Fracture-induced decomposition of a brittle high explosive: Pentaerythritol tetranitrate

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FRACTURE-INDUCED PENTAERYTHRITOL li.hl. HAUSER.

DECOMPOSITION

22 July 1983

PHYSICS LETTERS

OF A BRIlTLE

HIGH EXPLOSIVE:

TETRANITRATE

J.E. FIELD and V. KRISHNA hlOHAN of Solids. Cmwtdish Laboratory. Madittgley

Physics md Chcmirtr~

Road. Combridge

CB3 OFE. UK

t\ xtudy hat been nmde of the frxturc-induced decomposition of pentxrythritol terranitrate, a brittle high explosive. Txxo dlffcrcnt modecx ith w.r> inp cner~\ input x~crc used to produce the cr) stal fixture. Time-of-fli&t mascspectral chonctcrivtion of the dccornposirion products shov cd that the reaction p.Away is dependent upon the energy transmitted to the crack. In the energetic mode the primary procr~ invohes the rupture of one of the four central C-C bonds while in the icw enrrgr‘tic case‘. the rcxtlon proceeds ns in slog thermal decomposition. kia the &age of the much menkcr CHzO-NO2 bond.

I_ Introduction

energy (E) terms represent the contributions of elastic. surface. kinetic, plastic and chemical energies respec-

Pentaerythritol tetrdnitrate (PETN) is a van dor Waals type organic solid explosive which crystallizes w irh refragonal symmetry. Fracture surface energy

tively_ The partitioning of energy among the various terms differs from explosive to explosive depending

nieasurenwts 01; these crystals have shown that PETN is a weal. solid (IOU Young’s modulus) and undergoes brirrle fracture with some plastic flow [I]. In an earlier paper Winter and Field [2] pointed out that the loc4ization ofenergy by plastic flow can play a dominant part in the initiation of self-propagating reactions in an explosive. While an understanding of the role of mechanical properties of explosives in their initiation characteristics is imperative, the study of chemic.d reactions induced by mechanical processes_ such as fracrure, facilitates an assessment of its importance ill frlSl reaCtiOnS iI ellergetiC IIlateridS. One of the earliest investigations in this direction was that of Fox and Soria-Ruiz [3] who suggested the incorporation of 911 additional term f?,_t,,,, to the Griffith energy balance condition for the propagation of a crack. This means that some of the energy dissipated at the crack tip goes into promoting a chemical reaction. normally decomposition_ The criterion now takes on the form d(Eet - [Es + Eki + (Epl+ Echcm)Illdc~O, \\htae

66

c is the crack length

and the subscripts

(1) for the

upon its mechanical properties_ A characteristic of brittle fracture is that the cracks can reach very high velocities - several thousands of metres per second. The crack tip radius is of the order dimensions, and when it develops it does so by a process of breaking atomic bonds. Fox and Soria-Ruiz [3] studied the chemical decomposition produced by the release of elastic strain energy when a fast cleavage crack runs through brittle crystalline materials including metal carbonate and azides. These compounds decompose to produce a single gaseous species and thus the decomposition mechanism is chemically simple. Only preliminary results were reported by these authors for the more complex molecule, PETN. Recently hliles and Dickinson [4] examined the fractoemission from PETN and cyclotetramethylene tetranitramine (HMX) crystals using two-channel electron multipliers. They have shown that upon crushing these crystals produce both electrons and positive ions. These observations confirm the earlier suggestion [3] that fracture of these explosives is accompanied by bond breakage. of atomic

In this paper

we present

0 009~2614/83/0000-0000//s

results

of our experiments

03.00 0 1983 North-Holland

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22 July 1983

CHEMICAL PHYSICS LETTERS

carried out employing a time-of-flight (TOF)mass spectrometer to identify the species released due to the propagation of a crack in PETN. As described below two alternative techniques were adopted to generate the cracks. The decomposition spectra recorded were found to display differences thus indicating that the decomposition proceeds along separate routes depending on the mode of fracture_

