We report progress of a continuing effort to characterize and simulate the response of energetic materials (EMs), primarily HMX-based, under conditions leading to cookoff. Our experiments include mechanical-effects testing of HMX and FIMX with binder at temperatures nearing decomposition thresholds. Additional experiments have focused on decomposition of these EMs under confinement, measuring evolution of gas products and observing the effect of pressurization on the solid. Real-time measurements on HMX show abrupt changes that maybe due to sudden void collapse under increasing load. Postmortem examination shows significant internal damage to the pellets, including voids and cracks. These experiments have been used to help develop a constitutive model for pure HMX. Unconfined uniaxial compression tests were performed on HMX and LX-14 to examine the effect of binders on the deviatoric strength of EM pellets, and to assess the need of including deviatoric terms in the model. A scale-up experiment will be described that is being developed to validate the model and provide additional diagnostics

KANESHIGE,MICHAEL J.

RENLUND,ANITA M.

SCHMITT,ROBERT G.

WELLMAN,GERALD W.

UNT Digital Library

1’9 . qqi??ow-z 6L6 c“ c&p@opo”‘ q)(i? 2 @gKS1?Mechanical Rpsponse and Decomposition of Thermally Degraded Energ ti#Materials:Experiments and Model Simulations‘,:A. M. Renlund, M. J. Kaneshige, R. G. Schmitt and G. W. WellmanSandia National Laboratories*Albuquerque, NM 87185ABSTRACTWe report progress of a continuing effort to characterize and simulate the response of energeticmaterials (EMs), primarily HMX-based, under conditions leading to cookoff. Our experiments includemechanical-effects testing of HMX and FIMX with binder at temperatures nearing decompositionthresholds. Additional experiments have focused on decomposition of these EMs under confinement,measuring evolution of gas products and observing the effect of pressurization on the solid. Real-timemeasurements on HMX show abrupt changes that maybe due to sudden void collapse under increasingload. Postmo~em examination shows significant internal damage to the pellets, including voids and ,cracks. These experiments have been used to help develop a constitutive model for pure HMX.Unconfined uniaxial compression tests were performed on HMX and LX-14 to examine the effect ofbinders on the deviatoric strength of EM pellets, and to assess the need of including deviatoric terms inthe model. A scale-up experiment will be described that is being developed to validate the model andprovide additional diagnostics.INTRODUCTIONFor several years we have pursued a joint experimental and modeling effort to understand anddescribe the chemical and mechanical processes that occur as an energetic material (EM), primarily HMX(1,3,5,7-tetranitro-l ,3,5,7-tetraazacyclooctane) and its formulations, nears cookoff conditions”14 Inparticular, we wish to build a mathematical simulation of the state of the thermally damaged explosive.The microstructure and chemical composition at the time of ignition are very different from the pristinematerial. These properties will control the subsequent burning and together with confinement willdetermine the violence of the reaction. Our experimental approach has been to study small, confinedEMs, measuring in real time as many of its properties as we can. A companion paper at this meetingpresents application of two new techniques, Raman spectroscopy and ultrasound transmission, to aid indescribing the materials.5 The accompanying simulation efforts have focused on physically-based modelsof the observed processes, e.g., phase transition, creep, gas formation, porosity evolution andcompaction. We have presented the basic components of the model developed to describe the damagedstate.2 Our efforts are now aimed at populating the model with data of sufficient accuracy to set severalof the model parameters, and at developing an additional experiment that can be used to validate themodel. In addition, we continue to examine the mechanical response of the EMs to assure that the modelcontains and reflects the actual physical processes occurring during decomposition. Our main focus willbe on formulated EMs, and we are assessing how binder affects the physical phenomena being modeled.Approved for public release, distribution is unlimited.‘Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company,for the United States Department of Energy under contract number DE-AC04-94AL85000.1-..-—— .—.. .. .---?— . .,. .”