18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS Full-field Strain Mapping of C-SiC Composites for Hypersonic Applications S. Amini 1* and F.W. Zok 2 1, 2 National Hypersonic Science Center, Materials Department University of California, Santa Barbara, California, USA * Currently at the United Technologies Research Center, East Hartford, Connecticut, USA Keywords : ceramic matrix composites, strain mapping, hypersonic flight, digital image correlation 1. Introduction predicting macroscopic elasticity and local strains Sustained hypersonic flights at high Mach numbers remains a key issue for three-dimensional (3D) impose a range of high heat fluxes and heat loads textile composites including C-SiC composites, that vary with position on the vehicle from those that particularly in load-critical applications. The can be sustained by current materials to those that difficulty is inherently related to the strong influence cannot, even for brief times [1-2]. Ceramic matrix of the highly heterogeneous and locally anisotropic composites (CMCs) are the only class of materials character of a textile composite on the distribution of so far identified as potentially capable of satisfying stresses and strains. The physics of failure is also the system design requirements of high strength-to- intimately associated with the discrete nature of the weight ratio and operation with high surface interlaced fiber tows in a textile composite. Many temperatures [3-4]. Carbon fiber reinforced silicon empirical and theoretical studies have been carbide matrix composites (C-SiC) are the premier performed in order to identify the underlying materials targeted for use in high-temperature micromechanical origin of such intricate and applications of hypersonic flight vehicles, including complex composite failure mechanisms and to wing leading edges and scramjet liners as well as in propose solutions to improve the resistance against critical propulsion components such as turbine blisks, failure [8-11]. Better knowledge of the turbo-pump rotors, and nozzle exit ramps for micromechanics of these materials under load will advanced rocket engines [4-7]. The macroscopic not only make these materials more useful but also behavior of CMCs depends not only on the contribute to an improvement in safety and design of properties of their individual constituents but also on the engineering components made from them for the interaction between these phases, e.g. fiber and hypersonic applications. matrix, and also on their interface properties [8]. As material complexity evolves in CMCs to meet the In this work, tensile testing is coupled with the novel extreme challenges of hypersonics, major technique of surface strain mapping via digital shortcomings become critical in materials image correlation (DIC) utilized for resolving the characterization, e.g. failure mechanisms, obscured global mechanical behavior and spatial distribution by the difficulty of visualizing damage evolution of the strains in a 3-layer woven angle-interlock C- during tests due to high temperatures. More SiC composite. The DIC technique works by importantly, the various fields of application and the correlating the digital images of surface patterns complexity and heterogeneities in their before and after straining utilized as a powerful tool – microstructures that can also obscure to map strain distributions at different length scales. understanding the multiple interacting failure This allows conducting a detailed investigation of mechanisms – require better understanding of their complex micromechanical aspects that are micromechanical behavior under external loads. associated with the distribution of the strain in a C- When complex CMCs such as C-SiC are subjected SiC heterogeneous material. DIC is a practical, to external mechanical loads, the inhomogeneous effective and powerful tool for qualitative and strain localization due to waviness, may lead to the quantitative deformation measurement in 3D, nucleation of failure of the fibers, of the matrix specifically for polymer matrix composites [12-17]. material, or to de-bonding effects at the interfaces But there is little work done on woven ceramic between the matrix and the fibers preceding the matrix composites. The principal objectives of the actual global failure event [8-9]. The problem of present study are threefold: First, to implement a 3D
strain mapping system for probing the global fracture but warp orientation response exhibits mechanical response of a woven 3-layer angle significant non-linearity and greater strain to failure interlock C-SiC composite; Second, to identify the (0.95% vs. 0.6%). Also the elastic modulus (linear degree of inhomogeneity of strains and ascertain portion) in warp direction is considerably lower than whether periodic variations are consistent with the that in the weft direction. These effects (differences underlying tow architecture; Last, to identify in failure strain and stiffness knockdown) are differences in tensile behavior of such material attributed to the effects of warp tow waviness and subject to loading in the warp and weft directions straightening of the warp fiber tows during tensile and acquire strain distributions at different length loading. The amplitude of stiffness knockdown scales ranging from length scales as small as the depends strongly on the degree of fiber waviness, characteristic fiber tow dimensions up to the length being approximately proportional to the waviness scales as large as several unit cells of the weave [9]. More importantly, straightening of wavy tows in structure. this material (without a fully dense matrix) can occur more readily than what might be expected for 2. Experimental Details a woven fiber ceramic matrix composite with a stiff The composite samples are carbon fiber with SiC and fully dense ceramic matrix. matrix, in which the reinforcement is a three- dimensional 3-layer angle-interlock weave. The 3.2. Local Stiffness and Gauge Averaging (Weft composite was produced from a fiber preform Direction) consisting of carbon fiber tows (T300-6K) woven in Comparisons of strains measured along a single weft a 3-layer angle-interlock architecture (Fig. 1). The tow, averaged over a distance of either four entire preform was processed by chemical vapor unit cells (A-B) or over four individual unit cells (A- infiltration to deposit a thin coating of pyrolytic X, X-Y, Y-Z, Z-B [Fig. 2a]) shows that cell-to-cell carbon on the individual fibers and then to infiltrate local strain variations are about 15% (Fig. 3b). Note SiC partially within the individual fiber tows to that each unit cell is comprised of four characteristic build up a layer of about 10 µm thick SiC encasing tow widths. On the other hand, weft strains are each tow. This layer was sufficient to form a significantly greater than average by a factor of two continuous matrix that bonded all of the interwoven in segments situated beneath surface warp tows (m- fiber tows together where they touched one another, n, p-q, s-t and v-w [Fig. 2a]) as shown in Fig. 4a. while leaving distinct gaps between fiber tows in However, weft strains in segments that are other regions. The tensile tests were performed at positioned beneath surface warp crowns with higher room temperature on tabbed rectangular specimens, Z , e.g. v-w (Fig. 4b), exhibit smaller strains by ~ 25 mm in width and 140 mm in length, prepared by 40%. This knockdown is most likely attributable to laser-cutting. To determine full-field displacements, the smaller degree of fiber waviness in segments the specimens were covered with a random speckle such as v-w. More importantly, weft strains exhibit pattern obtained by spraying white and black paints wide variability in those segments residing on the on the surface. The optical images were used in the surface (n-o, o-X, X-p, etc. [Fig. 2a]) as shown in digital image correlation model to determine the Fig. 5a. Note that negative strains are attributable to displacement fields and to obtain in-situ full-field tow straightening during tensile testing. Large strain strain maps during tension testing. variability was also obtained in central row of weft tows (1-2, 3-4, 5-6, 7-8, 9-10, 11-12 [Fig. 2a]) as 3. Results shown in Fig. 5b. These tows reside beneath the near 3.1. Global Stiffness surface weft tows. The strain knockdown in these The Z -profile of the specimens tested in the weft and weft tow segments as opposed to the surface weft warp directions superimposed on the speckled tows (A-B) can be accounted for by the effects of surfaces are shown in Fig. 2. Global strains were tow waviness, being less in the former (e.g. 1-2, etc.). measured by DIC extensometry during tensile testing (Fig. 3a). The Initial elastic modulus is 3.3. Local Stiffness and Gauge Averaging (Warp dictated by the fibers in the loading direction. Weft Direction) orientation response remains essentially linear up to
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