Evaluation Of The Fracture Properties Of PIs And Their Hybrid Films

Polyimides (PIs) films and their hybrid films working as insulating materials have received great applications due to their low dielectric constant, excellent mechanical and thermal properties. Among all of them, the fracture toughness is one of the critical mechanical parameters for affecting their applications. Unfortunately, it is difficult to use the linear-elastic fracture mechanical methods and some well-developed elastic-plastic fracture methods (e.g. J-integral and COD) to evaluate the fracture toughness of such kind of ductile polymer films. Recently, a comparatively new method, essential work of fracture (EWF), has received more attentions since it is suitable to assess the fracture toughness of ductile film by which plain-stress condition is satisfied. The related theoretical research and potential applications are being taken.In this study, the method of essential work of fracture (EWF) was used to evaluate of the fracture toughness of PI/SiO₂ hybrid films. According to the EWF method, the total work of fracture (W_f) includes two parts, the essential work of fracture (W_e) and non-essential work of fracture (W_p), and the essential work of fracture can be deduced based on the relationship of the total work of fracture versus the ligament length (L). Therefore, according to the experimental results of double edge notched tension (DENT) specimens with different ligament lengths, the influence of silica doping levels on the fracture toughness of Pi/SiO₂ hybrid was given. Considering the equivalent relationship of the specific essential work of fracture (w_?e) and the critical J-integral (J_IC), the developing process and the shape of the plastic zone for Pi/SiO₂ hybrid films with different silica doping levels were investigated using finite element method. Some related calculating results are compared with the experimental values. By using the numerical method, the developing process of the plastic zone for mixed mode I/II and the maximal load of crack propagation were given. Meanwhile, the influence of the stagger angle (θ) on the maximal load of crack propagation, the critical volume and the shape factor of plastic zone were researched.