. — ,—. ,, . -—-— ,.,DISCLAIMERThis repoti was prepared as an account of work sponsoredby an agency of the United States Government. Neither theUnited States Government nor any agency thereof, nor anyof their employees, make any warranty, express or implied,or assumes any legal liability or responsibility for theaccuracy, completeness, or usefulness of any information,apparatus, product, or process disclosed, or represents thatits use would not infringe privately owned rights. Referenceherein to any specific commercial product, process, orservice by trade name, trademark, manufacturer, orotherwise does not necessarily constitute or imply itsendorsement, recommendation, or favoring by the UnitedStates Government or any agency thereof. The views andopinions of authors expressed herein do not necessarilystate or reflect those of the United States Government orany agency thereof.-m= -. . . . . . . . ..s?.73.77-.7,- .- :~.. .-—. . . , . .DISCLAIMERPortions of this document may be illegiblein electronic image products. Images areproduced from the best available originaldocument.—EXPERIMENTALOur experimental configuration has evolved as we have attempted to focus on different physicaland chemical processes. Briefly, all experiments have involved monitoring force, pressure andtemperature of a highly confined, small sample of EM. To recover and handle the damaged explosive,we have preferred to work with small samples, typically less than 400 mg, and usually 0.25-in-diameterpellets. While there are various configurations, which we term “hot cell” experiments, we usually performthe experiments in”~eitherdisplacement-controlled or load-controlled versions. The current, basic versionsare shown in Figs. 1 and 2. The EM is contained in a cylindrical stainless steel cell with thick walls.Opposing Invar pistons, sealed using O-rings, are used to confine the decomposition gases and transmitload along the central axis. The primary difference between the two configurations is that the top piston isallowed to move in the load-controlled experiments. A modification to the top piston that allows us to usea pressure transducer to monitor gas pressure has been previously described.l Some modifications havebeen made to that assembly to allow us to measure higher gas pressures than in the past.THERMOCOUPLES//I===I//WATER-PLATES, EM-BANDHEATERIiCELL.BAND -HEATERWATETOP PPLES1EM-WATER-COOLEDHEAT SINK ~RADIATIONSHIELD —BOTTOM PLATEFig. 1. Schematic of load-controlled hot-cell Fig. 2. Schematic of displacement-controlledexperimental configuration hot- cell experimental configuration.One variant of the load-controlled experiment was performed to examine the strength of the pelletin the radial direction when a load was applied in the axial direction. Instead of a close fit between thepellet and cell as is usual in our experiments, we heated a 0.25-in-diameter pellet inside a 0.5-in-diametercell, and applied a load using 0.5-in-diameter pistons. This is essentially an unconfined uniaxialcompression test of the EM pellets at temperature.EXPERIMENTAL RESULTSRefinement of the load-controlled experiment has led to higher precision in the measurements ofthe mechanical response of the heated EM. An example is shown in Figs. 3 and 4. The redesignedexperiment uses taller pellets (0.25-in rather than 0.125-in thick pellets used in the past). Also, by movingthe placement of the Linear Variable Displacement Transformer (LVDT) and adjusting the cooling andruggedness of assembly there is less noise in the displacement measurements. To give better data tofeed the model, we will be adding a servo controller to maintain the force at a more constant level. This2—-—. —.—— - ,.=—q ,+,,- ,. .=”+... . . ,,, -,. ‘—.... . . . ...200 0.135180 0.134~ 160 0.133z 140&0.132=:120 0.131~oL 100 0.130~~2 8020.129~g~ 60 0.128:Eg! 40 0.12720 0.1260 0.1250 50 100 150 200 250 300Time (mlnutss)Fig. 3. Data showing mechanical response of HMXat different loads in old experimental configuration.200Temperat renI ForcenPY\JiDispIacemt nt0.0220.0240.03ol I I I 1 0.0320 40 80 120 160Time (minutzs)Fig. 4. Data showing improvements in noise andresolution after redesign of experimental apparatus.will give us adequate precision to populate the model parameters so that subsequent validation tests willhave rigorous bounds.The results of the unconfined uniaxial compression test on HMX and LX-14 (95.5’%oHMX, 4.5’%Estane) are shown in Figs. 5 and 6, respectively. We maintained the temperature of the HMX test belowthe phase transition temperature, so the results show the deviatoric strength of the ~-HMX. Therecovered pellet had swelled somewhat, given that it was unconfined radially. By contrast, the LX-14pellet failed during the 40”C preconditioning soak that is part of our experimental procedure. Therecovered pellet was very fragile. Dimensions and masses of the pellets before and after are shown inTable 1.20000.4150.0140.0110.010 50 loa 150 200 250ThIO (mlwtos)Fig. 5. Results of unconfined uniaxialcompression test on HMX pellet.80 0.0270 0~ co -0.02~~ 50 4.04$; 40 .0.06~E 30 .4.08E; 20 -4.110 -0.120 .0.740510152025 303540Thm(mh)Fig. 6. Results of unconfined uniaxialcompression test on LX-14.TABLE 1. Results from uniaxial compression testsonHMXandLX-14 pellets.EM Pre-test Pre-test Pre-test Post-test Post-test Post-testmass (g) height (in) diameter (in) mass (g) Height (in) diameter (in)HMX .3628 .2500 .2505 .3625 .2484 .2548LX-14 .3624 .2503 .2494 .3619 .1023 .503——.