American Institute of Aeronautics and Astronautics 1 Fiber Bridging Strategies for Damage Tolerance Prasad Potluri 1* , Mubeen Arshad 2 , Payam Jamshidi 3 , Paul Hogg 4 North West Composites Centre, School of Materials, University of Manchester, Manchester M60 1QD, UK *corresponding author: Prasad.Potluri@manchester.ac.uk Amongst various toughening schemes, inter-laminar fiber bridging is considered to be the most effective method in improving the damage tolerance. This paper reviews various manufacturing techniques for creating fiber-bridging with a view to understand the influence of fiber architecture on the mechanical properties. Laminates with four different 3D fiber architectures have been subjected to various levels of impact followed by compression after impact tests. Compression after impact results have been analyzed after normalizing the data to 50% fiber volume fraction. I. Introduction amage resistance and damage tolerance have become important in primary airframe structures as well as in non-aerospace structures. Damage resistance is investigated by impacting test panels with predetermined impact energy levels and measuring the resulting damage using non-destructive or destructive methods. Damage tolerance, which is the ability of a structure to retain its load carrying capacity, is evaluated using in-plane tensile, compressive and shear tests [1]. Of these tests, compression after impact (CAI) is the most important test method. The need for damage tolerant composites transcends all parts of the composites industry and is common to structures made from glass fiber and carbon fiber, for aerospace to marine and wind turbine applications. Perhaps the most high profile and historically most critical applications have been carbon fiber composites in primary aerospace structures. Here the problem of damage is compounded by the fact that damage is commonly sub-surface, undetectable by eye, but can have a considerable detrimental effect on key laminate properties such as compression after impact [2]. The importance of damage tolerance in the aerospace sector is increasing given the trend to manufacture more primary civil aircraft structures from CFRP, including the fuselage which is susceptible to significant airfield abuse (baggage handling, vehicles etc) and from large hail stones. However damage tolerance in general is also of prime concern to the wind turbine industry as it is simply not economic to replace damage blades in service, especially if the turbines are deployed off-shore [3]. The industry has made considerable progress for a thirty year time period in improving the damage tolerance of all classes of composites. This is exemplified by the improvements in carbon fiber composites intended for service as primary structure in aerospace where the damage tolerance of the composite laminate is usually quantified by measuring the compression after impact strength according to some recognized standard [4]. Low energy impact is a significant in-service problem which results in difficult to detect damage, primarily consisting of delaminations, which can seriously reduce the compression strength of a composite laminate. The compression after impact properties have increased when measured using the Boeing standard test method from a level of about 170 MPa to almost 400 MPa in this period (figure 1). The improvements have been largely the result of increases in resin toughness leading to inter-laminar shear strength. The developments have included the introduction of formulated epoxies, followed by rubber toughened systems, thermoplastic toughened systems, through to thermoplastic, phase inverted thermoplastic-epoxies, monolithic thermoplastics and interleaf toughened epoxies. It is noticeable however that no significant improvements in damage tolerance have been developed over the last ten years, possibly as no specific new targets have been set by the industry. Boeing had stipulated a challenging CAI value of 350 MPa for selection of prepreg systems for use on the Boeing 777 empennage which was achieved. Most development work undertaken recently by industry has focused on developing composite systems (materials and preforms) that can be used in alternative out-of-autoclave processes to deliver equivalent damage tolerance to conventional autoclave processed materials. It might be concluded that the process of achieving increased toughness via improvements in resin toughening have reached the end of its life and that there is little room for further development. If this is indeed the case then alternative strategies for developing an enhanced 1 Reader in Textile Composites, AIAA Member 2 Graduate Student 3 Research Associate 4 Professor of Composite Materials, School of Materials, University of Manchester D 52nd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference<BR> 19th 4 - 7 April 2011, Denver, Colorado AIAA 2011-1983 Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.