Determining Fragment and Debris Hazards

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When the United States Department of Defense Explosives Safety Board (DDESB) determines fragment and debris hazards they use a 6-step process based on Technical Paper 12 (TP-12) 1. In summary, this process finds the range $R$ at which there is a probability $p$ of a person with an area $A_T=0.58\,m^2$ $(6.24\,ft^2)$ being struck by a fragment with a mass $m$ and kinetic energy $E_{CR}=58\,ft{\text -} lb\, \left( 79\,J \right)$. The process is,

  1. Determine initial fragment velocity either using the appropriate Gurney equations (or determine experimentally).

  2. Estimate the average fragment mass $m_0$ and number of fragments $Q_0$ by fitting the Mott distribution to a single-weapon arena test.

  3. Determine the mass $m$ of the lightest hazardous fragment reaching a specified distance $R$ either from the solution of:

    $2E_{cr}=mV^2e^{\left( \frac{-2R}{L_1m^{\frac{1}{3}}}\right)}\qquad(no\,gravity)$

    or from the solution of:

    $2E_{cr}=gL_1m^{\frac{4}{3}}\qquad(terminal\,velocity)$

    whichever gives the smaller value of $m$. The critical level of kinetic energy, $E_{cr}$, at impact is defined as $58\,ft{\text -}lb\, \left( 79\,J \right)$ and $L_1$ is defined as:

    $L_1=\frac{2\left(k^2m\right)^{\frac{1}{2}}}{C_D\rho}$

    where $m$ is the fragment mass, $k$ is a shape factor, and $C_D$ is the coefficient of drag.

  4. Determine the areal (relating to the area of something) density of fragments heavier than $m$ reaching distance $R$ from the inverse-square law and the Mott distribution:

    $q = \left(\frac{Q_0}{R^2}\right)e^{-\left(\frac{2m}{m_0}\right)^{1/2}}$

    where $Q_0$ is the number of fragments emitted from the bomb per unit solid angle,

    and the exponent term is the Mott distribution.

  5. Determine the areal density of hazardous fragments for an $x\%$ chance of being injured, where the proability is given by,

    $p=1-e^{\left(-qA_T\right)}$

  6. The final step is to solve the two critical energy equations, the areal density distribution, and probability of being injured simultaneously.

To complete this process we will need the following parameters,

  1. Initial Fragment Velocity $(V)$
  2. Average Fragment Mass $(m_0)$
  3. Total Number of Fragments $(Q_0)$
  4. Shape Factor $(k)$
  5. Drag Coefficient $(C_D)$
  6. Probability of Injury $(p)$

Some these will need to be calculated or determined experimentally other will be assumed based on conservative safety values.

Let’s start by loading the necessary libraries to conduct the analysis and standardize the plots:

# load libraries and set plot parameters
import numpy as np
import pandas as pd # puts data in in dataframe for analysis
from numpy import exp # exponential
from lmfit import Model # fitting routines
from pathlib import Path # library to go get the data stored in another directory

import matplotlib.pyplot as plt
%matplotlib inline

from IPython.display import set_matplotlib_formats
set_matplotlib_formats('pdf', 'png')
plt.rcParams['savefig.dpi'] = 75

plt.rcParams['figure.autolayout'] = False
plt.rcParams['figure.figsize'] = 1.61803398875*8, 8
plt.rcParams['axes.labelsize'] = 20
plt.rcParams['axes.titlesize'] = 22
plt.rcParams['font.size'] = 18
plt.rcParams['lines.linewidth'] = 2.0
plt.rcParams['lines.markersize'] = 8
plt.rcParams['legend.fontsize'] = 16

plt.rcParams['text.usetex'] = True
plt.rcParams['font.family'] = 'serif'
plt.rcParams['font.weight'] = 'regular'
plt.rcParams['mathtext.fontset'] = 'dejavuserif'
# function to read data using Pandas that allows me to skip header lines as needed.
def read_data(file,skip):
    df = pd.read_csv(file, skiprows = skip, index_col='t')
    return df

Initial Fragment Velocity - Parameter (1)

The initial fragment speed can be determined from high-speed camera footage. In this analysis two cameras were used to collect fragment speed data. Using the Tracker 2 software fragment speed can be calculated using the distance traveled (calibrated from a reference length in the video) and the number of frames the fragment was tracked. Loading this data stored in “comma-separated values” CSV files and plotting a histogram of the velocities measured we can see an outlier in the upper range but with no skewness.

