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 = ۱۰^۳ (LCF) and N_f = ۱۰^۶(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 = ۱۰^۳ and around 50% for N_f = ۱۰^۶; (iv) for notched samples fatigue strength was between 67 and 78% of the tensile strength for N_f = ۱۰^۳ and around 40% for N_f = ۱۰^۶; (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.

Introduction

Adhesively bonded structures are frequently used under cyclic loading conditions (e.g. rotating blades, engine vibration) making them sensitive to fail due to fatigue [1]. The phenomenon of fatigue, which involves the phases of crack nucleation and crack propagation, is very complex [2]. For instance, the presence of stress concentrations, i.e. notches, is a known factor that can severely alter the fatigue behavior of structural adhesives due to several reasons [3]:  formation of a multi-axial state of stress;  creation of non-uniform stress distribution;  increase in local strain rate; These factors combined make it difficult to accurately predict the fatigue lifetime of notched components. For this reason, designers often rely on large safety factors in order to ensure reliability of structural bonded structures against fatigue failure [4]. Since one of the driving forces for the use of structural adhesive bonding is the reduction of weight, part of this benefit is lost due to an over conservative design with subsequent cost and performance consequences [2].