Measurement of residual stress by load and depth sensing indentation with spherical indenters
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B. Taljat STEEL Group 31054 Motto di Livenza, Travois, Italy, and Faculty of Mechanical Engineering, University of Ljubljana, Slovenia
G.M. Pharr Oak Ridge National Laboratory, Metals and Ceramics Division, P.O. Box 2008, MS-6093, Oak Ridge, Tennessee 37831-6093, and the University of Tennessee, Department of Materials Science and Engineering, Knoxville, Tennessee 37996-2200 (Received 4 January 2001; accepted 26 April 2001)
A new experimental technique is presented for making measurements of biaxial residual stress using load and depth sensing indentation (nanoindentation). The technique is based on spherical indentation, which, in certain deformation regimes, can be much more sensitive to residual stress than indentation with sharp pyramidal indenters like the Berkovich. Two different methods of analysis were developed: one requiring an independent measure of the material’s yield strength and the other a reference specimen in the unstressed state or other known reference condition. Experiments conducted on aluminum alloys to which controlled biaxial bending stresses were applied showed that the methods are capable of measuring the residual stress to within 10 –20% of the specimen yield stress. Because the methods do not require imaging of the hardness impressions, they are potentially useful for making localized measurements of residual stress, as in thin films or small volumes, or for characterization of point-to-point spatial variations of the surface stress.
I. INTRODUCTION
The effects of residual stress on hardness measurement were first demonstrated in 1932 independently by Kokubo1 and Kostron.2,3 Twenty years later, Sines and Carlson suggested that these effects could be used to locally measure the residual stresses in the surface of a metal.4 Numerous studies have since been conducted to examine the relationship between hardness measurement and residual stress.5–12 In general, hardness decreases with tensile stress and increases with compressive stress, although the effects of compression are often not as large as tension and sometimes not observed. These phenomena are qualitatively explained by simple principles of plasticity. Since the principal stress of greatest magnitude imposed by indentation is compressive and directed normal to the surface of the specimen, a residual tensile stress parallel to the surface increases the magnitude of the local Mises stress, thereby enhancing plastic deformation and reducing the hardness.4,13 Conversely, if the material is stressed compressively parallel to the surface, the Mises stress is reduced and the hardness is increased. J. Mater. Res., Vol. 16, No. 7, Jul 2001
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To date, the development of hardness testing as a tool for measuring residual stress has been based largely on conventional Rockwell testing and Vickers microhardness testing. In these methods, the hardness is deduced either directly from optical measurement of the size of hardness impression or indirectly from the total depth of penetration and
