Resonant Inspection of Body Armour Inserts.

Every component has its own resonances or Eigen Frequencies, which depend upon the

dimensions and material properties of the component. For instance, a wine glass, when flicked

with a fingernail, will ring at a specific frequency. If the glass is not cracked and empty, it will ring

with a clear ringing sound whereas were the glass cracked it would ring less sharply, at a different

frequency.

If a different material were used, but the dimensions kept constant, the frequency of the ring

would also change, as would the quality of the sound: a mass produced wine glass made from

recycled glass will not have the same purity of tone as a crystal glass one manufactured from

new material. If the component is part of an assembly the frequency also changes: adding some

wine or water to the glass will change the tone as well as the quality of the resonance.

A body armour plate typically comprises a ceramic plate bonded with adhesive onto a composite

backing plate (Aramid or similar), all of which is enveloped by an outer coat of a ballistic nylon,

which in turn is held in place with yet another adhesive? Whilst all of the elements are important

to the functioning of the body armour, the ceramic plate itself is what carries the most

responsibility for protection from a projectile.

To satisfactorily inspect the ceramic plate, we need to generate and detect resonances in the

ceramic part of the plate. The layers of ballistic nylon and the adhesives used in its application

insulate the mechanical vibrations that are generated; however, we have found that by loading

pointed transducers onto the cover on the ceramic side of the plate we are able to generate

sufficient energy to perform an effective resonant inspection test.

This is not a definitive test that accounts for all risks to the wearer, but is a test with sufficient

integrity to identify those plates in service which pose an elevated risk to the wearer. This test can

be applied by semi-skilled personnel with easily portable equipment “in theatre” on a Go/No-Go

basis. Inspection frequency can be determined locally according to prevalent conditions.