Availableonlineatwww.sciencedirectcom ScienceDirect E噩≈RS ELSEVIER Joumal of the European Ceramic Society 27(2007)3301-3305 www.elsevier.com/locate/jeurceramsoc Strength enhancement of 2D-Sic/sic composites after static fatigue at room temperature A Morales-Rodrigueza M. Moevus,P. Reynaud, G Fantozzi a Departamento de Fisica de la Materia Condensada. Universidad de Sevilla, Apartado 1065, 41080 Sevilla, Spain b Groupe d'Enude de metallurgie Physique et de physique des Materiaux(GEMPPM, UMR CNRS 5510) INSa de Lyon. 20 Avenue Albert Einstein, 6962/ Villeurbanne. france Available online 2 April 2007 Abstract The fracture behavior of 2D-woven-SiC-fiber/SiC-matrix composites(2D-SiCe/SiC) has been studied under monotonic tensile test conditions in air at room temperature. The specimens statically fatigued at 90% OR showed a higher ultimate tensile strength (UTS) and failure strain than those of the original ones. Microstructural observations suggest that the static fatigue process enhances slow crack growth(SCG) mechanisms in fibers and the extensive fiber/matrix debonding after fatigue could be responsible of the enhanced final strength observed. Ultimate tensile stress has been evaluated from mirror radii of broken fibres, although this method turns up to overestimate experimental data. In contrast, the fracture behavior of Nicalon fiber bundles agrees with the results obtained in these composites. o 2007 Elsevier Ltd. All rights reserved. Keywords: Interfaces; Fatigue; Strength; SiC; Nicalon fibers 1. Introduction This work is focused on the influence of the static fatigue du ing long periods of time(about 120 h)on the residual strength Ceramic-matrix composites(CMCs), such as SiCr/SiC, are of Nicalon-SiCr/SiC composites and to determine if residual promising candidates for high-temperature applications as struc- strength of the composites is linked to the previous fatigue dam tural materials. The main disadvantages of monolithic ceramics age. Residual tensile tests after static fatigue at different stress for structural components are their brittleness and low relia- levels have been performed In situ fiber strength and shear stress bility; continuous-fiber-reinforced ceramic-matrix composites at the fiber/matrix interface have been estimated as a function of are specifically tailored to achieve a highly fracture-resistant loading conditions through the observation of fracture surfaces. material. CMCs combine the chemical stability of monolithic Special attention has been also paid to study the matrix damage ceramics with appropriate strength and damage tolerance by levels and more particularly the multiple cracking of matrix in means of reinforcing fibers that enhance the inherent ceramic longitudinal yarns It has been stated that the strength of Nicalon-SiCf/SiC com- 2. Experimental procedure tes, tested in air at room temperature, enhances aft fatigue. The origin of this enhancement is not yet well under- Specimens of 2D-Nicalon-fiber-reinforced SiC-matrix com- stood,but the authors suggest that it could be related to a decrease posites were provided by Snecma Propulsion Solide(france) of the fiber/matrix shear stress, T Low t values make the com- The composites were fabricated by densifying preforms of posite less sensitive to the nucleation of defects by decreasing 0/90 woven Nicalon NLM 202 fibers(from Nippon Carbon both the stress concentrations and the load transfer from matrix Co., Japan) with Sic deposited by chemical vapor infiltration Cr/SiC and Hi-Nicalon-SiC /Si, B, composites y observed in (CVI). The resulting composites were 15%0 porous, approxi- to fibers Similar strength enhancements have been mately, with large pores(macropores)located between the plies or at yarn intersections within the plies; much smaller pores are also present within the tows between the different matrix lay Corresponding author. Tel: +34 954 556029: fax: +34 954 612 097 ers. Details of fiber-reinforced SiC composites fabrication are E-mail address: amr@uses(A. Morales-Rodriguez described elsewhere 5 0955-2219/S-see front matter o 2007 Elsevier Ltd. All rights reserved. doi: 10. 1016/j-jeurceramsoc. 2007.02 180
