Strength (explosive)
Encyclopedia
In explosive material
s, strength is the parameter determining the ability of the explosive to move the surrounding material. It is related to the total gas yield of the reaction, and the amount of heat produced. Cf. brisance
.
The strength, or potential, of an explosive is the total work
that can be performed by the gas resulting from its explosion, when expanded adiabatically
from its original volume, until its pressure is reduced to atmospheric pressure and its temperature to 15°C. The potential is therefore the total quantity of heat given off at constant volume when expressed in equivalent work units and is a measure of the strength of the explosive.
Explosive strength is measured by, for example, the Trauzl lead block test
.
An explosion may occur under two general conditions: the first, unconfined, as in the open air where the pressure (atmospheric) is constant; the second, confined, as in a closed chamber where the volume is constant. The same amount of heat energy is liberated in each case, but in the unconfined explosion, a certain amount is used as work energy in pushing back the surrounding air, and therefore is lost as heat. In a confined explosion, where the explosive volume is small (such as occurs in the powder chamber of a firearm), practically all the heat of explosion is conserved as useful energy. If the quantity of heat liberated at constant volume under adiabatic conditions is calculated and converted from heat units to equivalent work units, the potential or capacity for work results.
Therefore, if
Then, because of the conversion of energy to work in the constant pressure case,
from which the value of Qmv may be determined. Subsequently, the potential of a mole of an explosive may be calculated. Using this value, the potential for any other weight of explosive may be determined by simple proportion.
Using the principle of the initial and final state, and heat of formation table (resulting from experimental data), the heat released at constant pressure may be readily calculated.
where:
The work energy expended by the gaseous products of detonation is expressed by:
With pressure constant and negligible initial volume, this expression reduces to:
Since heats of formation are calculated for standard atmospheric pressure (101 325 Pa, where 1 Pa = 1 N/m²) and 15°C, V2 is the volume occupied by the product gases under these conditions. At this point
and by applying the appropriate conversion factors, work can be converted to units of kilocalories.
Once the chemical reaction has been balanced, one can calculate the volume of gas produced and the work of expansion. With this completed, the calculations necessary to determine potential may be accomplished.
For TNT:
for 10 mol
Then:
Note: Elements in their natural state (H2, O2, N2, C, etc.) are used as the basis for heat of formation tables and are assigned a value of zero. See table 12-2.
As previously stated, Qmv converted to equivalent work units is the potential of the explosive. (MW = Molecular Weight of Explosive)
For TNT,
Rather than tabulate such large numbers, in the field of explosives, TNT is taken as the standard explosive, and others are assigned strengths relative to that of TNT. The potential of TNT has been calculated above to be 2.72 × 106 J/kg. Relative strength (RS) may be expressed as
Explosive material
An explosive material, also called an explosive, is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure...
s, strength is the parameter determining the ability of the explosive to move the surrounding material. It is related to the total gas yield of the reaction, and the amount of heat produced. Cf. brisance
Brisance
Brisance is the shattering capability of an explosive. It is a measure of the rapidity with which an explosive develops its maximum pressure. The term originates from the French verb "briser", which means to break or shatter...
.
The strength, or potential, of an explosive is the total work
Work (thermodynamics)
In thermodynamics, work performed by a system is the energy transferred to another system that is measured by the external generalized mechanical constraints on the system. As such, thermodynamic work is a generalization of the concept of mechanical work in mechanics. Thermodynamic work encompasses...
that can be performed by the gas resulting from its explosion, when expanded adiabatically
Adiabatic process
In thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which the net heat transfer to or from the working fluid is zero. Such a process can occur if the container of the system has thermally-insulated walls or the process happens in an extremely short time,...
from its original volume, until its pressure is reduced to atmospheric pressure and its temperature to 15°C. The potential is therefore the total quantity of heat given off at constant volume when expressed in equivalent work units and is a measure of the strength of the explosive.
Explosive strength is measured by, for example, the Trauzl lead block test
Trauzl lead block test
The Trauzl lead block test, also called the Trauzl test or just Trauzl, is a test used to measure the strength of explosive materials. It was developed by Isidor Trauzl in 1885....
.
An explosion may occur under two general conditions: the first, unconfined, as in the open air where the pressure (atmospheric) is constant; the second, confined, as in a closed chamber where the volume is constant. The same amount of heat energy is liberated in each case, but in the unconfined explosion, a certain amount is used as work energy in pushing back the surrounding air, and therefore is lost as heat. In a confined explosion, where the explosive volume is small (such as occurs in the powder chamber of a firearm), practically all the heat of explosion is conserved as useful energy. If the quantity of heat liberated at constant volume under adiabatic conditions is calculated and converted from heat units to equivalent work units, the potential or capacity for work results.
Therefore, if
- Qmp represents the total quantity of heat given off by a mole of explosive of 15°C and constant pressure (atmospheric);
- Qmv represents the total heat given off by a mole of explosive at 15°C and constant volume; and
- W represents the work energy expended in pushing back the surrounding air in an unconfined explosion and thus is not available as net theoretical heat;
Then, because of the conversion of energy to work in the constant pressure case,
- Qmv = Qmp + W
from which the value of Qmv may be determined. Subsequently, the potential of a mole of an explosive may be calculated. Using this value, the potential for any other weight of explosive may be determined by simple proportion.
Using the principle of the initial and final state, and heat of formation table (resulting from experimental data), the heat released at constant pressure may be readily calculated.
- m n
- Qmp = viQfi - vkQfk
- 1 1
where:
- Qfi = heat of formation of product i at constant pressure
- Qfk = heat of formation of reactant k at constant pressure
- v = number of moles of each product/reactants (m is the number of products and n the number of reactants)
The work energy expended by the gaseous products of detonation is expressed by:
- W = P dv
With pressure constant and negligible initial volume, this expression reduces to:
- W = P·V2
Since heats of formation are calculated for standard atmospheric pressure (101 325 Pa, where 1 Pa = 1 N/m²) and 15°C, V2 is the volume occupied by the product gases under these conditions. At this point
- W/mol = (101 325 N/m²)(23.63 l/mol)(1 m³/1000 l) = 2394 N·m/mol = 2394 J/mol
and by applying the appropriate conversion factors, work can be converted to units of kilocalories.
- W/mol = 0.572 kcal/mol
Once the chemical reaction has been balanced, one can calculate the volume of gas produced and the work of expansion. With this completed, the calculations necessary to determine potential may be accomplished.
For TNT:
- C6H2(NO2)3CH3 → 6CO + 2.5H2 + 1.5N2 + C
for 10 mol
Then:
- Qmp = 6(26.43) – 16.5 = 142.08 kcal/mol
Note: Elements in their natural state (H2, O2, N2, C, etc.) are used as the basis for heat of formation tables and are assigned a value of zero. See table 12-2.
- Qmv = 142.08 + 0.572(10) = 147.8 kcal/mol
As previously stated, Qmv converted to equivalent work units is the potential of the explosive. (MW = Molecular Weight of Explosive)
- Potential = Qmv kcal/mol × 4185 J/kcal × 103 g/kg × 1 mol/(mol·g)
- Potential = Qmv (4.185 × 106) J/(mol·kg)
For TNT,
- Potential = 147.8 (4.185 × 106)/227.1 = 2.72 × 106 J/kg
Rather than tabulate such large numbers, in the field of explosives, TNT is taken as the standard explosive, and others are assigned strengths relative to that of TNT. The potential of TNT has been calculated above to be 2.72 × 106 J/kg. Relative strength (RS) may be expressed as
- R.S. = Potential of Explosive/(2.72 × 106)