(DOS), antioxidant Lowinox and wetting agent lecithin, and was cured using isophorone diisocyanate (IPDI). These were combined in the quantities listed in ...
Towards a Fundamental Understanding of the Thermomechanical Response of Damaged Polymer Bonded Energetic Materials David M. Williamson∗, Daniel R. Drodge*, Ian G. Cullis**, Peter J. Gould**, Philip D. Church** *
SMF Group, Cavendish Laboratory University of Cambridge, CB3 0HE, UK **
QinetiQ Fort Halstead Sevenoaks, Kent, TN14 7BP, UK
Abstract. Polymer bonded energetic materials such as explosives and propellants can exhibit unsafe behaviour following mechanical and / or thermal insult. Of particular concern is the transition from burning to violent reaction, which has previously been shown to be aggravated by an increase in the surface area accessible to the flame front. To investigate the effect of microstructure on mechanical damage response, three RDX HTPB composites were produced, containing: (1) coarse RDX particles only, (2) fine RDX particles, and (3) a 2:3 mixture of both coarse and fine RDX particles. Samples of these materials were damaged by the application of dynamic uniaxial compression to various prescribed final strains. The dynamic storage modulus, thermal conductivity, density decrease and quasistatic stressstrain behaviour of these damaged materials were then measured. It was noted that the composite containing fine particles was the most resilient to damage, showing comparatively little deterioration in thermal conductivity or mechanical strength. The other two materials showed significant degradation. The mechanical response of the bimodal material transitioned from one resembling the coarse grained material to one resembling the finegrained material as the degree of damage intensity increased. This result was supported by the dynamic modulus data. Thermal conductivity and density appeared to show similar dependences on damage for all three composites, indicating a common underlying cause, thought to be a loss of connectivity following the formation of internal voids associated with debonding of the HTPB matrix from RDX particles.
Introduction The UKEnergetics Hazard Programme is concerned with understanding and reducing munition sensitivity, in particular to mechanical and thermal insult. Most polymer bonded explosives (PBXs) and polymer bonded propellants are particulate composites whose matrix is considerably softer than the filler. This
commonality of form makes the study of model particulate composite materials worthwhile, even where the behaviour of specific, commercial formulations may seem to be of more immediate relevance. In this study, three such model PBX formulations were subjected to damage by the application of dynamic compression. The resulting trends in their thermal and mechanical properties, as damage increases, are reported.
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Earlier research performed at the Cavendish Laboratory showed that PBX tensile failure occurs preferentially via crystalbinder debonding rather than crystal fracture when the binder is above its glasstransition temperature for a given rate of loading1,2,3. Even where failure initiated by binder cavitation, it soon progressed to adhesive failure rather than further cohesive rupture. Conversely, where experiments have taken place below the glass transition temperature of the binder, failure occurs through fracture of the composite as a whole, with cracks progressing largely undeflected through crystal and binder alike4. Both adhesive and cohesive microstructural failure can lead to the creation of new surface area within a PBX. When such a material burns, the increase in exposed surface area is expected to give rise to an increase in reaction rate, possibly leading to deflagration and detonation5. This effect has been demonstrated with mechanically and thermally damaged specimens of PBX95016,7. This socalled “Burn to Violent Reaction” (BVR) transition is the subject of ongoing study. The experiments reported here describe an approach towards producing a quantitative account of the relationship between microstructure, the extent of mechanicallyinduced damage, and the resulting strength, stiffness, density and thermal conductivity of PBX formulations. As well as offering validation data for modeling codes, the trends in property degradation offer insight into the underlying microstructural processes. Materials QRX214, QRX217 and QRX221 are three custom experimental PBXs formulated and produced at QinetiQ Fort Halstead to provide experimental data to aid BVR model development and validation. QRX214 and QRX217 are monomodal HTPBbound composites filled with fine RDX and coarse RDX respectively. QRX221 is a bimodal composite containing both of the aforementioned particle sizes, in the ratio of 70:30 of coarse to fine. Their compositions by volumetric fillfraction are shown in Table 1.
