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This paper is devoted to determination of damage mechanisms in isotropic and composite materials subjected to particle or projectile impact. The approach consists of three steps: (1) determination of time-dependent surface pressure distribution under an impacting particle, (2) determination of internal stresses in the target caused by the surface pressure, and (3) determination of failure modes in the target caused by the internal stresses. The pressure distribution under an impacting particle is obtained by analytically combining the dynamic solution to the problem of impact of bodies with the static solution for the pressure between two bodies in contact. Having the time-dependent surface pressure, available static analyses are used to obtain the time-dependent triaxial stresses in the targets made of isotropic materials, whereas, for targets made of composite materials the internal stresses are determined using finite-element computer solution. The internal stresses in targets made of isotropic and composite materials are expressed in terms of target and projectile properties, impact velocity, and impactor shape, and are used to determine the threshold velocities to initiate target damage. Types of target failures considered include tensile cracking, compression/crushing, and subsurface shear failure. Curves are presented that show the relationship between target properties (strength and modulus) and threshold velocity to initiate damage. The results are applied, in an approximate manner, to composites to establish their relative impact resistance. Types of composites considered include: fiberglass, boron-epoxy, and graphite-epoxy. Predictions are made of the threshold velocities to initiate tensile, compressive, and shear failures in the three types of composites and of the order of occurrence of these failure modes.
impact, composite materials, boron, fiberglass reinforced plastics, particles, graphite, failure, tension, compression, shear properties
Associate staff engineer, McDonnell Douglas Astronautics Company, Huntington Beach, Calif.