R. Naslain et al/Composites: Part A 30(1999)537-547 The third useful material data derived from tensile tests n model 1D-composites are related to the in-situ failure stength of the fibers. As in nD real composites, the failure of model 1 D-composites involves fiber pull-out and the fiber A failure surface displays the mirror-mist-hackle features classically observed on brittle materials. The mirror radius Im, is related to the in-situ tensile failure stress of the fiber OR, through the following empirical equation OR=A/(/m) where A is a constant depending on fiber fracture toughnes A=2.45 MPa mfor Nicalon fibers [27]. Mirror radii have been measured from the failure surfaces of Nicalon/PyC/ 5.05.1 SiC(CVI)minicomposites with Vf=0.27: 0.60 and 0.70 Om(Imm) in the SEm. The corresponding in-situ fiber failure stresses are presented in Fig. 8 together with the data for bare Nica Fig. 6. Weibull plot for the matrix failure stress in C(ex-PANYPyC/SiC lon single filaments and bare Nicalon fiber bundles (L= minicomposites, as derived from measurements performed with a tensile 75 mm). The value of the Weibull shape parameter, mt stage set in a scanning electron microscope, according to Ref [26] derived from the minicomposite data is 4.1-4.4. Further, he fact that the failure stress distributions for the minicom Furthermore, by performing a tensile test on a single mini- posites are almost identical to that for the Nicalon bundle omposite in a scanning electron microscope, the stresses at suggests that some degradation of the fibers may have which the successive cracks appear in the uncracked matrix occurred during minicomposite processing. The failure fragments have been measured. The statistical parameters stress distribution curves for the minicomposites are shifted were derived from the statistical distribution of the matrix towards higher stresses if the effective length is much cracking stresses taking into account the successive sizes of shorter than the gauge length(L= 75 mm)used in the the matrix fragments created at each step of the matrix bare bundle tests [27]. Therefore, these data may also cracking process. For the example shown in Fig. 6, the confirm that the fiber debonding was complete at failure statistical parameters for the SiC(Cvn)matrix, are mm and that the minicomposite failure is similar to the one of 8.5 and oom= 216 MPa(Vo=l mm). Statistical-probabil- fiber bundles istic models have been worked out to predict the stress- Although most mechanical tests have been pe train behaviour for model 1D-composites [26, 27, 32, 33]. under tensile loading in the present and related An example of such a prediction is shown in Fig. 7, for a other micromechanical tests are also of interest. Such is Nicalon/PyC/SiC minicomposite. The best fit between the the case for push-in or push-out tests conducted under model and the experimental data is observed for T. compressive loading. Such local tests (only one fiber is 80 MPa, providing thus an alternative method to get char- loaded) can be applied to either real nD-composites acteristics of the FM bonding [27, 33] model 1D-composites provided suitable specimen T=80 MP Strain (%o) Fig. 7. Simulation of the tensile stress-strain curve for a Nicalon/PyC/SiC minicomposite based on a probabilistic/statistical model, according to Ref, [27]Furthermore, by performing a tensile test on a single mini￾composite in a scanning electron microscope, the stresses at which the successive cracks appear in the uncracked matrix fragments have been measured. The statistical parameters were derived from the statistical distribution of the matrix cracking stresses taking into account the successive sizes of the matrix fragments created at each step of the matrix cracking process. For the example shown in Fig. 6, the statistical parameters for the SiC (CVI) matrix, are mm ˆ 8.5 and som ˆ 216 MPa (Vo ˆ 1 mm3 ). Statistical–probabil￾istic models have been worked out to predict the stress– strain behaviour for model 1D-composites [26,27,32,33]. An example of such a prediction is shown in Fig. 7, for a Nicalon/PyC/SiC minicomposite. The best fit between the model and the experimental data is observed for ti ˆ 80 MPa, providing thus an alternative method to get char￾acteristics of the FM bonding [27,33]. The third useful material data derived from tensile tests on model 1D-composites are related to the in-situ failure stength of the fibers. As in nD real composites, the failure of model 1D-composites involves fiber pull-out and the fiber failure surface displays the mirror–mist–hackle features classically observed on brittle materials. The mirror radius, rm, is related to the in-situ tensile failure stress of the fiber, sR, through the following empirical equation: sR ˆ A=…rm† 1=2 …2† where A is a constant depending on fiber fracture toughness. A ˆ 2.45 MPa m1/2 for Nicalon fibers [27]. Mirror radii have been measured from the failure surfaces of Nicalon/PyC/ SiC (CVI) minicomposites with Vf ˆ 0.27; 0.60 and 0.70 in the SEM. The corresponding in-situ fiber failure stresses are presented in Fig. 8 together with the data for bare Nica￾lon single filaments and bare Nicalon fiber bundles (L ˆ 75 mm). The value of the Weibull shape parameter, mf, derived from the minicomposite data is 4.1–4.4. Further, the fact that the failure stress distributions for the minicom￾posites are almost identical to that for the Nicalon bundle suggests that some degradation of the fibers may have occurred during minicomposite processing. The failure stress distribution curves for the minicomposites are shifted towards higher stresses if the effective length is much shorter than the gauge length (L ˆ 75 mm) used in the bare bundle tests [27]. Therefore, these data may also confirm that the fiber debonding was complete at failure and that the minicomposite failure is similar to the one of fiber bundles. Although most mechanical tests have been performed under tensile loading in the present and related studies, other micromechanical tests are also of interest. Such is the case for push-in or push-out tests conducted under compressive loading. Such local tests (only one fiber is loaded) can be applied to either real nD-composites or model 1D-composites provided suitable specimen R. Naslain et al. / Composites: Part A 30 (1999) 537–547 543 Fig. 6. Weibull plot for the matrix failure stress in C (ex-PAN)/PyC/SiC minicomposites, as derived from measurements performed with a tensile stage set in a scanning electron microscope, according to Ref. [26]. Fig. 7. Simulation of the tensile stress–strain curve for a Nicalon/PyC/SiC minicomposite based on a probabilistic/statistical model, according to Ref. [27]