PBX strength has been shown to be dependent upon particle size, both experimentally8 and in modeling9. This justifies the use of materials with different particlesizes. Furthermore, many PBX formulations have a bimodal particle size distribution (PSD) to allow their fillfractions to exceed the maximum possible for monomodal spheres, making the inclusion of a bimodal test material worthwhile. The particle size distributions of the coarse and fine RDX crystals, measured using laser diffraction, are shown in Figure 1. The HTPB binder contains the plasticiser dioctyl sebacate (DOS), antioxidant Lowinox and wetting agent lecithin, and was cured using isophorone diisocyanate (IPDI). These were combined in the quantities listed in Table 2 and cast into sheet moulds. Specimens were carefully cut or punched from these sheets as required. Table 1. Volume fractions of RDX components of the materials. Material QRX214 (fine) QRX217 (coarse) QRX221 (bimodal)
Coarse RDX 0.60 0.47
Fine RDX 0.54 0.20
Table 2. Contents of HTPB used to make QRX explosives.
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Item HTPB (uncured polymer) DOS (plasticizer) IPDI (curing agent) lecithin (wetting agent) Lowinox (antioxidant)
% by mass 60.3 32.9 5.6 0.8 0.4
2.5 mm, 2.8 mm and 3.2 mm thickness were used. A 20 cm long strikerbar was fired directly into each specimen at a velocity of 9 m s1, compressing them at strainrates of approximately 2000 s1. This high rate was necessary to ensure that the required strains were reached before any unwanted wave reflections reached the strain gauge on the Hopkinson bar. Stress was calculated using the 1wave method as described by Gray11, and strain was computed using the striker bar impact velocity and retardation force as described by Safford12.
0.14 Fine RDX Coarse RDX Bimodal RDX
0.12
Fraction
0.1 0.08 0.06 0.04 0.02 0
1
10
100
Median Bin Size / µm
1000
Specimen Collar
Striker Bar
Figure 1. Particle Size Distribution (PSD) of the fine and coarse RDX powders used in QRX214 and QRX217 respectively, measured at Fort Halstead10. A bimodal particle size distribution, calculated for QRX221 is also shown. Method I: Damage For the purposes of producing a controlled level of damage in PBX specimens, damage is taken to be a function of the work done on an unconfined specimen under bulk uniaxial compression, which will cause local areas of tension on length scales comparable to those of the filler crystals. Specimens were thus compressed to fixed strains at a linear strainrate, and their force extension histories recorded during the damage cycle. Three levels of final strain were chosen, giving three degrees of ‘damage intensity’. It was decided that dynamic rates of compression, of the order of 1000 s1, should be used; at these rates the mechanical response is stiffer, meaning that more work is done on the material in compressing it to a fixed strain. Dynamic rates of loading also better reflect those experienced in highrisk incident scenarios, such as fragment attack. The density and quasistatic compression specimens were 6 mm diameter cylinders. These were damaged using a direct impact Hopkinson Bar system, set up as shown in Figure 2. Dural bars were used: these were 12.7 mm in diameter. A rigid collar was placed around each specimen to limit the maximum strain imposed. Collars of
Hopkinson bar Strain Gage
v
20cm
25cm Light Gates
50cm
Figure 2. Schematic of Hopkinson Bar apparatus used to apply fixedstrain damage to specimens. The striker bar impacts with velocity v. Dynamic Mechanical Analysis (DMA) and thermal property specimens were larger than the diameter of the Hopkinson Bar. These were instead damaged using a dropweight system, which can accommodate larger specimens and essentially performs the same task. The apparatus consisted of a 6 kg weight, dropped from a height of 1.2 metres along a pair of guide rails onto an anvil mounted on a loadcell. Specimens were placed on the anvil, surrounded with rigid collars of the same thicknesses as described above to limit the final strain. A mechanical mechanism was in place which prevented the weight from striking the specimen more than once. The typical impact velocity was 5 m s1, compressing specimens at approximately half the Hopkinson bar strain rate. Both types of apparatus provide impact velocities and forcetime records, from which the stressstrain histories of the specimens can be computed. Integration of the stressstrain curves will give the specific work done on the samples during the damage process. Figure 3 shows an
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example forcetime trace, illustrating the weak transmitted stress prior to contact with the collar, indicated by a sharp rise. 10
0.8
F / kN
7.5
Full scale
0.7
5
0.6
2.5
0.5
0
0.4 Zoomin
2.5
0.3
5
0.2
7.5
0.1
10 0
100
200
300
400
Time / s
500
600
0 700
