Creep Behavior in Interlaminar Shear of a Hi-Nicalon™/SiC-B4C Composite at 1200°C in Air and in Steam

Creep behavior in interlaminar shear of a non-oxide ceramic composite with a multilayered matrix was investigated at 1200∘C in laboratory air and in steam environment. The composite was produced via chemical vapor infiltration (CVI). The composite had an oxidation inhibited matrix, which consisted of alternating layers of silicon carbide and boron carbide and was reinforced with laminated Hi-NicalonTM fibers woven in a five-harness-satin weave. Fiber preforms had pyrolytic carbon fiber coating with boron carbon overlay applied. The interlaminar shear properties were measured. The creep behavior was examined for interlaminar shear stresses in the 16–22 MPa range. Primary and secondary creep regimes were observed in all tests conducted in air and in steam. In air and in steam, creep run-out defined as 100 h at creep stress was achieved at 16 MPa. Similar creep strains were accumulated in air and in steam. Furthermore, creep strain rates and creep lifetimes were only moderately affected by the presence of steam. The retained properties of all specimens that achieved run-out were characterized. Composite microstructure, as well as damage and failure mechanisms were investigated. The tested specimens were also examined using electron probe microanalysis (EPMA) with wavelength dispersive spectroscopy (WDS). Analysis of the fracture surfaces revealed significant surface oxidation, but only trace amounts of boron and carbon. Cross sectional analysis showed increasing boron concentration in the specimen interior

EFFECTS OF TEMPERATURE AND ENVIRONMENT ON MECHANICAL PROPERTIES OF TWO CHOPPED-FIBER AUTOMOTIVE STRUCTURAL COMPOSITES

The Durability of Lightweight Composite Structures Project was established at Oak Ridge National Laboratory (ORNL) by the U.S. Department of Energy to provide the experimentally-based, durability-driven design guidelines necessary to assure long-term structural integrity of automotive composite components. The initial focus of the ORNL Durability Project was on composite materials consisting of polyurethane reinforced with E-glass. Current focus of the project is on composite materials reinforced with carbon fibers. The primary purpose of this report is to provide the individual specimen test date. Basic mechanical property testing and results for two chopped-fiber composite materials, one reinforced with glass- and the other with carbon fiber are provided. Both materials use the same polyurethane matrix. Preforms for both materials were produced using the P4 process. Behavioral trends, effects of temperature and environment, and corresponding design knockdown factors are established for both materials. Effects of prior short-time loads and of prior thermal cycling are discussed

EFFECTS OF TEMPERATURE AND ENVIRONMENT ON MECHANICAL PROPERTIES OF TWO CONTINUOUS CARBON-FIBER AUTOMOTIVE STRUCTURAL COMPOSITES

The Durability of Carbon-Fiber Composites Project was established at Oak Ridge National Laboratory (ORNL) by the U.S. Department of Energy to develop experimentally based, durability-driven design guidelines to assure the long-term (15-year) structural integrity of carbon-fiber-based composite systems for automotive structural applications. The project addressed characterization and modeling the durability of a progression of carbon-reinforced thermoset materials, each of which has the same urethane matrix. The primary purpose of this report is to provide the individual specimen test data. Basic mechanical property testing and results for a reference [{+-}45{sup o}]{sub 3S} crossply composite and a quasi-isotropic, [0/90{sup o}/{+-}45{sup o}]{sub S} version of the reference crossply are provided. The matrix and individual {+-}45{sup o} stitch-bonded mats are the same in both cases. Although the composite utilized aerospace-grade carbon-fiber reinforcement, it was made by a rapid-molding process suitable for high-volume automotive use. Behavioral trends, effects of temperature and environment, and corresponding design knockdown factors are established for both materials. The reference crossply is highly anisotropic with two dominant fiber orientations--0/90{sup o} and {+-}45{sup o}. Therefore properties were developed for both orientations

Strain Rate Dependence and Short-Term Relaxation Behavior of a Thermoset Polymer at Elevated Temperature: Experiment and Modeling,"

The inelastic deformation behavior of polymerization of monomeri

Diffusion of a fluid through a viscoelastic solid

This paper is concerned with the diffusion of a fluid through a viscoelastic solid undergoing large deformations. Using ideas from the classical theory of mixtures and a thermodynamic framework based on the notion of maximization of the rate of entropy production, the constitutive relations for a mixture of a viscoelastic solid and a fluid (specifically Newtonian fluid) are derived. By prescribing forms for the specific Helmholtz potential and the rate of dissipation, we derive the relations for the partial stress in the solid, the partial stress in the fluid, the interaction force between the solid and the fluid, and the evolution equation of the natural configuration of the solid. We also use the assumption that the volume of the mixture is equal to the sum of the volumes of the two constituents in their natural state as a constraint. Results from the developed model are shown to be in good agreement with the experimental data for the diffusion of various solvents through high temperature polyimides that are used in the aircraft industry. The swelling of a viscoelastic solid under the application of an external force is also studied.Comment: 26 pages, 7 figures, submitted to International Journal of Solids and Structure

A model for the degradation of polyimides due to oxidation

Polyimides, due to their superior mechanical behavior at high temperatures, are used in a variety of applications that include aerospace, automobile and electronic packaging industries, as matrices for composites, as adhesives etc. In this paper, we extend our previous model in [S. Karra, K. R. Rajagopal, Modeling the non-linear viscoelastic response of high temperature polyimides, Mechanics of Materials, In press, doi:10.1016/j.mechmat.2010.09.006], to include oxidative degradation of these high temperature polyimides. Appropriate forms for the Helmholtz potential and the rate of dissipation are chosen to describe the degradation. The results for a specific boundary value problem, using our model compares well with the experimental creep data for PMR-15 resin that is aged in air.Comment: 13 pages, 2 figures, submitted to Mechanics of Time-dependent Material

Matrix Cracking of Ceramic-Matrix Composites

In this chapter, the matrix cracking of fiber-reinforced ceramic-matrix composites (CMCs) is investigated using the energy balance approach. The relationship between the matrix cracking stress, fiber and interface oxidation, and fiber failure is established. The effects of the fiber volume, interface shear stress and interface debonding energy, fiber failure, and oxidation temperature on the time-dependent matrix cracking stress are analyzed. The experimental matrix cracking stress of different fiber-reinforced CMCs is predicted using the present models