B G. Nair et al. /Materials Science and Engineering 4300(2001)68-79 for such 2D composites. In ID ceramic matrix com- tial transient due to load transfer from the matrix to the posites with a distinctly thin(<O I um) viscoelastic fibers, leading to steady-state creep that is rate limited nterface separating the fiber and matrix, the loading by flow of the fibers. As o increases, the shear forces at geometry that produces the highest strain-rate is one the fiber-matrix interface increase, these result in a hat optimizes slip at the interface, i.e. that have fibers substantial contribution to the bulk composite strain by oriented at approximately 45 to the applied stress, or. interface sliding (or, depending on the composite sys- Thus, it is of fundamental interest to extend the study tem, interphase filow). The contribution due to this of fiber-orientation effects to the creep deformation of relative displacement at the interface is maximized at 2D composites and so investigate the feasibility of 45. However, geometrical constraints dictate that modeling high-T creep of these materials the matrix would still be rate-limiting as slip at the The specific rheologic response of the individual plies interphase must be accommodated by matrix shear in 2D composites depends on the stress-distribution flow. Further increase in op results in a high plasticity developed in the composite at steady state. Thus, the e.g. von Mises) potential le matrix around the recognition of a suitable paradigm that describes the fibers, leading to matrix flow around the fibers. This is elastic flow of 2D composites is predicated on under- accompanied by cavitation at the fiber-matrix interface standing how the stresses are partitioned between the due to the development of tensile tractions normal to various plies. This line of thought leads to the question the interface of whether a 2D composite could behave as a simple The present work focuses on characterizing the hi twophase laminate with each ply behaving as a thin temperature, low-differential-stress rheology of 2D(O/ section of a ID composite. In the remainder of this 90, cross-ply) laminates in off-axis loading geometries paper, any reference to 2D composites should be con- and comparing/contrasting their response with the case sidered to be meant for composites with 0/90o fiber for ID reinforcement. As a necessary requirement for reinforcement unless otherwise mentioned 2D composite specimens for the creep study under composites with an identical matrix composition. Be- taken here are designed such that the applied stress, a1 cause, the matrix chemical composition, phase distribu is parallel to the planes defined by the fiber directions in tion and morphology(and hence it is viscoelastic both sets of plies. These 2D specimens can be charac rheology) were different from that of the composites terized by a misorientation angle, y, which is the acute used in our previous experimental work [9], these ID angle between the direction of the applied stress and the composite baseline experiments also provided addi- set of plies is thus misoriented at(y-90%) from the oped earli cation to the Id rheological model devel- orientation of any one set of fibers(Fig. la); the other loading direction. Similarly, in ID composites, off-axis geometry can be characterized by a misorientation an- 2. Experimental design and procedure gle, o (Fig. 1b). Characterizing flow in 2D composites requires an analysis of their behavior in the context of low-well-understood creep behavior of ID composites; 2.1. Material specifications possible that fiber orientations in the 2D material The materials used in this study were Sic fiber orrespond behaviorally with ID material, i.e. with =vand90°一ψ. For ID composites with~0°,the Nicalon)-reinforced calcium aluminosilicate com- rheologic response is characterized by a significant ini posites fabricated and supplied by Coming, Inc. Both the bidirectionally reinforced(2D) and unidirectionally reinforced(ID)composite sheets were fabricated by uniaxial hot-pressing of prepreg plies at 1350C and 90°v 15 MPa [12]. Both ID and 2D materials consisted of 16 plies, each with a fiber volume fraction, Vr, of 0.3 the 2D composite had a [o/90s(sy D)lay-up The matrix was an anorthite-based glassceramic(Com- ing Code CAS-Il) with a grain-size of 3 um. The estimated oxide composition of the CAS-II matrix based on X-ray fluorescence spectroscopy (XRAL Labs, Ont., Canada)is shown in Table 1. X-ray diffrac tion of powder specimens indicated that the primary phases were anorthite (Cao: Al,O3: 2SiO2) and mullite (ALO3: 2SiO2 ): a calculation based on peak intensities Fig.1.Specimen geometry for off-axis'compression creep exp: indicated 85% anorthite and 11% mullite by weight ments.