A History of Research and a Review of Recent Developments

References 69
in Los Angeles Harbour, USA, at Christmas 1976, and in the Norwegian
tanker King Haakon VII.
3.5 THERMAL FLASH
A final characteristic of explosions must be examined before we end this brief
survey.
Structural damage of a quite different nature can result from the extremely
high temperatures created by the fireball of a nuclear explosion. The maximum
fireball diameter for a 1 megaton bomb yield is about 1.4 miles, and for a 10
megaton weapon it would be 3.4 miles. The thermal flash from bombs of this
magnitude can cause changes to properties of materials, and can weaken the
resistance of metal structures that contain material of relatively low melting
point. It might well be that an aluminium alloy structure would be robust enough
to withstand the shock pressures and blast winds from a large nuclear explosion,
but be seriously weakened because of a reduction in proof strength of the material.
For a low yield nuclear explosion, heat pulses at a range of 1000ft from
ground zero are of the order of 50 to 150 cals/cm 2 . A heat pulse of 1000 cals/
cm 2 is about equivalent to a maximum surface temperature in structural
aluminium of 250° C, and for aluminium plating between 0 and 0.5 inches
thick, this will lead to a reduction in strength by annealing. If a heat pulse of
100 cal/ cm 2 is applied to the front face of a slab inch thick, the front and back
faces will have both reached 250° C after 2 seconds. However, if the slab is
inch thick, it has been shown by Parkes [3.20] that the behaviour is different.
The front face, after reaching a maximum temperature of 200°C, falls gradually
in temperature to reach a final value of 120°C after 2.5 seconds. Meanwhile the
back face rises more gradually in temperature to reach the same final value.
By analysing structural members as a number of laminations, each with
varying strain and properties, Parkes showed that it is possible to estimate the
residual stress distribution after a period of uneven cooling. A high residual
tensile stress means that a subsequent tensile loading of the member will cause
failure at a relatively lower applied stress. Care should therefore be taken to
inspect apparently undamaged aluminium alloy structures after a nuclear
explosion to check for minor deflection and distortions that signify a noticeable
reduction in structural strength.
3.6 REFERENCES
3.1 Fletcher, R.F. (1968) Characteristics of liquid propellant explosions, Prevention
and Protection against Accidental Explosion of Munitions, Fuels and Other
Hazardous Mixtures, New York Academy of Sciences, 432.
3.2 High, R.W. (1968) The saturn fireball, Prevention and Protection against
Accidental Explosion of Munitions, Fuels and Other Hazardous Mixtures, New
York Academy of Sciences, 441.