investigated, the tan delta peak occurred at approximately 20°C in all three materials, but the storage moduli differed: with the QRX221 being the stiffer material at ambient temperature. This is as expected given its higher fillfraction (see Table 1). To avoid the complications introduced by differential thermal expansion of the PBX components, subsequent experiments were conducted at ambient temperature, the aim being to study the effect of damage. For these experiments the loading frequency was varied between 1 and 10 Hz, and the loading amplitude was set to a strain of 0.05%. Three specimens were measured per damage level per material type. F 1000
0.4
0.35
Storage Modulus / MPa
Specimen dimensions were measured, before and after the damage was applied. The difference was less than that implied by the total applied strain during the loading cycle, indicating that some amount of elastic recovery had occurred. Method II: Properties A screw driven Instron machine was used to compress undamaged and damaged specimens at a constant strainrate of 102 s1. A 2 kN loadcell was used to monitor the sample stress, and a 10 mm clipgauge was used to measure strain. The specimens were compressed between parallel anvils whose faces were lubricated using petroleum jelly grease. Dynamic modulus measurements were made using a TA Instruments Q2000, in the single cantilever bending mode. Initially, a temperature sweep was performed at a fixed frequency of 1 Hz on undamaged materials, the results of which are shown in Figure 4. Within the temperature range
0.3 100
0.25
Tan Delta
Figure 3. Example forcetime trace of a Hopkinson bar impacted specimen with restraining collar, shown at coarse and fine voltageresolutions. At full scale, the typical trapezoidal loading pulse is seen.
0.2 QRX214 QRX217 QRX221 10 60
40
0.15
20
0
Temperature / °C
20
0.1 40
Figure 4. 1 Hz DMA thermal scan of the three materials. The upper three curves show the tan delta response. Density was measured using a density bottle of volume 10.277 ml. Deionised water was used as the displacement fluid densities were measured immediately before and soon after each experiment. Specimens were weighed dry, and immersed in a beaker of deionised water. They were then agitated by stirring and shaking to remove any bubbles on the specimen surface. The wetted specimens were then placed in the density bottle, which was filled up with water and weighed. The specimen volume and can be calculated from the bottle volume, mass when empty, and masses when full of water and
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Results The stressstrain properties of the materials, starting from an undamaged state, i.e. virgin material, at Hopkinson Bar and quasistatic strain rates are shown in Figure 5. Compression at higher strainrate is seen to induce higher stresses, due to the timedependent nature of PBXs14. The quasistatic stressstrain curves, for the three damaged states, labelled by the imparted prestrain εp, are shown in Figure 6 through to Figure 8. For these experiments the damage was applied using the Hopkinson bar at a strainrate of approximately 2000 s1. Figure 9 shows the DMAmeasured storage moduli of the materials, at a frequency of 1 Hz, as a function of damage (prestrain). The relative decrease in density from that of the undamaged material is plotted as a function of damage pre strain in Figure 10. The variation in thermal conductivity is shown in Figure 11. Table 3 summarises the results, showing the degradation of the thermal and mechanical properties of all three compositions for a comparable damage intensity corresponding to a prestrain of 0.5.
5
True Stress / MPa
4
3
2 QRX214 (fine) QRX217 (coarse) QRX221 (bimodal)
1
0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
True Strain
Figure 5. Undamaged compressive response of all three materials, at 2000 s1 (upper curves) and 2×102 s1 (lower curves). 1
0.8
True Stress / MPa
containing water and specimen. Three specimens were measured per damage level per material type. A “Hot Disk Thermal Analyser” was employed to measure thermal conductivity and diffusivity of damaged and undamaged specimens. Square slab specimens, approximately 15 mm by 15 mm were used. The Hot Disk apparatus employs the Transient Plane Source method, developed by Gustaffson13. Two specimens of the same material are placed on either side of the dual function heating element and thermoresistive sensor. The assembly is held in intimate thermal contact by the application of a 0.3 N dead load. The measurements made are effectively undertaken at room temperature, given the low temperature rise associated with the technique.
0.6
0.4 ε = 0 p
ε = 0.19 p
0.2
ε = 0.36 p
ε = 0.51 p
0 0
0.1
0.2
0.3
0.4
True Strain
0.5
0.6
0.7
Figure 6. Compressive response of QRX214 (fine) at a strainrate of 2×102 s1 following damage (pre strain) to the levels indicated. Damage was applied by Hopkinson bar at a strainrate of approximately 2000 s1.
