Fiber Reinforced Composite Fracture Toughness Photoelastic Experimental Study

Fiber reinforced composite materials have wide applications in modern engineering structures, but there are a variety of initial defects and damage before loaded, which affects the failure strength of composite structures badly. Therefore, to measure the fracture toughness of composite materials is an main content for fracture mechanics of composite materials, and also an important means of understanding the fracture properties of composite materials.With reference to the standard (GB/T 4161-2007), fracture toughness of crack perpendicular to the fibers was tested using three-point bending test and the photoelastic coating method. Based on CA-CPE-3000 image acquisition card and MINTRON MTV-8055MK-II color CCD, continuous acquisition of photoelastic image was realized. The cracking process can be directly observed and the cracking critical point of material can be identified with P-V curve. The experimental results show the effectiveness of this method. The fracture edge is flat and the crack has been propagated under the condition of plane strain. Near the crack tip, the material has no plastic deformation, which satisfies the condition of small plastic region, and correspondingly the steady fracture toughness can be obtained. Cracks appear along the fiber direction, rather than the direction of original crack line. The interface of fiber and matrix prevents the propagation of cracks. And then, the photoelastic image could not reflect the stress field near the crack tip accurately.With three-point bending test and photo-elastic patch experiment, the stress field near the crack tip, the critical condition of cracking and the cracking point can be effectively determined. Using Finite Element Method and experimental analysis, P-V curves can be drawn according to the displacement of the indenter instead of crack opening displacement, and then energy release rate and fracture toughness were calculated. The method proves to be direct and effective. However, little research has been conducted on the fracture toughness of composite materials. This paper mainly focuses on experimental method and what we have observed needs to be proven by a large number of experiments.