1. Characterisation of mechanical properties
of polymers by indentation tests
C. Marotzke, G. Kalinka, T. Feldmann, BAM, Division 5.3, Mechanics of Polymers
indentation tests
determination of stiffness for small material volumes
determination of local material properties, e.g. local properties near interfaces
influence of different indentation depths on the measured mechanical properties
→ Hysitron 2µm nm, microdevice 2-50µm
local material behaviour
highly inhomogeneous stresses in contact zone between indentor and specimen
stress concentrations at edge of cylindrical indentor and tip of
conical or pyramidal indentors (Berkovic, Vickers)
highly plastic material behaviour in regions of high stress concentrations
additional viscoelastic effects for polymer matrices
Berkovic indentor – microindentation test
indentation tests with Berkovic indentor on different scales
nanoscale (200 nm) vs. microscale (2-50µm)
volume displaced by Berkovic indentor → volume is quadratic function of indentation depth
despite of nonlinear material behaviour → force displacement curve is approximately quadratic
main characteristics of force - displacement curve found on both, nano- and microscale
remaining indent of Berkovic indentor
- original indentation depth – 50µm
indentation test with microdevice indentation test with Hysitron - comparison force/displacement curve comparison influence of viscoelastic behaviour of two indentation depth (normalised) of nano- and microscale (normalised)
finite element simulation of indentation tests with Berkovic indentor
FEM simulation – linear elastic / multilinear plastic
material behaviour, large deformation analysis
material of substrate: epoxy, indentor: hardened steel
material data of epoxy resin measured with tensile test
high plastic strains in zone below the indentor → 40%
good correlation between simulation and
experiment for loading phase
residual strains after unloading overestimated by model
→ material behaves not only plastic but nonlinear elastic as well
remaining plastic indents of Berkovic indentor
left: nanoscale, right: microscale, identical magnification
vertical stresses in the vicinity total strain in the vicinity force-displacement curve for 50µm indent
of the indentor of the indentor experiment (blue) / FEM simulation (red)
Acknowledgements
Thanks to K. Elabshihy, M. Khater for leaving the experimental results and M. Bistritz for specimen preparation