Journal of the European Ceramic Society 27 (2007) 3301–3305 Strength enhancement of 2D-SiCf/SiC composites after static fatigue at room temperature A. Morales-Rodr´ıguez a,∗, M. Moevus b, P. Reynaud b, G. Fantozzi b a Departamento de F´ısica de la Materia Condensada, Universidad de Sevilla, Apartado 1065, 41080 Sevilla, Spain b Groupe d’Etude de M´etallurgie Physique et de Physique des Mat´eriaux (GEMPPM, UMR CNRS 5510), INSA de Lyon, 20 Avenue Albert Einstein, 69621 Villeurbanne, France Available online 2 April 2007 Abstract The fracture behavior of 2D-woven-SiC-fiber/SiC-matrix composites (2D-SiCf/SiC) has been studied under monotonic tensile test conditions in air at room temperature. The specimens statically fatigued at 90% σR showed a higher ultimate tensile strength (UTS) and failure strain than those of the original ones. Microstructural observations suggest that the static fatigue process enhances slow crack growth (SCG) mechanisms in fibers and the extensive fiber/matrix debonding after fatigue could be responsible of the enhanced final strength observed. Ultimate tensile stress has been evaluated from mirror radii of broken fibres, although this method turns up to overestimate experimental data. In contrast, the fracture behavior of Nicalon fiber bundles agrees with the results obtained in these composites. © 2007 Elsevier Ltd. All rights reserved. Keywords: Interfaces; Fatigue; Strength; SiC; Nicalon fibers 1. Introduction Ceramic-matrix composites (CMCs), such as SiCf/SiC, are promising candidates for high-temperature applications as structural materials.1 The main disadvantages of monolithic ceramics for structural components are their brittleness and low reliability; continuous-fiber-reinforced ceramic-matrix composites are specifically tailored to achieve a highly fracture-resistant material. CMCs combine the chemical stability of monolithic ceramics with appropriate strength and damage tolerance by means of reinforcing fibers that enhance the inherent ceramic brittleness. It has been stated that the strength of Nicalon-SiCf/SiC composites, tested in air at room temperature, enhances after cyclic fatigue.2 The origin of this enhancement is not yet well understood, but the authors suggest that it could be related to a decrease of the fiber/matrix shear stress, τ. Low τ values make the composite less sensitive to the nucleation of defects by decreasing both the stress concentrations and the load transfer from matrix to fibers.2 Similar strength enhancements have been observed in Cf/SiC and Hi-Nicalon-SiCf/Si,B,C composites.3,4 ∗ Corresponding author. Tel.: +34 954 556 029; fax: +34 954 612 097. E-mail address: amr@us.es (A. Morales-Rodr´ıguez). This work is focused on the influence of the static fatigue during long periods of time (about 120 h) on the residual strength of Nicalon-SiCf/SiC composites and to determine if residual strength of the composites is linked to the previous fatigue damage. Residual tensile tests after static fatigue at different stress levels have been performed.In situ fiber strength and shear stress at the fiber/matrix interface have been estimated as a function of loading conditions through the observation of fracture surfaces. Special attention has been also paid to study the matrix damage levels and more particularly the multiple cracking of matrix in longitudinal yarns. 2. Experimental procedure Specimens of 2D-Nicalon-fiber-reinforced SiC-matrix composites were provided by Snecma Propulsion Solide (France). The composites were fabricated by densifying preforms of 0/90◦ woven Nicalon NLM 202 fibers (from Nippon Carbon Co., Japan) with SiC deposited by chemical vapor infiltration (CVI). The resulting composites were 15% porous, approximately, with large pores (macropores) located between the plies or at yarn intersections within the plies; much smaller pores are also present within the tows between the different matrix layers. Details of fiber-reinforced SiC composites fabrication are described elsewhere.5 0955-2219/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2007.02.180
A. Morales-Rodriguez et al. Journal of the European Ceramic Sociery 27(2007)3301-3305 Table I List of MTT-deformed specimens indicating the solicitation conditions, strengths, final strains and Youngs moduli Specimen Loading rate(N/min) OUTs(MPa) Efna(%) Ec(MPa 0.48 The fracture behavior at room temperature in air has been investigated under different uniaxial tensile conditions. mono- tonic load (MTT, Monotonic Tensile Test) and after static fatigue #1 at various stress levels to explore the residual properties(RTT, Residual Tensile Test). The mechanical tests have been per #2 formed on a servohydraulic machine(INSTRON model 8502) using samples with bone-shape of rectangular section(calibrated length: 8 mm x 3 mm x 40 mm). Samples and test conditions ￡(% are summarized in Tables 1 and 2 Fractured specimens were examined using a conventional Fig. 1. Monotonic tensile behavior of original specimens loaded at 100N/min scanningelectronic microscope(SEM, model JEOL JSM-840A) (dashed line)and at 500N/min(solid line) at room temperature in air. to investigate the characteristic fracture surface morphology of the fibers in order to estimate in situ fiber strength. Pull out lengths have been measured to estimate the shear stress developed during fiber/matrix sliding before the final fracture. 