Exploration of a new affordable thermal protection system utilizing 2.5D silica/polysiloxane composite

Ablative materials are used as thermal protection systems (TPS) for reentry vehicles and solid rocket motor (SRM) nozzle applications. Phenolic and cyanate ester are the state-of-the-art (SOTA) resin systems used in many of the ablative composites today, including MX-2600 (silica/phenolic) from Cytec Solvay Group. While these ablatives have worked well, more demanding requirements drive the need for affordable lightweight advanced composites capable of handling high heat fluxes with minimal mass loss. These advanced ablative composites result in lighter reentry heat shields and solid rocket motors, increasing payload capabilities of spacecraft and rockets. Molding compound made of aerospace grade 99% SiO2 fabric and polysiloxane resin showed considerable improvement over MX-2600 in ablation properties in recent studies. In order to meet increased mechanical strength demands, NASA recently developed an ablative composite using a 3D quartz woven/cyanate ester composite material designed for the Orion spacecraft. While 3D woven composites provide excellent out-of-plane mechanical and ablation properties, they are very expensive, which limits their application. This research explores needle-punched silica fabric, sometimes referred to as 2.5D, which provides similar out-of-plane mechanical benefits to 3D woven composites in a more flexible VARTM manufacturing process at a much lower cost. The needle-punched silica fabric was infiltrated with polysiloxane resin and mechanical tests were performed. The needle-punched composites showed an increase of 181% in flexural strength, 27% in interlaminar shear strength, 2% in tensile strength, and 13% in compressive strength. In aerothermal ablation tests, the 2.5D out-performed the 2D laminate in char yield, mass loss, and recession rate; and in char yield and mass loss (%), the 2.5D out-performed the industry standard MX-2600 molding compound. The increased out-of-plane strength and char yield make it a promising and affordable ablative candidate for ablation performance with enhanced mechanical properties.

Improved method of manufacturing carbon nanotube infused multifunctional 3D woven composites

This research work aimed to develop smart multifunctional composites via a process for uniformly dispersing carbon nanotubes (CNT) on an orthogonal three-dimensional (3D) woven glass fabric with minimised filtering effect. These smart composites could detect strain under tensile and flexural loading by the piezoresistive response of the infused CNT network. Conventional vacuum assisted resin transfer moulding was modified to control the infusion of 0.25 wt% CNT on the 3D woven glass fabric by varying the vacuum pressure. Results showed that at 101.3 kPa vacuum pressure, the CNT percolated through the thickness of the orthogonal 3D woven glass fabric while being marginally filtered by the fibres and were suitable for sensing tensile strain, whereas at 30.4 kPa, the CNT were deposited only on the surface of the fabric preform without getting filtered and were suitable for sensing flexural strain.

EXPERIMENTALLY VALIDATED SIMULATIONS OF BLIND HOLE DRILLING IN 3D WOVEN CARBON/EPOXY COMPOSITE WITH PROCESSING-INDUCED RESIDUAL STRESSES

Manufacturing-induced residual stresses in carbon/epoxy 3D woven composites arise during cooling after curing due to a large difference in the coefficients of thermal expansion between the carbon fibers and the epoxy matrix. The magnitudes of these stresses appear to be higher in composites with high throughthickness reinforcement and in some cases are sufficient to lead to matrix cracking. This paper presents a numerical approach to simulation of development of manufacturing-induced residual stresses in an orthogonal 3D woven composite unit cell using finite element analysis. The proposed mesoscale modeling combines viscoelastic stress relaxation of the epoxy matrix and realistic reinforcement geometry (based on microtomography and fabric mechanics simulations) and includes imaginginformed interfacial (tow/matrix) cracks. Sensitivity of the numerical predictions to reinforcement geometry and presence of defects is discussed. To validate the predictions, blind hole drilling is simulated, and the predicted resulting surface displacements are compared to the experimentally measured values. The validated model provides an insight into the volumetric distribution of residual stresses in 3D woven composites. The presented approach can be used for studies of residual stress effects on mechanical performance of composites and strategies directed at their mitigation.

PROGRESSIVE DAMAGE RESPONSE OF 3D WOVEN COMPOSITES VIA THE MULTISCALE RECURSIVE MICROMECHANICS SOLUTION WITH TAILORED FIDELITY

Progressive failure simulations have been performed for orthogonal 3D woven composites consisting of RTM6 resin matrix and AS4 carbon fibers. The Multiscale Recursive Micromechanics approach has been used, which, while being computationally efficient, captures the primary effects of the microstructure at each considered length scale. This approach also enables use of any micromechanics theory at any length scale, and herein, the fidelity of the chosen theories across the scales has been tailored to strike a balance with computational efficiency. The Mori-Tanaka method is employed at the lowest length scale, the Generalized Method of Cells is used at intermediate scales, and the High-Fidelity Generalized Method of Cells is used at the highest woven composite repeating unit cell scale. Furthermore, two different damage models, also with different levels of fidelity and efficiency, have been used for the resin material at the lowest length scale. Results for the mechanical behavior in response to loading in various directions are compared for the two damage models and with available test data.