RH. Jones et al. I Journal of Nuclear Materials 307-311(2002)1057-1072 59 x) Acceptable structure lifetime in terms of sumed as the maximum allowable compressive stress fluence and plasma-first wall interaction;a since no damage is observed under compression. The cant experimental campaign is required to number of fibers through the thickness of the composite the sice/Sic limits is usually lower and their arrangement different. So, if one can accept the uncoupling of stresses in plane and 2.3. SiC /SiC thermo-mechanical model and design crite stresses through the thickness, the former can be eva rId uated using the von mises criteria, while the latter corresponds to the measured rupture value. Taking into The proposal for using SiC/SiC as structural mate- account the above remarks, for compressive stresses the rial for a nuclear component with a reasonably long limit is the rupture limits while for tensile stresses the lifetime is fairly recent, therefore no adequate modeling limit is the elastic limit. a margin of about 20% may be and design criteria are available yet. Some preliminary allowable on the elastic limit. work has recently been performed [2] aiming both to For example, in the case of the TaURo blanket identify appropriate models available in the aerospace based on the CERaSEP@ composites produced by research field and to theoretically define sound design SNECMA, the following limits have been assumed criteria to improve the design thermo-mechanical ana for normal stresses through the thickness, 110 MPa for tensile stresses(roughly corresponding to the ma- 2.3.1. Modeling trix tensile resistance limit outside the composite)and SiCr/Sic composites exhibit a complex nonlinear 420 MPa for compressive stresses(rupture limit for behavior combining brittle damage, residual strains CERASEP N2-1) and opening-closing of microcracks. These composites for shear stresses through the thickness, 44 MPa(as- present different properties, and therefore different sumed rupture limit, to be confirmed ) strengths, for different loading directions; moreover, for stresses in plane, 145 MPa for tensile stresses(be- tensile and compression strengths are very different. ginning of fiber/matrix debonding) and 580 MPa for Under loading, the interaction of fibers and matrix lead compressive stresses(rupture limit measured on the at first to matrix microcracking. then to matrix/fiber CERASEP N2-1). It appears clear that this design decohesion, followed by opening of the microcrack and criteria proposal is very preliminary; it needs to finally to fiber failure. This sequence corresponds to an further evaluated both theoretically and, more im- initial isotropic behavior in plane and then to a crack portant, experimentally through a systematic specific growth perpendicular to the fibers depending on the load direction. A relatively simple model, able to take into account such a behavior and based on continuum damage mechanics which consider the composites as a 3. Promise of Sicr/Sic composites continuous media, has been implemented in the FEM code CASTEM. Significant improvements on the results Composite materials made from continuous fibers of have been obtained when compared with models used SiC, can be woven into several variant fabric architec- for metals [1]. On the other end, damage description has tures and the matrix formed with a variety of infiltration been limited to scalar variables that is appropriate when methods. The se of Sic has been shown by damage is oriented in the fiber direction but not satis- merous studies [l] to have a saturation swelling value of factory when damage is loading oriented. A substantial about 0.1-0.2% at 800-1000oC. This suggests that effort is still required to develop and implement this for a composites of Sicr/Sic have the potential for excellent radiation stability. The continuous fiber architecture coupled with engineered interfaces between the fiber and 2.3.2. Design criteria matrix, provide excellent fracture properties and frac- To avoid degradation of the composite physical ture toughness values on the order of 25 MPam /.The properties, the elastic limit must be used as the maxi- strength and fracture toughness are independent of mum allowable stress. On the other hand, one of the temperature up to the limit of the fiber stability. With most attractive characteristics of SiCr/Sic composites is aprovements in fiber stability these materials exhibit that they are damage-tolerant, that is, they are capable. excellent mechanical properties to at least 1200C. Als of accommodating a high degree of deformation because these fiber/matrix microstructures impart excellent of crack arrest phenomena driven by the interface be- thermal shock and thermal fatigue resistance to these ween fibers and matrix. In principle, it can be assumed materials so plasma discharge and start-up and shut the limit for matrix microcracking saturation(beginning lown cycles should not induce significant structural of fiber/matrix debonding) as the maximum allowable damage In oxygen bearing environments, Sic will form tensile stress. The actual failure limit can instead be as- a protective layer of Sio, that greatly retards furtherii(ix) Acceptable structure lifetime in terms of neutron fluence and plasma-first wall interaction; a signifi- cant experimental campaign is required to define the SiCf/SiC limits. 