In the framework of the simplified Sivashinsky equation it is shown that the process of formation, propagation and interactions of cracks on flame fronts can be considered as the interference between nonlinear effects and instabilities of plane flame fronts. It is demonstrated that in a model with a constant growth rate cracks can form webs with triple vertices. At late times, the crack web consists of straight line segments. In agreement with recent observations [D. Breadly and C.M. Hamer, Comb. Flames 99 ( 1994) 562], the fourth order (and higher) vertices are rare and can appear as a result of scattering of triple vertices with each other. Within this model we predict the merging of small cells into neighboring larger cells.