# read some HS video data analysis with speed data
# path to files
root = Path('../hs_video_analysis')
path_to_data_a = root / "20140701_PropaneTank_Phase3_Shot2" / "shot2_fragment_a_velocity.csv"
path_to_data_b = root / "20140701_PropaneTank_Phase3_Shot2" / "shot2_fragment_b_velocity.csv"
path_to_data_c = root / "20140701_PropaneTank_Phase3_Shot2" / "shot2_fragment_c_velocity.csv"
path_to_data_d = root / "20140701_PropaneTank_Phase3_Shot2" / "shot2_fragment_d_velocity.csv"

skip = 1 # skip header lines in data files.
# read function
dfa = read_data(path_to_data_a, skip)
dfb = read_data(path_to_data_b, skip)
dfc = read_data(path_to_data_c, skip)
dfd = read_data(path_to_data_d, skip)

# combine all the speed data into one dataframe
frames = [dfa, dfb, dfc, dfd]
# all speed data in one file
result = pd.concat(frames)
ax = result.sort_values("v").v.plot(kind="hist", histtype='bar', rwidth=0.95, fill=False) #plot a histogram of the speed data.
ax.set_xlabel("Speed (ft/s)");

Histogram of Fragment Velocities

A useful Python function when you need summary statistics is the describe() function. As you can see below it provides a quick overview of velocity data.

result.describe()
v
count122.000000
mean1112.882862
std141.651055
min639.754710
25%1033.742678
50%1131.873717
75%1182.878358
max1602.978995

For parameter (1) the initial velocity is, $n=122$, $95\% CI$ $(1112\pm423)ft/s$.

There are times when it is not possible to conduct an arena test 3 to experimentally determine fragment speeds. In those cases the fragment speed can be determined from a wonderful series of formulas developed by Gurney 4. Gurney’s equation(s) convert the energy in the expanding detonation gases into the kinetic energy of the case fragments. Gurney’s equations assume a linear speed profile and a uniform but time-varying pressure and density in the detonation gases. The Gurney Equation is given by:

where $\sqrt{2E}$ is the experimentally determined gurney-energy, which is specific for an explosive, $M/C$ is the case-metal to charge ratio, and $n$ is a geometry-symmetry-factor for plane $(n=1)$, cylinderical $(n=2)$, spherical $(n=3)$ configurations. The plot below shows the variation of the metal velocity normalized with the gurney energy, $\sqrt{2E}$, as a function of $\left(M/C\right)$ 5. This plot shows that at small $M/C$ (small case weight compared to charge weight) the geometry of the bomb effects the fragment velocity.

MC = np.arange(0.1,10.,0.01)
VVG_plane = 1/(MC+1/3)
VVG_cylinder = 1/(MC+1/2)
VVG_sphere = 1/(MC+3/5)
fig, ax1 = plt.subplots()
ax1.semilogx(MC, VVG_plane, 'k-.', label="Plane")
ax1.semilogx(MC, VVG_cylinder, 'k-', label="Cylinder")
ax1.semilogx(MC, VVG_sphere, 'k--', label="Sphere")
ax1.set_xlabel(r'$(M/C)$')
ax1.grid(True)
ax1.legend()
ax1.set_ylabel(r'$\frac{V_0}{\sqrt{2E}}$');

Normalized Fragment Velocity vs. Charge Shape

The table below from Jacobs shows typical values for the Gurney Energy where the value of $A$ is an arbitrary dimensionless constant replacing $(n/n+1)$ 5.

