Evaluation of the Char Strength of Polymer Nanocomposites for Propulsion Systems

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Sponsor: The University of Texas at Austin

Date: Fall 2009

Requirements:
The mechanical properties of the char pose several challenges that impede the usage of traditional testing methods. The fragileness of the char impedes the use of hardness testing, where plastic deformation is used to measure properties. Also, the protocol must be automated so that repeatability can be assured, since a quantitative comparison of different materials is required and errors must be minimized.

Problem:
The aim of this project is to develop a quantitative testing protocol to measure the strength of the char layer of internal insulation materials for solid rocket motors. The development of this protocol is an unexplored area of materials testing that can result in valuable insights for thermal protection applications. Insights from the testing of thermoplastic polyurethane elastomer nanocomposites (TPUNs) can lead to a replacement of the state-of-the art Kevlar filled EPDM rubber currently used as a thermal insulation in rocket motor chambers. TPUNs are recyclable and easier to manufacture.

Solution:
Drawing insights from the limited previous literature available, a testing method was developed and a prototype created to evaluate this protocol. The testing involves a measurement of the forces generated during penetration of the char layer. These forces are then analyzed and compared to previous tests to quantify the performance of the polymers being tested. The prototype, constructed of parts adapted from a robotics kit, positions and lowers a testing probe that crushes the char. This probe is mounted on a beam. A strain gauge on the beam measures the forces during the test. LabVIEW is used to control the rig, as well as for data acquisition and processing. The team concluded that the proposed protocol is a viable solution for testing the strength of char in TPUNs.

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