Influences of stress on the measurement of mechanical properties using nanoindentation: Part I. Experimental studies in
- PDF / 268,225 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 17 Downloads / 200 Views
MATERIALS RESEARCH
Welcome
Comments
Help
Influences of stress on the measurement of mechanical properties using nanoindentation: Part I. Experimental studies in an aluminum alloy T. Y. Tsui Department of Materials Science, Rice University, 6100 Main Street, Houston, Texas 77005
W. C. Oliver Nano Instruments, Inc., 1001 Larson Drive, Oak Ridge, Tennessee 37830
G. M. Pharr Department of Materials Science, Rice University, 6100 Main Street, Houston, