, 1448 - fit to data nd 1473 K in argon ed 10 CG-C specimens tested at 1173, 1273, 1298, 1323, 5 1348,1373,and1398K gon. The Hi-C com-§610 sites were tested at temperatures higher than the CG-C materials because Hi-Nicalon fibers have greater thermal stability compared to Nicalon-CG 2I0 8105 t Composite fabrication performed by DuPont Lanxide Fig. 1. Displacement-time curve for a CG-C material(Table Corp, Wilmington, DE, USA 1)at 600 N constant load(corresp Composite fabrication performed by RCL, Whittier, m 2 )test at 1373 K in argon. Experi ve (solid line) CA, USA(RCI is no longer existent). is compared to equation(1)HENAGER et al.: SUBCRITICAL CRACK GROWTH: PART I 3729 Table 1. Physical properties of the materials testeda (variability given as 95% confidence intervals) Composite designation CG-C Hi-C Fiber architecture 2D plain weave fiber mats 2D plain weave fiber mats Processing conditions Isothermal CVI at
1050°C Isothermal CVI at
1050°C Fiber type Ceramic-grade Nicalon Hi-Nicalon Fiber coating thickness (µm) 1.0±0.1 1.0±0.1 Fibers/tow 420 321 Fiber diameter (µm) 15.0±0.5 14±2.3 Fiber vol. fraction 40 40 Composite coating CVD SiC CVD SiC Porosity 20±5% 6±1.4% Modulus (GPa)b 122±21 172±23 Fracture toughness, KQ (MPa m1/2) c 17.5 22.4±0.1 Interface sliding stress (MPa)d 8.4±6.6 72.1±33.5 a We do not have access to all the processing details for these vendor-supplied materials. However, the processing of these CVI-SiC/SiC materials is rather standardized. b The modulus is calculated from the specimen compliance measured from the linear portion of loading during flexural testing of SENB specimens at elevated temperature and corrected for fixture compliance. c Calculated using peak load as described in ASTM E399. d Measured by fiber push-in test at room temperature as described in [43]. mats.† Before matrix infiltration, a 1-µm-thick carbon interphase layer was deposited on the fibers by chemi￾cal vapor deposition (CVD). The outer surfaces of each bend-bar were coated with a 2-µm-thick layer of silicon carbide, deposited by CVD, to provide oxi￾dation resistance and to protect the surfaces from damage. These materials will be abbreviated as “Hi￾C” to indicate the fiber type and the interphase com￾position (Table 1). The materials studied previously were fabricated in a similar manner.‡ The Nicalon-CG fibers were also coated with either a 1-µm-thick carbon interphase layer, a 0.4-µm-thick carbon/boron nitride (C/BN) interphase layer, or a 1-µm-thick multi-layer interphase layer consisting of carbon/boron carbide/boron nitride (C/B4C/BN), with the carbon layer next to the fiber. These materials will be abbreviated “CG-C.” The time-independent mechan￾ical properties of the CG-C materials in argon were identical, within experimental uncertainties, inde￾pendent of the interphase chemistry and thickness. Composite properties and identification details are listed in Table 1. We tested Hi-C specimens at 1373, 1423, 1448, and 1473 K in argon and compared the results with CG-C specimens tested at 1173, 1273, 1298, 1323, 1348, 1373, and 1398 K in argon. The Hi-C com￾posites were tested at temperatures higher than the CG-C materials because Hi-Nicalon fibers have greater thermal stability compared to Nicalon-CG fibers. † Composite fabrication performed by DuPont Lanxide Corp., Wilmington, DE, USA. ‡ Composite fabrication performed by RCI, Whittier, CA, USA (RCI is no longer existent). 3. RESULTS 3.1. Displacement–time curves The displacement–time curve for a representative CG-C specimen tested at a constant load of 630 N at 1373 K for 8×105 s in argon is a nonlinear curve (Fig. 1), suggestive of a transient creep curve, but accompanied by subcritical, time-dependent crack growth in a multiply-cracked damage zone (see below) that extends from the notch root much as a mode I crack. The data shown in Fig. 1 are the midpo￾int displacement of the notched bar as a function of time, t, and are compared to a functional fit of the form: f a[t exp(
b/T)]c (1) which corresponds to a Sherby–Dorn creep equation where a, b, and c are independent fitting parameters Fig. 1. Displacement–time curve for a CG-C material (Table 1) at 600 N constant load (corresponding to a Ka = 9.6 MPa m1/2) test at 1373 K in argon. Experimental curve (solid line) is compared to equation (1) (dashed line).