Scratch Hardness
When two materials of different hardness are forced against one another, the softer material will give way by deforming or scratching while the harder material is unaffected. A scale of relative hardness can be established based on the ability of one material to scratch or deform another. A kit containing files of various known hardness is usually used for scratch testing. The files are small and portable and allow access in tight spaces.
Scratch testing provides qualitative test results and can be very helpful in the early stages of a failure investigation, especially when a comparative evaluation of material "A" against material "B" will suffice. It is also a great way to evaluate the hardness of heat-treated components potentially damaged from the temperatures of a fire.
Dynamic Hardness
Dynamic hardness tests determine material properties by bouncing an object of standardized mass and shape against the material of interest. The indenter rebound height and velocity is a function of the energy absorbed during the deformation process and can be calibrated to an equivalent material hardness. The rebounding indenter method of measurement does not require a standardized load and lends itself to light portable devices. These devices provide improved precision and accuracy as compared to scratch testing and lend themselves to again evaluating fire damage. Dynamic hardness should only be used on fairly massive components that will not move when impacted.
Static Hardness
Static hardness tests slowly force a standardized indenter into the test material under a given load. The relationship of load and indentation area or depth determines the hardness value for a given scale, that is, hard materials will allow minimal penetration and smaller resulting indentation areas. Varied combinations of indenter size, shape, and load define the various static hardness scales. Each scale is relatively narrow in its applicability and can only be used for certain materials and material conditions. For example, hardened 1040 steel and soft 6061-O aluminum require two vastly different hardness scales. A Rockwell C scale is required for determining the hardness of the heat-treated steel. The soft aluminum would be best measured using the Brinell hardness scale. Each scale, in this case, is associated with a different indenter shape, indenter load, and test machine. Static hardness test machines provide the most precise and accurate measurements of the methods summarized, but are not portable.
Static hardness testing allows the failure analyst the flexibility to inspect small components like bearings, chains, blades, and sectioned parts with maximum precision and accuracy. The static hardness test is the best way to identify and quantify manufacturing defect.
The most common static type hardness tests are associated with Rockwell, Brinell, and Vickers/Knoop scales. These scales will be summarized and discussed in upcoming articles.