Experimental investigations on the path dependence of the fracture behavior of structural adhesive joints in mixed mode

• Applicant: Professor Dr.-Ing. Stephan Marzi
• Subject Area: Mechanics ; Polymer Materials
• Term: since 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 329889536

Adhesive bonds are an established joining method in many industrial areas. In addition to the requirements for stiffness or damping properties of the connection, the failure behavior of an adhesive joint under mechanical load is often of great interest. In automotive engineering, for example, the realistic prognosis of failure in the event of a crash is an essential design criterion for the structural components. An experimental characterization of the joint failure is carried out in fracture mechanics tests, from the results of which suitable material models and associated parameters can be obtained. The type of loading is of great importance, whereby a combination of peel and shear stress is referred to as mixed-mode. Currently, the experimental characterization of bonded joints under mixed-mode loading is established under monotonic loading conditions. The combined peeling and shearing stress is realized by a suitable specimen clamping or load jigs and the specimen is loaded in a testing machine until breakage. Any influence of the load path on the fracture behavior cannot be investigated with these established test procedures. In a completed research project, two new test setups were developed to analyze the fracture behavior of adhesively bonded joints under mixed-mode loading. In these tests, the test specimen is loaded by two independently controllable actuators, which can each apply a tensile and torsional load or a tensile and bending load. This allows a targeted variation of the relationship between shear and peel loading in the test and thus an experimental investigation of the dependence of the fracture behavior on the load history. The two actuators are currently controlled via contributions from the J-integral, which is based on the assumption of a possible additive decomposition of the fracture energy with respect to the individual modes. The aim here is to calculate the traction at the crack tip and to determine a constitutive law for cohesive zone modeling of the adhesive layer. Finally, the dependency of the fracture behavior on the load path will be described experimentally for the first time by targeted variations of the mixed-mode loading. Based on this state of knowledge, the dependence of the fracture behavior on the loading path will now be experimentally described for the first time through targeted variations of the mixed-mode loading during testing. The results shall then be used to develop suitable cohesive zone models, including the associated parameter identification.

• DFG Programme: Research Grants