unit and an oscilloscope

2. Experimental

3. Results

PETN crystals grown from acetone solution were used in this work. These crystals had dimensions up to e-3 X 5 X 20 mm and were semi-regular hexagonal in shape. The fracture experiments were performed in a conventional ultra-high vacuum (UHV) system connected to Bendix RGA-1 A TOF mass spectrometer_ Details of the apparatus have been given in refs. [3,5]. The crystals weremounted on a stage in the centre of a six-way stainless steel section of the UHV system_ The stage could be moved up and down by a linear motion drive, An adjustable chisel was introduced into the system from above and centred on the crystal surface. Then the linear motion drive was used to lift the crysta1 until its upper surface touched the edge of the chisel. The fracture itself was initiated in two ways: (i) the chisel was explosively driven by firing a detonator inside it against the crystal, (ii) by raising the crystal against the stationary chisel using the linear motion drive. In the first series of runs synchronisation of the event with the mass spectrometer could be achieved by a time delay between the firing pulse and the start of the first mass spectrum. This delay was set to 350, 750 and 1000 JLSrespectively. The time delay unit was not used in the second series of experiments_ The recording of spectra was begun as soon as the crack was produced,which gave a time delay of =l s. ATOF mass spectrometer was employed in the present work in place of the quadrupole instrument in ref. [3] _ The fomler provides a better time and mass resolution which is important when studying complex moiecules such as PETN. However, with this equipment data acquisition is more difficult- In order to facilitate data recording, it was necessary to build three electronic units, namely a trace offset controller, and time delay

3.1. Ekpenmmts

bright-up unit. These are de-

scribed in detail in ref. [5] _A 7603 Tektronix OS&Oscope and a C27 Polaroid camera were used to obtain a photo~aph~c record of the spectra taken at an ionisation voltage of 70 V, The fracture-induced decomposition caused a sudden increase in pressure by two orders of magnitude from low7 Torr (=10-5 Pa), the normal operating pressure, to 10ds Torr (*IO-3 Pa)_

wirfi eneeetic

fracture

The fracture of PETN initiated by an explosively driven chisel produces the following decomposition peaks of mass-tocharge ratio (&e): 15 (0), 18 (H,O), 28 (CO), 39 (HCO), 30 (NO), and 44 (CO2 or NzO) in the low-mass region and nt/e 60 {CHzONO) on& in the high-mass region. Typical spectra are presented in fig. 1 which clearly reveal the steady build up of vari1L

(a)

4 . I

(b) Fig. 1, Mass spectra taken during energetic fracture: (a) Iowmass region with a time deIay of 350 gts,and (b) hi&-mass re$on with a time deIay of 700 MS_

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ous pedks from the step-wise presentation of the spectra. Each step consists of the superposition of S spectra and therefore representsa time span of 210~s. The rise of the peaks corresponds to ~4 steps. i.e. 1000 J.G. This can be explained by the diffusion of the decatnposed molecules from the site of production to the ionisatron region of the mass spectrometer. Although the distance between these two locations is only 50 mm. u hich can be covered by a molecule. travelling typically 3t 200 m/s. in 50 Ms. most molecules will collide with the ~.tlls 3 number of times before they reach the ionisation region. Since the walls of the vacuum s> stem are dk0 2 5 0 nun apart, only four such collisions are nccrssary to produce the observed gradual I ix in the spcctr21 peaks+ Fig. 1a shows the peaks in the low-mass region @z/c G 50) with prominent ones at 15 (CH3). ZS (CO). -t-t (CO, OT X,0) rind 60 (CH,ONO). Fig. 1b depicts the spt-ctrutn corresponding to the hit@mass region (ntfc > 50): the only significant feature is the peak at ,Fl/ti

=

60.

-76

,

1.

I I I (3) 60

176 I

I

1

i

I

f

I

I 1

W

22 July 1983

PHYSICS LETTERS

3.2. Experimenrs

with less energetic fracture

The less energetic fracture of PETN produces a number of different spectra which appear to depend on the amount of energy transmitted to the crack. Three types of spectra have been recorded: (i) a large intense peak at m/e = 76 plus other peaks as revealed in fig. ?a. (ii)a high-intensity peak at rm/e = 6Oplus others. or (iii) the simultaneous presence of both peaks at 111Je 76 and 60 in addition to others as presented in fig. 3b.

4. Discussion frt the case of energetic fracture. the mass spectrum exclusively contains the species CH,ONO in the highmass region. This species can only be derived from the fragmentation of “CH,ONO,. an unstable radical formed by the breakage of the C-C bondin the parent molecule. C(CH,ONOZ)q_ The unstable radical is not detected by the TOF spectrometer presutnably because of its short lifetime compared with the 30 us time resolution of the instrument. Two plausible degradation routes of the ~zfe = 76 fragment are:

-CH,ONO,

+ ‘Cl-l20 + ‘NO?.

