One possible design of stretchable integrated circuits consists of functional islands of stiff thin films on a polymer substrate. When such a structure is stretched, the substrate carries most of the deformation while the islands experience little strain. However, in practice, the island/substrate interface can never cohere perfectly. Existing experiments suggest that, interface debonding occurs if the island is larger than a certain size. I am now studying the critical size of stiff islands on stretchable polymer substrates due to thin film delamination, using finite element simulations. We show that the maximum energy release rate of interfacial cracking goes down as island size or substrate stiffness decreases. As a result, the critical island size can be enhanced if the substrate is chosen to be more compliant. An approximate formula is given to predict the energy release rate for the configuration of stiff islands on very compliant substrate.
Very interesting. What is a typical size of an island?
Yuval
Typical Size
Thanks. The typical size of an island is in micron-scale. To determine the critical size we have to know the interface toughness at given mode mixity and compare with our calculated driving force.
In reply to Typical Size by Nanshu Lu
Continuum?
Yuval
Continuum
The length of the islands is in 100 micron and the thickness is in 1 micron. To be precise, we used continuum mechanics to analyze such structure and after normalization the thickness doesn't really matter. So the results will be valid for structures as small as continuum mechanics can apply.
I didn't do experiments to measure the interface toughness. What I did was to just provide the calculated energy release rate.
Any reference?
Do you have any reference about that? Thanks.
Tianlei Li
Basic references
1. Hutchinson 1996, Stresses and Failure Modes in Thin Films and Multilayers
2. Nix, 353 Class Notes on Thin Film Mechanics
3. Hutchinson and Suo 1992, Mixed mode cracking in layered materials
Actually we have a very good course of thin film fracture and delamination (ES 242r). Very completed resource there.
In reply to Basic references by Nanshu Lu
Thanks
Thank you very much. It is a big help.
Can I ask how do you get the stress field in your project? FEM? I meet this problem when I am calculating the chemical potential of a void growing in a finite grain. I don't knwo what your model looks like, but I guess you may meet the same problem. Right now I'm trying to get the stress using traditional stress function method since the my model, represented as a rectangle within a circular hole, is not very complicated. Are you dealing your model using stress function too?
Regards
In reply to Thanks by Tianlei Li
ABAQUS
You are welcome.
I am now using the commercial FEM code ABAQUS v6.5 to do the calculation because it includes modeling and postprocessing capabilities for fracture mechanics analysis. I don't think ABAQUS is able to do chemo-mechanical coupled problems so you have to develop other feasible methods.
critical size of stiff islands
Can your finite element calculations match with the theoretical predications, or experimental results, on the critical size of stiff islands?
Comparison with experiments and theories
There's no very relavent experiments available for comparison. But the calculation does match our theory quite well. Please pay attention to our upcoming paper. Thanks.