Observations Of Rate-Dependent Fracture Of Locally Weakened Interfaces In Adhesive Bonds

Studies of the fracture behavior of adhesive joints can provide scientific understanding of failure processes as well as properties required for engineering design purposes. The focus of the present paper is to discuss the role that locally weakened interfaces in adhesive bonds can have on the fracture behavior of double cantilever beam specimens loaded in both mode I conditions and in mixed (mode I/II) conditions in a dual actuator load frame that permits independent control of the applied loads. Locally weakened areas are created by several methods of contamination of one aluminum adherend, including physical vapor deposition of copper through a mask perforated with the desired size, spacing, and pattern. Results from this experimental study have provided evidence of the size of a weakened zone that is required to be detected by a growing cohesive crack for a commercial adhesive system. The detection size depends on the mode mixity applied, with opening shear conditions rendering detection of smaller weakened zones and closing shear conditions detecting only larger weakened zones. In addition, an interesting rate dependence will be described in which rapidly growing cracks are more likely to detect locally weakened zones than more slowly growing cracks for several systems studied. Possible mechanisms will be suggested.