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0.5
Storage Modulus at 1Hz / MPa
25
True Stress / MPa
0.4
0.3
0.2 ε = 0 p
εp = 0.45
0.1
ε = 0.65
QRX214 (fine) QRX217 (coarse) QRX221 (bimodal)
20
15
10
5
p
ε = 0.73 p
0
0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
True Strain
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Prestrain
Figure 7. Compressive response of QRX217 (coarse) at a strainrate of 2×102 s1 following damage (prestrain) to the levels indicated. Damage was applied by Hopkinson bar at a strain rate of approximately 2000 s1.
0.8
Figure 9. 1 Hz DMAmeasured storage moduli, at ambient temperature, as a function of damage intensity (prestrain). Damage applied by drop weight at a strain rate of approximately 1000 s1.
2
0.8 0.7
0
Change in density / %
True Stress / MPa
0.6 0.5 0.4 0.3 ε = 0 p
0.2
2
4
ε = 0.27 p
QRX214 (fine) QRX217 (coarse) QRX221 (bimodal)
6
ε = 0.51
0.1
p
ε = 0.60 p
0 0
8
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 True Strain
0
0.1
0.2
0.3
0.4
0.5
Prestrain
Figure 8. Compressive response of QRX221 (bimodal) at a strainrate of 2×102 s1 following damage (prestrain) to the levels indicated. Damage was applied by Hopkinson bar at a strain rate of approximately 2000 s1.
0.6
0.7
0.8
Figure 10. Percentage change in specimen density from undamaged values as a function of damage (prestrain). Damage was applied by Hopkinson bar at a strainrate of approximately 2000 s1.
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QRX214 (fine) QRX217 (coarse) QRX221 (bimodal)
0.26
1
Thermal Conductivity / W m K
1
0.27
0.25 0.24 0.23 0.22 0.21
0
0.1
0.2
0.3
0.4
0.5
Prestrain
0.6
0.7
0.8
Figure 11. Thermal Conductivity, measured using the Hot Disk Thermal Analyser, as a function of prestrain damage. Damage was applied by drop weight at a strain rate of approximately 1000 s1. Table 3. Relative change in properties for a damage intensity corresponding to a comparable prestrain of 0.5. N.B. density and proof stress measurements were damaged at 2×103 s1, and dynamic modulus and thermal conductivity at 1×103 s1. % reduction in: Material ρ κ M’ σ0.2ε QRX214 ~ 0 ~ 0 ~ 40 ~ 30 QRX217 ~ 3 ~ 10 ~ 57 ~ 58 QRX221 ~ 3 ~ 6 ~ 57 ~ 58
Discussion Some immediate observations can be made of the mechanical response of the undamaged materials (Figure 5). The stress required to de bond a particle is predicted to decrease as its radius increases15,16. Thus the largest crystals in a composite are expected to debond first. This will cause a significant reduction in stiffness as these particles will no longer be fully loadbearing. This is seen more obviously in the stressstrain curves of QRX217 and QRX221 without predamage than the less affected QRX214. The former, coarse particle materials show distinct peak stresses at
strains of 10 %, indicating that the modulus degrades catastrophically when the local tensile stresses have reached a level sufficient to cause debonding of a significant population of the coarse grains. The fineparticle composite curves show significantly less modulus degradation with pre damage, and one interpretation would be that the local tensile stresses simply do not reach a level sufficient to cause debonding of the fine particles to the same degree as that which occurs to the coarse grains. This is despite the stresses in this composite being higher. The bimodal QRX221 can be thought of as a coarsegrained material with a stiffer binder; hence the higher stresses reached. The deterioration in stressstrain response with increasing damage offers a fuller picture. As damage increases, QRX221 and QRX217 tend towards a stressstrain response similar in shape to that of QRX214; when the larger particles detach from the composite we are left with the mechanical response of the remaining structure. In the idealised case of fully damaged QRX217 only the binder remains, and in fully damaged QRX221 only the fineparticle filled binder, of similar composition to QRX214 remains‡. (The volume fraction of the filledbinder in QRX221, computed from Table 1 is 0.61, as compared to 0.54 in QRX214). The structural voids left by the notionally ‘absent’ debonded particles weaken the material as a whole, hence the lower plateau stresses seen in damaged QRX221 as compared to QRX214. However, it is clear from Figure 12 that even following a prestrain of 0.5, we are still some way from realising such an idealized state; the stress straincurves of all three compositions lie significantly above that of the binder material implying all three have matrices that are still reinforced to some extent, and even for QRX217 the debonding process has not gone to completion. It follows that a realistic description of the processes occurring clearly needs to describe the fractional populations of bonded and debonded particles as a function of damage intensity.
