Influence of mode mixity and loading conditions on the fatigue crack growth behaviour of an epoxy adhesive

Abstract

Adhesive joints usually experience mixed mode and mostly cyclic stresses conditions during their service life. The aim of the current research is to investigate the fatigue behaviour of a structural epoxy adhesive. Pure modes I and II and mixed mode tests were carried out to study the fracture and fatigue crack growth (FCG) behaviour of the adhesive. Compliance‐based beam method was considered for experimental fracture energy measurement. The effects of load level and load ratio on the mode I FCG behaviour and Paris law parameters were also investigated. Result showed that the effect of load level on fatigue crack propagation is more pronounced for lower R ratios. It was found that when the crack faces are closer during the unloading process, the difference between the R2 and Gmin/Gmax is higher. Some possibilities are the crack closure phenomenon, difficulty in measuring the Gmin, and the employed data reduction approach.

The influence of mode mixity and adhesive system on the fatigue life of adhesive joints

“…One important parameter which should be considered is the threshold fracture energy (G th ), which is defined as the energy release rate below which the fatigue crack growth is negligible. Based on the previous results already published by the current authors, 10 it was shown that for adhesive A, G th in mode I is higher than mode II as shown in Table 3. This associated with the fact that the energy introduced in the system per cycle increases with the presence of a shear component, aggravates the reduction of the fatigue strength as the load angle increases.…”

“…A reason can be the stiffness of the joint that is lower for adhesive B and as stated previously, this affects the introduced energy per cycle. According to previous studies, 10,26 the threshold fracture energy obtained for adhesive A is higher than the one obtained for adhesive B as presented in Table 3. These two factors associated may be the cause for a lower fatigue resistance.…”

“…9 Fracture mechanics-based methods have been experimentally and numerically considered by several authors. [10][11][12][13][14][15][16][17][18] In these works, the influence of different geometrical and loading parameters has been analysed on fatigue crack growth of adhesive materials. Ajdani et al 15 presented a new test method to measure the fracture energy of brittle and semibrittle adhesive joints subjected to mixed mode loading conditions.…”

Experimental and numerical evaluations on the effects of adhesive fillet, overlap length and unbonded area in adhesive‐bonded joints

“…Generally, the total fatigue failure of an adhesive‐bonded joint consists of crack initiation 16,17 and subsequent crack growth 18–21 . It has been reported that the proportion of crack growth life will increase with increasing load levels 22–24 .…”

“…One of the reasons is the complexity of loading conditions in adhesive‐bonded joints. Most adhesives are under multiaxial stress conditions 21 in practice, for example, peel, shear and axial stresses in the adhesive layer, even for the simplest single lap joint, 29 which bring challenges to growth life estimation. Furthermore, the crack initiation phase was omitted in these research studies, which would lead to conservative predicted results 30 .…”