High Explosive (HE)Best Estimate (ft/sec) A = 0.5Best Estimate (ft/sec) A = 0.3HE Density (g/cc)
HMX908087601.89
PETN899086701.76
RDX894086201.79
TNT (cast)726070101.60
TNT (pressed)72607010-
COMP B (64/36)828079901.71
COMP B (60/40)821079201.70
CYCLONTOL (77/23)955092101.75
CYCLONTOL (75/25)850082001.72
OCTOL (78/22)868083701.82
PENTOLITE (50/50) (cast)810078201.69
PENTOLITE (50/50) (pressed)81007820-
Nitromethane738071201.14
H-6 (RDX/TNT/AL/Wax):(47/31/22/5)838080801.71

Mass Distribution - Parameters (2) and (3)

Fragment mass distributions are typically presented as a cumulative distribution function (CDF) i.e. the number of fragments $N$ heavier than mass $m$. The common analytical solution to this distribution is the Mott distribution 6.

where $M_T$ is the total mass,

$m_0$ is the average mass.

If we fit the Mott Distribution to test fragmentation data we have,

# Read Data
x = pd.read_csv('PROPANE TCV FRAG.csv')

data = x.wt_g

# sort the data:
data_sorted = np.sort(data)

# calculate the proportional values of samples
p = (1 - 1. * np.arange(len(x)) / (len(x) - 1))*100


# fragment data
x = data_sorted
y = p


def mott(x, MT, m0):
    """Mott Distribution"""
    return (MT/m0)*exp(-(2*x/m0)**0.5)


gmodel = Model(mott)
#result = gmodel.fit(y, x=x, MT=sum(x), m0=sum(x)/len(x))
result = gmodel.fit(y, x=x, MT=1000, m0=10)

print(result.fit_report())
#print(result.eval_uncertainty())
[[Model]]
    Model(mott)
[[Fit Statistics]]
    # fitting method   = leastsq
    # function evals   = 16
    # data points      = 104
    # variables        = 2
    chi-square         = 737.358898
    reduced chi-square = 7.22900881
    Akaike info crit   = 207.703120
    Bayesian info crit = 212.991902
[[Variables]]
    MT:  6908.69001 +/- 159.538790 (2.31%) (init = 1000)
    m0:  62.9744260 +/- 1.80888571 (2.87%) (init = 10)
[[Correlations]] (unreported correlations are < 0.100)
    C(MT, m0) =  0.980

For parameter (2) the average fragment mass collected is $63\,\pm\,1.8\,g$ and parameter (3) the total number of fragments is $6908\,\pm\,159$.** We can now plot the data and the fitted Mott distribution,

dely = result.eval_uncertainty(sigma=3)
fig, ax1 = plt.subplots()
ax1.plot(x, y, 'k.', label = "Fragment Data")
ax1.plot(x, result.best_fit, 'k-', label = "Fit of Mott Distribution")
ax1.fill_between(x, result.best_fit-dely, result.best_fit+dely, color = "gray", alpha=0.4, label="3$\sigma$ Uncertainty")
ax1.legend()
ax1.set_xlabel('Fragment Mass (g)')
ax1.set_ylabel('Cumulative Fraction $(m\geq m_{dist})$');

Cumulative Distribution of Fragment Mass

Simultaneously Solve Equations - Parameters (4), (5), and (6)

For parameters (4) and (5) we will be specifying them based on conservative values,

Parameter (6) the probability of being struck by a hazardous fragment will be solved for $p=1\%,\,10\%,\,25\%$, and $50\%$. The code to solve simultaneous equations is,

from scipy.optimize import fsolve
import math
'''
This code solves two sets of nonlinear equations for the
hazardous fragmentation distance (HFD).  The mass of the
hazardous fragment m = x[0] and the range R = x[1] are
solved for using the optimize.fsolve function.  This is
not designed to solve for a munition with multiple
sections.
'''
def equation1(x):
    # solving for m = x[0] and R = x[1]
    # for the non-freefall fragment energy
    # equation
    C_D = 0.8
    rho = 0.076 # lb/ft
    v_0 = 1063 # ft/sec
    g   = 32.2 # ft/sec^2
    A_T = 6.24 # ft^2
    N_T = 104
    m_0 = fragment_mass*0.00220462 # lb
    k = 162.9 # lb/ft^3
    P = 0.01 # probability

