Nonlinear Material Models for Design of Carbon-Carbon Composites

Advanced Navy reentry vehicles will use 2D and 3D carbon-carbon (CC) thermal protection system (TPS) components in order to survive conditions imposed by high speed flight. The TPS structures are exposed to severe temperatures, pressures and thermal gradients that the designer must address to prevent structural failure. In addition to the severe loads, the design problem is further complicated by the fact that the CC composites are orthotropic, nonlinear, and temperature dependent. Existing components are analyzed and validated in pre-flight preliminary design reviews (PDR) and critical design reviews (CDR) using sophisticated finite element models. Existing 3D CC components are analyzed using a material property subroutine that simulates nonlinear tensile, compressive, and shear stresses-strain curves for temperature dependent, orthotropic materials. The existing CC models are based on an extensive database of measured properties that have characterized materials as functions of temperature, direction, and load. Upcoming designs will not have the luxury of an extensive database or even a stable material supply. Recent experience has shown that commercial availability of carbon fibers and matrix precursors is driven by market forces. Carbon-carbon constituents can disappear before TPS components are qualified and fielded. Additionally recent budget reductions mean that program resources needed to create a database are now withheld until after the design has demonstrated feasibility during PDR and CDR. Thus there is a need to define accurate nonlinear material models for future 2D and 3D carbon-carbon composites using limited experimental data. Additionally while existing finite element solutions are capable of computing accurate temperatures, stresses, strains, and safety margins, they do not address effects of damage accumulation or predict catastrophic failure. Fortunately recently developed composite material theories are now able to address nonlinearity, physically realistic failure modes, and damage accumulation. These theories coupled with more efficient computation methods provide the foundation to develop nonlinear carbon-carbon material model to address the needs TPS designers. In this Phase I program Material Research & Design (MR & D) proposes to develop separate 2D and 3D nonlinear carbon-carbon material models for design of TPS components. Each model will capture the unique characteristics of the composite. The 3D model will include the effects of fiber reinforcement in three orthogonal directions, while the 2D model will simulate the very different response of the crossply and inplane properties. The proposed modeling effort will follow a building block approach. Limited, but well defined test data will be used to: 1) Compute a complete consistent set of orthotropic temperature dependent elastic properties; 2) Extrapolate orthotropic temperature dependent nonlinear stress-strain curves; 3) Compute tensile, compressive, and shear strengths using composite failure theories that account for fiber damage and matrix damage; 4) Account for damage accumulation due to non-catastrophic failure modes, and 5) Define input parameters that support a material property subroutine for use in ABAQUS. The Phase I Base effort will focus on software to compute elastic properties and nonlinear stress-strain curves. The feasibility of the Phase I Base effort will be tested by comparing theoretical stress-strain curves to available test data. The Option will extend the composite model to include damage accumulation, link the results to an ABAQUS UMAT subroutine, and create a graphical user interface to facilitate use by TPS design engineers.