Predicting failure of bonded patches using a fracture mechanics approach Harald Osnes a,n , Dag McGeorge b , Jan R. Weitzenb ¨ ock b , Geir O. Guthu c a Mechanics Division, Department of Mathematics, University of Oslo, P.O. Box 1053, Blindern, NO-0316 Oslo, Norway b Det Norske Veritas, NO-1363 Høvik, Norway c COWI AS, Kr˚ akerøy, NO-1601 Fredrikstad, Norway article info Available online 31 January 2012 Keywords: Composites Finite element stress analysis Fracture mechanics Lap-shear abstract Adhesively bonded joints have found important application areas in the marine and offshore industry during the last years. One particular application is the use of bonded patches to repair steel structures such as floating production storage and offloading units (FPSOs). Experience has shown that FPSOs develop corrosion and cracks during service. Welding, which is frequently applied when repairing such kinds of defects, is hot work that is not allowed for FPSOs during production. Closing down the production can be very expensive. Thus, composite patch repair using adhesives is an attractive alternative, which means that there is a need to investigate the strength of bonded steel-composite joints. In the present paper, the strength of adhesively bonded lap-shear joints has been studied. Failure loads obtained experimentally have been presented and compared with theoretical predictions. Capacity estimates provided by traditional strength of materials approaches do not agree with experiments. On the other hand, results obtained by a recent inelastic fracture-based analysis represent measured strength values well. Furthermore, finite element analysis using cohesive elements for the adhesive bondline is shown to be a powerful tool in strength predictions of adhesively bonded joints. In addition to provide accurate estimates of the ultimate failure loads, the fracture process can be modelled, and the analysis method is applicable to a wide range of joint geometries. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction Adhesively bonded joints have found important application areas in the marine and offshore industry during the last years. One particular application is the use of bonded patches to repair steel structures such as floating production storage and offloading units (FPSOs) and other kinds of offshore storage units. Ships and offshore installations are being operated in harsh environments, and experience has shown that such structures develop corrosion and cracks during service. That kind of defects is no serious problem for ordinary tankers—the defects are simply welded and possibly reinforced when the ship calls into the next harbour. Welding is, however, hot work that is not allowed for FPSOs and similar offshore installations during production. Closing down the production can be very expensive. In some cases this can be many 100s of thousands of dollars per day. The actual repair of the structure is only a small fraction of the total cost. The main cost factors are loss of revenue due to the fact that the vessel is taken out of service, and loss of production if process plants have to be stopped, tanks and pipes cleaned and ventilated due to danger of explosion because of welding. Hence, there is considerable inter- est in parts of the maritime and offshore industry to develop repair methods that do not involve hot work, and composite patch repair using adhesives is an attractive alternative. Thus, there is a need to investigate the strength of bonded steel- composite joints. For the past 10 years, Det Norske Veritas (DNV) has been conducting research into developing a cold repair method for FPSOs using bonded patches [1,2]. This is a well established method for, e.g., fighter aircraft. However, for ships and floating units no guidelines or rules exist. The research has culminated in a new Recommended Practice (RP) [3] on bonded patches for repair of steel structures. A crucial part of the research activity leading to the RP has been on the strength of adhesively bonded lap-shear joints. The authors of the present paper have been involved in several experimental studies [1,2,4] and theoretical investigations [5–9] on the strength of bonded joints. It has been shown that results obtained by traditional strength of materials approaches do not always agree with experiments [6,8,9]. During the last decade several fracture-based finite element formulations have been developed with the aim of predicting the strength of adhesive joints [10–16]. Various models describing the initiation and propagation of cracks within the cohesive zone have been introduced and implemented as user-defined functionality in a Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/ijadhadh International Journal of Adhesion & Adhesives 0143-7496/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2012.01.015 n Corresponding author. E-mail address: osnes@math.uio.no (H. Osnes). International Journal of Adhesion & Adhesives 37 (2012) 102–111