Cylindrical Explosive Charge Research Articles

Controlled blasting in jointed rocks

This paper presents the results of some small-scale experiments on rock blasting, whose experimental procedure is described in two other papers by the author. Long cylindrical explosive charges were placed inside 6.2mm diameter vertical holes drilled in 600mm x 300mm x 300mm blocks of limestone. A large canvas collecting chamber was used, enabling the collection of broken fragments and the study of post- blast rock conditions. To demonstrate the influence of an explosive- induced fracture plane on the perimeter face for the stable remaining rock mass, many observations were obtained from test blocks, blasted with various orientations of the joint planes. The results show: (1) the influence of joint orientation on the development of fracture planes on the perimeter face; (2) significant disturbance to the remaining rock faces. The effects of the following types of rock joints are given: (1) horizontal joints; (2) perpendicular vertical joints; (3) perpendicular 60 degrees and 30 degrees down and up dip joints; and (4) parallel 60 degrees and 30 degrees dip joints.

Experimental and analytical study of early time material processing, in a collapsing shaped - charge liner, using “softly - recovered” partially - collapsed copper liners

It is not yet clear what detailed deformation mechanisms enable copper shaped charge jets to exhibit the extraordinarily high ductility, which characterizes their dynamic behavior. The study described in this paper seeks to find some of these answers, by stopping the liner collapse process at various intermediate stages, and examining the grain structures in the partially collapsed liners. Well characterized OFE copper shaped-charge liners, assembled into a cylindrical polycarbonate case, of constant length and volume, were partially collapsed, with reduced-weight cylindrical explosive charges. A series of increasing explosive charge weights were used to obtain progressively greater partial deformations on individual copper liners. The shock waves from the varying length explosive charges were coupled to the copper liners through intermediate water fill, which was in direct contact with the rear of the liners. The series of partially collapsed copper liner was captured by “soft recovery” in low density polystyrene. Flash radiography prior to liner recovery, confirmed that the unexpected shapes of the recovered partially collapsed liners, actually existed prior to their entering the recovery medium and were not the result of the recovery process itself. This was an early concern when the unusual shapes of the recovered liners were first seen. These shapes were also independently confirmed by a series of computations at Los Alamos National Laboratory, using MESA 2D. A comparison of the photomicrographs of undeformed virgin copper liners andd the series of partially collapsed liners, shows regions on the inner apex near the liner axis where plastic flow has occured, with very substantial modifications (refinement and elongation in grain structure even for the small deformations which barely change the overall liner shape. Time dependent strain and strain rate computations, using LaGrangean tracer markers, indicate very large strain rates, between 3×10 7/sec. and 4.7×10 7/sec. in those regions with plastic flow where grain refinement and elongation are seen, even with very small overall deformation. It is believed likely that this early time material processing and grain refinement, arising from the localized plastic deformation of the liner, plays a key role in preparing the liner material structure, so that it can exhibit the high dynamic ductility, characteristic of copper shaped-charge jets.

Controlled dynamic cracking of hardened cement paste specimens

A new method of studying the dynamic development of microcracks in hardened cement paste (HCP) plates subjected to dynamic loads is developed. The dynamic loading is produced by special cylindrical explosive charges containing about 10 mg PETN each. These charges, when initiated, produce a dynamic pressure pulse for about 1–2 μ/s. The charges are inserted inside boreholes perpendicular to the hardened cement paste plate faces in predefined locations. The dynamic loading causes cracking of the plates, enabling one to measure the crack propagation velocity and to investigate the crack interaction phenomenon. Crack propagation velocities were found to be in the range of 60 to 200 m/sec, much lower than the theoretical values. Crack arresting phenomena of adjacent cracks, which were observed during the experiments, could explain delays in crack propagation as well as the increase in hardened cement paste specimen strength under dynamic stresses. The new method enables simple measurements of crack propagation velocities in HCP samples of different shapes and dimensions under dynamic tensile stresses, including small samples, in which crushing of concrete is observed in common methods during similar tests.

