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.
Fracture toughness
The problem can also be formulated in terms of stress instead of energy, leading to the terms stress intensity factor "K" (or "K" for mode I) and critical stress intensity factor "K" (and "K"). These "K" and "K" (etc.) quantities are commonly referred to as fracture toughness, though it is equivalent to use "G". Typical values for "K"are 150 MN/m for ductile (very tough) metals, 25 for brittle ones and 1–10 for glasses and brittle polymers. Notice the different units used by "G" and "K". Engineers tend to use the latter as an indication of toughness.