Dual-actuator mixed-mode bending tests on structural adhesive joints

Abstract

In this article, a novel mixed-mode bending (MMB) test setup is presented and used to investigate the fracture behavior of the structural adhesive SikaPower®-498. The novelty lies in the implementation of two actuators that can be controlled independently of each other. This opens up investigation possibilities that go beyond those of the MMB standard configuration. For example, a varying mode-mixity can be prescribed during testing, allowing identification of an entire fracture envelope from a single experiment. Additionally, an optical fiber strain measurement (OFS) system is used to determine the curvatures of the substrates, allowing for a detection of the actual crack tip position and an analysis of the loading conditions behind the crack tip. The presented work focuses on the experimental calibration of the compliance change during crack growth, on tests performed at constant (external) mode-mixity using a mode partitioning approach based on the J-integral, as well as on tests carried out under varying mode-mixity. Additional aspects of the test setup such as stable crack growth regime or differences between the results of different evaluation methods are discussed. Finally, the obtained fracture envelopes are compared with references from literature, including for the first time a comparison between mixed-modes I+II and I+III.