Blast energy-attenuation (EA) seats, although not new to the market, have not been fully tested with respect to energy attenuation capability and the resulting effects on occupant protection. The Ground Systems Survivability (GSS) Interiors Seat Team tested and evaluated EA seats over a one-year period using a drop tower test method.

The fleet of ATDs all contained the same instrumentation, which included accelerometers in the head, thorax, and pelvis.
To understand the current blast EA seats on the market and in development, 12 seat models were evaluated on the drop tower located at the TARDEC Occupant Protection Laboratory (OP Lab). Testing blast mitigation seats on a drop tower has been established as a preliminary evaluation of seat assets without introducing the variability or cost associated with a full-scale blast test. A matrix was developed to assess the seats with a simulated blast input with test variables including two severities (200 g or 350 g peak acceleration pulse), three anthropomorphic test devices (ATDs, or crash test dummies), and with or without personal protective equipment (PPE). The seats were tested in their recommended use range. Several of the seats were designed specifically for the lower input velocities. Efforts were made in the matrix development to maximize information gained with a limited number of seat assets.

Each test included an instrumented and ballasted ATD to measure forces, moments, and accelerations imparted to the occupant. The fleet of ATDs all contained the same instrumentation, which included accelerometers in the head, thorax, and pelvis (see figure). Load cells to measure forces and moments were located in the upper neck, lumbar spine, femur, upper tibia, and lower tibia. The data recorded off each transducer was compared to the ARL/SLAD crew injury criteria for accelerative events for the 50th percentile male.

The primary focus of the testing was to evaluate the test methodology developed for EA seat analysis via drop tower; namely, the ability of commercially available or prototype seats to produce occupant injury values below the internal Occupant Centric Platform (OCP) thresholds for all body segments for all size occupants. Each ATD channel was reviewed to determine if the maximum or minimum value exceeded the associated injury assessment reference value (IARV) limit.

A review of the data showed compliance with the OCP IARV limits for some of the seats in the tested configurations, leading to the conclusion that the target loads and accelerations set by OCP were attainable and appropriate. Some of the recorded data from the platform was questionable due to various issues with the accelerometers over the full series, including accelerometer mounting problems due to rough or imprecise mounting surfaces, and cable tiedown issues resulting in damaged or severed cables or cable “whip.”

Throughout the test series and accompanying data analysis, several lessons were learned. Although all ATD data channels were reviewed for exceeding IARVs, an analysis of the ATD trends allowed for the formation of general observations of “go/no-go” channels to review if time is limited. Lumbar compression seems to be the go/no-go injury criteria when evaluating the seat as a survival system.

A review of lower extremity injury values led to the conclusion that some type of flooring system should be included to mitigate lower leg injuries during a blast event, as confirmed by comparing tibia IARVs between seats that featured footrests or blast mats, relative to those without.

Seat manufacturers currently design their systems for optimization during a blast event with an occupant representative of a 50th percentile male, and many seats were tuned for approximately a 200-g peak acceleration event. Consequently, the majority of the seats passed the lumbar compression load for the 50th percentile male at this test condition. A review of the lumbar compression data for the 95th percentile male demonstrates that the additional weight of the occupant and higher IARV thresholds leads to passing numbers for almost all seat models. As expected, the seats were not designed for the lightest occupant, leading to lumbar compression limits over the threshold of the 5th percentile female for 83% of the seats tested.

The purpose of testing with and without PPE was to determine if the additional weight, in the case of the 95th percentile male, would cause a seat to “bottom out,” or if the lack of weight, as in the unencumbered 5th percentile female, was too light to cause the seat to stroke as designed. However, due to the limited data sets, it was difficult to complete comparative analyses between ATDs with and without PPE.

The drop tower testing and evaluation performed on commercial and developmental seats provided an objective assessment of the seats’ performance with respect to the injury criteria. The test methodology and OCP IARV assessment criteria were evaluated and deemed acceptable for future use. Data analysis was performed for a quality check of the data and was used to determine general trends in ATD performance.

This work was done by Katrina Harris, David Clark, and Risa Scherer of the Army TARDEC; Kelly Bosch of Booz Allen Hamilton; and Joseph Melotik of the Naval Air Systems Command. ARL-0178


Aerospace & Defense Technology Magazine

This article first appeared in the February, 2016 issue of Aerospace & Defense Technology Magazine.

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