June2000ateofSmenghDecreaseofFiberReinorcedceramic-MarixComposiesdhuringanigue 1471 Table Summary of Monotonic Tensile 140 Test Results 133.7±3.4GPa 51±8MPa 08±31MPa 1.05±0.17% 154±17m gn3°0 0.35±003 100 F [oJ] SIC/CAS ≈04mm g =240 MPa 200 Hz. RT 03 0= 240 MPa 60200Hz,RT Imm W025 Fig. 2. SEM micrograph showing the fracture surface of a spe Cycles tested in monotonic tension Fig. 3. Recorded damage indicators for two typical specimens cycle evolution of the hysteresis modulus as a function of the number of load temperature, AT, of the specimen(due to frictional cycles N;(b)temperature rise, AT, as a function of the number of cycles. The hysteresis modulus, E, was calculated from the stress-strain data. Figure 3(a)shows how E changes as a function of the number Table l characteristies of of cycles N for specimens that were cycled to failure. The Specimens Cycled to Failure evolution of E for all four specimens follows the same trend: The 87 7GPa modulus decreases significantly, to about 87 GPa within approx- lately 2 x 10 cycles. Thereafter E remains nearly constant(a 0.08±0.01% slight modulus recovery, about I GPa, was found for two of the l44±33pm specimens) L Values for the characteristic parameters are listed in Table Il 1.3×105-3.5×105 Note that the values of e and e are the values that were measured 69±14K just before localization occurred; s was measured away from the calized region after fatigue failure. These parameters can thus be used in micromechanical models. which are based on intact fibers Figure 3(b)shows the temperature rise curves recorded for the( Fig. 4(a)suggests that the center region failed before final specimens cycled to failure. For these specimens fatigue failure overload of the remainder cross section. At some locations(within occurred outside the 5 mm spot size of the infrared pyrometer. The the core region with no fiber pull-out)significant debris was heating curves follow the same trend; they increased slowly within present(Fig. 4(b)) the first 10 cycles, but increased rapidly during additional cycling Like the fracture surface, the broad faces of the specimens were This rapid increase is attributed to an increasing number of matrix found to have an inhomogeneous appearance. Along the sides of cracks and accompanying slip zones. The peak temperatures of the the specimens there were zones with larger matrix crack openings fact that the specimens fail at different numbers of cycles, while L, was about 6-10 times the matrix crack spacing. This the temperature is increasing. If they had failed at the same number considerably larger than the localized zone found after monoton- of cycles, the maximum temperature rise of each specimen likel ically loading virgin specimens to failure(see Table I). The larger would have been roughly the same. For specimens that failed L of the cyclically loaded specimens indicates a lower value of T within the spot size of the pyrometer a very rapid temperature rise In contrast, in the middle of the broad face( where the core region occurred within the last few seconds before failure was close to the surface), there was no such localized zone, the The fracture surfaces of the specimens cycled to failure had two matrix crack opening was similar to that remote from the fracture distinctively different regions. One part of the fracture surface site. This indicates that no global load sharing and no fiber pull-out displayed fiber pull-out, while the other area had no fiber pull-out had taken place near the core region during specimen failure (Fig. 4). The area without fiber pull-ou ocated in the core of the specimen cross section; fiber pull always present in region near the specimen edges. This ap nce is opposite to 1 (3) Specimens Cycled to 10 Cycles found at the fracture surfaces of specimens that have been exp The evolution of hysteresis modulus and the temperature rise of to external oxidation. The appearance of the fracture su specimens cycled to 10. cycles are included in Fig. 3. Aftertemperature, DT, of the specimen (due to frictional energy dissi￾pation). The hysteresis modulus, E#, was calculated from the stress–strain data. Figure 3(a) shows how E# changes as a function of the number of cycles N for specimens that were cycled to failure. The evolution of E# for all four specimens follows the same trend: The modulus decreases significantly, to about 87 GPa within approx￾imately 2 3 105 cycles. Thereafter E# remains nearly constant (a slight modulus recovery, about 1 GPa, was found for two of the specimens). Values for the characteristic parameters are listed in Table II. Note that the values of E# and ε* are the values that were measured just before localization occurred; s was measured away from the localized region after fatigue failure. These parameters can thus be used in micromechanical models, which are based on intact fibers. Figure 3(b) shows the temperature rise curves recorded for the specimens cycled to failure. For these specimens fatigue failure occurred outside the 5 mm spot size of the infrared pyrometer. The heating curves follow the same trend; they increased slowly within the first 104 cycles, but increased rapidly during additional cycling. This rapid increase is attributed to an increasing number of matrix cracks and accompanying slip zones. The peak temperatures of the four specimens differ somewhat. This difference is attributed to the fact that the specimens fail at different numbers of cycles, while the temperature is increasing. If they had failed at the same number of cycles, the maximum temperature rise of each specimen likely would have been roughly the same. For specimens that failed within the spot size of the pyrometer a very rapid temperature rise occurred within the last few seconds before failure. The fracture surfaces of the specimens cycled to failure had two distinctively different regions. One part of the fracture surface displayed fiber pull-out, while the other area had no fiber pull-out (Fig. 4). The area without fiber pull-out was located in the core of the specimen cross section; fiber pull-out was always present in a region near the specimen edges. This appearance is opposite to that found at the fracture surfaces of specimens that have been exposed to external oxidation.11 The appearance of the fracture surface (Fig. 4(a)) suggests that the center region failed before final overload of the remainder cross section. At some locations (within the core region with no fiber pull-out) significant debris was present (Fig. 4(b)). Like the fracture surface, the broad faces of the specimens were found to have an inhomogeneous appearance. Along the sides of the specimens there were zones with larger matrix crack openings (Fig. 5). The length (in the fiber direction) of this localized zone, L, was about 6–10 times the matrix crack spacing. This is considerably larger than the localized zone found after monoton￾ically loading virgin specimens to failure (see Table I). The larger L of the cyclically loaded specimens indicates a lower value of t. In contrast, in the middle of the broad face (where the core region was close to the surface), there was no such localized zone; the matrix crack opening was similar to that remote from the fracture site. This indicates that no global load sharing and no fiber pull-out had taken place near the core region during specimen failure. (3) Specimens Cycled to 105 Cycles The evolution of hysteresis modulus and the temperature rise of specimens cycled to 105 cycles are included in Fig. 3. After Table I. Summary of Monotonic Tensile Test Results Ec 133.7 6 3.4 GPa s0.02 351 6 8 MPa su 508 6 31 MPa εu 1.05 6 0.17% s 154 6 17 mm vf 0.35 6 0.03 L '0.4 mm Fig. 2. SEM micrograph showing the fracture surface of a specimen tested in monotonic tension. Fig. 3. Recorded damage indicators for two typical specimens cycled to failure (solid lines) and specimens cycled to 105 cycles (dashed lines): (a) evolution of the hysteresis modulus as a function of the number of load cycles N; (b) temperature rise, DT, as a function of the number of cycles. Table II. Characteristics of Specimens Cycled to Failure E# 87 6 7 GPa ε* 0.08 6 0.01% s 144 6 33 mm L '1.0 mm Nf 1.3 3 105 –3.5 3 105 (DT)max 69 6 14 K June 2000 Rate of Strength Decrease of Fiber-Reinforced Ceramic-Matrix Composites during Fatigue 1471