The effect of mixed mode I/III on crack evolution in brittle solids

A modification of Sommer's classical experiment [4] has been used to fracture rods of a brittle epoxy resin in mixed mode I/III stress conditions. The nucleation and growth of cracks in an increasing K_III/K_I stress field have been investigated, particularly in relation to the formation of river lines and the evolution of multiple cracks between river lines to form smooth helicoid surfaces. The fractographic features associated with the progressive development of river line patterns involving crack bowing at river line steps, interaction between arrays of cracks and the coalescence of river lines are described. As K_III/K_I increases the scale of the river line patterns increases but the patterns remain self similar. Using principles from differential geometry it is shown that helicoid surfaces can be generated entirely by crack evolution involving tilting without twiting.Emeritus Goldsmiths' Professor of Metallurgy, University of Cambridge, U.K.