6 Surfaces parallel to the loading axis have been cut using a slow speed saw, polished up to 1 um diamond paste and chem ically etched by the Murakami procedure to exhibit the matrix 3. Experimental results ig. I presents a-e plots corresponding to the initial monotonic tensile behavior of 2D-SiC/Sic composites. The mechanical properties obtained from these tests are collected in Table 1. It is observed that these composites are able to achieve UTS values of 228 MPa and final deformations up to 0.7%.Fig. 2 shows a-E plots corresponding to RTT after static loading at two different stress levels of 70%(dashed line) and 90% or(solid line) where the maximal strength measured in MTT has been Fig. 2. a-c plots from RTT after two different loading conditions in fatigue of considered as the reference strength, oR, for these composites 70%(dashed line)and 90%0 OR(solid line) at room temperature in air. These The changes observed in o-E slopes after static fatigue tests, Et, plots show the loading, the static fatigue and the final residual tensile curves comparing with the first loading slopes, EC, indicate that dam- aging processes are developed during static fatigue, decreasing for as-received specimens. In contrast, an enhancement of the the elastic modulus(EC< EF). Youngs modulus evolution is residual properties is observed after loading at 90% of rupture negligible up to 60% OR static loading conditions(Fig. 2, #5). stress level; higher strength and larger final deformation have The mechanical properties obtained from RTTs are collected been achieved in"high-stress"post-fatigued composites(solid in Table 2. Note that the strength after static loading at low line). This enhancement of residual properties has not been noted stress(dashed line) is limited to the monotonic strength found previously after static fatigue Table 2 List of RTT-deformed specimens indicating the static fatigue conditions, strengths, final strains and Youngs moduli(F: first loading and D: damaged after static Specimen Fatigue conditions OUTS(MPa) Final(%o) (MPa) (MPa) 100h,70%aR 6 h,90% Loading rates of 500N/min were imposed during monotonic tensile steps
3302 A. Morales-Rodr´ıguez et al. / Journal of the European Ceramic Society 27 (2007) 3301–3305 Table 1 List of MTT-deformed specimens indicating the sollicitation conditions, strengths, final strains and Young’s moduli Specimen Loading rate (N/min) σUTS (MPa) εfinal (%) EC (MPa) #1 500 228 0.66 189 #2 100 202 0.48 178 #3 100 225 – – The fracture behavior at room temperature in air has been investigated under different uniaxial tensile conditions: monotonic load (MTT, Monotonic Tensile Test) and after static fatigue at various stress levels to explore the residual properties (RTT, Residual Tensile Test). The mechanical tests have been performed on a servohydraulic machine (INSTRON model 8502) using samples with bone-shape of rectangular section (calibrated length: 8 mm × 3 mm × 40 mm). Samples and test conditions are summarized in Tables 1 and 2. Fractured specimens were examined using a conventional scanning electronic microscope (SEM, model JEOL JSM-840A) to investigate the characteristic fracture surface morphology of the fibers in order to estimate in situ fiber strength. Pullout lengths have been measured to estimate the shear stress developed during fiber/matrix sliding before the final fracture.6 Surfaces parallel to the loading axis have been cut using a slow speed saw, polished up to 1 m diamond paste and chemically etched by the Murakami procedure to exhibit the matrix cracks. 3. Experimental results Fig. 1 presents σ–ε plots corresponding to the initial monotonic tensile behavior of 2D-SiCf/SiC composites. The mechanical properties obtained from these tests are collected in Table 1. It is observed that these composites are able to achieve UTS values of 228 MPa and final deformations up to 0.7%. Fig. 2 shows σ–ε plots corresponding to RTT after static loading at two different stress levels of 70% (dashed line) and 90% σR (solid line) where the maximal strength measured in MTT has been considered as the reference strength, σR, for these composites. The changes observed in σ–ε slopes after static fatigue tests, ED C, comparing with the first loading slopes, EF C, indicate that damaging processes are developed during static fatigue, decreasing the elastic modulus (ED C < EF C). Young’s modulus evolution is negligible up to 60% σR static loading conditions (Fig. 2, #5). The mechanical properties obtained from RTTs are collected in Table 2. Note that the strength after static loading at low stress (dashed line) is limited to the monotonic strength found Fig. 1. Monotonic tensile behavior of original specimens loaded at 100 N/min (dashed line) and at 500 N/min (solid line) at room temperature in air. Fig. 2. σ–ε plots from RTT after two different loading conditions in fatigue of 70% (dashed line) and 90% σR (solid line) at room temperature in air. These plots show the loading, the static fatigue and the final residual tensile curves. for