100 16 
(16) Damaging Power of different Pressure Waves. 
For the practical application of the results described in the foregoing sections it 
is necessary to know what it is that determines damage, whether it is simply the 
maximum intensity of the pressure, or the whole time-integral of pressure, or some 
intermediate function such as the energy flux. The best way to investigate this 
question is to determine the relative distances at which big and little charges inflict 
the same damage on a given structure. It has been shown in Section 9 that the 
maximum pressure from a charge is proportional to 
Wi 
—D’ 
W being the weight of explosive and D the distance from the centre of the charge: 
consequently if damage 1s determined simply by the maximum pressure the damaging 
range of a charge will be proportional to W'; the energy flux is proportional to oe 80 
that if damage is determined by energy flux the damaging range will be propor- 
tional to W*: the time-integral of pressure is proportional to 
wi 
Dp’ 
so that if damage is determined by time-integral of pressure the damaging range will 
be proportional to W}. 
During the first half of the experiments a great many observations were made by 
Mr. Robert Royds (on behalf of the Mining School) on the damage sustained by H 4 
mine cases at different distances from various charges ; afterwards these observations 
were extended by the writer to a greater range of charges, and a bottom line was 
used, as described in Section 21, to ensure accuracy of distance. An H 4 mine 
case is a sphere of mild steel 4th inch thick and 31 inches in diameter; it is 
made in two halves. joined by an equatorial weld, and the lower half is fitted with a 
heavy plate carrying the mechanism of the mine. These mine cases, used as damage 
gauges, gave more constant results than had been anticipated ; the central weld was 
the most variable part of the structure, and no great weight was attached to its 
behaviour, The damage varies very rapidly with the distance from the charge; 
there is a critical distance at which moderate damage is inflicted (damage 2 on the 
scale given below) and this distance can be estimated with a fair degree of accuracy, 
because a small change one way or the other gives either heavy damage or no damage 
at all; if 100 represents the distance at which the mine case is just not damaged at 
all, it is only necessary to bring it in to distance 75 or 70 to inflict fairly heavy 
damage (3 or 4 on the scale below). The damage begins with the formation of 
shallow dents, or places where the surface is pushed in and its curvature reversed ; at 
closer range these dents become larger and deeper; finally they overlap each other 
and produce a general crumpling and folding of the mine case, and at this stage the 
metal is often more or less extensively torn ; it should be noted, however, that (apart 
from the weld) the damage never shows any sign of beginning with a crack—when 
the metal is torn it is always as a result of excessive folding. The different degrees 
of damage were measured by the following scale :— 
0. No damage (Fig. 63). 
1. Shallow dents, not more than 1} inches below the original surface. 
2. Large deep dents, say 3 or 4 inches below the original surface. 
3. Very ea overlapping dents, producing crumpling and folding of the 
metal. 
4. Mine case very severely crushed and the metal torn (Fig. 64). 
riments were made with these mine cases at different distances from 40-Ib. 
charges of 40/60 amatol, 300-lb. charges of 40/60 amatol, 1,600-lb. charges of 
30/20 amatol, and 1,900-Ib. charges of 50/50 amatol. The differences in the 
composition of the amatol can be left out of account, for the corresponding differences 
