The Static Characteristics And Fatigue Properties Of The Center-cracked Aluminum Plates Bonded With Composite Patches

Composite repair to damaged structures is an advanced technology for recovering performances of the structures, which mainly includes repair design, repair materials, repair processes and properties evaluation of the repaired structures. Bonded composite repair and reinforcement of cracked aluminum structures are focused on in this paper.Firstly, based on the ANSYS software, a reinforced two-dimension finite element model (2D FEM) for the bonded composite repair of cracked aluminum structures (BCRCAS) was developed. The adhesion layer between the composite patch and the cracked aluminum plate was treated as an elastic solid body, instead of a spring element which can only transfer shear loadings. The 2D FEM can also carry out analysis of thermal-mechanical loadings, so the thermal residual stresses involved in the curing process of the bonded structures can be simulated in the analysis. Results obtained by the 2D FEM were verified by results from 3D FEM and experimental data. Effects of types of composite patches, geometric sizes of the BCRCAS and curing temperature of the bonded structures on the properties of repaired structures were then investigated by the 2D FEM. Further studies show that properties of the repaired structures are influenced significantly by combined loading conditions. Results also show that stresses near the crack tip will increase under tensile loading along the crack orientation.Secondly, in order to analyze the fatigue behavior of the BCRCAS, a parametric program based on the ANSYS was developed. For the cracked aluminum structures, the general form of the Paris equation can be expressed as: ddNa = C(?K)m, where, ddNa is the propagation rate of the crack and can be measured at specific fatigue cycles, ?K is an amplitude of the stress intensity factor (SIF) at the crack tip during a fatigue cycle, and can be obtained by FEM, and C and m are constants which relate to the mechanical properties of the materials involved in the cracked structures, loading conditions and testing environment, and can be obtained by fitting the data of dNda and ?K . For BCRCAS, two modified forms of the Paris equation were used to characterize the fatigue properties of the bonded structures. One form is assumed that the Paris equation is valid but has different values of C and m compared to cracked aluminum structures, so it is called material parameters modified method (MMM). Another form is assumed that the Paris equation is still valid but has different values of ?K , and it is called SIF modified method (SMM). Results show that the MMM is more suitable for the cracked aluminum structures bonded with fiber reinforced epoxy composite patches, and the corresponding parameters for carbon fiber reinforced epoxy composite patches are C=6.76×10-10 and m=2.27, respectively, while for glass fiber reinforced epoxy composite patches, C=7.89×10-10 and m=2.33, respectively.Thirdly, a new anodizing treatment method for the adhesion surface of the cracked aluminum structures was developed in this paper. Optimum conditions for the anodizing treatment were obtained, which are aqueous solutions of phosphoric acid of 100g/L, process time of 30min, voltage of 15V and solution temperature of 50℃, respectively. The cracked aluminum structures bonded with unidirectional carbon fiber/epoxy composite patches were concerned. Effects of anodizing treatment on static and fatigue properties of the bonded structures were investigated. Results show that the anodizing treatment produces porous and micro-rough film on the adhesion surface of the aluminum structures. The anodizing treatment promotes the lap-shear strength of the specimen, wich is 104% higher than that of the normal specimen. Static and fatigue properties of the bonded structures are improved after anodizing treatment. The failure strength of the bonded structures reaches up to 418.13MPa which is 93.42% of that of the perfect aluminum structures. Fatigue life of the bonded structures is 1.42 times longer than that of the cracked aluminum structures. Residual strength of the bonded structures after specific fatigue cycles is also improved significantly.Lastly, the fracture mechanism of the cracked aluminum structures bonded with composite patches was investigated experimentally. Generally, there are two main fracture modes during the failure process of the bonded structures under fatigue loadings, which are the crack propagation of the pre-crack and the debonding of the interface between the composite patch and the cracked aluminum plates, respectively. Final fracture of the bonded structures is due to one of them. An experimental technique was developed in this paper to distinguish the fracture modes of the bonded structures under a specific fatigue loading. The experimental technique is based on the crack-opening-displacement (COD) measured by strain gauges and the de-bonding area measured by dyed techniques for the debonding layers. The critical crack length and the critical debonding area, which correspond to the unstable development of the fracture, were then obtained.