Experimental Study of the Influence of a Tensile Preload on Thin Woven Composite Laminates Under Impact Loading T. Langella , A. Rogani, P. Navarro, J.F. Ferrero, V. Lopresto, and A. Langella (Submitted September 20, 2018; in revised form December 6, 2018) In this work, the influence of a tensile preload on the impact behavior of thin woven composite laminates is investigated. These kinds of laminates are usually parts of more complex structures, for example, skins of structural sandwich panels or helicopter blades. These structures are generally subjected to external loads, and can also be impacted in operation. The objective of this study is to identify a correspondence between the preload of the structure and the resulting impact damage. This preliminary work is a first step of a more global study that aims at predicting impact damage in structural composites in order to optimize the design process. Experimental tests are carried out on carbon/epoxy and glass/epoxy mixed thin laminates and on sandwich structures. The samples are loaded uniaxially using a specially designed test rig. The effect of the preload on the impact damage size and shape and the peak load is experimentally quantified and discussed. Keywords low-velocity impact, preload, sandwich structure, woven composite 1. Introduction This paper deals with low-velocity impacts on prestressed thin woven composite laminates. The work focuses on the influence of a tensile preload on the impact damage within thin woven composite laminates that can be representatives of the skin of helicopter blades (Ref 1, 2). The presented study is an exploratory work. It aims at observing and quantifying how the impact response is influenced by a preload. It is part of a more global work that concerns the development of predictive models that can help for the design of such complex composite structures (Ref 3, 4). Due to their high strength-to-stiffness ratio, composite materials are widely used. These materials are particularly interesting in the transport industry such as helicopter factories, for the manufacturing of major structural parts. Hence, a minor weakness in the material can have catastrophic consequences. Thus, understanding impact damage mechanisms in this material is required (Ref 1, 2, 5). Many studies have dealt with the behavior of composite structures under impact loading. These investigations, per- formed for different materials and configurations, have been well summarized in comprehensive review papers (Ref 6-8). When impacted, composite laminates present a brittle behavior with extensive matrix cracking, delamination and fiber break- ages. These damages can lead to a loss of stiffness, and eventually to the complete failure of the structure. The impact behavior and resulting damage are influenced by several parameters. The most influencing are the stacking sequence (Ref 9, 10), the geometry (Ref 11, 12), the nature of the constitutive materials (Ref 13-15) and the impactor mass, geometry and velocity (Ref 16-18). The architecture of the reinforcement is also a key parameter (Ref 19). In the specific field of woven laminate composites, the characteristic structure of the plies leads to a complex behavior (Ref 20, 21). Local mechanisms have an important influence on the global response, damage initiation and propagation. Many studies deal with the influence of a tensile preload on impact damage of composite laminates. In the work of Whittingham et al. (Ref 22), who investigated 8 ply quasi- isotropic carbon fiber epoxy resin composite panels [0, + 45, 45, 90]s, the prestress has not significantly affected the peak force of impact, similar to the findings of Mitrevski et al. (Ref 23). For Schoeppner and Abrate (Ref 24), the tension preload has no influence on the peak load of impact force although this preliminary investigation on graphite/epoxy and graphite/PEEK concerning the effects of preload on the delamination threshold level suggested that the delamination threshold level may be independent of the in-plane tensile load. In Ref 25, experimental studies, carried out on unidirec- tional prepreg (T800S/M21) manufactured in an autoclave and on a biaxial Non Crimp Fabric (NCF) with two part infused epoxy resin (NCF/LY3505) manufactured using the VARI process, have shown that tensile preloading does significantly influence impact damage processes and also leads to earlier catastrophic failure of the composite. The authors observed an This article is an invited submission to JMEP selected from presentations at the International Symposium on Dynamic Response and Failure of Composite Materials (Draf2018) held June 12-15, 2018, on the Island of Ischia, Italy, and has been expanded from the original presentation. T. Langella, V. Lopresto, and A. Langella, Department of Chemical, Materials and Production Engineering, University of Naples ‘‘Federico II’’, P.le Tecchio 80, 80125 Naples, Italy; and A. Rogani, P. Navarro, and J.F. Ferrero, Universite ´ de Toulouse, Institut Cle ´ment Ader, UMR CNRS 5312, UPS/INSA/ISAE/Mines Albi, 3 rue Caroline Aigle, 31400 Toulouse, France. Contact e-mail: tania.langella@unina.it. JMEPEG ÓASM International https://doi.org/10.1007/s11665-019-03916-4 1059-9495/$19.00 Journal of Materials Engineering and Performance