Dynamic Fragmentation Experiments Under Plane Strain and Uniaxial Stress Conditions

Abstract

The explosively loaded cylinder is further studied as an experimental method to improve dynamic fracture and fragmentation modeling. Details of the cylinder configuration are investigated to prescribe controlled loading conditions of uniaxial stress and plane strain. Commonly used fracture models, e.g. Johnson-Cook, are calibrated with strain at fracture under such controlled conditions. Earlier works by Goto, et al [1] had used thin-walled tubes to provide plane strain loading and shorter “rings” to establish uniaxial stress conditions. This paper extends on that work to look at alternative cylinder dimensions and metals of interest. A tungsten alloy, Aero 224, and a high strength steel, Eglin Steel (ES-1), are the subject metals. Dynamic, continuum-mechanics based modeling and simulations evaluated whether the stress triaxiality conditions are being met as design parameters of cylinder wall-thickness, explosive type, and initiation configuration. Experiments conducted for this effort, reported in greater detail by Weiderhold [2], provided precise measurement of the cylinder expansion process and fragmentation distributions. An explosively driven metal event is usually considered highly transient and multi-dimensional in stress; however, selective design of the system can result in a controlled experimental configuration. The analysis shows that the ductile ES-1 steel cylinder and rings do establish the desired plane strain and uniaxial stress conditions, respectively, as the cylinder expands to failure. Ultra-high speed photography experiments verify the time of fracture and correlate casewall expansion and velocity measurements. The analysis of the tungsten alloy had verified that if the material was capable of achieving at least a 25% strain to failure then the cylinder and rings would be viable controlled loading paths. However, fragments recovered from the explosively driven rings verified that the strain to failure was less than 14% and the triaxiality condition of uniaxial stress was not achieved by then. The data of this fragmentation under controlled loading conditions are to be used to determine coefficients for fracture-models and serve as benchmarks of relevant, dynamic fragmentation processes for future explosive/metal design opportunities.