GelMan: A Physical Model for Measuring the Response to Blast

K.E. Simmonds and P. Matic
Materials Science and Technology Division
M. Chase and A. Leung
NOVA Research, Inc.

Introduction: On the battlefield, ground forces are at great risk from nonpenetrating blunt trauma forces and blast dynamics that may cause injury to vital organs. The biodynamics describing the mechanisms of these injuries is not well understood. The objective of NRL's research is to develop tissue surrogate materials that simulate the mechanical and acoustical properties of biological tissues. These are then assembled into an experimental test system of the human thorax, called "GelMan," for assessing blunt forces and blast dynamics. With accurate surrogate systems, we can determine the manner in which the blast wave interacts with the thorax and any protective equipment worn, as well as how distinct pressure time histories and strain rates produce loads on tissues. Beyond the obvious military applications, potential civilian applications include understanding automobile crash injuries, nonlethal projectiles used by law enforcement personnel, and the performance of blast-resistant office structures for protection against terrorist attacks.

GelMan Thoracic Model: The NRL Multifunctional Materials Branch has developed, in coordination with Office of Naval Research (ONR) Medical S&T, the GelMan thoracic model that is instrumented for studying the biodynamics of blunt force trauma. The main objective of this research is to develop a surrogate model with more-accurate material properties and geometries than current thoracic models. One focus is to develop materials that duplicate the acoustical and mechanical responses of biological tissues. By using modified ordnance gelatin, tissue simulant formulations have been developed that have the necessary acoustical and mechanical bulk properties for the human lungs and heart. Figure 4(a) shows the stress/strain relationship measured for the surrogate lung material, actual human lung tissue, and ordnance gelatin. Molds of standard lungs and the heart are used to make anatomically correct organs (Fig. 4(b)) used in the GelMan model.

Figure 4 Image
(a) Modified gelatin stress/strain prpoerties designed to match lung tissue properties up to 24% strain, which covers the ragne of interest for lung damage. (b) Instrumented anatomically shaped lung and heart surrogates.

Impact Experiments: Experimental and finite-element analysis was applied to study the dynamic response of GelMan to low velocity impacts. The experiment was designed to relate impactor velocity, size, and associated frequencies to material damage. The experiment measured displacements, wave propagations, and stress distribution by using embedded accelerometers, pressure sensors, and photoelastic techniques. These measurements provided the basis for understanding the GelMan response to subsequent exposures to vibrations, shock tube pressure pulses, and blasts.

A GelMan finite-element model consisting of cylindrical lungs and surrounding tissue was constructed. The computational model predicted surface and internal displacements as well as stress field distributions similar to those found in experimental drop tests using photoelastic visualization techniques. This computational model will be used for developing and understanding more complex thoracic models.

Shaker Table and Shock Tube Experiments: With the use of the Spacecraft Engineering Division's vibration/shaker table at NRL (Fig. 5(a)), tests were performed on a GelMan with constant peak accelerations of 0.25, 0.50, and 1.00 g, and frequencies from 5 Hz to 1 kHz. In the frequency domain, resonance peaks of acceleration and pressures for the critical components of the complex system were measured for the first four modes of vibrations. The GelMan system was also subjected to dynamic pressure pulse conditions using the shock tube facility at the Walter Reed Army Institute of Research.

A steady-state modal analysis of the GelMan (Fig. 5(b)) was performed to analyze the mechanical vibration characteristics of the thorax; this compared well with measured responses. An advantage of this model is that it provides local stress, strain, pressure, and acceleration data that can be used to develop a damage criterion and optimize sensor locations for the thorax model. These additional shock impulse loading experiments will be used to identity the frequencies and modes of vibration that cause injury to the organs.

Figure 5 Image
(a) Thoracic model during vibration mechanical testing. (b) Finite-element modeling of thoracic model on shaker table.

Field Trial Experiments: The NRL GelMan thoracic model has been deployed at various blast test sites to measure free field and confined-space responses to blast. The information from these tests and the shock tube tests will be used to develop transfer functions relating blast waves to the body response.

During one of the free field test series (Fig. 6(a)), GelMan was used to evaluate the effectiveness of body armor (Fig. 6(b)) against blast. Body armor is a soft vest used with and without hard ceramics inserts. These types of body armor systems are designed to provide effective protection against ballistic weapons threat. The free field blast data, in conjunction with shock tube tests (Fig. 6(c)), showed that the soft and combined soft and hard body armor systems do, however, slightly increase the peak lung pressures when subjected to blast. This suggests that future body armor designs should attempt to protect against blast as well as to preserve the ballistic protection capabilities. The GelMan system can be used to guide and evaluate new body armor designs by linking the external ballistic and pressure loads to the internal body dynamics.

Figure 6 Image
(a) Blast detonation at test sight. (b) Thoracic model with body armor at test sight. (c) Instrumented thoracic model undergoing shock impulse from pressurized shock tube.

Summary and Significance: The GelMan surrogate provides valuable information useful for warfighter protection by supplementing medical injury models for blasts and resolving the local biodynamic response of tissue that may lead to injury. In the future, this may lead to the development of better armor to protect soldiers and sailors against blunt trauma and blast injuries.

Acknowledgments: The authors acknowledge the assistance of D. Bowers and D. VanDerLoo (Materials Science and Technology Division); the assistance of
P. Peffers and B. Haynes (NRL Spacecraft Engineering Department); A. Jurrus (Armed Forces Institute of Pathology) and J. Morris (Walter Reed Army Institute of Research).

[Sponsored by ONR, DTRA, and NATICK]