___________________ ‡
Here we are guided by the evidence of Figure 5, and assume it is not energetically favourable to debond fines.
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0.6 QRX214 (fine) QRX217 (coarse) QRX221 (bimodal) Binder
True Stress / MPa
0.5 0.4 0.3 0.2 0.1 0 0
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 True Strain
Figure 12. Quasistatic stressstrain curves for the three composites all at a damage state of 50 % pre strain, with HTPB binder for comparison.
The thermal conductivity and density show significant deterioration with increasing damage in QRX217 and QRX221, whilst QRX214 shows none. However the DMA modulus of QRX214 does show a degree of deterioration. There is also evidence of this in Figure 6: the initial slope and highest achieved stresses of the damaged quasi static curves are lower than in the undamaged material, albeit to a much lesser extent than seen in the other two composites. During density and conductivity measurements the materials are not under any significant load, so any internal cracks opened during the damage process may be closed by elastic recovery. This makes the material indistinguishable from its undamaged form to the more ‘passive’ density and conductivity techniques. However, under ‘active’ mechanical testing the debonded surfaces in damaged QRX214 reopen and the material is measurably weaker. In the coarsegrained materials, the permanent density decrease with damage is most likely due to the formation of internal voids in the sample, which cannot close under elastic recovery. The lack of recovery may be due to geometric interference induced by rotation of the debonded particles. The partial closure of cracks in QRX217 has been noted in a previously reported investigation17.
The trends in conductivity and density deterioration appear correlated, so one might conclude that void formation inhibits the conduction of heat by reducing the effective cross sectional area. Conclusions Three model PBX compositions were subjected to interrupted dynamic compressive loading to fixed strains, introducing damage whose ‘intensity’ metric is the degree of ‘prestrain’. The resulting effects on thermal and mechanical properties were measured. The finegrained QRX214 composite showed little change in density or thermal conductivity as damage increased, but a reduction in dynamic storage modulus and the 20 % proof stress at a quasistatic loading rate was noted, indicating that some damage had indeed occurred. The coarsegrained QRX217 and bimodal QRX221 composites illustrated similarities in their trends for all measured properties: their dynamic moduli, densities and thermal conductivities decreased with increasing damage, thought to be due to internal void formation when their coarse filler particles debonded. It was noted that the quasistatic response of QRX221 became increasingly like that of QRX214 as damage increased. Of the three compositions, the monomodal fine particle material QRX214 was found to be the most resilient to damage. This is believed to be because the composite did not develop local stresses sufficient to debond its filler particles to the extent that occurred in QRX221 and QRX217. Acknowledgements This research was performed as part of the UKEnergetics HAZARD research programme, funded by the Defence Technology and Innovation Centre, UK MoD. W.G Proud of Imperial College London is thanked for overseeing the initial Cavendish Laboratory research. We are grateful to
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R. Cornell of the Department of Materials Science and Metallurgy at the University of Cambridge for performing the Hot Disk measurements, staff at the Cavendish Laboratory Mott Workshop for their assistance, and to K. Cox of QinetiQ for producing the formulations and measuring the RDX PSD. References 1. Palmer, S. J. P., Field, J. E., and Huntley, J. M., “Deformation, strengths and strains to failure of polymer bonded explosives” Proc. R. Soc. London, Ser. A, Vol. 440, pp.399-419, 1993. 