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 180Journal 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 struc￾tural materials.1 The main disadvantages of monolithic ceramics for structural components are their brittleness and low relia￾bility; 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 com￾posites, tested in air at room temperature, enhances after cyclic fatigue.2 The origin of this enhancement is not yet well under￾stood, but the authors suggest that it could be related to a decrease of the fiber/matrix shear stress, τ. Low τ values make the com￾posite 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 dur￾ing 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 dam￾age. 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 com￾posites 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, approxi￾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￾ers. 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