Nanoindentation

Nanoindentation

Why nanoindentation over tension/ compression testing? Tension and Compression are destructive to the specimen Clinical Applications Macroscopic vs microscopic failure

Principles of Indentation A small stiff probe applies a small load into the specimen, and the deformation is observed Time Displacement Hold Load Unload

Larger contact area, averaged material property, assumes homogeneity Traditional hardness testing Nanoindentation Millimeter scale Larger contact area, averaged material property, assumes homogeneity Measure: Projected area Material Properties: Hardness Nano- micrometer scale Smaller contact area, finer spatial precision for heterogeneous materials Measure: Indentation depth, force response Material Properties: Hardness, load- displacement data, Young’s Modulus Additional tests: creep, stress relaxation, strain rate sensitivity, fracture toughness

How do we get this extra data? MEMS Probe Cantilever Probe

Difficulties in Nanoindentation Cannot image during indentation Limited depth of indentation Sensitivity and accuracy of measurements Complex calibration Difficult to detect the surface of the sample in soft tissues Material pile-up

Indenter Geometries te Berovich Tip used in Lab maybe Conincal and pyramid get down to 20 nanometer, a few micron Spherical and cylindrical typically 100-1000 micron Interface Focus. 2014 Apr 6; 4(2): 20130055 Berovich Tip used in Lab maybe http://www.microstartech.com/

Specimen Parameters As level of surface as possible Boundary Conditions Width and height Thickness Typically 1:10 indentation depth to thickness As level of surface as possible As Smooth of surface as possible, minimize surface Roughness

Resulting curve Load Displacement S = dP/dh Hold Unload Pmax hmax S = dP/dh Resulting curve Small strain within elastic limit continuous and non-conforming elastic half-space Frictionless surface Time Displacement Hold Load Unload Can get force, displacement, and elastic properties of a material Assumptions of elastic contact theory Small strain within elastic limit Surfaces are continuous and non-conforming (i.e. semi-infinite plate) Each body can be considered an elastic half-space Frictionless surface

ε = 0.75 Geometric const. S = contact stiffness

Young’s Modulus Contact Stiffness Effective Modulus The slope of the unloading curve evaluated at Pmax We will give you Effective Modulus Young’s Modulus Other Material properties: Hardness, Strain Rate sensitivity, Creep, Stress Relaxation, Fracture toughness β = 1.03 geometric constant Assume Poisson’s Ratio of 0.3

Intermediate Calculations: Test Data: Force displacement Intermediate Calculations: Contact Stiffness Contact Height Contact Area Effective Modulus Final Analysis: Young’s Modulus Indenter parameters