2. Rae, P. J., Palmer, S. J. P., Goldrein, H. T., Field, J. E., and Lewis, A. L., “Quasi-static studies of the deformation and failure of PBX 9501” Proc. R. Soc. A, Vol. 458 pp. 2227-2242, 2002. 3. Rae, P. J., Goldrein, H. T., Palmer, S. J. P., Field, J. E., and Lewis, A. L., “Quasi-static studies of the deformation and failure of beta-HMX based polymer bonded explosives” Proc. R. Soc. London, Ser. A, Vol. 458, pp.743-762, 2002. 4. Williamson, D. M., Palmer, S. J. P., and Proud, W. G., “Brazilian disc testing of a UK PBX below the glass transition temperature”, Proceedings of the 15th APS Topical Conference on Shock Compression of Condensed Matter, pp. 803-806 Waikoloa HI, July 2007. 5. Asay, B. W., Son S. F., and Bdzil, J. B., “The role of gas permeation in convective burning” Int. J. Multiphase Flow, Vol. 22, pp. 923-952, 1996. 6. Berghout, H. L., Son, S. F., and Asay, B. W., “Convective burning in gaps of PBX 9501” International Symposium on Combustion, Vol. 28 pp. 911-917, 2000. 7. Berghout, H. L., Son, S. F., Skidmore, C. B., Idar, D. J., and Asay, B. W., “Combustion of damaged PBX 9501 explosive” Thermochim. Acta, Vol. 384, pp. 261-277, 2002. 8. Balzer, J. E., Siviour, C. R., Walley, S. M., Proud, W. G., and Field, J. E., “Behaviour of ammonium perchlorate-based propellants and a polymer-bonded explosive under impact loading” Proc. R. Soc. London, Ser. A, Vol. 460, pp.781806, 2004. 9. Tan, H., Huang, Y., Liu, C., and Geubelle, P. H., “The {MoriTanaka} method for composite materials with nonlinear interface debonding” Int. J. Plast, Vol. 21, pp. 1890-1918, 2005. 10. QinetiQ Unpublished results, 2008.
11. Gray III, G. T., “Classic Split Hopkinson Bar Testing” in ASM Handbook – Mechanical Testing and Evaluation Vol. 8, pp. 463-476, ASM International, 2000. 12. Safford, N. A., “High Strain Rate Studies with the Direct Impact Hopkinson Bar” PhD Thesis, University of Cambridge, 1988. 13. Gustafsson, S. E., “Transient plane source techniques for thermal conductivity and thermal diffusivity measurements of solid materials”, Rev. Sci. Instrum. Vol. 62 pp. 797-804, 1991. 14. Williamson, D. M., Siviour, C. R., Proud, W. G., Palmer, S. J .P., Govier, R., Ellis, K, Blackwell, P. and Leppard, C. “Temperature-time response of a polymer bonded explosive in compression (EDC37)”, J. Appl. Phys. D, Vol. 41, 085404, 2008. 15. Gent, A. N. “Detachment of an elastic matrix from a rigid spherical inclusion” J. Mat. Sci, Vol. 15, pp. 2884-2888, 1980. 16. Nicholson, D. W. “On the detachment of a rigid inclusion from an elastic matrix” J. Adhesion, Vol. 10, pp. 255-260, 1971. 17. Drodge, D. R., Chapman, D. J., and Proud, W. G. “Mechanical response of damaged explosive compositions” Proceedings of the 16th APS Topical Conference on Shock Compression of Condensed Matter, pp. 1249-1252 Waikoloa HI, June 2009. Discussion Matt Lewis, LANL Have you considered micrography of your specimens? We have performed computational microtomography but were unable to bring those results with us. Reply by Daniel Drodge We performed sectioning and optical micrography on the specimens but the results were unsatisfactory as the polishing process caused grain pull-out. X-ray computer tomography was attempted in the early stages of this investigation, but damage was found to be unresolvable in all but the coarse-grained material (QRX217), and even then a compression rig is required to hold the cracks open [1]. The additional volume of the rig reduces the pixel resolution of the technique. In future studies we intend to make use of higherresolution apparatus, such as that used in a previous study on a sugar-based composition [2].
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[1] D.R. Drodge, D.J. Chapman, and W.G. Proud. Mechanical response of damaged explosive compositions. in Shock Compression of Condensed Matter, volume 1195 of AIP Conference Proceedings, pages 1249–1253, 2009.
[2] P R Laity, C R Siviour, P D Church, and W G Proud. High strain-rate characterisation of a polymer bonded sugar, in Shock Compression of Condensed Matter, number 845 in AIP Conference Proceedings, pages 905–908, 2005.
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