Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
CP620, Shock Compression of Condensed Matter — 2001 edited by M. D. Furnish, N. N. Thadhani, and Y. Horie 2002 American Institute of Physics 0-7354-0068-7 EXPERIMENTAL SIMULATIONS OF DYNAMIC STRESS BRIDGING IN PLASTIC BONDED EXPLOSIVES Keith M. Roessig and Joseph C. Foster, Jr. Air Force Research Laboratory/Munitions Directorate 101 W. Eglin Blvd. Ste. 135 EglinAFB, Florida 32542 Abstract. This work investigates the role of the particle/binder interface in the formation of stress bridges within bonded particulate materials. The photoelastic technique is exploited to examine the dynamic stress states within three systems: a binderless particle bed, a particle bed with binder, and a particle bed with a binder bond strength of zero. In a binderless system, stress concentrations form readily due to the fact that the stress must be transferred through specific contact points. The particle bed with binder is shown to have a much more diffuse stress state because shear stresses are transferred at the interface between crystal and binder. In the system with bond strengths of zero, stress concentrations redevelop due to stress transfer only near the contact point between disks. Stress chains are seen to develop in front of the bulk wave in the zero bond strength condition. plasticized binder holds all the particles together. At the mesoscale, there are three distinct components of the particulate plastic bonded explosive: the crystal, the binder, and the crystal/binder interface. The contact mechanics within the mesoscale of this material are such that under a compressive loading, shear stresses will develop. Damage to the material will include debonding of the crystals and binder, crystal fracture and binder tear. The damage accumulation is not uniform, but is concentrated along certain paths. INTRODUCTION Initiation of energetic materials by mechanical loading is important for many safety and performance issues. Of specific interest in this work is the ignition behavior of cure cast plastic bonded explosives (PBXs), particulate materials consisting of explosive crystals in a plastic binder. Heat generation through bulk mechanical shear comes from the yield strength of the material, which is very low in the PBXs of interest (<10 MPa), and not thought high enough to start a reaction. But within single crystals, localization of the shear deformation within the crystals on certain slip planes can lead to an increased heat generation within the particles due to dislocation motion. Stress bridging has been shown to increase stresses in crystals loaded in a dry particle bed by localizing the volume of material loaded to a small percentage of the whole (1,2). A micrograph of a post-test impact test specimen made of a modified PBXN-109 cure cast plastic bonded explosive is shown in Fig. 1. Energetic HMX crystals, substituted for the RDX crystals found in the standard PBXN-109 formulation, are surrounded by small aluminum particles. A 829 FIGURE 1. Damage pattern of a modified PBXN-109 after an unconfined impact test. This work examines the effect of the particle/binder interface on the stress state within a participate material. Of specific interest is the ability of the material to form stress concentrations along specific paths within the material. Three conditions are examined: a binderless system, a particle/binder system with an approximate acoustic wave speed ratio of one in which both compressive and shear stresses can be transferred at the boundary, and a system where shear stress is not allowed to be transferred at the boundary. This last case corresponds to a system with bond strength equal to zero. RESULTS AND DISCUSSION Dynamic fringe patterns for the particle bed without a binder are shown in Fig. 2. In this configuration, the load must be distributed through the disks due to the lack of a binder. This allows for large stress concentrations to develop at the contact points between the disks, shown by the number of fringes occurring at these points. The fringe patterns are symmetric, showing that there is little to no shear being transferred across the contact points (4). There are two distinct paths that support the most load. Other disks have a much lower loading state, while two disks remain completely unloaded. By adding the acrylic binder, the stress state changes dramatically. The fringe patterns in Fig. 3 show a much more diffuse stress state than that of Fig. 2. The general circular wavefront of the fringe patterns is due to the loading conditions described in the experimental methods section and the fact that the binder can support the load. Each disk is loaded in this case, and the stress concentrations do not develop. Towards the front of the fringe wavefront, the contact points are seen to be supporting shear by the antisymmetry of the fringe patterns. Similar fringe shapes are seen in simpler geometries designed to promote shear deformation (3). The fringe patterns are also seen to be propagating at a much faster speed than the case without binder. This is not a change in the acoustic properties of the materials, but rather an increase in the maximum shear at points farther away from the impact at a later time. Photoelasticity is sensitive to shear stresses, and with the disk/binder interface allowing the transfer of shear, fringe patterns form earlier in disks further from the impact. The final case with the acrylic binder and greased interfaces is shown in Fig. 4. While the distributed circular wave pattern forms as in the bonded acrylic binder case, stress concentrations form at the disk contact points as in the binderless case. The fringes are symmetric, indicating a low level of shear stress at the contact points. The stress must be transferred as compressive stress near the contact points as the disks now act as wave