Slow growth damage assessment of bonded metal-composite joint using a numerical approach

The applications of composite materials and adhesive-bonded joints in the aviation industry have significantly increased in recent years and their damage assessment attracts a lot of research interest. This study investigates the slow growth damages of bonded metal-composite joints and develops a finite element-based numerical procedure to capture mechanical behaviors of those damages in the joints. The residual static strength and strain energy release rates (SERRs) (which determine the damage growth rates) of the joints are the two quantities that can be determined to predict the fatigue life of adhesive bonded joints. The absence of extensive experimental and numerical assessments for predicting the slow growth damage of bonded metal-composite joints necessitates the development of a numerical procedure to determine these quantities. Toward this end, a three-dimensional (3D) finite element (FE) model was created. From the FE analyses, the residual static strength and SERRs of bonded metal-composite joints were calculated as a function of varying disbond crack lengths following the adhesive element failure criterion and fracture mechanics approach, respectively. Additionally, the residual strength and SERR for joints with delamination of the composite adherend were also calculated by implementing a linear fracture mechanics-based failure index. The results suggested that the stable crack length of the joint with 180mm long overlap length was about 140mm and 100mm when the crack was initiated from taper and gap end, respectively. These lengths were consistent for both disbond and delamination scenarios. Residual strength values were 89kN and 65kN for joints with 10mm disbonds initiated from the taper end and gap region, respectively. With the material properties considered, the delamination propagation strengths for delaminated joints were notably lower than their disbonded counterparts. Based on the findings of this study's analysis, a systematic strategy was developed for applying the slow growth technique for bonded joints, which would aid in the creation of an actual sample for experimental validation. The experiment in turn will provide calibration for the parameters in the model for practical applications.