——. “..-The recovered LX-14 pellet was sectioned and examined using scanning electron microscopy (SEM).Results are shown in Figs. 7 – 9 for a pristine LX-14 pellet and the top and sectioned sutfaces of therecovered pellet. The recovered pellet was very fragile, and showed that the binder had flowedsubstantially. The pellet was essentially held together by a thin binder coating on the sutface.*Fig. 7. SEA4of cleaved suiface ofunheated LX-14 pellet.Fig. 9. SEM of cleaved swface of LX-14 pelletfrom unconfined uniaxial compression test.4-, - ,.., .. . .,r----- ...... r e ..- ,... . . . . . .Z’JL ..-. .-.,:~.!,u-f,ll . ,- —-.--{, , ,..In addition to the mechanical properties described above, we have also examined the decomposition ofHMX and LX-14 to higher degrees of decomposition while measuring the gas pressure in a displacement-controlled arrangement. Results are shown in Figs. 10 and 11. It is clear that the materials behavedifferently, with discrete steps of force (combined response) reduction and gas pressurization beingobserved for the HMX, but not for the LX-14.7C4m6302041C40\ IiJ --!Ter Iperat me I11~ Prt ssure2CC.I0050 lm15a2m2s4300Worn (mlnules)Fig. 10. Force response and gas pressureevolution from thermally degraded HMX.700600100070CU60C0mm2W0Iccoo0 20406000100120 140160Tim (minutes)Fig. 11. Force response and gas pressureevolution from thermally degraded LX-14.DISCUSSIONThe improvements to the load-controlled experiment that led to the difference shown in Figs. 3and 4 were necessary to achieve the precision required to establish model parameters for the heatedEMs. Based on these refinements we are in the process of performing several experiments at varioustemperatures and loads, following a recipe provided by the modelers. These inputs will define thematerial response for the baseline, heated, but undecomposed materials. These mechanical propertieswill be evaluated only on materials at temperatures below the onset of significant reaction.It was obvious from the results of the unconfined uniaxial compression tests (Figs. 5 and 6) thateven a small amount of binder (4.5Y0Estane in the LX-14) can dramatically alter the mechanicalproperties of formulated EMs. Clearly, the HMX pellet retained much of its strength under the deviatoricstress state. At first glance of the LX-14 result, it may appear that the binder acted as a lubricant causingthe solid to behave as a fluid. A closer look, however, indicates that what we observed was solidresponse leading to failure, with accompanying shear-induced void formation (see SEMS in Figs. 8 and9). We are currently evaluating what this implies for the detail of modeling required to adequatelydescribe the deviatoric response of formulated EMs near cookoff conditions. It should be noted that theconditions used in the test are unrealistic for any practical applications.The effects of gas pressurization also show differences for theHMXan$theLX-14. Theexperiment was designed-to help decouple the gas from the solid response. The load cell measures thecombined response of the system: the mechanical processes including expansion, phase transition,creep and compaction, while the pressure transducer measures only the evolved gas pressure. For bothmaterials, there were times when the combined response was decreasing while the gas pressurizationincreased. This implies that a scalar pressure model cannot capture the response. The solid and gaspressure responses are clearly coupled. In the HMX, there were stepwise decreases in the combined(load cell) force that occurred simultaneous with the stepwise increases in gas pressure. We postulatethat this was due to collapse of gas-filled voids inside crystals, since pressure internal to crystals wouldnot be measured by the pressure transducer. In tests on HMX pellets from different particle size-- ., .,_ .,...--—.,. . ,.,,.. .. ,.*,77’.,.distributions of the powder, we have found that the steps are more likely to occur for the larger particles.By contrast, LX-14 and other formulated EMs tend to show more uniform changes in both pressure andload. We believe that the local forces are distributed more uniformly on individual crystals in those cases.Our original model was developed to capture many of the features we observed experimentally.Improvements to the diagnostics will allow us to determine some parameters more accurately. We areinterested in measuring the phase change kinetics and deviatoric creep rates. Our present experimentalconfiguration does not facilitate deviatoric stresses; our validation experiment, discussed below, will allowus to assess this effect directly. The complication arises because phase change and deviatoric creep arecoupled to bulk response