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IntroductionOutstanding properties of fiber reinforced polymer composite materials, lead to a wide application in all the sectors such as aerospace, automotive, marine, sports industries and even in the civil infrastructures, etc. Since the last decades, an enormous use of composite materials in the aerospace sector has caused an increasing need for repair technology of damaged component rather than replacement with a new component [1][2][3][4]. Composite structures in service experience damage that comes from the accidental impact and mechanical or environmental condition [5,6]. The main environmental threats are related to the effect of temperature and moisture absorption, which can affect the strength of composite structures and reduce their service life [4,7,8]. The temperature and moisture levels could vary throughout the day (see Fig. 1), from take off to the landing or vice versa, during seasonal change or geographical difference [9]. This cyclic temperature and moisture could even further deteriorate the structures and then lead to premature failure of the structure. AbstractOver the last two decades, the repair of existing engineering structures using fiber reinforced polymer composites has attracted a great attention by aerospace industry, as it is more economical than replacing new. With an increased use of composite material in aerospace field, it is thus essential to restore the structural integrity by repair of damaged part. Concerns regarding the long term durability of composite repair bonded joints have been a major obstacle for critical component of aerospace structures. This paper reviews the current research on the environmental durability of adhesive bonded repair of composite structures to focus on the durability concerns and suggestion on the research needed in this area. The most important environmental factors (moisture and temperature) are reviewed thoroughly and also combined environmental effect. Finite element methods used to predict the environmental influence on the composite bonded joints are briefly reviewed. Finally, the paper concludes with key findings, opportunities and future research topics in order to develop cost effective, better quality and reliable composite repair bonded joints.

Surface treatment of the adherends prior to adhesive bonding plays an important role in the enhancing of strength and durability of bonded joints. In this work, an investigation on effect of adherend surface roughness on adhesive bond strength was performed. Single strap joints with different adherends (mild steel and aluminium) bonded with an epoxy resin (Araldite® 2015) were tested. The adherend surface was treated by mechanical abrasion process using an emery paper. Contact angle measurement and SEM analysis to understand the wettability and the failure mechanism of the joints were performed. It was found that an optimum surface roughness exists for a maximum bonding strength and the roughness range depends on the adherend material. The joint strength changes are associated not only simply by the increased bonding area, surface texture or mechanical interlocking, but also by the chemical characteristics of the surface and the chemical bond between them.

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