as-received specimens. In contrast, an enhancement of the residual properties is observed after loading at 90% of rupture stress level; higher strength and larger final deformation have been achieved in “high-stress” post-fatigued composites (solid line). This enhancement of residual properties has not been noted previously after static fatigue. Table 2 List of RTT-deformed specimens indicating the static fatigue conditions, strengths, final strains and Young’s moduli (F: first loading and D: damaged after static fatigue) Specimen Fatigue conditions σUTS (MPa) εfinal (%) EF C (MPa) ED C (MPa) #4 100 h, 70% σR 225 0.6 197 115 #5 120 h, 90% σR 265 0.9 171 82 #6 120 h, 90% σR 256 – – – Loading rates of 500 N/min were imposed during monotonic tensile steps.
A. Morales-Rodrfgnez er al. /Journal of the European Ceramic Society 27(2007)3301-3305 3303 Microstructural observations have been devoted to charac e the fiber fracture surfaces, pull-out phenomena and matrix cracking to account for the composite mechanical response. The main results obtained are summarized as follows, (i)The 72% and 82% of fibers in RTT specimens exhibit mirror surfaces after static fatigue at 70% and 90% or respectively, whereas about the half of fibers in MTT deformed composites presents mirror surfaces(56%). The previous result suggests that deformation under constant load enhances SCG processes in fibers. The mirror sur faces are well-defined and, in general, the original defect in these Nicalon fibers is associated to the fiber surface. but is too small to be observed(Fig 3). Measurements of mir- ror surfaces radii indicate that similar critical crack sizes are observed in both testing conditions (ii)The pull-out length is shorter after static fatigue RTT=(34+2)um, than for original specimens, Fig. 4. Pulled-out fibers emerging from the fracture plane in specimen #4. MTT=(58+4) um. Interfacial debonding between fibers and matrix has been systematically observed for pulled-out fibers. Fig. 4 shows a typical micrograph of pulled-out fibers from a RTT-deformed composite (iii) The distribution of the intercracking distance of the Sic matrix has been studied to compare multicracking damage of composites. Fig. 5 shows that the mean matrix intercracking distances decreases with increasing final stress levels acting on the specimens. Data from two HA日 additional tests performed to compare the influence of monotonic and static loading on the matrix damage are also included in Fig. 5(specimens were loaded below to final fracture stress values until 80% R in case of MTT(#7)and r during 90h for the static f the intercracking distribution was studied for these non- fractured specimens). No differences with global behavior have been deduced from these both loading conditions induce similar matrix damage char- d (um) correlation between maximum final stress applied and matrix damage: the higher istics where the key role is played by the final stress 4. discus 4.1. In situ fiber strength and fiber/ matrix interfacial shear stress of 2 D-SiCySiC composites from the measured mirror radii using the following empirical Fig 3. Distinct mirror surface composites where it is where Im is the mirror radius, or the in situ fiber tensile strength ot possible to identify the initial defect. Note the interfacial debonding between and Am is the mirror constant. The value of Am=3.5MPam-I fiber and matrix has been considered in this study following the work of Thouless
A. Morales-Rodr´ıguez et al. / Journal of the European Ceramic Society 27 (2007) 3301–3305 3303 Microstructural observations have been devoted to characterize the fiber fracture surfaces, pull-out phenomena and matrix cracking to account for the composite mechanical response. The main results obtained are summarized as follows: (i) The 72% and 82% of fibers in RTT specimens exhibit mirror surfaces after static fatigue at 70% and 90% σR, respectively, whereas about the half of fibers in MTTdeformed composites presents mirror surfaces (56%). The previous result suggests that deformation under constant load enhances SCG processes in fibers. The mirror surfaces are well-defined and, in general, the original defect in these Nicalon fibers is associated to the fiber surface, but is too small to be observed (Fig. 3). Measurements of mirror surfaces radii indicate that similar critical crack sizes are observed in both testing conditions. (ii) The pull-out length is shorter after static fatigue, lRTT = (34 ± 2)m, than for original specimens, lMTT = (58 ± 4)m. Interfacial debonding between fibers and matrix has been systematically observed for pulled-out fibers. Fig. 4 shows a typical micrograph of pulled-out fibers from a RTT-deformed composite. (iii) The distribution of the intercracking distance of the SiC matrix has been studied to compare multicracking damage of composites. Fig. 5 shows that the mean matrixintercracking distances decreases with increasing final stress levels acting on the specimens. Data from two additional tests performed to compare the influence of monotonic and static loading on the matrix damage are also included in Fig. 5 (specimens were loaded below to final fracture stress values until 80% σR in case of MTT (#7) and 70% σR during 90 h for the static fatigue test (#8) and then, the intercracking distribution was studied for these nonfractured specimens). No differences with global behavior have been deduced from these experiences meaning that both loading conditions induce similar matrix damage charFig. 