of the material. We hope that the constitutive model will be able to separate outsome of these effects to allow us to determine the relative importance of these phenomena. The outputof a constitutive model including reaction chemistry will be the damage state of the material which is in “turn the initial state for a burn model to assess hazards. We postulate that an accurate description ofporosity evolution is crucial to the burn model, and results of our tests to date indicate that this maybeaffected by shear response of the heated material.VALIDATION EXPERIMENTWe have designed and are beginning to perform experiments to validate the constitutive modeldeveloped from the small-scale tests. This validation test is in part a scale-up of the load-controlled hotcell experiment, but also allows measurement of the effects of deviatoric stresses. The radialconfinement is much less, using a thin-walled aluminum sleeve rather than the thick stainless steel cellused in the small-scale experiments. We use strain gages attached to the walls of the Al sleeves tomeasure radial strain, a parameter that is not measured and which is not applicable for the small-scaleexperiments.F/Ii+wpne”maticcyunder~II——r&——7+IIFig. 12. Whole and section views of scale-up hot cell apparatus6,. .’.The apparatus consists of a 1” diameter by 1” tall EM pellet contained inside a metal sleeve andbetween two metal pistons. Redundant O-rings on each piston seal the containment. initially, the sleeveand pistons will be aluminum 2024-T4. Aluminum was chosen because it will provide a strong strainsignal on the surface of the sleeve while containing reasonably high internal pressures. Al 2024-T4 wasselected because the strain gage temperature compensation is calibrated for that type. Each piston isthreaded and carries a flat Invar plate, to which one or two LVDTS are mounted. Invar was chosen tominimize warping due to thermal gradients, which could adversely affect the LVDT readings. Each lnvarplate is jammed in:place by an aluminum nut. At the end of each piston is an annular Invar coupler,which serves to interface and insulate the pistons from the adjacent components (Invar is about 10 timesless conductive than aluminum), and to accommodate instrumentation (pressure transducer,thermocouple) leads and connections. On one end, the coupler sits on a load cell. On the other end, thecoupler contacts the ram of a pneumatic cylinder, in the load-controlled arrangement. An aluminum cagecontains the forces generated by the pneumatic cylinder and/or by pressure within the cell. A rope heaterwrapped around the outside of the sleeve will provide heating. This heater can generate a maximum of250 W. Control is achieved by monitoring the temperature at the surface of the sleeve and feeding it intoa feedback controller. No active cooling is provided. Natural convection and radiation to the laboratoryenvironment will constitute heat losses.The hardware for this experiment has been fabricated and initial tests are in progress to calibratethe response using inert materials. Initial EM tests will be used to validate the constitutive model, andfollow-on tests will be conducted as full cook-off tests.ACKNOWLEDGEMENTSWe gratefully acknowledge the expert technical assistance of J. C. Miller and L. G. Rice for theirassistance with these experiments. We are also grateful to Jon Maienschein of Lawrence LivermoreLaboratories for supplying LX-14 pellets on very short notice.REFERENCES1. Renlund, A. M., Miller, J. C., Wellman, G. W. and Schmitt, R. G., “Characterization of ThermallyDegraded Energetic Materials: Mechanical and Chemical Behaviot’, JANNAF Propulsion SystemsHazards Subcommittee Meeting, Tucson, AZ (1998).2. Schmitt, R. G., Wellman, G. W., Renlund, A. M. and Miller, J. C., “A Constitutive Model for ConfinedHMX During Cookoff, JANNAF Propulsion Systems Hazards Subcommittee Meeting, Tucson, AZ(1998).3. Renlund, A. M., Miller, J. C., Trott, W. M., Erickson, K. L., Hobbs, M. L, Schmitt, R. G., Wellman, G.,and Baer, M. L., “Characterization of Thermally Degraded Energetic Materials,” Proceedings of 11thInternational Detonation Symposium, Snowmass,CO(1998).4. Hobbs, M. L., Schmitt, R. G., Renlund, A. M., “Analysis of Thermally-Degrading, Confined HMX,”JANNAF Propulsion Systems Hazards Subcommittee Meeting, Monterey,CA(1996).5. Tappan, A. S., Renlund, A. M., Gieske, J. H., and Miller, J. C., “Raman Spectroscopic and UltrasonicMeasurements to Monitor the HMX ~-b Phase Transition, JANNAF Propulsion Systems HazardsSubcommittee Meeting, Cocoa Beach, FL (1999).7

Mechanical Response and Decomposition of Thermally Degraded Energetic Materials: Experiments and Model Simulations

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Mechanical Response and Decomposition of Thermally Degraded Energetic Materials: Experiments and Model Simulations

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