Failure mechanisms of impact-damaged sandwich panels under uniaxial compression

Abstract

An experimental program was conducted to study the failure mechanisms of impact-damaged sandwich panels under uniaxial compression. Facesheets in (0190) and (k45) configurations were made from Hercules AW193PWl3501-6 plain weave graphitelepoxy prepreg and placed on a core material of Ciba-Geigy Nomex honeycomb. The panels had impact-type damage, inflicted via static indentation, and also damage to the core only or to the facesheet only which simulated the damage observed in the statically indented specimens. Specimens were tested to failure in compression, the damage propagation monitored, and the failure strengths and modes noted. The results show that the manner in which the damage propagates and the final manifestation of the failure mode is dependent upon the relative magnitudes of the two types of damage (core and facesheet). The key to the damage propagation is the propagation of a dimple in the panel which occurs only if core damage is present. However, final failure is manifested as a fracture of the facesheet and is therefore dependent upon the properties of the facesheet. It is therefore necessary to include both types of damage in experimental work or analytical modelling in order to properly assess the damage tolerance of composite sandwich specimens with impact damage. Composite sandwich construction has been used in a wide range of structural applications because of its superior mechanical properties1-5 such as high stiffness-toweight ratio, good energy absorption capability, provision of multiple load paths, and low production part count. However, sandwich construction increases the level of structural complexity over that in monolithic or discretely stiffened structures due to the presence of the core and the adhesive layer. Due to the ever-increasing mage of composite sandwiches in different engineering disciplines, this construction needs to be critically evaluated within the design philosophy to ensure structural integrity and safety. The damage-tolerant design philosophy has been the foundation for aircraft design since the 1 9 7 0 ~ ~ ~ 3 ~ . Damage tolerance is a measure of the ability of a damaged materiallstructure to perform, * Professor, AIAA Associate Fellow Copyright O 1994 by P.H. Wilson Tsang and Paul A. Lagace. Published by the American Institute for Aeronautics and Astronautics, Inc. with permission. given particular requirements, with damage present. This is in contrast to the property of damage resistance, which is a measure of the ability of the damage incurred by a material or structure due to a particular event such as