guides. The fringe pattern propagation speed has decreased. As shear stress cannot be transferred through the disk/binder interface, one would expect the shear stress states to develop at later times. EXPERIMENTAL METHODS The determination of dynamic stress at the mesoscale within an explosive during an impact event is very difficult, if not impossible, with current testing methods. Therefore, both temporal and spatial scalings must be performed to understand the mechanics at this scale (3). The simulation of explosive crystals in a hard binder took the form of circular PMMA disks 6.4mm thick, 50mm diameter in an acrylic binder with similar acoustic properties to PMMA. This binder material forms a bond to the disks to allow the transfer of both compressive and shear stresses across the disk/binder interface. To examine the case where only compressive stress is transferred across the interface, silicon grease was placed on the edges of the disks to prevent bonding of the binder to the disks during the curing process. Both of the binder systems are compared to a binderless system consisting exclusively of PMMA disks. The loading cell for the tests consists of a 4340 steel frame. Dynamic tests were conducted by placing the load frame into a compression Hopkinson bar apparatus. The loading pulse could then be controlled by the striker bar length and velocity. A 6.4 mm thick loading pin transfers the loading pulse from the Hopkinson bar to a 1018 steel bar that spans the entire specimen impact surface. In this way, planar impact conditions could be simulated. The photoelastic technique was used to generate fringes of constant maximum shear stress within the disks. High speed photographs of the fringe patterns were taken with an Imacon 460 digital camera. 830 FIGURE 2. Stress chains form in particle beds without any binder, Concentrations occur at each contact point between disks. pictures were taken at 200 and 600|j,s after impact. The FIGURE 3. By adding an acrylic binder, the dynamic stress state changes dramaticlaly. There are very few concentrations and the load is distributed across both the binder and the disks. Pictures were taken 45 and 115|as after impact. FIGURE 4. Placing grease interfaces between the disks and binder creates a different stress state than shown in Figure 4 because shear stress is not allowed to pass across the interfaces. While a more distributed stress state develops, stress concentrations at disk contact point appear in the pictures. The pictures were taken 45 and 150|as after impact. 831 The interesting aspect of the second picture in Fig. 4 is the stress bridge that propagates three disks in front of the bulk fringe pattern. Figure 5 schematically shows the stress chain. This behavior has been seen in computer simulations by Bardenhagen and Brackbill (5). In the simulations, the mechanical wave speed ratio of the binder to disk material causes the stress chain. In this experiment, the chain is due to the interface condition of pure compressive stress transfer similar to the binderless case. There are two different mechanisms that both inhibit transfer of shear from one crystal to the next causing these stress chains. In the experiments described in this work, shear stress is prevented from traveling across the boundary. In the computer simulations, shear is not transmitted due to the long transit time of the shear wave within the binder. In either case, shear information is not transferred to interact with the compressive wave at the contact points between disks. The stress chain also uses the binder in this case to propagate. The arrow in Fig. 5 goes through the binder to the last disk. There are no fringes at the contact between the last two disks in the chain, implying no contact load between the two. This behavior is seen in real explosives also. The fracture in the specimen in Fig. 1 many times propagates from crystal to crystal but also propagates along the crystal/binder interface. is seen to be essential in eliminating the stress concentrations within the sample. FIGURE 5. The stress chain formed in the acrylic binder system with greased interfaces extends out from the fringe wave front in the rest of the material. REFERENCES 1. Foster, Jr., J. C., Christopher, F. R., Wilson, L. L., Osborn, J., "Mechanical Ignition Of Combustion In Condensed Phase High Explosives," Shock Compression of Condensed Matter 1997, edited by S.C. Schmidt et al., AIP Conference proceeding 429, pp. 389-392. 2. Roessig, K.M., and Foster, J.C., Jr., "Dynamic Stress Chain Fracture in Particle Beds," in Plastic and Viscoplastic Response of Materials and Metal Forming, edited by A.S. Khan et al., proceedings of Eighth International Symposium on Plasticity and Its Current Applications, July, 2000, pp. 437-439. 3. Roessig, K.M., "Mesoscale Mechanics of Plastic Bonded Explosives," Shock Compression of Condensed Matter 2001. 4. Shukla, A. and Higam, H., Journal of Strain Analysis 20,241-245(1985) 5. Bardenhagen, S.G. and Brackbill, J.U., Journal of Applied Physics 83, 5732-5740 (1998). CONCLUSIONS Dynamic photoelasticity is used to examine the stress states within three systems: a binderless particle bed, a particle bed with binder, and a particle/binder system with a bond strength of zero. In the binderless system, stress concentrations form readily at the specific contact points between the crystals. The particle bed with binder is shown to have a much more diffuse stress state because shear stresses are transferred at the contact points as well as along the rest of the interface. In the system with bond strengths of zero, stress concentrations redevelop. In this case, only purely compressive stress may be transferred anywhere along the interface. Stress chains are seen to develop in front of the bulk wave in the zero bond strength condition. The transfer of shear along the interface 832