Fracture mechanical stability of DCB and 4ENF tests complemented with linear springs

In this work the stability of the DCB and 4ENF fracture specimens is analyzed using some beam theories and the compliance calibration method. The main goal is that the systems are complemented with linear springs and it is investigated how the springs influence the stability of crack propagation from the theoretical standpoint. Large number of experiments were carried out including crack initiation tests providing the experimental load displacement responses, the compliance datasets and the qualitative observation of crack initiation. Apart from the analytical method the compliance calibration method was employed to evaluate the experimental data and to determine the fit curves of the compliance datasets, the R-curves and finally the bifurcation curves or in other words the critical displacement and crack length relationships including several additional spring stiffnesses for both test configurations. The results indicate that - as expected - the smaller the spring stiffness is the worst the stability of the DCB specimen will be, leading to the possibility of destabilization. On the contrary the analysis and experiments confirm that the 4ENF test cannot be destabilized independently of the stiffness of additional springs.