In recent years, fracture mechanics has been used extensively in micro devices (such as MEMS interface, thin film, coating, etc.) and in applications subjected to multi-physics (such as thermal, piezoelectric, corrosion, etc.) environments. Therefore, this requires a computationally efficient and accurate tool to perform fracture analysis.

“This paper will have a significant impact on the fracture analysis of any layered material system, particularly in the emerging MEMS and NEMS reliability analysis sector, and in adhesively bonded structural composite joints.”

The virtual crack closure technique (VCCT) method has attracted attention due to its simplicity. It computes strain energy release rate (SERR) by using the nodal forces and displacements, which are the primary variables in finite element analysis (FEA) and only a single FEA is required. It avoids the integral over stresses and makes the computation much simpler and easier in conjunction with FEA without much extra post-processing.

Many numerical "experiments" have shown that the VCCT is not sensitive to the FEA mesh size. It generally can yield accurate values of SERR even with coarse finite element meshes with regular and lower order elements and can be used equally well with singular and collapsed elements. However, before the publication of our paper, there was no unified development in using a one-step VCCT for cracks that kink. I think people were looking forward to this development. Therefore, once it was published, several others followed in using it.

  Does it describe a new discovery, methodology, or synthesis of knowledge?

This paper first presented a new VCCT formulation for kinking cracks and demonstrated that a kinking crack cannot be treated as an inclined crack. For inclined cracks, both force and displacement components are projected as a vector to the crack orientation in order to compute SERR by VCCT. However, for kinking crack, the force can be projected as a vector while displacement should be projected by angle functions (not vector projection).

  Would you summarize the significance of your paper in layman’s terms?

This paper will have a significant impact on the fracture analysis of any layered material system, particularly in the emerging MEMS and NEMS reliability analysis sector, and in adhesively bonded structural composite joints.

  How did you become involved in this research and were any particular problems encountered along the way?

This development was an outgrowth from a project to study the failure of adhesively bonded composite joints sponsored by the DOE. During these tests, we observed crack kinking. With inclined crack formulations, we could not explain the test results clearly. Therefore, we were aware of the need to re-examine the computation of SERR, as has been done in the past.

  Where do you see your research leading in the future?

Well, we can use this formulation to perform a fracture analysis of many problems associated with layered materials. We are now working on other related developments such as the introduction of a discrete cohesive element (DCZM) for other fracture problems. We also welcome suggestions from other researchers.