The use of explosives for forming metals. In explosive forming, rapid movement of the metal through its elastic range and into the plastic range produces a permanent set, or shape, within microseconds. This high rate of stress application is the essential difference between explosion metal-farming and conventional methods. Initial stress loads using conventional methods are considerably lower.
Understanding of the changes that occur in metal subjected to explosive forces is largely theoretical. However, it is known that it is the cohesive and repulsive forces acting between atoms in a metal crystal which are disturbed and rearranged during explosion forming.
This deformation, termed a slip, occurs along many planes in the crystal grain of the metal. Total deformation of a specimen is the integration of many small displacements along many slip planes. When the grain boundary is encountered, slip interference (braking) takes place, causing atom ad-justment (healing) due to cohesive forces between atoms.
Theoretically, under high stress-strain rates associated with explosion forming, atoms go through a series of slip, brake, and heal events very rapidly. Failure will occur when the healing process or cohesive forces between atoms are unable to cope with the continued process of slipping.
An evaluation of data obtained from explosion forming programs indicates that the capacity of a metal to be formed by explosives is a direct function of one mechanical property: percentage of elongation.
Four factors are critical in determining the success of an explosion forming or welding operation: type, amount, shape, and location of the charge. A charge placed in water, or in some other media (e.g., talc, clay, plastic, or oil) that will transmit the force of the blast, can be 1/10 of the size of a charge used in open air to do the same job.
The explosive is adjusted to provide a force having correct distribution and quantity for a specific forming.
Patents were granted on the explosion forming process to British and German engineers as early as 1900; this process is presently used in the aircraft and aero-
space industry.