John R. Koenig

The characterization of low cost fiber reinforced thermoplastic composites produced by the DRIFT™ process

Abstract A new, low cost process for hot-melt impregnation of continuous reinforcing fibers with thermoplastic polymers is described. This technique can be used to fabricate various product forms including discontinuous, long-fiber products for compression molded parts, continuous fiber products for pultrusion, filament winding, and woven fabric applications. Mechanical data are presented for composites with various fiber and polymer combinations. Effects of fiber orientation and length on mechanical properties are discussed, and the effect of fiber–polymer bonding on impact strength and microstructure are shown. It is shown that the low cost and high performance achieved with this approach has the potential to expand applications of thermoplastic composite materials.

Development of Design Analysis Methods for Carbon Silicon Carbide Composite Structures

The stress—strain behavior at room temperature and at 1100° C (2000°F) is measured for two carbon fiber-reinforced silicon carbide (C/SiC) composite materials: a two dimensional (2D) plain-weave quasi-isotropic laminate and a 3D angle interlock woven composite. Previously developed micromechanics-based material models are calibrated by correlating the predicted material property values with the measured values. Four-point beam-bending subelement specimens are fabricated with these two fiber architectures and four-point bending tests are performed at room temperature and at 1100°C. Displacements and strains are measured at the mid-span of the beam and recorded as a function of load magnitude. The calibrated material models are used in concert with a nonlinear finite-element solution using ABAQUS to simulate the structural response of the two materials in the four-point beam bending tests. The structural response predicted by the nonlinear analysis method compared favorably with the measured response for both materials and both test temperatures. Results show that the material models scale-up fairly well from coupons to subcomponent level.

Design and Characterization of a Variable High Temperature/Low Pressure Facility for Evaluating Thermal Protection Systems in Reentry Environments

A variable high temperature / low pressure facility has been developed to characterize the thermal response of candidate thermal protection materials in reentry environments. Transient, non-linear finite element models were developed to optimize the design of the test facility. All models used temperature dependent material properties and time dependent boundary conditions based on thermocouple data obtained during checkout runs. These experimental runs were based on simulated reentry profiles. A one-dimensional model was developed considering only the axial heat flow through the thickness of the test stack. It was used as a benchmark when considering multi-dimensional heating effects. Axisymmetric and three-dimensional models were developed to minimize transverse heating and edge effects through a unique combination of material selection and geometry. The final, optimized configuration is capable of temperatures as high as 4500 °F with pressures as low as 100 mTorr in inert atmosphere. Heating rates that produce surface temperature changes of 300 °F / min are attainable. Pyrometers are used to measure the element temperature and the specimen hot face temperature. Up to thirty-five thermocouples are used to measure internal specimen, cold-face, and insulation temperatures within the stack. A PID algorithm is used to control the furnace, while a data acquisition system is used to log all data.Copyright

Lightweight Nonmetallic Thermal Protection Materials Technology

To fulfill President George W. Bush’s “Vision for Space Exploration” (NASA, 2004) — successful human and robotic missions to and from other solar system bodies in order to explore their atmospheres and surfaces — the National Aeronautics and Space Administration (NASA) must reduce the trip time, cost, and vehicle weight so that the payload and scientific experiments’ capabilities can be maximized. The new project described in this paper will generate thermal protection system (TPS) products that will enable greater fidelity in mission/vehicle design trade studies, support risk reduction for material selections, assist in the optimization of vehicle weights, and provide materials and processes templates for use in the development of human‐rated TPS qualification and certification plans.

Characterization of the Fatigue Damage of Advanced Ceramic Composites by Scanning Acoustic Microscopy

In this report we demonstrate the use of the scanning acoustic microscope (SAM) for non-destructive characterization of fatigue damage in SiC fiber composites. Four continuous fiber ceramic composites (CFCC) samples, subjected to fatigue damage, were studied by scanning acoustic microscopy. The damaged samples were inspected by the high-frequency Ernst Leitz Scanning Acoustic Microscope (ELSAM) and the lowfrequency time-resolved acoustic microscope developed at the Institute of Biochemical Physics (IBCP), Moscow. It has been demonstrated that the high-frequency microscope provides a powerful means of detecting subsurface cracks in composite matrix. In a previously published paper (Manghnani et al. 2000), the low-frequency acoustic microscope was chosen to study the internal fatigue damage in samples without any preparation for SAM investigation. The surface of the fiber sample was rough because of the thick fiber bundles breaking at the surface. It was shown that internal cracks (several millimeters in length) propagated parallel to the sample surface and could be detected by the time-resolved acoustic microscope (Manghnani et al. 2000). In this report we show that a small subsurface crack of several millimeters in length becomes visible after polishing the sample surface. Most of the observed cracks run perpendicularly to the bundles of fibers. Samples with different fatigue loading cycles were thus investigated with SAM, and findings are presented here.

Characterization of Cracks in Oxidation-Protective Coatings

Carbon-carbon materials are being developed for high temperature use in gas turbine engines and other applications. They have high specific strength and stiffness at elevated temperature, as well as thermal shock resistance. Silicon carbide based coatings are commonly used to protect the material from oxidation.