1738 Journal of the American Ceramic Sociery-Holmqguist and Lange Vol. 86. No. 10 (a) N720.1200°c/00h N720.1200°c2h N720.1300c/10oh 300pm Nominal Flexure Strain (%) 0.1200c00h 10.1300c/100h (b 300pm Nominal Flexure Strain(%) Fig. 7. Flexural stress versus flexural strain plots for unnotched compos- Fig 8. SEM micrograph of fracture surfaces of N720 composites: (a)as ites reinforced with (a) N720 fibers and(b) N610 fibers, in as-processed processed, and (b) heat-treated at 1300C for 100 h. condition and after aging In-plane mechanical properties of the notched specimens are room temperature. The results are also summarized in Table lI and reported in Table Ill. Net-section strengths were evaluated by Fig. 7. There is no significant effect on either stiffness or strength assuming that the bar dimensions used to calculate the maximum after thermal exposure at 1200 C for 100 h. The fracture surfaces tensile stress at failure(Eq (2)do not include the notched portion are similar to those of the as-processed specimens, showing a of the bar; i.e., the length of the notch is subtracted from the height fibrous nature. However, the pull-out lengths of the broken fibers of the beam, and this value is used as the height used to determine appear to be somewhat shorter. Such a trend was also observed by Mattoni et al. 2 when the matrix strength was increased by obtained by comparing the net-section strengths of the notched repeated precursor impregnations to reduce the porosity. The specimens to the strength of the unnotched specimens(strength proposed explanation was that the reduced matrix porosity induced ratio, Table IID). The strength ratio was -0.75-0.90 for the N720 higher stress concentrations around fiber breaks and thereby the composites and-0.70-0.8.5 for the N610 composites, indicating failure probability of adjacent fibers was increased. A similar moderate notch sensitivity for the material. These values are in as-processed specimens tended to fail in a mixed mode rather than oth other processes. ,0 Load-displacement data in Fig. 9 shows that in a pure tensile mode the fracture occurs in a stable manor after the load maximum has Despite the higher thermal stability of the N720 fibers, these opposites exhibited a significantly lower strength at room tem- perature compared with those fabricated with N6l0 fibers: the lower strength of the N720 fibers is expected to be the cause of the Table Ill. Properties of In-Plane Notched Composite lower strength composite. Three- Point Bend Test Increasing the heat treatment to 1300 C/(100 h) had a more Net-section posites and -35% for N610 composites, The degree of fiber Heat treatment pull-out was significantly reduced. as shown in Fig. 8(b), A small AN7201200°C/2h) 1666 0.90 increase in stiffness could be observed for the N720 composite BN7201200°C12h) The N610 fibers are known to loose strength rapidly at high temperatures, which might explain the decrease in composite BN7201200°C/100h)18190.9 16 strength. Also the N720 composites are affected, but to less extent CN6101200°C/(2h) because of the higher thermal stability of N720 fibers compared CN6101200°C/100h)254 with N610 fibers. The heat treatment will also give a denser and DN6101200°C/100h) stronger matrix as described above aW=0.50±002