The fracture properties of constrained Ag and Ni interlayers were investigated by experimental techniques as well as by finite element modelling. The interlayers were thin pieces of pure Ag, or Ni, joined on either side to a high strength steel. The state of stress in the interlayer was a function of the interlayer diameter to thickness ratio, D/T. Increasing D/T resulted in high triaxial tension in the interlayer due to the constraining influence of the steel. Therefore, the interlayer geometry was a good model system to study the role of triaxial tension on the ductile fracture mechanisms.

Tensile testing of the interlayers indicated that the failure stress increased, and the failure strain decreased, with increasing D/T. The failure stress, however, did not increase linearly with D/T. The failure stress reached a limit of approximately 10 times the uniaxial yield strength of the interlayer material and did not increase further with increases in D/T beyond approximately 50. It was also observed that the average fracture surface dimple size in the thin Ag and Ni interlayers was larger by a factor of approximately 10 than the dimple size in the thick interlayers. The fracture process in the interlayers was observed to be initiated by void nucleation from inclusions in the interlayer near the steel interface. The decrease in fracture stress and strain resulting from increased inclusion size and density was studied with two groups of Ag interlayers containing different inclusion populations.

Finite element analysis was used to determine the general stress and strain state in the various thicknesses of Ag interlayers as well as the local stress state around rigid inclusions located in the Ag interlayer. The results of this analysis corresponded closely to the experimentally observations. The finite element model was able to predict the deviation in linearity in the failure stress versus D/T relationship if fracture was assumed to occur, by a critical stress criterion, at a rigid inclusion interface. The analysis of the local stresses around a rigid inclusion indicated that the most important factor influencing the stress at the inclusion was its proximity to the steel / Ag interface. This was confirmed by a simple mechanical model which approximated the interfacial stresses resulting from the limitation of the secondary plastic zone size around the inclusion by the presence of a rigid interface.

The observation of larger dimple sizes on the fracture surfaces of the thin interlayers, as well as the observed sudden nature of the fracture process, suggests that the constrained interlayers fractured by unstable void growth at the final stages of loading. An analysis of the void growth mechanism showed that in the thin interlayers, the high triaxial stress made it energetically favorable for unstable void growth to occur between widely spaced voids. This accounted for the large dimple sizes in the thin interlayers.

This study has shown that the fracture strength of the interlayers is determined not only by the local stresses around individual particles, or defects, in the interiayer but also by the general stress state in the interlayer which influences the critical spacing between voids below which unstable transverse void growth is energetically favorable. This description of the fracture process is supported by the experimental results reported here for Ag and for Ni interlayers. The analysis represents a novel approach to studying the ductile fracture process in states of high triaxial tension. Theories dealing with the processes of void nucleation c..id growth in these stress conditions have previously suffered from a lack of experimental verification. Thus, the results from the combined experimental and computational approach presented here have a wide range of applications in various constrained ductile systems.