FACTA UNIVERSITATIS Series: Mechanical Engineering Vol. 10, N o 2, 2012, pp. 91 - 104 EQUIVALENT STIFFNESS AS MEASURE OF LOW VELOCITY IMPACT DAMAGE OF COMPLEX COMPOSITE STRUCTURES  UDC 669.1; 620.175 Giangiacomo Minak 1 , Snežana Ćirić Kostić 2 , Zlatan Šoškić 2 , Nenad Radović 1 1 Alma Mater Studiorum University of Bologna, Department for Industrial Engineering, Italy 2 University of Kragujevac, Faculty of Mechanical and Civil Engineering in Kraljevo, Serbia E-mail: soskic.z@mfkv.kg.ac.rs Abstract. This paper presents an analysis of the application potentials implied in the equivalent stiffness changes of complex mechanical structures as a measure of damage caused by low-velocity impacts. In a series of drop-weight tests of a commercial composite component, the impact force time variation is measured, and by calculating the respective displacement of the impacted structure, the stiffness of the structure in impacted points before and after the impacts is calculated. The determined stiffness change serves as the basis for defining a measure of damage extent that is compared to the damage extent measures based on characteristic values of the measured force. It is shown that the measure of damage based on change of stiffness has correlation with the measure of damage based on reduction of loading capacity of the structure, but it does not have correlation to the measure of damage based on ratio of maximal impact force and force needed for initial material failure. Key Words: Composite Structures, Low Velocity Impacts, Damage 1. INTRODUCTION Composite materials are widely used in aerospace and automotive sectors both for their material properties (low density, high stiffness and strength, resistance to chemical and environmental agents), structural design potentials (ability to design material with de- sired anisotropic mechanical properties) and manufacturing advantages (energy saving due to low production temperatures and pressures, complicated shaped components can be molded in one process rather than being assembled from components), successfully re- placing not only steel, but also light alloys in mechanical structures. Received February 21, 2012  Acknowledgements. Authors wish to acknowledge their gratitude to the "Magneti Marelli" company for experimental samples and to Dr Daniele Ghelli and Dr Ana Pavlović from DIEM department of University of Bologna for their help in performing the experimental impact tests. Z. Šoškić and S. Ćirić Kostić wish also to acknowledge the support of the Ministry of Science of Republic of Serbia through research projects No.37020 and No.35006