Rotation control with dual actuators: A new device to extract mixed-mode traction–separation relations

Abstract

This paper addresses the extraction of traction-separation relations associated with the mixed-mode interactions at the interfaces between two materials in the context of cohesive zone modeling for large scale bridging during delamination. A direct approach that provides the normal and shear components of the traction-separation relation at any mode-mix is pursued using a novel rotation-controlled loading device. In the past, multiple laminated beam specimen geometries have been used to vary the mode-mix. However, when using the same specimen geometry is desirable, dual actuation provides the most general solution to mixed-mode loading path control. Based on our previous work, rotation control of laminated beams has been proposed as being optimal from crack growth stability and mode-mix control standpoints. This paper describes the implementation of this concept using laminated beams consisting of an epoxy sandwiched between aluminum strips. Digital image correlation was used to determine the location of the crack front and measure the normal and shear components of the crack tip separations. The specimen geometry allows the normal and shear components of the J-integral to be determined separately via measurements of the reactive torques. The normal and shear components of the traction–separation relations for five different mode-mixes are then presented. The results challenge some of the commonly accepted trends regarding the initiation and evolution of damage. The change of mode-mix as the damage evolves is also discussed as it relies heavily on the definition of the mode-mix.