Investigation of the lumpiness of a blasted mass for various diameters of a cylindrical explosive charge

The experiments that we conducted made it possible to establish the effect of the diameter of cylindrical charges and, accordingly, the scale of blasting on the granulometric composition of the blasted material. The size of the reduced material increased in both the solid and jointed medium with increasing charge diameter, other influencing factors the same, including the relative (in charge diameters) magnitude of the line of least resistance and the specific explosive consumption. The radius of reduction to a specific maximum size of material is approximately proportional to the cube root of the charge diameter raised to the second power and to the cube root of the dimension G of the maximum lumps. Use of similitude criterion in the form (G/d)1/3 makes it possible to obtain cumulative plots of blasted-material size, which are generalized for different blasting scales, as well as the linear relationship between the relative (in charge diameters) radius of reduction to the specific maximum size of the material and the similitude criterion.

Time-histories of principal strains generated in rock by cylindrical explosive charges : Anderson, D A; Winzer, S R; Ritter, A P In: Rock Mechanics in Productivity and Protection (Proceedings of the Twenty-Fifth Symposium on Rock Mechanics, Evanston, Illinois, 25–27 June 1984)P959–968. Publ New York: AIME, 1984

Time-histories of principal strains generated in rock by cylindrical explosive charges : Anderson, D A; Winzer, S R; Ritter, A P In: Rock Mechanics in Productivity and Protection (Proceedings of the Twenty-Fifth Symposium on Rock Mechanics, Evanston, Illinois, 25–27 June 1984)P959–968. Publ New York: AIME, 1984

Modeling the explosion of a system of borehole charges in a scarp

On the basis of the mathematical model here proposed for the action of the explosion of two cylindrical (borehole) explosive charges in the scarp of a rock mass, the scope for investigating the correlation between wave propagation parameters with scarp rock cutting in a pit is expanded. This offers the possibility of solving a series of problem associated with the correction of wave propagation parameters and their optimization in conditions of open-pit mining of minerals.

Velocity and density fields in reaction products of a detonating cylindrical charge

Experiments are described for the determination of the spacial and temporal behavior of the detonation products present during and after the detonation of a cylindrical explosive charge. The charge consisted of an explosive column of “Composition B” (40 mm diameter, 70 mm height) which was divided into cells by horizontal and vertical imbedding of gold foils of 10 μ m thickness. In order to ascertain the time behavior of momentum transfer from the detonation products to a metallic layer, a 2 mm thick copper plate was attached to the base of the column. The temporal changes of the size and of the position of the individual cells was followed by X-ray flash photography. In this way, the density, the flow direction, and the flow velocity were determined as functions of time for each cell. The results are presented in the form of velocity and density fields for times lasting from 3 μ sec before until 5 μ sec after the arrival of the detonation front at the copper plate. The time history for the pressure was determined approximately by using a simple theoretical model.

Radiation from an Explosion in a Non-uniformly Pre-stressed Medium

It is known that both an increase of the cavity size and rupture propagation from an explosion in a pre-stressed medium will release strain energy stored in the initially strained material thus affecting the seismic radiation field. Under uniaxial tension, a distribution of shear stresses was produced in the rhomboid shaped plexiglas plate. The static stress analysis was done by the photoelastic and analytic methods. A cylindrical explosive charge was used as a source to generate seismic waves. The termination of the crack propagation produced a detectable secondary seismic arrival, corresponding to the stopping phase. The direction of cracks corresponded to the theoretically computed average orientation of the maximum shear stresses and close to the normal to the applied tension. The velocity of the crack propagation was determined as 0.833 km s−1, 0.61 times the shear wave velocity in the Plexiglas. The radiation pattern of P-wave was altered from its cylindrical symmetry. It is concluded from our experiments that no shear waves are generated from a cylindrically symmetric line source in the absence of pre-stress and significant inhomogeneity. An increase of S-wave amplitudes might be suggested as the criterion for determining the effect of stored strain energy release and of radial cracking. Distribution of ambient stresses in the medium produced an observable effect on the frequency spectrum of an explosion.

