An Investigation of Compressive and Shear Strength of Char from Polymer Nanocomposites for Propulsion Applications

Abstract

Ablative material is used in solid rocket motors (SRMs), since the formation of a char layer provides thermal protection to key structural components during motor firings. The char strength of thermoplastic polyurethane elastomer nanocomposites (TPUNs) is of particular interest due to its potential as a replacement for the current industry standard, Kevlar®-filled EPDM. TPUNs are being considered to replace Kevlar®-filled EPDM for a number of reasons, primarily because: TPUNs exhibit superior ablation and insulative characteristics, easy fabrication, and are recyclable. Due to the fragile nature of the charred TPUNs, ordinary testing methods (e.g., Rockwell hardness) are not feasible; thus necessitating the creation of a testing protocol and sensor. This study encompasses the improvement of a compressive testing protocol and sensor for evaluating the strength of the char layer of SRM insulation materials and also the early stages of a shear testing protocol. The compression protocol that was developed is the continuation of previous work which has shown potential in determining the comparative strengths between different SRM insulation materials. Building on prior art, a crushing test method was further developed and a sensor platform was assembled to perform crushing tests as well as the addition of a shear test. The test procedure consists of measuring the amount of force required to crush an area of the charred sample for a specified penetration depth. The shear test involves shearing off the char from a horizontal direction. During the shear test, the force required to shear off the char completely is measured. The test is repeated for different types of TPUNs that consist of carbon nanofibers, montmorillonite organclays, and multi-walled carbon nanotubes; and the current industry standard Kevlar®-filled EPDM. For both the crushing and shearing tests, the energy dissipated is quantified to determine which TPUN exhibited the best performance. The test is fully automated to ensure repeatability of each measurement and to remove the potential for human-induced error.

Compressive char strength of thermoplastic polyurethane elastomer nanocomposites

“…The clamping mechanism should not only adapt to the sample shape but also create little or no bending stress on the sample as the material is very fragile. The clamping mechanism concept has been implemented on the shear strength version of the developed tester . New protocols to elaborate data could be considered: as an example, because the material that has thicker char layers will inherently require more energy to crush the char, then in order to compare the char strength of different specimens more accurately, the energy can be normalized by the thickness of the char.…”