On the validity of linear elastic fracture mechanics methods to measure the fracture toughness of adhesive joints

The analysis of large-scale fracture processes, such as those involved in the fracture of adhesive joints, falls outside the scope of Linear Elastic Fracture Mechanics (LEFM). However, experimental data produced in testing adhesive joints are usually reduced with LEFM methods. The consequent error has not yet been evaluated. In this work, an experimental characterization under pure-mode loading of an FM-300 epoxy film adhesive is presented for different adhesive and adherend thicknesses. The experimental data is analyzed using both LEFM-based and J-integral-based data reduction methods in order to study their suitability to analyze adhesive joints. LEFM-based data reduction methods are shown to entail a relevant deviation in the fracture toughness results that heavily depends on the size of the fracture process zone. It is concluded that LEFM methods are not suitable to characterize adhesive joints and that their use is restricted, at best, to the measurement of initiation values. The effect that the adhesive and the adherend thicknesses have on the fracture toughness and the R-curve of the material is studied. Adhesive and adherend thicknesses are shown to have a significant influence on the bond fracture toughness and the source of such influence is discussed.