3. Distinct mirror surface developed in 2D-SiCf/SiC composites where it is not possible to identify the initial defect. Note the interfacial debonding between fiber and matrix. Fig. 4. Pulled-out fibers emerging from the fracture plane in specimen #4. Fig. 5. Matrix-intercracking distance (d) vs. final stress level plot showing a correlation between maximum final stress applied and matrix damage: the higher stress, the shorter intercracking distance. acteristics where the key role is played by the final stress level carried. 4. Discussion 4.1. In situ fiber strength and fiber/matrix interfacial shear stress of 2D-SiCf/SiC composites In situ fiber strength in the composites was evaluated from the measured mirror radii using the following empirical relationship7: σfr1/2 m = Am, (1) where rm is the mirror radius, σf the in situ fiber tensile strength and Am is the mirror constant. The value of Am = 3.5 MPa m−1/2 has been considered in this study following the work of Thouless
A. Morales-Rodriguez et al. Journal of the European Ceramic Sociery 27(2007)3301-3305 where A is a m-dependent prefactor that has been taken for a value equal to 1 based on and r is the mean radius of the fiber (r=7.25 um for NM 202 Nicalon fibers) The difference in fiber pull-out lengths measured results in apparent interfacial shear stress values of 80 and 120 MPa for MTT- and RTT-specimens, respectively. There is no phys- 0.6 ical explanation of such increase of interfacial shear stress due to static fatigue. On the other hand during RTT an additional matrix multicracking can be introduced leading to a saturation of the matrix cracking: in that case. the pull-out length is not representative of the actual value of #2 the interfacial shear stress. Complementary tests are now in #4 progress, using the microindentation technique, to confirm this hypothesis 4.2. A fiber tows-based approach to explain the 2D-SiCrSic composites tensile strengt Fig. 6. Statistical distributions of in situ fiber tensile strength Although the in situ properties of fibers and fiber/matrix interfaces in the composite material is commonly used to infer et al. 8 By plotting the cumulative distribution of of(Fig. 6) the macroscopic mechanical properties of CMcs as the tensile measured on many fibers(more than 20 fibers/specimen), the strength, UTS, from the strength of the fibers and the shear stress shape factor, m, and the scale parameter of the fibers, o, has developed during fiber/matrix sliding before the final fracture, 6 been ascertained from the Weibull function given by Eq (2): the previous analysis of our results is not sufficient to describe the UTS experimentally obtained P(or)=1-exp o Regarding Figs. I and 2, the final linearity of a-E curves (except for #2 case)and the large final strains achieved suggest that fiber tows could be the entity responsible These statistical parameters have been used to predict the ulti- mate failure in these composites, better than individual fiber mate strength of composites following Eq. (3)proposed by characteristics. The extensive fiber/matrix debonding stated at microstructural observations supports the previous hypoth esis. Debonding phenomena could be strongly determined by (3) matrix-intercracking distances after static loading: the shorter intercracking level, the stronger debonding. Then, the final where Ve is the volume fraction of fibers in the loading direc- unloading-reloading performed on fatigued-specimens favours ion(in this study V=0. 2). A comparison between predicted fill fiber/matrix debonding in high-stress fatigued-specimens and measured values of oUTS is presented in Table 3. Even if a achieving larger deformations Under lower loading conditions significant fraction of the fibers exhibit well-defined mirror su it is expected to achieve higher matrix-intercracking distances faces, it seems that the fracture mirror approach overestimates making more difficult the full fiber/matrix debonding and, the UTs values experimentally obtained. therefore, hindering the tows or individual fibers elonga The in situ scale parameter, o, has been also used to cal- tion. The previous argument justifies the improvement in final culate the fiber/matrix shear stress, t, using the pull-out length strength obtained after static fatigue at 90%o respect to 70%OR measured on fractured specimens by means of the following Case expression Recently, Calard and Lamon have presented a model for the oUTs of composites considering the force-strain curves 入(m)roo (4) for Nicalon NLM 202 fiber bundles. The final tensile strength of composites, oUTS, is approximately given by the following Table 3 Comparison between ultimate strengths predictions obtained from in sin fiber strength, single fibers and fiber tows approaches PUTS (MPa) arTs(MPa) OUTS (MPa) (MPa) 34 24 The statistical parameters obtained from data plotted in Fig. 6 are included
