Void formation in defeated aluminum alloy targets

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Void Formation in Defeated Aluminum Alloy Targets R. L. WOODWARD When a metal target is impacted by a flat-ended cylindrical projectile, target failure frequently occurs by the ejection of a plug of material. In some materials indications of adiabatic shearing parallel to the d i r e c tion of projectile motion are associated with this plug and failure o c c u r s by catastrophic shear along these bands 1 In other situations the separation of the plug from the target can involve plastic shearing followed by a shear fracture which is associated with the growth and coalescence of voids. This latter case is of interest in the present work where it is demonstrated that fracture is initiated by voids formed in association with inclusion particles and that the use of target materials of low inclusion content can r e suit in significantly more plastic shearing at the plug// target interface before shear fracture occurs. A section of 7039 aluminum a r m o r which has failed by plugging is shown in Fig. l(a). In this and all other cases below the projectile was a 25.4 mm long cylinder with a length to diameter ratio of 4. The section A of the micrograph is the target and the plug has separated into two segments, B and C. High magnification metallography showed the cone, B, was s e p a r a ted from the remainder of the plug, C, along a line of intense shear; separation was not complete throughout the plug. The formation of cones of this type by adiabatic shear has been noted before. ~ The s e p a r a tion of the plug from the target occurs firstly by compression of the plug which is accompanied by plastic shearing. This is followed by unstable shear fracture of the plug from the target and is associated with the growth and coalescence of voids. The d i s t r i bution of voids in the target and plug is clear from the unetched section of Fig. l(b). As has bee