Tech Briefs

Ensuring consistent test methods could reduce the risk of head injuries.

As the medical community learns more about brain injury, the importance of blunt impact mitigation becomes more apparent. As such, it is critical to make sure that research labs are not only capable of performing testing in this field, but also show inter-laboratory consistency and reproducibility. This study is a comparison between the two validated blunt impact testing labs (Aberdeen Test Center (ATC) and National Technical Systems (NTS) Chesapeake Testing Services (CTS)), and Natick Soldier Research Development and Engineering Center (NSRDEC).

Figure on the left depicts a typical acceleration response. A first and second impact can be seen on the rear location of a hot conditioned ACH. Most impact locations result in a single, mostly parabolic, response as shown. Variations from impact to impact are seen due to geometry and pad interaction dependent on the impact location. Figure on the right depicts a complete data set of peak accelerations for a single “hot conditioned” ACH in the test method. The column on the left denotes the first impact at each impact location and the column on the right of each impact location denotes the second impact, tested 60 to 120s later. The second impact generally results in a higher peak acceleration.

NSRDEC has acquired new blunt impact test equipment including the Cadex uniaxial monorail drop tower, which has become the unofficial standard for military blunt impact testing. The primary objectives of this study were to validate and verify that NSRDEC's new equipment and personnel are conforming to the current standard operating procedure and to ensure that inter laboratory data are similar.

Each laboratory was provided with 8 Advanced Combat Helmets (ACHs) of each size (small, medium, large and X large) for a total of 32 helmets. This provided each test laboratory with the six required helmets of each size for the test and two contingency helmets of each size.

The purchase description CO/PD 05 04 for the ACH specifies the use of DOT FMVS218 with some exceptions. This test method leaves a lot of room for interpretation and the variation in test results can be seen in historical data. The recently developed Internal Operating Procedure (IOP) from ATC incorporates all requirements from the ACH purchase description, while removing a significant amount of user interpretation from the test method. This effectively improves the reproducibility of the procedure. All tests and collected data for this effort complied with ATC's IOP.

Head form acceleration and drop velocity data were collected for each impact. Velocity immediately preceding impact is collected as a single data point through a laser time gate while acceleration is collected continuously throughout the event. The Cadex data acquisition system collects data at a frequency of 33 kHz and filters the data through the CFC 1000 filter. The CFC 1000 is a 4 pole 1650 Hz low pass Butterworth filter specified for head impact acceleration data by Instrumentation for Impact Test, SAE standard J211-1. Although the entire curve is collected, the interest for this project lies solely in the peak or maximum acceleration the head form experiences during impact. The Test Laboratories followed the official test procedure (ATC-MMTB-IOP-029-Blunt Impact Testing).

Six helmets of each size were prepared prior to testing. The helmets had to be weighed, labeled and the Team Wendy pads had to be placed into their corresponding locations as specified in the procedure. Two of each size helmet had to be placed into a cold (10 ± 3 °C) environmental chamber and two of each size helmet had to be placed into a hot (54.4 ± 3 °C) environmental chamber for at least 12 hours. The last two helmets had to be conditioned at ambient (21 ± 10 °C) for at least 12 hours.

On test day, just prior to the test, the Cadex drop tower was verified by using a calibration check procedure. The Cadex software was programmed for the test plan with correctly identified test sample nomenclature.

The helmets were tested in groups by size. Each helmet was impacted in seven different locations, twice. The helmets impact a hemispherical anvil, apex to apex. The seven locations were impacted in order as follows: crown, front, rear, left side, right side, left nape, and right nape. The second impact occurred between 60 and 120 seconds after the first.

The helmet was fitted onto its corresponding Department of Transportation (DOT) head form incorporating a foam chin. The front straps were tightened halfway and the back straps were tightened until the helmet was snug. The helmet was positioned to what is known as Helmet Position Index (HPI), a measured distance between the brim of the helmet and the first line on the DOT head form. The hot and cold conditioned helmets shall not be left outside their respective environmental chambers for more than 5 minutes. Any helmet that is left out for more than 5 minutes must undergo the full conditioning process again prior to continuing the test.

All impacts were conducted at a velocity of 10 ft/s (3.048 m/s) with a tolerance of ±0.3 ft/s (±0.091 m/s). A laser gate velocity detector was used to record the velocity at every impact and a uniaxial accelerometer (vertically located at the head form's center of gravity) was used to record acceleration during impact. Only the peak acceleration was evaluated.

This work was done by Tony J. Kayhart, Charles A. Hewitt, and Jonathan Cyganik for the Natick Soldier Research, Development and Engineering Center. ARL-0213