Cullis, I

Gould, P

Taylor, N

Williamson, D

Apollo (Cambridge)

High-Rate Experiments on a Nitrocellulose/NitroglycerinePropellantNicholas Taylor1,a), David Williamson1, Peter Gould2 and Ian Cullis21SMF Group, Cavendish Laboratory, University of Cambridge, CB3 0HE, United Kingdom2QinetiQ, Cody Technology Park, Farnborough, GU14 0LX, United Kingdoma)Corresponding author: net20@cam.ac.ukAbstract. The mechanical behaviour of a rubbery nitrocellulose/nitroglycerine double-base propellant was probed at a range ofstrain rates. The propellant was relatively soft, and inhomogeneous on the millimetre scale, presenting a few experimental diffi-culties. These difficulties were overcome in a shock experiment using a plate reverberation technique. The resulting informationon the shock Hugoniot and release isentrope of the propellant was compared against a hydrocode using a group interaction basedmaterial model.INTRODUCTIONNitrocellulose/nitroglycerine (NC/NG) double-base propellants are a major component of some solid rocket motors.Solid rocket motors are vulnerable to a phenomenon known as unknown-to-detonation transition (XDT). In XDT, afragment striking a rocket motor initiates detonation of the motor after a short delay [1].The mechanical properties of a propellant at high rate will directly influence its XDT response. A physically-based material model for the propellant is valuable for predicting these properties in the hot, highly-strained materialthat contributes to XDT. An NC/NG propellant is essentially a plasticized polymer. For such materials, group interac-tion modelling (GIM) [2] is a useful approach to material modelling.This work consists of high-rate experiments on an NC/NG propellant, probing both singly-shocked and off-Hugoniot states. The results are compared to the predictions of a GIM model for the same propellant.This work was part of the Insensitive Munitions for Rocket Motors (IMRM) programme of the Weapons Scienceand Technology Centre (WSTC) of the UK Ministry of Defence. IMRM aims to develop an understanding of XDT inrocket motors and the role of propellant properties in the observed responses to fragment impact. The University ofCambridge was a partner in this programme, led by QinetiQ and including Imperial College London and Roxel Ltd.METHODExperimentalTo exercise the model, it was desirable to probe off-Hugoniot states. A reverse-impact plate reverberation techniqueprovides information on a single shocked state and several points on the release isentrope from that state, in a singleexperiment [3].As a cast double-base propellant, the material under investigation has some heterogeneity on the millimeterscale [4]. However, the plate reverberation technique has been used successfully on concrete, a much more heteroge-neous material [5]. Plate reverberation is therefore likely to be usable here.Figure 1 shows the layout of the experiment. The projectile was launched using the single stage 2′′ light gas gun atthe Cavendish Laboratory. Rear-face velocity was recorded using a three-point push-pull VISAR [6, 7]. Measurementpoints were positioned on the experiment axis, 12 mm above the axis, and 12 mm right of the axis. These threemeasurement points served three purposes. First, the arrival time of the initial shock at each point indicated the angleCoppershimVISARviAluminiumsupportPropellantPropellantareaCoppertarget Copper target12 mm12 mm35 mmVISAR probelocations(a) (b)FIGURE 1. Layout of plate-reverberation experiment. Not to scale. All measurements approximate; precise measurements variedbetween experiments. (a) Side view. Aluminium support 3 mm thick Al-6082 alloy, propellant 5 mm thick, copper shim 25.4 µmthick C101 high conductivity copper, copper target 1 mm thick C101 high conductivity copper. Propellant glued using OttocollP520 two-part polyurethane cement. (b) Rear view, showing VISAR probe locations.of that shock relative to the target plate. This in turn allowed the angle between the projectile and the target plate to becalculated. Second, the separate measurement points provided an assessment of the effect of material heterogeneity.Third, the off-axis points are affected by lateral release during the experiment, providing a further probe of the materialmodel.Figure 2 illustrates the propagation of shock and release waves in this experiment. The thin target plate expe-riences multiple shock-release cycles before the initial shock has propagated through the thicker propellant sample.Each release probes a single point on the propellant’s release isentrope: the odd-numbered states in Fig. 2b. The even-numbered states are directly observed by the VISAR. The two sets of states are coupled by the loading and unloadingcurves of the copper target plate.ModellingGroup interaction modelling was used to construct a material equation of state, constrained by dynamic mechanicalanalysis (DMA [8]) of the propellant. A constitutive model was constructed from quasi-static cyclic tensile testing ofthe propellant at temperatures from −30 ◦C to 20 ◦C.The equation of state and constitutive model were used as input for GRIM, an Eulerian hydrocode. GRIM’snumerical scheme cannot accommodate polymer relaxation. Relaxation was therefore neglected. Existing materialmodels for the copper, aluminium and glue were used.RESULTS AND DISCUSSIONFigure 3 compares the experimental results for the central VISAR probe with model predictions for the same location,ignoring the glue layer. Agreement is quite close early in the experiment. The simulation shows a slightly stiffermaterial response than