Page 262 - Dynamic Loading and Design of Structures

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A compression wave from an explosion in air expands as a three-dimensional blast wave
propagating at maximum velocities well above that of low amplitude sound waves. It reflects
and refracts from solid surfaces and from atmospheric discontinuities.
Explosions also produce high temperatures, which are more locally concentrated than the
pressure and decay rapidly, and also produce high velocity fragments from any confining
structure, which may impact with a surface before the blast wave arrives. The synergistic
effects of blast and fragment impact are not well understood.
If an explosion occurs in contact with a solid it produces stress of the same order of
magnitude as the elastic modulus of the solid. The air pressure produced at close range has an
initial peak, which is orders of magnitude larger than normal atmospheric pressure, but
decreasing with distance travelled. Behind the peak the pressure is still above atmospheric but
decreasing with time and falls below atmospheric. The potential of this underpressure to
produce structural damage is not certain and in part depends on synchronization with the
rebound of the structure.
An impact produces a localized application of pressure on the surface of the structure,
which can only spread into the structure from the point of application. This is in contrast to
explosions where the blast pressures rapidly engulf the entire surface of the structure. The
important parameters of an impact, for diagnostic or forensic purposes, are the shape, velocity
and mass of the impactor, and whether or not the impactor deformed.
When pressure is applied very rapidly to the surface of a structure then strain waves are
generated which transfer the local dynamic surface deformations into overall structural
deformations. An analysis of the transient stress state is necessary when the applied pressure
changes more rapidly than the time taken for the strain waves to travel between the
boundaries of the structure and establish a state of equilibrium between overall structural
resistance and applied pressure effects. During this transition period the transient strain and
stress conditions may produce local failures that are decoupled and of different shape from the
failures that can occur due to overall structural deformation.
The strain waves propagate at characteristic velocities for the material and transfer
momentum into the structure by dynamic displacements of the boundary surface. The rates of
strain and stress that are produced locally in the material are orders of magnitude greater than
those produced in the overall structural deformation, which are again orders of magnitude
greater than under slowly applied loading. Most construction materials have enhanced
properties at these high rates of strain.

6.2 BLAST PHENOMENA

6.2.1 Explosive sources
A detonation wave travels through high explosives at 5,000–10,000 m/s. At a free air
boundary the gaseous products expand at high velocity, pressure and temperature