Research Summary
Finite Element Modeling of Nanoindentation Experiment
Authors: Andrew Caulfield, Bazle Z. (Gama) Haque, and John W. Gillespie Jr.
OBJECTIVES
• Develop finite element models of a Kevlar fiber at the fibril level with different fibril sizes
• Develop a finite element model of nanoindentor with conical nose shape
• Model Van der Wal’s interactions between fibrils via spring elements connecting fibril center of gravities
NANOINDENTATION BACKGROUND
• Nanoindentation experiments provide insight into the deformation mechanisms of fibrillar networks
• Past nanoindentation experiments provided data on the average mechanical responses and deformations of a number of fibers
• Kevlar 49 surface fibril diameters were found to vary between 10-50 nm
FE MODELING OF NANOINDENTATION EXPERIMENT
• Finite element model (FEM) developed considered the fiber as an assembly of nanofibrils
• Two models were developed to investigate effect of fibril size on indentation response
• Half-symmetric model with DFibril = 200 nm
• Quarter-symmetric model with DFibril = 50 nm
UNIT CELL MODELING STRATEGY
• Original models developed used semi-spherical fibrils:
• Approximate fibril shape
• Higher void space
• New models will use hexagonal packing
• Much greater fibril packing efficiency
• Void space can be varied parametrically
• Use same fibril diameter and compare overall deformation
FINITE ELEMENT MODELING STRATEGY
• Develop 2D fibril unit cell of cross-section as basis for model
• Copy unit cell to create rectangle basis for fiber
• Remove fibrils at edges to round off fiber at specified diameter
• Extrude 2D base to desired fiber length
HALF-SYMMETRIC FE MODEL OF FULL FIBER CROSS-SECTION
• Fibril diameter, DFibril = 200 nm
• Large fibril diameter allows modeling majority of fiber
• Depth of fiber modeled was determined to allow for maximum visibility of deformation
• Bottom two rows of fibrils were constrained in x, y, and z directions
QUARTER-SYMMETRIC FE MODEL DFibril = 50 nm
• The extent of the deformation reached all boundaries, indicating the need for a larger model leading to 50 nm fibril diameter
• Good indenter contact was established with 50 nm model
SUMMARY
• Participated in high impact research and was able to contribute to a journal article
• Gained experience with LS-DYNA modeling and analysis
• Learned about fundamentals of explicit FEM
ACKNOWLEDGEMENTS
This work is supported by the Army Research Laboratory through the Composite Materials Research program
