Progressive Damage Analysis On The Design Of Repairs To Aircraft Composite Structure

The weight ratio of advanced composite structure becomes larger and larger in modern aircraft design. In the currently developing civil aircraft and other aircrafts in China, a lot of composite is also used in the aircraft structures. According to the heavy repair workload of the mass of metal aircraft structure at present, it can be foreseen that the huge and hard repair work of composite aircraft struc-tures in future. The technology of composite structure repair and analysis has become one of the im-portant study aspects of composite material technology. This paper mainly studied the analysis me-thod of the typical aircraft composite structure repair.Firstly, a three-dimensional (3D) finite element model (FEM) of composite laminates progressive damage analysis (PDA) has been established based on continuum damage mechanism (CDM). It is assumed that the internal damage of composite lamina is orthotropic. The internal damage status va-riable, which is in the format of the second order symmetry tensor, presents the status of composite damage in the three directions of material coordinates, and controls the degradation of material stiff-ness matrix at integration point. The evolution of the internal damage status variable is controlled by the critical fracture strain energy release rate of composite material, which is one of the composite material properties. Therefore, the presented FEM in this paper has an advantage of less dependence on the experience and experiments, comparing to the traditional PDA with the constant factors of ma-terial elastic stiffness degradation. The damage initiation criterion in the presented PDA model adopts 3D HASHIN criterion, which has been used widely in the field of engineering. The damage evolution criterion adopts the second order energy criterion. The proposed composite lamina constitutive equa-tion, damage initiation criterion and damage evolution criterion have been implemented in the finite element analysis (FEA) process, through coding the user material subroutine (UMAT) of the finite element analysis software ABAQUS. Because of the the damaged area of laminates usually machined into circle shape in the preparation process of repair, the laminates with central hole in different di-ameter and layup are analyzed under single axis tension and compression loads respectively. The re-sults of FEA have a good agreement with experiments. It validates the proposed 3D FEM.Secondly, the paper implemented two FEMs of adhesive layer based on continuum damage me-chanics (CDM) and cohesive zone model (CZM) respectively. The FEM based on CDM assumes that the damage in the adhesive layer is isotropic and is presented by the internal damage status variable in the format of scalar. The damage initiation criterion adopts second order strength criterion in strain. The internal damage status variable is controlled by the critical fracture strain energy release rate. In the FEM based on CZM, adhesive layer is simplified as a virtual interface layer. The damage initia-tion criterion adopts the second order strength criterion in the format of relative displacement of two surfaces. The damage evoluation criterion adopts second order energy criterion. The adhesively bonding single lap joints (SLJ) in different configuration are analyzed with both presented FEMs. Comparing the calculation results with experiments, the FEM based on CDM is more suitable to the analysis of the composite adhesive joint failure in medium scale model established in this paper.Thirdly, the 3D FEM is proposed for the PDA of adhesively bonding patch repaired composite laminates in this paper. The modeling of laminates and adhesive layer are based on the assumption of orthotropic damage and isotropic damage in the scope of CDM respectively. The interaction of the damage initiation and evolution of all the laminates, patch and adhesive layer are considered simul-tantly during the entire analysis process. The paper analyzed the lamintes with bonding patch repaired in different configuration under single axis tension and compression loads respectively. The good agreement between analysis results and experiments shows the validity of the proposed FEM in this paper. The repair parameters were also evaluated on the repair strength with the proposed FEM. Addi-tionally, the calculation condition and cost of 3D FEM of composite bonding patch repair is much higher than the adhesively bonding SLJ configuration. Therefore, it is studied preliminary that the engineering method to estimate the strength of the adhesively bonding patch repaired laminates with the strength of standard adhesively bonding SLJ for the purpose of convenience of engineers.Finally, the paper presented the 3D FEM for the PDA of laminates adhesively bonding repaired with stepped patch. The FEM can predict the damage initiation and progression of laminates, adhesive layer and patch until structure final failure. The modeling of laminates and adhesive layer are based on the assumption of orthotropic damage and isotropic damage in the framework of CDM respectively. The paper analyzed the laminates, which is repaired with adhesively bonding stepped patch under single axis tension. The analysis results have good agreement with experiments. Based on the FEM, the main parameters of the stepped patch repaired laminates were analyzed on the effect of ultimate strength, including step width, extra layup configuration, direction and thickness of extra layup. As same as the purpose of the bonding patch repaired laminates to reduce the dependence on the calcula-tion resources and to increase convenience of engineers using, the preliminary relationship between the strength of the stepped patch repaired laminates and the strength of stepped SLJ was formulated, with which the strength of stepped patch repair can be estimated.