‘CH,0N02

- ‘CH,ONO

-I- ‘0 _

(I) m

Reaction (I) is ruled out as the species NO2 is not observed in the mass spectra. Support for reaction (II) conies from the large peak at w/e = 30 (NO+) which reflects the easy cleavage of the CH,O-NO bond and the presence of the species CH,ONO in the spectra. Thus energetic fracture results in the rupture of one of the four central C-C bonds. PETN crystallises out from acetone in a tetragonal lattice and belongs to the space group P?Zr C. The cleavage planes are (110) and (li0) since only the wedk van der Waals bonds between the molecules need to be broken along these planes. The violent fracture initiated by a detonator, however, is not restricted to these planes but penetrates into many other ones sometimes bifurcating in the process and producing a rough surface (fig. 3a). With regard to the fracture of PETN by the use of the linear motion drive,much smaller fracture energies are involved. This is supported by fig. 3b which shows

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21 July I.983

Fig. 3. Surfaces produced during (a) energetic fracture depicting rough features, and (b) less energetic fracture showing a smooth profile. The length of tlte horizon bar equals 1 mm. the fracture surface of a crack produced in this mode. A smooth surface following the (110) cleavage plane can be seen which contrasts with fig_ 3a. The mass spectra also reveal differences compared with those recorded during energetic fracture_ As mentioned earlier, both the peaks at 76 and 60 are observed either singularly or together_ If we interpret the former peak to represent CH20N02 ion, then this must originate from the molecule CH,ONO,. Methyl nitrate is shown

to be produced during the thermal decomposition of PETN; the decomposition proceeding via the initial cleavage of one CH,O-NO, bond foifowed by the elimination of a neutral for&ddehyde molecule 161. The mechanism of the fracture-induced decomposition in the less energetic case closely parallels that for thermal decomposition_ The species CH30N0, possesses a much longer lifetime compared to the radical ‘CH,ONO and gives rise to CH,ONOf upon eIectron 69

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When the spectra

contain

PHYSICS

only this species,

tfle fracture surface is found to be smooth. But in some cases more energy is transmitted to the crack, the surface ase

is rou$

and clearly

deviates

22 July 1983

LETTERS

is at least temporarily,

more sensitive. This has important implications in shock or impact loading as emphasized earlier [9,10]_

from the cleav-

plane. This behaviour shows a mass spectrum simi-

lar to that obtained in energetic fracturing showing the prominent nrje = 60 peak. Finally, a hybrid form of the above two cases also occurs when the crack produces 3 partly rough and partiy smooth surface. During this both the peaks at n~fe 60 and 76 are observed. The present

work,

therefore_ pathways,

establishes depending

decomposition

This work has been supported and the Procurement Executive, UK.

References 111 J-T_ Hagan and MX. Chaudhri. Mater. Sci. Letters 12 (1977) 1 OS5 [ 21 R.E.1VinterandJ.E.

Field,Proc.Roy.Soc.A338(19?4)

(31 ~kFosandi.Soria-Ruiz,Proc.Roy.Soc.A317(lV70) 79. f4f M-H. &fifes and J-T_ Dickinson, Appl. Pbys. Letters 41 (1982) 924. IS] H.hf. Hawser, Ph.D. Thesis, University of Cambridge, Cambridge (I 977) p_ 24. i6] W.L. Ng. J-E. Field dnd H.M. Hauser. J. Chem. Sot. Perkin Trans. II (1967) 637. I7 J V. Krishna hlolmn andT_B.Tang, J. Chem. Pbys. (1982). submitted for publication_ IS] Xhl. Chaudhri, Combust. Flsnte 19 (1971) 419. [ 91 h1.M.

Chaudhri and J.E. Field, Proc. Roy. Sot. A340

(1974) I lo]

113.

J-E. Field. Proceedingz of the International Conference on Primary Explosives, E.R.D.E., \valthdm Abbey, hiarch (1975)_

70

by grants from SERC Ministry of Defence,

that dif-

upon the energy associated with the crack are followed during the fracture of PETN crystals. In the less energetic mode. the decomposition mechartism is similar to the one for thermal decomposition. viz. the rupture of the Cli,O-NO, bond. It is only with considerable surplus energy in the crack that another route involving the breakage of the much stronger C-C bond is folfnwed. SimiIar differences in the reaction pathways are found in the thermal decomposition and explosion of lead azotetrazole. N4C-CN,*Pb(OH)2_ Decomposition proceeds via the formation of CN, species, while during explosion CN, is additionally produced ;is ZIresults of the N=N bond scission [71_ It 113s IO be pointed out here that afthough fracture can 0wnerAte gaseous products and surfaces of different lcactlvity. no evidence has been fVound thus far that fracture olo~le can Ie.td to initiation of fast reaction 1S]. However. as a consequence the esplosive sample ferent

Acknowledgement