Nanofractography Of Composition B Fracture Surfaces with AFM
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Nanofractography Of Composition B Fracture Surfaces with AFM Y. D. Lanzerotti*, J. Sharma**, R. W. Armstrong***, R. L. McKenney***, and T. R. Krawietz*** *U. S. Army ARDEC, Picatinny Arsenal, NJ 07806-5000 **Naval Surface Warfare Center, Carderock Division, West Bethesda MD 20817-5700 ***AFRL-MNME, 2306 Perimeter Rd., Eglin AFB, FL 34542-5910 ABSTRACT The characteristics of TNT (trinitrotoluene) crystals in the fracture surface of Composition B (a melt-cast mixture of TNT and RDX) have been studied using atomic force microscopy (AFM). The size of TNT crystals has been examined by analyzing the surface structure that is exhibited after mechanical failure of the Composition B. The failure occurs when the material is subjected to high acceleration in an ultracentrifuge and the shear or tensile strength is exceeded. AFM examination of the topography of the Composition B fracture surface reveals fracture across columnar grains of the TNT. The width of the columnar TNT grains ranges in size from ~ 1 µm to ~ 2 µm. Their height ranges in size from ~ 50 nm to ~ 300 nm. Flat TNT columns alternate with TNT columns containing river patterns that identify the direction of crack growth. Steps in the river patterns are a few nanometers in depth. The TNT constitutent fracture surface morphology is shown to occur on such fine scale, beginning from adjacent columnar crystals only 1-2 µm in width, and including river marking step heights of only a few nanometers, that AFM-type resolution is required.
INTRODUCTION Energetic materials are of interest for scientific and practical reasons in the extraction (mining) industry, structure demolition, space propulsion, and ordnance. In such applications the materials can be subjected to high, fluctuating, and/or sustained acceleration. The nature of the fracture process of such materials under high acceleration is of particular interest, especially in ordnance and propulsion applications. For example, explosives in projectiles are subjected to setback forces as high as 50,000 g (g = 980.6 cm/s2) during the gun launch process. These high setback forces can cause fracture and premature ignition of explosives. Fundamental understanding of the behavior of energetic materials subjected to high acceleration is a key to better practical ordnance designs that solve the problems of abnormal propellant burning and premature ignition of explosives during gun launch. An energetic material will experience a pressure gradient during acceleration in the gun, as well as under gloading in a laboratory experiment in an ultracentrifuge. The pressure gradient that is experienced by the explosive during acceleration in the gun and under g-loading in the ultracentrifuge is unique and will produce different kinds of behavior and failure than under other material test conditions. This work is particularly relevant to the future development of insensitive energetic materials to be used in devices with higher acceleration. Because of the general importance of the topic, the mechanical behavior of energetic ma
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