(a) 2D composites: The outer ply ()is cut away to reveal its complementary ply (D);(b) ID composites Electron-microprobe analysis(Cameca SX51) showedB.G. Nair et al. / Materials Science and Engineering A300 (2001) 68–79 69 for such 2D composites. In 1D ceramic matrix com￾posites with a distinctly thin (B0.1 mm) viscoelastic interface separating the fiber and matrix, the loading geometry that produces the highest strain-rate is one that optimizes slip at the interface, i.e. that have fibers oriented at approximately 45° to the applied stress, s1. Thus, it is of fundamental interest to extend the study of fiber-orientation effects to the creep deformation of 2D composites and so investigate the feasibility of modeling high-T creep of these materials. The specific rheologic response of the individual plies in 2D composites depends on the stress-distribution developed in the composite at steady state. Thus, the recognition of a suitable paradigm that describes the inelastic flow of 2D composites is predicated on under￾standing how the stresses are partitioned between the various plies. This line of thought leads to the question of whether a 2D composite could behave as a simple twophase laminate with each ply behaving as a thin section of a 1D composite. In the remainder of this paper, any reference to 2D composites should be con￾sidered to be meant for composites with 0/90° fiber reinforcement unless otherwise mentioned. 2D composite specimens for the creep study under￾taken here are designed such that the applied stress, s1 is parallel to the planes defined by the fiber directions in both sets of plies. These 2D specimens can be charac￾terized by a misorientation angle, c, which is the acute angle between the direction of the applied stress and the orientation of any one set of fibers (Fig. 1a); the other set of plies is thus misoriented at (c−90°) from the loading direction. Similarly, in 1D composites, off-axis geometry can be characterized by a misorientation an￾gle, 8 (Fig. 1b). Characterizing flow in 2D composites requires an analysis of their behavior in the context of now-well-understood creep behavior of 1D composites; it is possible that fiber orientations in the 2D material correspond behaviorally with 1D material, i.e. with 8=c and 90°−c. For 1D composites with 80°, the rheologic response is characterized by a significant ini￾tial transient due to load transfer from the matrix to the fibers, leading to steady-state creep that is rate limited by flow of the fibers. As 8 increases, the shear forces at the fiber-matrix interface increase, these result in a substantial contribution to the bulk composite strain by interface sliding (or, depending on the composite sys￾tem, interphase flow). The contribution due to this relative displacement at the interface is maximized at 845°. However, geometrical constraints dictate that the matrix would still be rate-limiting as slip at the interphase must be accommodated by matrix shear flow. Further increase in 8 results in a high plasticity (e.g. von Mises) potential in the matrix around the fibers, leading to matrix flow around the fibers. This is accompanied by cavitation at the fiber-matrix interface due to the development of tensile tractions normal to the interface. The present work focuses on characterizing the high￾temperature, low-differential-stress rheology of 2D (0/ 90°, cross-ply) laminates in off-axis loading geometries and comparing/contrasting their response with the case for 1D reinforcement. As a necessary requirement for such a comparison, experiments were done on 1D composites with an identical matrix composition. Be￾cause, the matrix chemical composition, phase distribu￾tion and morphology (and hence it is viscoelastic rheology) were different from that of the composites used in our previous experimental work [9], these 1D composite baseline experiments also provided addi￾tional verification to the 1D rheological model devel￾oped earlier. 2. Experimental design and procedure 2.1. Material specifications The materials used in this study were SiC fiber (Nicalon)-reinforced, calcium aluminosilicate com￾posites fabricated and supplied by Coming, Inc. Both the bidirectionally reinforced (2D) and unidirectionally reinforced (1D) composite sheets were fabricated by uniaxial hot-pressing of prepreg plies at 1350°C and 15 MPa [12]. Both 1D and 2D materials consisted of 16 plies, each with a fiber volume fraction, Vf , of 0.3; the 2D composite had a [0/90°]4S (symmetrical) lay-up. The matrix was an anorthite-based glassceramic (Com￾ing Code CAS-II) with a grain-size of 3 mm. The estimated oxide composition of the CAS-II matrix based on X-ray fluorescence spectroscopy (XRAL Labs, Ont., Canada) is shown in Table 1. X-ray diffrac￾tion of powder specimens indicated that the primary phases were anorthite (CaO:Al2O3:2SiO2) and mullite (3Al2O3:2SiO2); a calculation based on peak intensities indicated 85% anorthite and 11% mullite by weight. Electron-microprobe analysis (Cameca SX51) showed Fig. 1. Specimen geometry for ‘off-axis’ compression creep experi￾ments. (a) 2D composites: The outer ply (I) is cut away to reveal its complementary ply (II); (b) 1D composites.