On the fatigue behavior of notched structural adhesives with considerations of mechanical properties and stress concentration effects

In this work, three types of structural modified epoxy adhesives were used to investigate the effect of stress concentrations on the fatigue behavior of notched bulk specimens. SN curves of un-notched and notched specimens were determined at constant amplitude and R = 0.1 in the range between N_f = 10³ (LCF) and N_f = 10⁶ (HCF). The following key conclusions were made: (i) fatigue strength was reduced due to the presence of notches, especially at the HCF; (ii) adhesives showed different values of notch sensitivity with values for the adhesives lower than typical values of metals; (iii) for un-notched samples fatigue strength was between 62 and 78% of tensile strength for N_f = 10³ and around 50% for N_f = 10⁶; (iv) for notched samples fatigue strength was between 67 and 78% of the tensile strength for N_f = 10³ and around 40% for N_f = 10⁶; (v) fractography evidenced the presence of voids and shear yielding around the notches, (vi) unnotched samples showed the same fracture behavior for both LCF and HCF with crack formation at the external surface. For notched samples there was a significant distinction between LCF and HCF with cracks forming at the notch root.