Crack growth analysis of a composite/adhesive interface toughened by in-mold surface preparation Takuya Suzuki a , Ryosuke Matsuzaki b,n , Akira Todoroki a , Yoshihiro Mizutani a a Department of Mechanical Sciences and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8552, Japan b Department of Mechanical Engineering, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan article info Article history: Accepted 2 January 2013 Available online 11 January 2013 Keywords: B. Interface B. Composites B. Surface modification C. Fracture mechanics Nanoimprint lithography abstract The present study performs in-mold surface preparation by imprinting microstructures to achieve high toughness of adhesive bonding during the curing of composites. Since the microstructures are fabricated on composites during curing, this technique saves the time required for and costs involved in conventional surface preparations such as sand blasting and chemical etching. To predict the effect of the microstructures on the composite/adhesive interface toughness, crack growth analysis of the interface with microstructures was performed using the finite element method and a cohesive zone model. The effects of the microstructure size and shape were investigated. It was found that the toughness is characterized by the microstructure shape, whereas the effect of the microstructure size is insignificant. From these results, a simplified formula was constructed to predict the toughness. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction Adhesive bonding is widely applied to joints of composite materials as its use leads to weight reduction and less stress concentration and prevents galvanic corrosion of the structure compared with the use of mechanical joints [1–3]. To obtain high adhesive bonding strength, appropriate surface preparation is required. For example, sanding using emery papers, sand blasting, chemical etching or plasma etching is employed for composites. Sanding or sand blasting mechanically removes weak surface layers and thus increases the adhesive area and exposes the inner surface with high surface free energy [4]. On the other hand, when applying plasma etching, the surface is chemically modified by a functional group with high adhesive force [4]. These surface preparations, however, add to manufacturing processes of materials since they are performed after curing. Dust discharged during sanding or chemicals used in plasma etching may worsen the working environment. It is also difficult to apply these techniques to a large adhesive area in the case of structural materials such as those used for aircraft; uniform surface rough- ness of large structures is hardly achievable by manual sand blasting and a vacuum chamber is usually required for plasma etching, which limits the structure size. Moreover, productivity is currently a critical issue when considering using composite materials in automobile industries, and surface preparation methods that are more efficient are highly sought after. To solve these problems, the authors [5,6] proposed a surface preparation during the forming of composites using nanoimprint lithography (NIL) [7–9]; microstructures fabricated on a forming mold are pressed on a low-viscosity matrix during curing and the patterns are transferred by demolding at low temperature. By preparing a roughened surface during this process, the effect is expected to be the same as that for sand blasting [10–12]. Performing this ‘‘in-mold surface preparation’’ allows us to save time and costs compared with employing conventional methods such as sand blasting or plasma etching. We have experimentally shown the effect of microstructures on the tensile strength of butt joints [5] and the apparent mode I fracture toughness of a double-cantilever beam (DCB) speci- men [6]. The tensile strength of butt joints was found to be 52% higher than that of untreated joints. The apparent mode I fracture toughness increased with an increase in the aspect ratio of microstructures. The effects of microstructures of the surface on the adhesive strength were also investigated by other researchers such as Refs. [13–15]. Critchlow and Brewis [13] introduced grid blasting on the aluminum joint surface and concluded that the rougher surface did not improve the durability of joints. Harris and Beevers [14] studied the effects of grid blasting with different alumina grits on the surface property of adhesion. They concluded that a rougher surface improves the durability of steel joints, which is attributed to the change in chemical condition of the surface as well as the physical change. Shahid and Hashim [15] revealed that the roughness introduced by grid blasting improves Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/ijadhadh International Journal of Adhesion & Adhesives 0143-7496/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijadhadh.2013.01.008 n Corresponding author. Tel./fax: þ81 4 7124 1501. E-mail address: rmatsuza@rs.tus.ac.jp (R. Matsuzaki). International Journal of Adhesion & Adhesives 42 (2013) 36–43