Anvil effect in spherical indentation testing on sheet metal
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A spherical indentation test is considered to be invalid if there is presence of a visible mark on the side of the sheet metal facing the anvil and exactly below the indentation. With the available standard loads of the conventional testers such as Brinell and Rockwell hardness testers, it is difficult to avoid this anvil effect while dealing with the sheet metals. The penetration depth increases when the thickness of the sheet is reduced at constant indentation pressures. The reason behind this is the change in mode of deformation. When the thickness of the sheet metal is reduced, and the indentation test is carried out on it, then the sheet metal experiences first indentation, then bending, followed by lifting of the sheet from the anvil which leads to a forging mode of deformation. The modes of deformation were identified using a finite element simulation of the indentation process. Plots of normalized depth against normalized thickness were created for the same indentation pressure, and a second order polynomial curve was fitted to the data points. The equation of this curve quantifies the anvil effect. The anvil effect was identified as a function of sheet thickness, indenter radius, indentation load and two material constants. A method to correct this anvil effect was also developed using the equation representing the anvil effect. It is possible to obtain the equivalent geometry of indentation without anvil effect. A MATLAB program is developed to obtain the parameters defining the curve for the anvil effect. Indentation test on a sheet using three different indenters and corresponding loads is required for this method. For accurate prediction of the equivalent depth of indentation, a lower limit of 10 % and upper limit of 80 % for penetration depth (ratio of depth of indentation and thickness of sheet metal) was identified for the spherical indentation testing on the sheet metals. Verification of the curve fitting model was carried out with the indentation experiments on commercially available Niobium, Al2024-T3, Al7075-T6 and 1020 low carbon steel sheets. These tests show good agreement between fit, prediction, and experiments for the anvil effect.
Dhaigude, Mayuresh Mukund (2006). Anvil effect in spherical indentation testing on sheet metal. Master's thesis, Texas A&M University. Available electronically from