Composite materials are used in various industrial fields due to their excellent specific strength and specific rigidity. When fastening a composite structures, a mechanical fastening method and an adhesively bonded method are used. Compared to mechanical fastening, the adhesively bonded joints are light weight and does not cause stress concentration at the fastening part. However, when bonding, adhesion may vary depending surface treatment, operating environment, and the aging period of the adhesive film. These various bonding condition affect fatigue and fracture, and the adhesively bonded joints of the structure is an important part of the determination of the structure integrity.
In this study, a Double Cantilever Beam (DCB) test was performed to confirm the fracture characteristics under mode I load, which is opening mode, for composite adhesively bonded specimen with non-uniform adhesion. The test was performed according to the ASTM D5528 standard. Through the test, the failure behavior of the structure against the in-plane vertical stress was confirmed and derived mode I fracture toughness. Due to the non-uniformity of the bonding condition, stable/unstable crack growth were shown even within the same bonded joints, and various mode I fracture toughness was calculated. The mode I fracture toughness of specimen is the average value in the section excluding the section where cracks are initiated and unstable cracks grow. After the test, the failure mode of each section was confirmed through the failure surfaces of the specimen and the load-displacement curve.
Finite element analysis was performed using a cohesive zone model to simulate the initiation and growth of cracks. The behavior of cohesive element determined by the traction-separation law defined by the initial stiffness(K), maximum traction(Tmax) and fracture energy(Gc). To confirm for each variable affects the crack growth, initial stiffness, maximum traction and fracture energy were analyzed as variables. When analyzing the initial stiff ness as a variable, the reference values were applied for the maximum traction and fracture energy, and the reference initial stiffness was selected through the analysis. When interpreting the maximum traction and fracture energy as variables, two variables other than the variable were assigned a reference value. The analysis of maximum traction and fracture energy confirms that the load change and the slope change of the load-displacement curve as the maximum traction and fracture energy change.
The cohesive zone was subdivided based on crack length measured in the test. Based on the test results, the fracture energy was defined differently for each section, and the maximum traction was also changed to analyze the composite adhesively bonded specimen with non-uniform adhesion. Similar tendencies were confirmed by comparing the load-displacement curve of tests and analysis, and the finite element analysis confirmed that the mode I failure behavior of adhesive bonded joints with non-uniform adhesion can be simulated.