Repeatability and Reproducibility of a Standard Test Method for Measuring Head-Supported Mass Properties

During development of military helmets and HMDs, materiel developers need a valid and repeatable test methodology for measuring their mass properties. DEVCOM SC and USAARL reviewed existing mass properties measurement procedures and identified critical items to incorporate into a unified Army methodology.

August 1, 2023

Army Combat Capabilities Development Command Soldier Center, Natick, MA

Headform and fixture (Izz orientation) mounted to the KSR 330-20 at DEVCOM SC. The FaroArm is positioned next to the mass properties instrument for digital measurements. (Image: Army Combat Capabilities Development Command Soldier Center)

U.S. military service members are provided protective head gear for use in training and operational environments. This headgear is typically in the form of a helmet, at a minimum consisting of a rigid outer shell and an individual fitting and retention system, which is an essential personal protective equipment (PPE) item. Many different helmet configurations are used by the U.S. soldier, depending on their military occupational series, the anticipated threat, and the operational environment.

Helmet designs have evolved to provide protection beyond fragmentation, to include head protection from blunt head impact events and maxillofacial protection. Military aviation helmets also include protective visor systems, hearing protection, and integrated communication systems. The helmet shell also provides an ideal mounting surface for ancillary equipment or advanced tactical systems such as night vision systems, weapon targeting displays, and other visual augmentation systems. As technology continues to advance, the use of helmet mounted devices (HMDs) has become increasingly common to enhance capabilities.

The cumulative effect of adding components to the helmet is increased mass and a change in weight distribution of the total head-borne system (HBS) being supported by the wearer’s neck. These head-borne systems have the potential to increase the risk of cervical spine injury and to decrease wearer performance through accelerated fatigue rates. During development of military helmets and HMDs, materiel developers need a valid and repeatable test methodology for measuring their mass properties.

The U.S. military has been measuring aviation helmet mass properties since the early 1990s. However, all three Services (Army, Navy, and Air Force) developed unique measurement methodologies and reference coordinate systems which were closely aligned with and supported their respective recommendations or requirement thresholds for mass and center of mass (CM) (Albery 1997, Deavers 1992, Loverro 2012, Thornton 1992). A fundamental difference between the three services lies in the coordinate system definition. Other differences include headforms, test fixtures and procedures for measuring the CM and moment of inertia (MOI) with mass property instruments.

As a result of these differences, test results generated by one service are not accurately translatable against the requirements of another service. Combat Capabilities Development Command Soldier Center (DEVCOM SC) and the U.S. Army Aeromedical Research Laboratory (USAARL) reviewed the Army, Navy and Air Force mass properties measurement procedures and identified critical items to incorporate into a unified Army methodology.

This included:

Utilization of a mass properties instrument (MPI) capable of measuring both CM and MOI
A coordinate system consistent with the mass properties instrument
Headforms with defined anatomical markings to allow for consistent monitoring of fixture and test article alignment
A fixture used to mount the headforms to the MPI, which reduces operator error
Minimization of headform and fixture weights to improve fidelity of mass property measurements relative to the test article 2
Systematically account for the contribution to head-borne mass properties from cables that extend from the helmet system

Once the measurement tools, equipment items and method were finalized, a gage repeatability and reproducibility study was performed to assess the validity of the system and procedures.

This work was performed by Marina Carboni, B. Joseph McEntire, and Nathan Flath for the Army Combat Capabilities Development Command Soldier Center. For more information, download the Technical Support Package (free white paper) below. TSP-08231