Technical report | Development of a Low Strain-Rate Gun Propellant Bed Compression Test and its Use in Evaluating Mechanical Response
The mechanical integrity of the propellant bed is a key factor in safe and consistent gun performance. To inform in this regard, a low strain-rate bed compression test was developed, primarily for use at low temperatures and, in conjunction with the time-temperature superposition principle, to simulate the high strain rates that exist in the gun chamber during ignition. A range of single-base propellants was used to determine the appropriate test temperature, strain rate and maximum load to, as close as possible, simulate the mechanical response of the propellant bed during ignition in the gun. Results of this testing are given in terms of visual fracture categorisation, applied stress versus bed density, relative vivacity (following burning of crushed and reference samples) and stress relaxation. Artificial ageing programs were also employed to develop relationships between propellant mechanical integrity and propellant molecular weight distributions as a function of age.
The low strain-rate gun propellant test described in this report was developed under the auspices of Project JP2086 for the comparative assessment of propellants from the Mulwala Existing Facility (MEF) and Modernised Mulwala Facility (MMF) with respect to the effect of age on propellant bed mechanical integrity. Two test configurations were designed, for small and large calibre natures respectively, based on similar designs found in the literature. Test fixture dimensions were selected such that bed response was able to be repeatably measured whilst also being able to apply representative loads to the propellant bed. The repeatability of test results was assessed for a range of propellant grain sizes and found to be satisfactory, although the largest grain form tested, BS-NACO, produced results indicating that its grain size was at the upper limit of that which could be repeatably tested. Procedures that were developed to ensure accurate and repeatable results are presented in this report, namely: bed packing methodology for optimal initial bed density; temperature conditioning duration determination to ensure homogenous bed conditions; and data analysis procedures to extract the propellant bed response from the measured combined bed and apparatus response. The determination of the most appropriate test conditions, in terms of: strain rate (0.02 s-1), maximum applied pressure (100 MPa for the small and 40 MPa for the large diameter assemblies) and test temperature (-60˚C) was effected via a study of a range of single base propellant types of interest to Project JP2086, namely AR2211, AR2220, FNH-025 and BSNACO over a range of temperatures (ambient, -15˚C, -40˚C and -60˚C). The prime consideration in this determination was replication, as far as was possible, of the conditions expected to be present during ignition in the gun, keeping in mind the low operational temperature requirements of the aforementioned propellants in service. As brittleness is characteristic of propellant at the high strain rates involved in gun firings, the degree of brittleness was assessed via: visual inspection and categorisation of crushed samples to determine levels of fracture; inspection of applied stress versus bed density; bulk modulus versus applied stress; bulk modulus versus bed density; Heckel plots derived from the measured force versus piston displacement data; relative vivacity analysis of crushed samples fired in a closed vessel; and computation of time-temperature shift factors derived from stress relaxation measurements.