The fracture resistance of structural adhesives is characterized by the strain energy release rate. Most of the methodologies developed until now to measure are based on Linear Elastic Fracture Mechanics (LEFM), and have shown that strongly depends on the load ratio applied at the crack tip. An example can be found in [1-2]. However, it is well known that many of the structural adhesives used in industry have considerable non-linear behaviour. Recently, it has been demonstrated in [3] that seems to be independent of the mode ratio when including the non-linear behaviour of the crush-optimized adhesive SikaPower®-498 in the evaluation of the fracture results. In order to investigate further on this matter, the influence of the loading speed on the fracture properties of a toughened epoxy adhesive is studied here. The adhesive was submitted to TDCB (Tapered Double Cantilever Beam) and MMB (Mixed Mode Bending) tests at three loading speeds: 3 µm/min, 300 µm/min and 30000 µm/min. At first, the results of the fracture tests are analysed using the LEFM theory; this showed important influence of the loading speed on the measurements of when moving towards the mode II (pure shear) load state. In order to identify its non-linear behaviour, the adhesive was loaded by means of the modified Arcan fixture [4] under monotonic loading, at the same conditions as the ones used for the TDCB-MMB tests. Different phase angles were examined at each loading speed: tension (0°), tension/shear (45°), shear (90°) and compression/shear (135°). Therefore, for each loading speed, the elastic-plastic behaviour of the joint was determined by means of inverse identification. This non-linear behaviour was then introduced to evaluate the TDCB-MMB test results and investigate possible differences in the evolution of the fracture energy in the mixed-mode I+II plane. This has been done numerically by using volume elements to represent the adhesive and cohesive elements to describe damage initiation and propagation. The results of the present research shall also be used to evaluate the limits of the LEFM theory to analyse the experimental data of Fracture Mechanics tests like the TDCB-MMB presented here.

References

[1] G. Stamoulis, N. Carrere, J.Y. Cognard, P. Davies, C. Badulescu, Int. J. Adhes. Adhes., 51, 148-158 (2014).

[2] G. Stamoulis, N. Carrere, J.Y. Cognard, P. Davies, C. Badulescu, Int. J. Adhes. Adhes., 66, 147-159 (2016).

[3] G. Stamoulis, N. Carrere, Eng. Fract. Mech., 179, 260-271 (2018).

[4] J.Y. Cognard, L. Sohier, P. Davies, Compos Part A-Appl S, 42(1), 111-121, (2011).