Stochastic Consideration of Cased Munitions for Casing Breakup and Airblast

OBJECTIVE: To develop an analytic method to predict fragment characterization and the resulting airblast propagation from the detonation of a cased explosive. Current procedures are heuristic, based on empirical analysis of test data and simplified analytic expressions. While these work satisfactorily for traditional munitions that detonate in conditions similar to the testing arenas, there is considerable uncertainty for new types of munitions and/or munitions that are partially embedded at detonation. The intent of this effort is to provide an innovative physics-based alternative, using explosive, casing, and environmental properties to produce stochastic fragmentation and airblast load. DESCRIPTION: Casing breakup fragmentation is a phenomenon that is not well analyzed by deterministic approaches such as conventional finite element method because of their randomly distributed characteristics. As a result, a number of heuristic methods based on observations have arisen for characterizing the fragments produced by the casing"s breakup. These methods need to be revisited and upgraded because present day problems related to the detonation of embedded munitions, the increasing use of small precise munitions, and the practice of more precise targeting has given rise to the need for better tools to characterize casing fragments and fragment effects. Since the characterization of fragments and their impact on the behavior of structural components struck by them is an important aspect of characterizing target response, it is important that the present stochastic methodologies (e.g., the Mott equation) and other simplified equations associated with fragment loadings be revised or replaced to reflect the present day problems. Equally important and apparently steeped in even more uncertainty are the methods used to develop the airblast resulting from the detonation of cased munitions. The present reliance on the uncertainties in using the Fano equation, especially for smaller and embedded munitions, needs to be addressed. The present methodology for the generation of fragment and airblast loadings to assess the response of structural components struck relies on relatively simplistic equations to calculate the fragment masses, their distribution, and other properties. Similarly, the Fano equation is generally used to determine the amount of explosive available for airblast generation. In both situations, improvements are warranted. The current assumption for characterizing casing fragments and their application as loading of a structural component is that the fragments are ejected at monotonically increasing angles from the warhead axis as the point of ejection moves from the nose end to the tail end of the warhead, such that no fragments cross paths and the distribution of fragments by polar zone matches JTCG/ME-approved fragmentation data for each warhead. Fragment properties (number of fragments/area, mass distribution, speed, and the time of arrival) are calculated at each gage point, where fragment loads are computed. PHASE I: Develop methodologies for the required functionality to characterize the loading imparted to structural components both from airblast and casing fragments for a broad range of munition detonation scenarios. Review different methodologies and data for different detonation scenarios, especially as they pertain to small and embedded munitions. Identify the most feasible and applicable methods in the Phase I research for possible implementation in a Phase II effort. PHASE II: Develop methods and tools for the simulation of the casing breakup and airblast generated resulting from the detonation of a cased explosive. Develop methods for characterizing the casing fragments in the form of loadings suitable to perform high-fidelity physics-based (HFPB) analyses to compute the structural response of components impacted by fragment, including components in which the munition is embedded. Gather data suitable for evaluating the characterization methods. Evaluate the capability of each aspect of the methods by comparisons to actual data. Use these comparisons to characterize the uncertainties associated with using these methods. PHASE III/DUAL USE APPLICATIONS: Transition HFPB capabilities developed in Phase II into engineering level fast running models (FRMs). Provide these models in a stand-alone software module and other forms for use by those interested in characterizing weapons effects in DoD and commercial entities of force protection and weaponeering/targeteering areas. Commercial applications would assist in integrating these FRMs, including inherent uncertainties associated with the methodologies, into fast running planning and assessment tools such as Integrated Munitions Effects Assessment (IMEA), Vulnerability Assessment and Protection Option (VAPO), and Modular Effectiveness and Vulnerability Assessment (MEVA). Capabilities for commercial computer codes would be enhanced by adopting these applications. REFERENCES: 1."Structures to Resist the Effects of Accidental Explosions,"Departments of the Army, the Navy, and the Air Force, Army TM 5-1300, Navy NAVFAC P-397, Air Force AFR 88-22, Washington, D.C., November 1991. 2."A Manual for the Prediction of Blast and Fragment Loadings on Structures,"Department of Energy, DOE/TIC-11268, August 1981. 3. Smith, P. D. and J. G. Hetherington,"Blast and Ballistic Loading of Structures,"1994. 4. Dallriva, F. and J. L. Davis,"Data Report for Multiple-Fragment Impact Experiments on Reinforced Concrete Slabs,"ERDC/GSL TR-02-7, U.S. Army Corps of Engineers, Engineer Research and Development Center, Vicksburg, MS, June 2002. 5. Joint Munitions Effectiveness Manual (JMEM), Air-to-Surface, Test Procedures for High Explosive Munitions, JMEM, 1975.