Tech Briefs

Modeling and simulation were used to analyze the effects of underbody blasts on moving vehicles.

Recently, modeling and simulation (M&S) engineers have made impressive strides in improving ground vehicle reliability and soldier safety. This work involved live-fire testing and evaluation (LFT&E) of the effects of underbody improvised explosive device (IED) blasts on moving ground vehicles. A multi-fidelity, multi-temporal M&S methodology was developed and successfully applied towards reconstruction of theater IED events.

(Left) The red region represents the combinations of charge size and offset that will result in a vehicle flipping over when moving at a certain speed. (Right) The region shaded between the two curves shows the effects of forward velocity. This region represents scenarios where the same charge size and center-ofgravity offset will result in a moving vehicle being flipped over, but not the same vehicle when stationary.
An IED blast event from blast-off to return-to-ground (RTG) lasts for about 500 to 2500 milliseconds (ms) depending on the vehicle, threat, and threat location. Since occupant injuries can happen in both stages of the event, it is imperative to analyze both of them in a multi-temporal fashion. For a successful analysis of such an event, innovative computational modeling is essential in understanding underbody blast effects on a moving vehicle structure and its occupants because it provides in-depth information on the overall physics of the event, with access to tremendous amounts of data and visualization.

Theater IED events involving vehicles moving in a convoy have always sparked considerable concern because the effects of IEDs are seemingly accentuated by the vehicle’s forward velocity, especially as it pertains to vehicle flipovers and rollovers. All LFT&E has been done to date on stationary vehicles, while the majority of the blast events in theater operations occur on moving vehicles. The computational methodology developed in this work will be able to analyze vehicle performance not only during the blastoff phase, but through the entire event (blastoff through RTG).

A two-phased, multi-temporal strategy was developed in which a high-fidelity M&S model was used to simulate the blast-off phase, and a Reduced Order Model (ROM) was utilized to capture the vehicle free flight phase including vehicle flip-overs. In the first phase of the analysis, high-fidelity models including detailed vehicle structures, along with occupants, were used, and the effects of the vehicle’s forward velocity during blast-off were analyzed. This model captured the complex phenomena that occur during this very brief time; namely, the interaction between the charge’s detonation, soil, air, and the vehicle’s underbody. The vehicle structural performance including hull and floor deformations and occupant injury responses were analyzed.

Using the same high-fidelity approach from the first blast-off phase for the longer second phase is a prohibitively expensive and a time-consuming proposition from a computational viewpoint. Therefore, for the second phase of the analysis, a ROM was used to simulate the vehicle free flight until RTG. The innovative manner in which the geometry and the blast loading were modeled to a reduced order to obtain accurate predictions of vehicle global behavior in a timely manner is critical to the success of this methodology.

During this second phase, the vehicle’s flip-over tendencies and the effect of forward velocity on these flip-overs were analyzed in detail. The concept of “flip-over characteristic curves” was introduced — vehicle-specific descriptors of the combinations of the three variables (speed, charge size, and charge offset) that will flip the vehicle over in an underbody blast. For example, on the left side of the figure, the red region conceptually represents the combinations of charge size and offset that will result in a vehicle flipping over when moving at a certain speed. On the right side, the region shaded between the two curves shows the effects of forward velocity. This region represents scenarios where the same charge size and center-of-gravity offset will result in a moving vehicle being flipped over, but not the same vehicle when stationary.

This work was done by Ravi Thyagarajan, Jaisankar Ramalingam, Sanjay Kankanalapalli, and Madanmohan Vunnam of Army TARDEC. ARL-0188