An Experimental Study on the Tensile Behavior of the Cracked Aluminum Plates Repaired with FML Composite Patches

Repairing of the cracks by fiber metal laminates (FMLs) was first done by some aeronautical laboratories in early 1970s. In this study, experimental investigations were done on the effect of repairing the center-cracked aluminum plates using the FML patches. The repairing processes were conducted to characterize the response of the repaired structures to tensile tests. The composite patches were made of one aluminum layer and two woven glassepoxy composite layers. Three different crack lengths in three crack angles and different patch lay-ups were examined. It was observed for the lengthen cracks, the effect of increasing the crack angle on ultimate tensile load in the structure was increase. It was indicated that the situation of metal layer in the FML patches had an important effect on the tensile response of the tested specimens. It was found when the aluminum layer is farther, the ultimate tensile load has the highest amount. Keywords—Crack, Composite patch repair, Fiber metal laminate (FML), Patch Lay-up, Repair surface, Ultimate load Ι. INTRODUCTION IBER–METAL LAMINATES (FMLS) are hybrid structures based on thin sheets of metal alloy and plies of fiberreinforced polymeric materials. These hybrid material systems combine the excellent specific strength and stiffness, and fatigue properties of composites and the machinability and toughness of metals [1] They were initially developed at the National Aerospace Laboratory in Nether-lands, after fatigue studies on the centre wings of a Fokker F-27 showed that bonded metal laminates presented promising fatigue properties [2]. Subsequent enhancement of their mechanical properties at the Technological Delft University resulted into commercially available FMLs under the trade name of ARALL (aramid fibre/aluminium) and GLARE (glass fibre/aluminium) [2]. A. Pourkamali Anaraki is with the Mechanical Engineering Department, Shahid Rajaee Teacher Training University, Tehran, Iran (phone: +98-02122970025; e-mail: [email protected]). G. H. Payganeh is with the Mechanical Engineering Department, Shahid Rajaee Teacher Training University, Tehran, Iran (phone:+98-021-22970052; e-mail: [email protected]). F. Ashena Ghasemi was with the Mechanical Engineering Department, Shahid Rajaee Teacher Training University, Tehran, Iran (phone: +98-02122970060; e-mail: [email protected]). A. Fallah is with the Mechanical Engineering Department, Shahid Rajaee Teacher Training University, Tehran, Iran (phone: +98-021-22970060; e-mail: [email protected]). Current applications of FMLs include fuselages, leading edges, etc. in the Airbus A380, where weight reduction and improved damage tolerance ability are critical [3]. One of the serious shortcomings of the current-generation FML is that it has a low Young’s modulus because woven glass-fabric has a fairly low tensile modulus. As a result, the modulus of glass/epoxy composite layer is low, nearly 26GPa, which is lower than that of the aluminum layer. Nevertheless, woven glass-fabric used in the current generation of the FML has a high tensile strength and strain-to-failure. The combination of lower modulus glass/epoxy layers and aluminum layer inevitably produce a laminate with a Young’s modulus lower than that of the monolithic aluminum alloy [4]. This may limit the applications of the FML in aircraft structures where the stiffness is a predominant design requirement. The lower modulus of glass/epoxy composite layer also leads to another serious concern, that is, the load carried by the aluminum layers is proportionally higher due to its higher modulus than the composite layers. The presence of high stress leads to a shorter fatigue crack initiation life of the aluminum layer [5–7]. In order to reduce the stress level and to improve the fatigue crack initiation life in the aluminum layer, it is necessary to increase the modulus of the composite layer. The beneficial effect of mingling boron and glass fibers on improving the Young’s modulus and the yield strength in the hybrid boron/glass/aluminum FMLs has been successfully demonstrated in Ref. [8]. In this study, the tensile behavior of the cracked aluminum plates repaired with FML composite patches, were tested by tensile tests. In these tests, three factors are changed, i.e, crack length, crack angle and the lay-up of FML layers. Each factor contains three levels. Fourteen tests with three replicates were done. The results of tests were discussed and the effects of mentioned factors were compared. ΙΙ. SPECIMEN PREPARATION In this section, the method of specimens preparation is explained. The specimens include the cracked aluminum plates and the FML patches. A. Cracked Aluminum Plates In this study, the specimens were made of AL plate AA1035 having dimensions [7] as shown in Fig 1 and Fig 2. The mechanical properties of Al plate were given in table 1. F World Academy of Science, Engineering and Technology 61 2012