Mixed mode fracture of glued-in rods in timber structures

Glued-in-rods in timber structures lead to overcome the use of traditional bolted connections, preserve a large part of the original timber and offeraesthetic benefits. Several research programs were achieved to improve the mechanical knowledge of this technique, exhibiting experimentally the influence of materials and the effect of the geometric configuration. From these experimental results, some design rules predicting the axial strength are available, but a common criterion is still lacking. This paper relates to experimental investigations and finite element computations on glued-in rods, with the aim of providing a better knowledge about their mechanical behavior until failure. An experimental campaign is carried out on single glued-in rod connections. The finite element modeling reproduces the experimental configuration: it exhibits significant normal stress (to the interface)at the onset of the bonding, in comparison with shear stress. Within the framework of equivalent linear elastic fracture mechanics, resistance curves in mode I and mode II are established for each specimen. Finally, a mixed mode fracture criterion (I/II) is used to describe the fracture process zone development at the woodadhesive interface (failure zone). An analytical formulation is then proposed allowing the evaluation of peak load of each specimen, which highlights a new approach for the design of such connections.