Mixed-mode I–II–III delamination toughness determination via a shear–torsion-bending test

A new “shear–torsion-bending” (STB) test is introduced that allows for delamination toughness testing under any ratio of mode I, II and III loading. STB test specimens are similar to those that are used for existing mode I, II and mixed-mode I–II test methods. Three-dimensional finite element analyses are used to show that the distributions of energy release rates across the delamination front are relatively uniform, with the variations being reasonably similar to those that occur in other widely accepted test methods. Closed-form data reduction methods are introduced and are verified for a wide range of material and geometric properties. Thus, the STB appears to be a promising test for evaluating mixed-mode toughnesses of laminated composites under conditions where mode III is present. Use of the STB is illustrated by mode III delamination toughness testing on two different carbon/epoxy materials.