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DETONATION PROPERTIES
3. DETONATION PROPERTIES
3.1 Detonation Velocity and Diameter Effect. The velocity with which a steady
detonation travels through an explosive is measured by using a broomstick-shaped
piece of the explosive, called a rate stick. A standard rate stick is a right cylinder,
usually composed of a number of shorter cylinders that have been cast, pressed, or
machined to a predetermined diameter. The stick is detonated at one end, and the
progress of the detonation is measured at discrete points along the stick length. The
locations of the measurement points are determined with a micrometer. The times
at which the detonation front reaches these points are determined by using the high
conductivity or pressure at the detonation front to close an electrical switch called a
pin. The switch closure allows a capacitor to discharge, and the associated signal is
recorded on a fast oscilloscope. The detonation velocity can be calculated from the
measured distances and times by using an appropriate numerical procedure.
Sometimes optical records of the detonation trajectory along the stick have been
obtained with a smear camera, but this less precise method is used only in special
circumstances, as for very small diameter sticks in which pins might perturb the
detonation wave significantly.
When a rate stick is detonated initially, there usually are velocity transients for
some distance along its length. Therefore, the data from the first part of the run, a
distance equal to six rate stick diameters, usually are discarded. Detonation
velocities in plastic-bonded explosives pressed to more than 95% of theoretical den-
sity commonly are measured to within 0.1% by these techniques.
Liquid-explosive rate sticks must be contained in rigid cylinders. The way in
which t.his container affects the detonation velocity and the explosive diameter at
which failure occurs is called the confinement effect. When measuring the detona-
tion velocity of a confined explosive, one should make the container walls thick
enough to represent infinitely thick walls, to simplify data interpretation.
Details of these techniques are given in A. W. Campbell and Ray Engelke, Sixth
Symposium (International) on Detonation, San Diego, California, August 1976, Of-
fice of Naval Research Symposium report ACR-221, and in other works cited
therein.
Diameter-Effect Curve
The velocity of a detonation traveling in a cylindrical stick decreases with the
stick diameter until a diameter is reached at which detonation no longer
propagates. That is called the failure diameter. The steady detonation velocity as a
function of the rate stick radius is given by D(R) = D(m)[l - A/(R - R,)], where
D(R) and D( ~0) are the steady detonation velocities at rate stick radius R and at in-
finite radius, respectively. A and R, are fitting parameters. Campbell and Engelke
discuss this fitting form. Table 3.01 lists nonlinear least squares fits of this function
to empirical data. The fits can be used to interpolate the detonation velocity at any
diameter that will allow detonation to propagate.
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