Model for Converging Detonations in Solid Explosives

The fluid-dynamic equations which describe converging detonation fronts in high explosives are solved over a coordinate system that changes from circular to a pleated pattern as the detonation converges. There are three reasons for introducing such a coordinate system rather than making the usual assumption that when a spherical or cylindrical explosive charge is initiated simultaneously on its curved surface the detonation front maintains its symmetry as the detonation converges to the center: (1) Calculations made elsewhere show that a converging circular shock wave is unstable. (2) Calculations in this paper show that if spherical or cylindrical symmetry is maintained, density and velocity become infinite, but if the detonation front folds these quantities remain finite except at a few points which can be removed by cuts. The average fluid velocity remains almost constant over 99% of the detonation front. (3) Experiments indicate that a cylindrical detonation front becomes rather symmetrically deformed.

Compression waves generated in rock by cylindrical explosive charges: A comparison between a computer model and field measurements

A computer model is used to simulate the compressive strain waves generated in rock from the end detonation of long and short cylindrical explosive charges. The model synthesizes the strain wave by superposing the waves from detonation of a number of concentrated explosive charges. In situ strain data were recorded by strain gages mounted in a granite gneiss at various distances and positions with respect to 2 1 2 ft and 27 ft charges of a high-pressure gelatin (HPG) and a more slowly detonating mixture of prills and fuel oil (PFO). The good agreement between the strain data obtained in the field and the computer-generated data shows that the model can be used to predict the strain waves generated in rock by cylindrical explosives charges. The paper also shows that for relatively short end-initiated HPG charges, the resulting strain is not spherically symmetric even at large distances in the rock. Furthermore the peak strains due to both long and short charges are shown to be greatly influenced by the detonation velocity of the explosive.

Analysis of Optical Determination of Detonation Velocity in Short Charges

This paper discusses some applications of the smear camera, one of the primary tools used to measure the velocity of rapid reaction fronts in an explosive or detonation reaction. Solid cylindrical explosive charges are normally initiated at a small area on the end of the axis. Due to the nature of propagation of a reaction front, the optical records, normally obtained from the periphery of a charge, are distorted and do not give a true measure of the front's velocity. For accurate measurement, long charges are ordinarily used and only a small portion of the total smear record is used to obtain the true velocity. A transformation equation has been derived whereby the data from smear camera records of the peripheral detonation of a short (L/D of 2.5) tetryl charge can be interpreted to obtain a detonation velocity measurement. The results are in excellent agreement with the values measured along the axis as well as with those published in the literature.

Coupling between unconfined cylindrical explosive charges and rock

The stresses generated in Yule marble by unconfined cylindrical explosive charges were determined for various geometries of charges and various interface conditions between the explosive and the marble. A pellet technique was used in which the momentum imparted to a pellet placed on a free surface of a marble specimen gave a measure of the momentum of a portion of the shock wave normally incident to the free surface. Stress-time curves were determined by using pellets of several thicknesses. For a contact charge, the charge diameter affected the peak stress, measured within marble 2·00 in. from the charge, much more strongly than the charge length. The angle between the axis of the charge and the surface of the marble had little effect on peak stress if the contact area between the explosive and the marble was held constant. The peak stress within the marble decayed from 271,000 psi 0·50 in. from the contact charge to 15,200 psi 4·00 in. from the contact charge, the decay being slightly greater than a theoretical 1/ r decay. Partial confinement of the charge substantially increased the peak stress. The shock attenuation in a layer of dissimilar material between the explosive and the marble was found to be less for water than for marble itself and greatest for air. The attenuation rate in a layer of sand, cardboard, or foam rubber was also high, being almost equal to that of air.