Fractography of Fracture Surface of Unidirectional CFRP under Three-Point Bending

Abstract

It is well known that the fracture mode changes depending upon the span-to-depth ratio in three-point bending tests of unidirectional fiber reinforced composites. The characteristic change in fracture modes under three-point bending is clearly observed especially in unidirectional CFRP (Carbon Fiber Reinforced Plastics) because of its strong anisotropy in strength.
In this study, the change in three-point bending behavior of unidirectional CFRP has been examined over a wide range of span-to-depth ratio. The changes in apparent maximum bending and shear stresses have been correlated to that in fracture modes.
It has been shown that a unidirectional CFRP breaks into two pieces at the loading point in a certain range of span-to-depth ratio, and the regions of tension and compression side can be obviously distinguished in the broken fracture surface. The microscopic configurations of each region were examined by fractography using a scanning electron microscope and the results were discussed in conjunction with the fracture mechanism.
It has also been shown that the broken fracture surface changes significantly with the strain rate or crosshead speed. A phenomenological correlation has been given between the strain rate dependency of apparent bending strength and the fracture surface based on the microfractographic examinations.