What indentation stress–strain tells you
Where a material yields, and how it behaves on the way there, decides whether a thin film, a small component or a local region will deform under load. Conventional tensile tests need a machined coupon and give a bulk average. Indentation stress–strain analysis measures the response locally: on coatings, near welds, in individual phases, or on parts too small to test any other way. The nanoindentation page explains the technique and the instrument behind it.
How spherical indentation works
A spherical tip first makes purely elastic contact, described by Hertzian contact mechanics. As load increases, the stress under the tip reaches the elastic limit and plastic deformation begins. By converting load and contact area into indentation stress, and contact radius relative to tip radius into indentation strain, the test produces a stress–strain curve with a visible elastic–plastic transition.
The tip radius matters: it must be small enough to reach stresses beyond the elastic limit within the available load. NANOVEA’s cyclic stress–strain study used a 5 µm-radius sphere on steel and found the elastic–plastic transition near 0.076 strain, with an elastic limit of 1.45 GPa.
Properties at every depth
With continuous cyclic loading, contact stiffness is measured repeatedly as the tip penetrates (over 100 oscillations during loading in the study above). Hardness and modulus are then known as functions of depth. This reveals gradients in surface-treated or coated materials and shows how properties develop as deformation increases.
Compression and bending at small scale
The same load–depth platform runs other mechanical tests with different tips:
- Flat-punch compression of microparticles, pillars and powders. NANOVEA has published compression strength of salt microparticles, with loads down to 0.1 mN available for fragile particles.
- Three-point bending of small beams, using a flat tip as the central load point, to obtain stiffness and the critical load at fracture.
- Probe tack of pressure-sensitive adhesives, based on ASTM D2979, with controlled contact and separation.
