This research encompasses the study of testing protocols and the design of sensors for evaluating the compressive strength of the char layer of ablative material used in solid rocket motors (SRMs). The testing protocol that has been developed is the continuation of previous work for determining the compressive strengths between different SRM insulation materials. A crushing test method was further developed, and a sensor platform was assembled to perform the tests. The test procedure consists of measuring the amount of force required to crush a given area of the charred sample for a specified depth. The test was repeated for the industry standard Kevlar®‐filled ethylene propylene diene monomer rubber and thermoplastic polyurethane elastomer nanocomposite with different weight loading of multi‐walled carbon nanotubes, montmorillonite nanoclay, and carbon nanofibers. The energy of destruction or energy dissipated was quantified to determine which ablative exhibited the best performance. Maximum force was also recorded as a secondary quantity to determine char strength. The proposed test method is fully automated to ensure repeatability of each measurement and to remove the potential for human‐induced error. Because char layer thickness varies depending on the material, a method of differentiating neat material from char was proposed and explored. The introduced procedure also represents a novel and unique approach to solve the problem of the determination of the char strength. Copyright © 2014 John Wiley & Sons, Ltd.
Thermoplastic polyurethane nanocomposites (TPUN) show potential as an internal solid rocket motor (SRM) insulation material by acting as a sacrificial layer. Samples of TPU filled with different weight combinations of multi-walled carbon nanotubes (MWNT) and montmorillonite (MMT) nanoclays (NC) are mixed in a twin screw extruder and tested on an oxyacetylene test bed (OTB). Peak temperatures measured at 5mm incremental lengths from the impingement surface support that MMT nanoclays are the best form of insulator. However, through visual observation and char compression strength testing, it is also found that MMT nanoclays are the most unstable of all the char layers tested. On the other hand, MWNT provide better char stability and strength while showing higher peak temperatures. Consistently, the recession depths of the TPUN filled with MMT nanoclays are greater than those containing only MWNT. Also, while the addition of polyamide 11 (PA11) to TPU shows improvements in mechanical performance (from rheology and dynamic mechanical analysis), it has an adverse effect on the ablative properties of the resultant TPUN.
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.
Ablative materials, such as thermoplastic elastomer nanocomposites (TPUNs) are used as internal insulation materials for solid rocket motors. These TPUNs are thermoplastic elastomer reinforced by montmorillonite nanoclays, carbon nanofibers, and multi‐walled carbon nanotubes. When these TPUN materials are exposed to an extreme heat flux, a char layer forms along the surface as it ablates. This research aims to use the newly developed shear strength sensor to evaluate the shear strength of this char layer, a characteristic that is important to evaluate ablative materials. This device consists of a method to apply a transverse loading on a test specimen, while measuring the reaction force and the induced strain. This device was used on several types of TPUN test specimens to demonstrate its effectiveness. As a means to determine which ablative exhibited the best performance, the energy of destruction or the energy of dissipation was developed. The maximum force was also accounted for as a secondary quantity for determining the char shear strength. This new shear char strength sensor is fully automated to ensure that each test is repeatable. This guaranteed reliability when collecting the data and eliminated the potential for human error. Copyright © 2017 John Wiley & Sons, Ltd.
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