The Role of Clay Backing During Hard Body Armor Plate Testing
The Role of Clay Backing During Hard Body Armor Plate Testing
Some social media channels use ballistic clay in their armor testing videos. As a viewer, I enjoy watching this - but, the reality is that the BFD "measurements" in the clay are most valid in this case when considering the relevance of one shot to the next during the same test. Not really knocking the practice, but folks need to understand that BFD measurements from the lab are performed under strict conditions and will usually vary (sometimes significantly) from field testing outdoors with clay blocks.
Lots of variables have an influence over the accuracy of the clay to include the period of time the clay is conditioned and the temperature of the material (should be constant through the block). Variations in clay temperature will significantly impact the pre and post drop clay calibration testing that is required in NIJ-certified lab testing. What does this mean?
Before a professional ballistics test, a new clay block must be conditioned in an atmospherically isolated area for a minimum of 24 hours. Existing clay blocks must be atmospherically conditioned for a period equal to three times the length of time removed from a conditioning chamber and no less than three hours. But, why all the rules regarding clay backing during ballistic tests?
On a ballistics test report, you will see at least a pre-drop test report like the one below:
But what do these drops mean? It's important to understand that higher clay temperatures increase the viscosity of the clay, making the material less resistant to pressure/energy and resultant backface deformation (BFD). So, you could shoot sample A with clay temperatures at 105F and get much more BFD that the same test performed with a uniform clay block temperature at say, 99F for example.
To standardize the impact of clay temperature and resistance on ballistic test results, the NIJ uses ASTM E 3004-22 Standard Specification for Preparation and Verification of Clay Blocks Used in Ballistic-Resistance Testing of Torso Body Armor1 which are standards that NIJ certified labs follow when conditioning clay blocks for testing. Part of this "calibration" requirement for the clay involves dropping a steel ball (2") with specified weight and size into the clay from around 80" vertically through a guide tube (see Section 5.1.5). The resultant impact craters should be 19mm +/- 2mm (the goal being 19mm). When you see a ballistics test report showing pre-drops in the upper end of the range (21mm) you will see higher BFD readings for the sample that you would see at the lower end of the range (17mm). I have seen BFD fluctuate for the same materials layup at either end of this 17mm-21mm range fluctuate by as much as 8mm. The higher the drop test average in the clay block, the less pressure resistant the material is = higher BFD (given the same sample).
I'd also like to mention (just because I think it's helpful) that just because you don't see physical BFD on the armor plate wear face during a YouTube field video - does NOT mean that there is no or minimal BFD. When we test our armor that incorporates carbon fiber or other wear face stiffeners, there is almost no visible BFD... but it does show up in the ballistics test reports.
The point of this post is not to unfairly criticize valuable social media personalities and their "testing." It's to point out that NIJ-approved ballistics testing is very important prior to purchasing armor. I would also recommend that you insist on seeing the lab reports, even from armor that is (or will be) listed on the NIJ Compliant Product List (CPL). There is a lot of information on those reports - to include the above considerations and overall BFD measurements. You want to buy armor that has a proven track record of performing over a period of time.
Some social media channels use ballistic clay in their armor testing videos. As a viewer, I enjoy watching this - but, the reality is that the BFD "measurements" in the clay are most valid in this case when considering the relevance of one shot to the next during the same test. Not really knocking the practice, but folks need to understand that BFD measurements from the lab are performed under strict conditions and will usually vary (sometimes significantly) from field testing outdoors with clay blocks.
Lots of variables have an influence over the accuracy of the clay to include the period of time the clay is conditioned and the temperature of the material (should be constant through the block). Variations in clay temperature will significantly impact the pre and post drop clay calibration testing that is required in NIJ-certified lab testing. What does this mean?
Before a professional ballistics test, a new clay block must be conditioned in an atmospherically isolated area for a minimum of 24 hours. Existing clay blocks must be atmospherically conditioned for a period equal to three times the length of time removed from a conditioning chamber and no less than three hours. But, why all the rules regarding clay backing during ballistic tests?
On a ballistics test report, you will see at least a pre-drop test report like the one below:
But what do these drops mean? It's important to understand that higher clay temperatures increase the viscosity of the clay, making the material less resistant to pressure/energy and resultant backface deformation (BFD). So, you could shoot sample A with clay temperatures at 105F and get much more BFD that the same test performed with a uniform clay block temperature at say, 99F for example.
To standardize the impact of clay temperature and resistance on ballistic test results, the NIJ uses ASTM E 3004-22 Standard Specification for Preparation and Verification of Clay Blocks Used in Ballistic-Resistance Testing of Torso Body Armor1 which are standards that NIJ certified labs follow when conditioning clay blocks for testing. Part of this "calibration" requirement for the clay involves dropping a steel ball (2") with specified weight and size into the clay from around 80" vertically through a guide tube (see Section 5.1.5). The resultant impact craters should be 19mm +/- 2mm (the goal being 19mm). When you see a ballistics test report showing pre-drops in the upper end of the range (21mm) you will see higher BFD readings for the sample that you would see at the lower end of the range (17mm). I have seen BFD fluctuate for the same materials layup at either end of this 17mm-21mm range fluctuate by as much as 8mm. The higher the drop test average in the clay block, the less pressure resistant the material is = higher BFD (given the same sample).
I'd also like to mention (just because I think it's helpful) that just because you don't see physical BFD on the armor plate wear face during a YouTube field video - does NOT mean that there is no or minimal BFD. When we test our armor that incorporates carbon fiber or other wear face stiffeners, there is almost no visible BFD... but it does show up in the ballistics test reports.
The point of this post is not to unfairly criticize valuable social media personalities and their "testing." It's to point out that NIJ-approved ballistics testing is very important prior to purchasing armor. I would also recommend that you insist on seeing the lab reports, even from armor that is (or will be) listed on the NIJ Compliant Product List (CPL). There is a lot of information on those reports - to include the above considerations and overall BFD measurements. You want to buy armor that has a proven track record of performing over a period of time.