Modeling the elastic modulus of 2D woven CVI SiC composites (original) (raw)

2006, Composites Science and Technology

The use of fiber, interphase, CVI SiC minicomposites as structural elements for 2D-woven SiC fiber reinforced chemically vapor infiltrated (CVI) SiC matrix composites is demonstrated to be a viable approach to model the elastic modulus of these composite systems when tensile loaded in an orthogonal direction. The 0 o (loading direction) and 90 o (perpendicular to loading direction) oriented minicomposites as well as the open porosity and excess SiC associated with CVI SiC composites were all modeled as parallel elements using simple Rule of Mixtures techniques. Excellent agreement for a variety of 2D woven Hi-Nicalon TM fiber-reinforced and Sylramic-iBN reinforced CVI SiC matrix composites that differed in numbers of plies, constituent content, thickness, density, and number of woven tows in either direction (i.e, balanced weaves versus unbalanced weaves) was achieved. It was found that elastic modulus was not only dependent on constituent content, but also the degree to which 90 o minicomposites carried load. This depended on the degree of interaction between 90 o and 0 o minicomposites which was quantified to some extent by composite density. The relationships developed here for elastic modulus only necessitated the knowledge of the fractional contents of fiber, interphase and CVI SiC as well as the tow size and shape. It was concluded that such relationships are fairly robust for orthogonally loaded 2D woven CVI SiC composite system and can be implemented by ceramic matrix composite component modelers and designers for modeling the local stiffness in simple or complex parts fabricated with variable constituent contents. class of materials for a variety of high temperature air-breathing, space, and nuclear applications [1]. Future SiC/SiC components will possess rather complex shapes requiring various architectures, differences in local thickness, and local curvature, which may also lead to processing non-uniformities throughout a given component. Needless to say, in order to design with such materials, the effect of geometry, fiber-orientation, architecture, and constituent properties and content on stress-strain response needs to be well understood. The first and the foremost property required for component design is the effect of these factors on tensile elastic modulus. A significant amount of work has been performed towards understanding the mechanical properties of 2D [2-8] and 3D [5] woven CVI SiC matrix composites primarily with regard to the occurrence of non-linear stress-strain behavior due to matrix cracking. However, little has been done to quantify the elastic properties of these composites as a function of architecture, constituent properties, and constituent content. Therefore, the objective of this study was to determine relationships for the elastic modulus of 0/90 2D woven CVI SiC composites in the orthogonal directions for a wide variety of composite parameters by carefully accounting for all those parameters. The parameters included architectural variations, which included composite thickness, number of plies, balanced and unbalanced weaves, and tow size, and constituent variations, which included fiber-type, interphase composition, and constituent contents. In a companion paper [9], the effects of these same architectural and compositional parameters on matrix cracking were studied. EXPERIMENTAL Composite specimens with different CVI SiC matrix content were obtained from several processing approaches/anomalies representing vintages that span over 10 years from the same vendor (currently General Electric Power Systems Composite, Newark, DE). The three general processing approaches are shown schematically in Figure 1 and indicated in Table I. " Standard specimens" went through typical CVI fabrication and machining at General Electric Power Systems Composites, Newark, DE (Figure 1a). This consisted of woven fiber lay-up, CVI interphase infiltration, initial CVI SiC infiltration, specimen machining and a final CVI SiC infiltration step to further densify the specimen. Specimens consisting of eight to 36 plies were fabricated in this manner. Some