One of the critical requirements in soft ballistic testing is measurement of “back side signature” (i.e. energy delivered to tissue by a non-penetrating projectile) in a deformable backing material placed behind the targeted vest. The majority of military and law enforcement standards have settled on an oil/clay mixture for the backing material, known as Roma Plastilena. Although harder and less deformable than human tissue, Roma represents a “worst case” backing material when plastic deformations in the oil/clay are low (less than 20 mm) (Armor placed over a harder surface is more easily penetrated.) The oil/clay mixture of “Roma” is roughly twice the density of human tissue and therefore does not match its specific gravity, however “Roma” is a plastic material that will not recover its shape elastically, which is important for accurately measuring potential trauma through back side signature.
The selection of test backing is significant because in flexible armor, the body tissue of a wearer plays an integral part in absorbing the high energy impact of ballistic and stab events. However the human torso has a very complex mechanical behavior. Away from the rib cage and spine, the soft tissue behavior is soft and compliant. In the tissue over the sternum bone region, the compliance of the torso is significantly lower. This complexity requires very elaborate bio-morphic backing material systems for accurate ballistic and stab armor testing. A number of materials have been used to simulate human tissue in addition to Roma. In all cases, these materials are placed behind the armor during test impacts and are designed to simulate various aspects of human tissue impact behavior.
One important factor in test backing for armor is its hardness. Armor is more easily penetrated in testing when backed by harder materials, and therefore harder materials, such as Roma clay, represent more conservative test methods.
| Backer type | Materials | Elastic/plastic | Test type | Specific gravity | Relative hardness vs gelatin | Application |
|---|---|---|---|---|---|---|
| Roma Plastilina Clay #1 | Oil/Clay mixture | Plastic | Ballistic and Stab | >2 | Moderately hard | Back face signature measurement. Used for most standard testing |
| 10% gelatin | Animal protein gel | Visco-elastic | Ballistic | ~1 (90% water) | Softer than baseline | Good simulant for human tissue, hard to use, expensive. Required for FBI test methods |
| 20% gelatin | Animal protein gel | Visco-elastic | Ballistic | ~1 (80% water) | Baseline | Good simulant for skeletal muscle. Provides dynamic view of event. |
| HOSDB-NIJ Foam | Neoprene foam, EVA foam, sheet rubber | Elastic | Stab | ~1 | Slightly harder than gelatin | Moderate agreement with tissue, easy to use, low in cost. Used in stab testing |
| Silicone gel | Long chain silicone polymer | Visco-elastic | Biomedical | ~1.2 | Similar to gelatin | Biomedical testing for blunt force testing, very good tissue match |
| Pig or Sheep animal testing | Live tissue | Various | Research | ~1 | Real tissue is variable | Very complex, requires ethical review for approval |
Stab
Stab and spike armor standards have been developed using 3 different backing materials. The Draft EU norm calls out Roma clay, The California DOC called out 60% ballistic gelatin and the current standard for NIJ and HOSDB calls out a multi-part foam and rubber backing material.
- Using Roma clay backing, only metallic stab solutions met the 109 joule Calif. DOC ice pick requirement
- Using 10% Gelatin backing, all fabric stab solutions were able to meet the 109 joule Calif. DOC ice pick requirement.
- Most recently the Draft ISO prEN ISO 14876 norm selected Roma as the backing for both ballistics and stab testing.
This history helps explain an important factor in Ballistics and Stab armor testing, backing stiffness affects armor penetration resistance. The energy dissipation of the armor-tissue system is Energy = Force × Displacement when testing on backings that are softer and more deformable the total impact energy is absorbed at lower force. When the force is reduced by a softer more compliant backing the armor is less likely to be penetrated. The use of harder Roma materials in the ISO draft norm makes this the most rigorous of the stab standards in use today.