Metal Finishing Guide Book

2013

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plastic deformation even at very small loads, and they are easier to manufacture precisely as they have only three sides. Cube corner indenters are even sharper than the Berkovich, causing higher stresses and strains. They can be used to estimate fracture toughness at relatively small scales. While using spherical indenters produces only elastic deformation at low loads, they could be used to examine yielding and work hardening, and to generate the entire uniaxial stress-strain curve 4. Hardness, Modulus and Creep During a nanoindentation measurement the indenter is driven into the material as shown in Figure 2, both elastic and plastic deformation processes occur. This produces an impression with a projected area Ap and surface area As of contact that depends on the shape of the indenter to a contact depth, hc. The nanoindentation measurement includes a loading and unloading cycle. Figure 3 shows indentation load (F) plotted against the displacement (h) relative to the surface before deformation, where the data was obtained for one complete indentation cycle. The important quantities are the maximum depth (hmax) of penetration, the peak load (Fmax), and the final depth after unloading (hr). The slope of the upper portion of the unloading curve, S is known as the contact stiffness. The contact depth and stiffness are determined using the Oliver-Pharr method as described in ISO 14577 and ASTM E2546. The hardness and elastic modulus are derived from these quantities. In nanoindentation the Martens Hardness is determined from the loading portion of the load-displacement curve and includes the materials resistance to both plastic and elastic deformation. The Martens Hardness can be plotted as a function the indentation depth. Martens Hardness is given by, HM = F A s (h ) Instrumented Indentation Hardness correlates to traditional forms of hardness as it is a measure of the resistance to plastic deformation. Instrumented Indentation Hardness is given by HIT = Figure 2: Schematic of indenter (blue) deforming test material (green). Fmax Ap Reduced elastic modulus, Er that is indicative of the stiffness of the sample is given by Er = π S 2β Ap is a constant that depends on the geometry of the indenter. The reduced elastic modulus 452 Figure 3: Load-displacement curve measured on a nanoindentation tester.

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