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Fractographic analysis, using scanning electron microscopy, is an important method to study polymer fracture behavior. There are several distinctive patterns on
fracture surfaces, such as radial striations, regularly spaced ‘rib’ markings, and conic-shaped patterns. The conic-shaped pattern is the intersection locus of a moving planar crack front and a radially growing circular craze or secondary crack front. In this paper, the effects of the ratio of crack velocity to growing craze or  secondary crack velocity on the shape of intersection loci are discussed using computer simulations. It is shown that when the ratio of crack velocity to craze or secondary crack velocity (a˙/c˙) increases progressively, the fracture surface pattern changes from a parabola or a prolate parabola to an ellipse and finally to an approximate circle. Therefore, the crack growth velocity can be estimated based on the fracture surface morphology and then related to the fracture processes as well as to the ductile–brittle transition or toughening mechanism of brittle polymers.