Rate effects in mode-II fracture of plastically deforming, adhesively bonded structures

Abstract

Results from a combined experimental and numerical investigation into the effects of rate on mode-II fracture of a plastically deforming, adhesively bonded joint are presented. It is shown that a cohesive-zone model has to be modified to include coupling between normal and shear modes of deformation when there is extensive shear deformation of the adhesive layer. A suitable cohesive-zone modeling strategy is described, and the mode-II cohesive parameters determined from the model are presented as a function of loading rate. Previous studies of the same system showed that the effects of rate in mode-I were limited to the probability that a crack growing in a toughened quasi-static mode would spontaneously make a transition to a brittle mode of fracture. No such transitions were found for mode-II fracture. Crack growth always occurred in a quasi-static fashion. While there was some evidence that rate might affect the mode-II fracture parameters, these effects were very limited even up to crack velocities of about 1,000 mm/s. Any possible effects was limited to a very minor increase in toughness and strength with increased loading rates. However, the magnitude of these possible increases were comparable to the magnitude of the uncertainties in the measured values.

Rate effects for mixed-mode fracture of plastically-deforming, adhesively-bonded structures

“…No transition to a brittle mode was observed in the mode-II joints, but the mode-II cohesive parameters appeared to increase slightly with rate up to crack velocities of about 2.5 m/s. The values of mode-II toughness were associated with relatively large uncertainties, but varied from a minimum of 8 kJ/m 2 to a maximum of 24 kJ/m 2 , while the shear strength varied from about 21 MPa to about 50 MPa (Sun et al, 2008c).…”

“…The cohesive-zone model for mixed-mode fracture is based on that originally developed by Yang and Thouless (2001), with an adaptation for the mode-II component made necessary by the extensive shear in the present geometry (as described in Sun et al 2008c). The mode-I and mode-II traction-separation laws are developed and described independently, but linked through a simple mixed-mode failure criterion 1 :…”

“…2 The details of the mode-II law are given in Sun et al (2008c). The form of the "adhesive" mode-I law was set equal to that of the "cohesive" mode-I law established in the prior work (Sun et al, 2008b).…”

“…Numerical simulations of this geometry were performed using the full range of the mode-II cohesive parameters appropriate for this system (Sun et al, 2008c). The mode-II toughness was varied between 8 kJm -2 and 24 kJm -2 , and the cohesive shear Furthermore, the calculations showed that the stress in the cohesive zone was essentially uniform along the bond, which meant that the nominal strength of the joint was identical to the interfacial strength.…”

“…Numerical simulations for the quasi-static tests with this geometry were performed using the full range of cohesive parameters appropriate for the mode-II cohesive law (Sun et al, 2008c). Different values of the interfacial mode-I strength and toughness were used in the cohesive-zone model to compute load-displacement curves, which were then compared to the experimental results.…”