3304 A. Morales-Rodr´ıguez et al. / Journal of the European Ceramic Society 27 (2007) 3301–3305 Fig. 6. Statistical distributions of in situ fiber tensile strength. et al.8 By plotting the cumulative distribution of σf (Fig. 6) measured on many fibers (more than 20 fibers/specimen), the shape factor, m, and the scale parameter of the fibers, σ0, has been ascertained from the Weibull function given by Eq. (2): P(σf) = 1 − exp − σf σ0 m . (2) These statistical parameters have been used to predict the ultimate strength of composites following Eq. (3) proposed by Curtin6: σmirror UTS = Vfσ0 2 m + 2 1/m+1 m + 1 m + 2 , (3) where Vf is the volume fraction of fibers in the loading direction (in this study Vf = 0.2). A comparison between predicted and measured values of σUTS is presented in Table 3. Even if a significant fraction of the fibers exhibit well-defined mirror surfaces, it seems that the fracture mirror approach overestimates the UTS values experimentally obtained. The in situ scale parameter, σ0, has been also used to calculate the fiber/matrix shear stress, τ, using the pull-out length measured on fractured specimens by means of the following expression6: τ = λ(m)rσ0 4l , (4) where λ is a m-dependent prefactor that has been taken for a value equal to 1 based on6 and r is the mean radius of the fiber (r = 7.25m for NLM 202 Nicalon fibers). The difference in fiber pull-out lengths measured results in apparent interfacial shear stress values of 80 and 120 MPa for MTT- and RTT-specimens, respectively. There is no physical explanation of such increase of interfacial shear stress due to static fatigue. On the other hand during RTT an additional matrix multicracking can be introduced leading to a saturation of the matrix cracking; in that case, the pull-out length is not representative of the actual value of the interfacial shear stress. Complementary tests are now in progress, using the microindentation technique, to confirm this hypothesis. 4.2. A fiber tows-based approach to explain the 2D-SiCf/SiC composites tensile strength Although the in situ properties of fibers and fiber/matrix interfaces in the composite material is commonly used to infer the macroscopic mechanical properties of CMCs as the tensile strength, UTS, from the strength of the fibers and the shear stress developed during fiber/matrix sliding before the final fracture,6 the previous analysis of our results is not sufficient to describe the UTS experimentally obtained. Regarding Figs. 1 and 2, the final linearity of σ–ε curves (except for #2 case) and the large final strains achieved suggest that fiber tows could be the entity responsible for the ultimate failure in these composites, better than individual fiber characteristics.9 The extensive fiber/matrix debonding stated at microstructural observations supports the previous hypothesis. Debonding phenomena could be strongly determined by matrix-intercracking distances after static loading: the shorter intercracking level, the stronger debonding. Then, the final unloading-reloading performed on fatigued-specimens favours full fiber/matrix debonding in high-stress fatigued-specimens, achieving larger deformations. Under lower loading conditions it is expected to achieve higher matrix-intercracking distances making more difficult the full fiber/matrix debonding and, therefore, hindering the tow’s or individual fiber’s elongation. The previous argument justifies the improvement in final strength obtained after static fatigue at 90% respect to 70% σR case. Recently, Calard and Lamon10 have presented a model for the σUTS of composites considering the force–strain curves for Nicalon NLM 202 fiber bundles. The final tensile strength of composites, σtows UTS, is approximately given by the following Table 3 Comparison between ultimate strengths predictions obtained from in situ fiber strength,6 single fibers and fiber tows10 approaches Specimen σexp UTS (MPa) m σ0 (GPa) σmirror UTS (MPa) σtows UTS (MPa) σfiber UTS (MPa) #1 228 3.2 2.3 298 265 238 #2 202 3.6 2.0 262 192 172 #4 225 4.2 2.1 286 221 216 #5 265 3.4 2.4 312 292 324 The statistical parameters obtained from data plotted in Fig. 6 are included.
