Nanoindentation technique is used to assess the mechanical properties of materials at nano-level. A very small tip (usually diamond) produces indents at the surface of the material to be tested. A load vs. deflection curve is generated and is used to study the elastic properties of materials. Generally, it is used for obtaining the hardness and Young's modulus of materials at nano-meter scale. Currently, the method to evaluate the mechanical properties by nanoindentation is restricted to homogeneous materials. Cement-based materials are heterogeneous in nature. Therefore, nanoindentation study of cement-based materials is critical and requires several important steps, which need to be performed accurately. This chapter provides a review of the theory of nanoindentation, instruments being used for nanoindentation, sample preparation techniques, indentation strategy, and determination of nanomechanical properties and data analysis for cement-based materials.
TopHeterogeneity Of Cementitous Composites
Cementitious materials, like many other materials exhibit heterogeneous features over a wide range of length scales, from the nano-scale of the elementary chemical components to the macroscopic scale of the aggregate-mortar composite. This multi-scale heterogeneity ultimately determines the in vivo mechanical performance (stiffness, strength), and degradation (damage, fracture, failure) of cementitious materials. While most codes of practice in concrete engineering account for this heterogeneity through probability theory to achieve certain macroscopic material properties with some certainty, current trends in concrete science and engineering aim at a better representation of this heterogeneity at multiple length scales, to ultimately identify the scale where physical chemistry meets mechanics. The rationale behind this approach is that the different chemical components of cement-based materials are defined by specific chemical equilibrium states, for which the probability that some solid chemical compounds go into the solution is smaller than the probability that the same chemical species in the solution precipitates onto the solid. Such an equilibrium state is associated with a stable material state. Hence, if it were possible to break down the heterogeneities of cement-based materials to this scale, where the solid material manifests itself in a chemically stable state, and to assess, at this scale, the mechanical material properties, it would be possible to translate with high confidence chemical equilibrium states into macroscopic material properties. The expected outcome of such an endeavour is a blueprint of the elementary chemo mechanical components of cementitious materials, which do neither change in time, nor from one cementitious material to another.
The indentation test provides a P - h curve, and the extraction of material properties requires an inverse analysis of these data. The theoretical foundation of elastic indentation is set by Boussinesq's problem and the Hertz contact problem: Boussinesq's stress and displacement solution of an elastic half-space loaded by a rigid, axisymmetric indenter, which was subsequently extended for conical and cylindrical indenter geometry, provides a linear P - h relation. Hertz's elastic contact solution of two spherical surfaces with different radii and elastic constants provides a means of evaluating the contact area of indentation, and forms the basis of much experimental and theoretical work in indentation analysis based on contact mechanics. Subsequently, Sneddon (1965) derived general relationships among load, displacement and contact area for any indenter describable as a solid of revolution.