Measuring Mixed Mode Cohesive Laws for Interfaces in Sandwich Structures

Abstract

The purpose of this work is to establish a test procedure that allows fracture mechanics characterization of interface cracking in sandwich structures. This often entails large scale fiber bridging, which increases the size of the process zone beyond a point where linear elastic fracture mechanics (LEFM) is applicable. Instead of assigning the fracture processes to a single point at the crack tip (as in LEFM) the fracture process zone is represented by a cohesive zone in which the traction separation law (cohesive law) needs to be determined. The cohesive law can be measured, either indirect by modeling experiments and fitting parameters until numerical and experimental results coincide, Li et al. [1], or by direct measurement by e.g. the J integral approach, Sorensen and Kirkegaard [2]. In this study mixed mode cohesive laws are measured directly by relating the J integral to the normal and tangential opening of the initial crack tip. The test method is based on a double cantilever beam (DCB) specimen loaded by uneven bending moments, see Fig. 1. The test is conducted under displacement control in a tensile test machine, and the moment ratio is kept constant throughout one test. By varying the moment ratio the crack opening can be varied from pure normal opening to pure tangential crack surface displacements. A detailed description of the test method can be found in [2].