Measurement of pure mode I fracture toughness at a sandwich interface and parametrization of the R-curve for a cohesive element

Abstract

A comprehensive procedure for the measurement of the pure mode I fracture toughness of a sandwich interface and the implementation of the measured property into a finite element model is presented. The measuring method is founded on the conventional practice of the double cantilever beam (DCB) test in conjunction with a modified sandwich specimen. The modification intends to equalize the asymmetric properties of the upper and lower parts with respect to an initially cracked interface in order to avoid mode mixity and secure the pure mode I fracture behavior during the test. For the equalization, two metal plates of different thicknesses are attached to the top and bottom surfaces of the sandwich. The dimensions of each equalizer can be precisely determined based on classical lamination plate theory. The effectiveness of the equalizers is validated through finite element analysis. The test result on the sandwich DCB specimens shows that the crack growth resistance initially increases before reaching steady state, which may be caused by the mixture of the stiff and soft core materials in the fracture process zone. The present study discusses the modeling methodology for taking into account the two-phase R-curve behavior in a finite element model through a tri-linear traction separation law based on the measurement data. It is shown that the finite element model reproduces load-delta response as well as delamination growth very accurately. The delamination extension in this study is measured on a scale of micro-millimeters using a digital-image based technique.