    L_1 = (2*k**(2/3))/(C_D*rho)
    ecr1 = 0.5*x[0]*(v_0**2)*math.exp(-(2*x[1])/((L_1)*x[0]**(1/3))) - 58
    q = (N_T/(4*x[1]**2))*math.exp(-(math.sqrt(2*x[0]/m_0)))
    p = 1 - math.exp(-q*A_T) - P # multiple q's for each munition section

    return ecr1, p

def equation2(x):
    # solving for m = x[0] and R = x[1]
    # for the freefall fragment energy
    # equation
    C_D = 0.8
    rho = 0.076 # lb/ft
    v_0 = 1063 # ft/sec
    g   = 32.2 # ft/sec^2
    A_T = 6.24 # ft^2
    N_T = 104
    m_0 = fragment_mass*0.00220462 # lb
    k = 162.9 # lb/ft^3
    P = 0.01 # probability

    L_1 = (2*k**(2/3))/(C_D*rho)
    ecr2 = 0.5*g*((2*k**(2/3))/(C_D*rho))*x[0]**(4/3) - 58
    q = (N_T/(4*x[1]**2))*math.exp(-(math.sqrt(2*x[0]/m_0)))
    p = 1 - math.exp(-q*A_T) - P

    return ecr2, p

x1 = fsolve(equation1, [0.01, 50])
x2 = fsolve(equation2, [0.01, 50])

if x1[0] <= x2[0]:
    m = x1[0]
    R = x1[1]
    C_D = 0.8
    rho = 0.076 # lb/ft
    v_0 = 1063 # ft/sec
    g   = 32.2 # ft/sec^2
    A_T = 6.24 # ft^2
    N_T = 104
    m_0 = fragment_mass*0.00220462 # lb
    k = 162.9 # lb/ft^3
    L_1 = (2*k**(2/3))/(C_D*rho)
    E_cr = 0.5*m*(v_0**2)*math.exp(-(2*R)/((L_1)*m**(1/3)))
    eqn = 1
else:
    m = x2[0]
    R = x2[1]
    C_D = 0.8
    rho = 0.076 # lb/ft
    v_0 = 1063 # ft/sec

    g   = 32.2 # ft/sec^2
    A_T = 6.24 # ft^2
    N_T = 104
    m_0 = fragment_mass*0.00220462 # lb
    k = 162.9 # lb/ft^3
    L_1 = (2*k**(2/3))/(C_D*rho)
    E_cr = 0.5*g*((2*k**(2/3))/(C_D*rho))*m**(4/3)
    eqn = 2

print("1% Probability of Being Struck by HF")
print("HFD = %d ft" % R)
print("mass = %f lb" % m)
print("Energy = %f ft-lb" % E_cr)
print("Equation %d" % eqn)
1% Probability of Being Struck by HF
HFD = 119 ft
mass = 0.001091 lb
Energy = 58.000000 ft-lb
Equation 1

Rerunning the code above for all four probabilities we have the following hazardous fragmentation distances:

Probability of HFDHFD Range (ft)Mass (lb)
1%1190.001091
10%370.000319
25%230.000223
50%140.000177

The Jupyter Notebook file is available here.

References

1. T. A. Zaker, “Fragment and Debris Hazards (TP-12),” Washington, D.C., 1975.

2. D. Brown, “Tracker 5.0” Open Source Physics, 2018.

3. Picatinny Arsenal, “Fragmentation Testing Procedures,” Dover, NJ, 1950.

4. R. W. Gurney, “The Initial Velocities of Fragments from Bombs, Shells, and Grenades,” Aberdeen Proving Ground, MD, 1947.

5. S. J. Jacobs, “The Gurney Formula: Variations on a Theme by Lagrange,” Silver Spring, MD, 1974.

6. N. F. Mott, “Fragmentation of shell cases,” Proc. R. Soc. London. Ser. A. Math. Phys. Sci., vol. 189, no. 1018, p. 300 LP-308, May 1947.

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