experiment, which is an expected consequence of GRIM’s inability to model relaxation. Thislack of relaxation also contributed to a higher propellant shock velocity in simulation than in experiment, shown bythe early arrival of the reflected shock from the aluminium back plate.Later, model and experiment begin to diverge. At this stage in the simulation the propellant experiences tensileload, and fails. The fall in reported velocity corresponds to the strength of the propellant. The most likely reason thatthis is not observed experimentally is that some other interface in the experiment, with negligible tensile strength,fails. The most likely candidate for this is the interface between the copper shim and the copper target.xtAluminiumPropellantCopper(a) (b)12345678upσPropellant HugoniotPropellant releaseCopper HugoniotCopper releasevi123456 87FIGURE 2. Shock propagation in the plate-reverberation experiment. (a) As an x-t diagram: solid lines are shock, dashed lines arerelease. (b) Illustrating stress vs. particle velocity relationship.Interestingly, while the timings and levels of the velocity plateaux show good agreement between model andexperiment, the evolution of their gradients does not.While agreement between model and experiment is good in the early stages of the experiment, it is during thesestages that the hitherto-neglected glue layer has the most influence. Figure 4 shows the effect of modelling the gluelayer on the two higher-velocity experiments. Agreement in the plateau velocities is noticeably improved by modellingthe glue. However, in the experiment at 713 m s−1, the model diverges from the experiment after only three plateaux(state 6 in fig. 2). This divergence remains unexplained.Off-axis effectsFigure 5 shows the output of all three VISAR measurement points in a single reverberation experiment. The mostobvious feature is the spread in shock arrival times. This corresponds to an angle of 8.1 milliradians between thenormals to the flyer and the target plate. In subsequent experiments improved alignment procedures reduced thismisalignment to between 1 mrad and 2 mrad.It is also interesting to note that the reported velocity at the off-centre points, while initially matching that in thecentral point, falls off over time. This almost certainly corresponds to lateral release from the edge of the propellantsample. Unfortunately, while the relative position of the three measurement points was well-known, the offset betweenthe central measurement point and the centre of the propellant sample was not well constrained. This complicatedcomparison between the off-centre probe points and simulation.CONCLUSIONSPlate reverberation is a useful experimental tool for testing the high-rate behaviour of material models, provided themodel for the target plate material is already well-validated.A group interaction model for high-rate material behaviour of the NC/NG propellant under investigation couldbe constructed and parameterized using only dynamic mechanical analysis and quasi-static tensile experiments.The predictions of the group interaction model matched the results of the plate reverberation experiment well.ACKNOWLEDGEMENTSThe authors wish to acknowledge the financial support of Dstl, UK MOD, under a Weapons Science and TechnologyCentre contract. The authors acknowledge the support of Richard Johnson for the supply of propellant specimens anduseful technical discussions.0100200300400500600700800−1 0 1 2 3 4 5 6AAABBBVelocity/ms−1Time / µsExpt, 275 m s−1Model, 275 m s−1Expt, 496 m s−1Model, 496 m s−1Expt, 713 m s−1Model, 713 m s−1FIGURE 3.Results, experimental vs. modelling, centre probe, no glue. Data time-shifted to separate traces. Note arrival of reflectedshock from aluminium support (A), and breakdown of model (B).0100200300400500600700800900−1 0 1 2 3 4 5CCDVelocity/ms−1Time / µsExpt, 496 m s−1Model, 496 m s−1Expt, 713 m s−1Model, 713 m s−1FIGURE 4. Results, experimental vs. modelling, centre probe, glue layer simulated. Data time-shifted to separate traces. Noteagreement in first plateau level (C) and rapid divergence of model and experiment for higher-velocity data (D).0100200300400500600−1 0 1 2 3 4 5Velocity/ms−1Time / µsCentre(13.4,−0.8) mm(0.1, 13.0) mmFIGURE 5. Output of all three VISAR probes in experiment at 496 m s−1. Note different shock arrival times due to impact tilt.Poor signal-to-noise after approx. 3 µs makes interpretation of off-centre gauges uncertain past that time.REFERENCES[1] S. A. Finnegan, J. K. Pringle, J. C. Schulz, O. E. R. Heimdahl, and A. J. Lindfors, Int. J. Impact Engng. 14,241–254 (1993).[2] D. Porter, Group Interaction Modelling of Polymer Properties (CRC Press, 1995).[3] P. C. Lysne, R. R. Boade, C. M. Percival, and O. E. Jones, J. Appl. Phys. 40, 3786–3795 (1969).[4] R. Steinberger and P. D. Drechsel, in Propellants Manufacture, Hazards, and Testing, Advances in Chemistry,Vol. 88, edited by C. Boyars and K. Klager (American Chemical Society, 1969) Chap. 1,, pp. 1–28.[5] C. A. Hall, L. C. Chhabildas, and W. D. Reinhart, Int. J. Impact Engng. 23, 341–351 (1999).[6] L. M. Barker and R. E. Hollenbach, J. Appl. Phys. 43, 4669–4675 (1972).[7] W. F. Hemsing, Rev. Sci. Instrum. 50, 73–78 (1979).[8] K. P. Menard, Dynamic mechanical analysis: a practical introduction, 2nd ed. (CRC Press, Boca Raton, FL,2008).

High-rate experiments on a nitrocellulose/nitroglycerine propellant

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High-rate experiments on a nitrocellulose/nitroglycerine propellant

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