Indentation hardness
Hardness measurements quantify the resistance of a material to plastic deformation. Indentation hardness tests compose the majority of processes used to determine material hardness, and can be divided into two classes: "microindentation" and "macroindentation" tests. Microindentation tests typically have forces less than . Hardness, however, cannot be considered to be a fundamental material property. Classical hardness testing usually creates a number which can be used to provide a relative idea of material properties. As such, hardness can only offer a comparative idea of the material's resistance to plastic deformation since different hardness techniques have different scales.
Stiffness matrix
By defining the vector "F" with components "F" = formula_5, the coefficients "u" are determined by the linear system "AU" = "F". The stiffness matrix is symmetric, i.e. "A" = "A", so all its eigenvalues are real. Moreover, it is a strictly positive-definite matrix, so that the system "AU" = "F" always has a unique solution. (For other problems, these nice properties will be lost.)