2.3. SiCf /SiC thermo-mechanical model and design crite￾ria The proposal for using SiCf /SiC as structural mate￾rial for a nuclear component with a reasonably long lifetime is fairly recent, therefore no adequate modeling and design criteria are available yet. Some preliminary work has recently been performed [2] aiming both to identify appropriate models available in the aerospace research field and to theoretically define sound design criteria to improve the design thermo-mechanical ana￾lyses. 2.3.1. Modeling SiCf/SiC composites exhibit a complex nonlinear behavior combining brittle damage, residual strains and opening-closing of microcracks. These composites present different properties, and therefore different strengths, for different loading directions; moreover, tensile and compression strengths are very different. Under loading, the interaction of fibers and matrix lead at first to matrix microcracking, then to matrix/fiber decohesion, followed by opening of the microcrack and finally to fiber failure. This sequence corresponds to an initial isotropic behavior in plane and then to a crack growth perpendicular to the fibers depending on the load direction. A relatively simple model, able to take into account such a behavior and based on continuum damage mechanics which consider the composites as a continuous media, has been implemented in the FEM code CASTEM. Significant improvements on the results have been obtained when compared with models used for metals [1]. On the other end, damage description has been limited to scalar variables that is appropriate when damage is oriented in the fiber direction but not satis￾factory when damage is loading oriented. A substantial effort is still required to develop and implement this for a complete design model. 2.3.2. Design criteria To avoid degradation of the composite physical properties, the elastic limit must be used as the maxi￾mum allowable stress. On the other hand, one of the most attractive characteristics of SiCf /SiC composites is that they are damage-tolerant, that is, they are capable of accommodating a high degree of deformation because of crack arrest phenomena driven by the interface be￾tween fibers and matrix. In principle, it can be assumed the limit for matrix microcracking saturation (beginning of fiber/matrix debonding) as the maximum allowable tensile stress. The actual failure limit can instead be as￾sumed as the maximum allowable compressive stress since no damage is observed under compression. The number of fibers through the thickness of the composite is usually lower and their arrangement different. So, if one can accept the uncoupling of stresses in plane and stresses through the thickness, the former can be eval￾uated using the Von Mises criteria, while the latter corresponds to the measured rupture value. Taking into account the above remarks, for compressive stresses the limit is the rupture limits while for tensile stresses the limit is the elastic limit. A margin of about 20% may be allowable on the elastic limit. For example, in the case of the TAURO blanket, based on the CERASEP composites produced by SNECMA, the following limits have been assumed: • for normal stresses through the thickness, 110 MPa for tensile stresses (roughly corresponding to the ma￾trix tensile resistance limit outside the composite) and 420 MPa for compressive stresses (rupture limit for CERASEP N2-1); • for shear stresses through the thickness, 44 MPa (as￾sumed rupture limit, to be confirmed); • for stresses in plane, 145 MPa for tensile stresses (be￾ginning of fiber/matrix debonding) and 580 MPa for compressive stresses (rupture limit measured on the CERASEP N2-1). It appears clear that this design criteria proposal is very preliminary; it needs to be further evaluated both theoretically and, more im￾portant, experimentally through a systematic specific experimental campaign. 3. Promise of SiCf/SiC composites Composite materials made from continuous fibers of SiC, can be woven into several variant fabric architec￾tures and the matrix formed with a variety of infiltration methods. The b-phase of SiC has been shown by nu￾merous studies [1] to have a saturation swelling value of about 0.1–0.2% at 800–1000
C. This suggests that composites of SiCf /SiC have the potential for excellent radiation stability. The continuous fiber architecture, coupled with engineered interfaces between the fiber and matrix, provide excellent fracture properties and frac￾ture toughness values on the order of 25 MPa m1=2. The strength and fracture toughness are independent of temperature up to the limit of the fiber stability. With improvements in fiber stability these materials exhibit excellent mechanical properties to at least 1200
C. Also, these fiber/matrix microstructures impart excellent thermal shock and thermal fatigue resistance to these materials so plasma discharge and start-up and shut￾down cycles should not induce significant structural damage. In oxygen bearing environments, SiC will form a protective layer of SiO2 that greatly retards further R.H. Jones et al. / Journal of Nuclear Materials 307–311 (2002) 1057–1072 1059