A Morales-Rodriguez er al. Journal of the European Ceramic Society 27(2007)3301-3305 3305 Acknowledgements gITS= Kotow The authors would like to thank Snecma Propulsion Solide ( France)and the programs of Ayudas de perfeccionamiento de Doctores de la junta de andalucia and avudas de Ayudantes de la Universidad de Sevilla(spain) for supporting where Vr is the volume fraction of longitudinal fiber tows in the this research. We are grateful to Dr M. R'Mili for his helpful composites,Otow the tow strength, F the force operating on the discussion tow, N the nominal number of fibers/tow, ac the critical value of the ratio of the number of broken fibers N to the total number References of fibers N(when the surviving fibers are unable to carry the applied load )and r is the mean fiber radius 1. Naslain, R, Design, preparation and of non-oxide cmcs for Table 3 presents the values of ultimate strength calcu- application in engines and nuclear reactors: an overview. Comp. Sci. Tech ated from previous equations. The critical fraction of broken no.,2004,64,155-170. fibers has been calculated considering that the neighbour 2. Mizuno, M. Zhu. S, Nagano, Y. Sakaida, Y, Kagawa, Y. and Watan- overloading of fibers interact (global load sharing). The ulti- abe, M, Cyclic-fatigue behavior of Sic/SiC composites at room and high temperatures.J. Am. Ceram Soc. 1996, 79, 3065-3077 mate strength has also been calculated from single sic-fibers 3. Shuler, S. F, Holmes, J. W and Wu, X, Influence of loading frequency data(E= 180 GPa), oUTS, considering the final deformations the room-temperature fatigue of carbon-fiber/SiC-matrix composites. JAm achieved by the composites. Results from predictions collected Ceran.Soc.,1993,76,2327-2336 in Table 3 show that both tows and single fibers are accurate 4. Penas, O, Etude de composites SiC/SiBC a matrice multisequencee fatigue cyclique a hautes temperatures sous air. PhD Thesis. INSA Lyon, to predict tended illeurbanne. france. 2002. obtained than considering the mirror approach(see Sec- 5. Stinton, D P, Caputo, A.J. and Lowden, R.A, Synthesis of fiber-reinforced tion 4.1) SiC composites by chemical vapor infiltration. Am. Ceram Soc. Bul, 1986, 65,347-350. 5. Conclusions 6. Curtin, W.A, Theory of mechanical properties of ceramic-matrix compos- s.JAm.cerm.Soc.,1991,74.2837-2845 7. Melchosky, J. J, Freiman, S. W. and Rice, R. w Fracture surface analysis This work points out that the extensive fiber/matrix debond- of ceramics. J Mater Sci. 1976. 11, 1310-1319 ing could be related to the strength enhancement observed in 8. Thouless, M D, Sbazeiro, O, Sigl, L S and Evans, A G, Effect of interface statically fatigued 2D-SiCrSiC composites. The a-E curves mechanical properties on pullout in SiC-fiber-reinforced ceramic matrix indicate that multicracking of the matrix is saturated near the composites.JAm Ceram Soc., 1989, 72, 525-532. 9. Lamon, J.A., Amicromechanics-based approach to the mechanical behavior fracture of the composite. Hence, UTS of these composites of brittle- matrix composites. Comp. Sci. Technol., 2001, 61, 2259-2272. is well described by fracture behavior of bundles and statisti- Calar, V. and Lamon, J, Failure of fiber bundles. Comp. Sci. Technol. cal distribution of individual fiber failures. However, analysis 2004, 64, 701-710 based on the mirror zone of the fibers'fracture surface is less 11. Fantozzi, G, Reynaud, P and Rouby, D, Thermomechanical behaviour of ong fibres ceramic-ceramic composites. Sil. Ind. 2002, 66, 109-119
A. Morales-Rodr´ıguez et al. / Journal of the European Ceramic Society 27 (2007) 3301–3305 3305 equations10: σtows UTS = Vfσtow, (5) σtow = F Nt(1 − αC)πr2 , (6) where Vf is the volume fraction of longitudinal fiber tows in the composites, σtow the tow strength, F the force operating on the tow, Nt the nominal number of fibers/tow, αC the critical value of the ratio of the number of broken fibers N to the total number of fibers Nt (when the surviving fibers are unable to carry the applied load) and r is the mean fiber radius. Table 3 presents the values of ultimate strength calculated from previous equations. The critical fraction of broken fibers has been calculated considering that the neighbour overloading of fibers interact (global load sharing).11 The ultimate strength has also been calculated from single SiC-fibers data (E = 180 GPa10), σfiber UTS, considering the final deformations achieved by the composites. Results from predictions collected in Table 3 show that both tows and single fibers are accurate to predict the final composite strength tendency experimentally obtained, better than considering the mirror approach (see Section 4.1). 5. Conclusions This work points out that the extensive fiber/matrix debonding could be related to the strength enhancement observed in statically fatigued 2D-SiCf/SiC composites. The σ–ε curves indicate that multicracking of the matrix is saturated near the fracture of the composite. Hence, UTS of these composites is well described by fracture behavior of bundles and statistical distribution of individual fiber failures. However, analysis based on the mirror zone of the fibers’ fracture surface is less accurate. Acknowledgements The authors would like to thank Snecma Propulsion Solide (France) and the programs of Ayudas de Perfeccionamiento de Doctores de la Junta de Andaluc´ıa and Ayudas de Movilidad de Ayudantes de la Universidad de Sevilla (Spain) for supporting this research. We are grateful to Dr. M. R’Mili for his helpful discussion. References 1. Naslain, R., Design, preparation and properties of non-oxide CMCs for application in engines and nuclear reactors: an overview. Comp. Sci. Technol., 2004, 64, 155–170. 2. Mizuno, M., Zhu, S., Nagano, Y., Sakaida, Y., Kagawa, Y. and Watanabe, M., Cyclic-fatigue behavior of SiC/SiC composites at room and high temperatures. J. Am. Ceram. Soc., 1996, 79, 3065–3077. 3. Shuler, S. F., Holmes, J. W. and Wu, X., Influence of loading frequency on the room-temperature fatigue of carbon-fiber/SiC-matrix composites. J. Am. Ceram. Soc., 1993, 76, 2327–2336. 4. Penas, O., Etude de composites SiC/SiBC a matrice multis ` equenc ´ ee en ´ fatigue cyclique a hautes temp ` eratures sous air. PhD Thesis. INSA Lyon, ´ Villeurbanne, France, 2002. 5. Stinton, D. P., Caputo, A. J. and Lowden, R. A., Synthesis of fiber-reinforced SiC composites by chemical vapor infiltration. Am. Ceram. Soc. Bull., 1986, 65, 347–350. 6. Curtin, W. A., Theory of mechanical properties of ceramic-matrix composites. J. Am. Ceram. Soc., 1991, 74, 2837–2845. 7. Melchosky, J. J., Freiman, S. W. and Rice, R. W., Fracture surface analysis of ceramics. J. Mater. Sci., 1976, 11, 1310–1319. 8. Thouless, M. D., Sbazeiro, O., Sigl, L. S. and Evans, A. G., Effect of interface mechanical properties on pullout in SiC-fiber-reinforced ceramic matrix composites. J. Am. Ceram. Soc., 1989, 72, 525–532. 9. Lamon, J. A., A micromechanics-based approach to the mechanical behavior of brittle-matrix composites. Comp. Sci. Technol., 2001, 61, 2259–2272. 10. Calard, V. and Lamon, J., Failure of fiber bundles. Comp. Sci. Technol., 2004, 64, 701–710. 11. Fantozzi, G., Reynaud, P. and Rouby, D., Thermomechanical behaviour of long fibres ceramic-ceramic composites. Sil. Ind., 2002, 66, 109–119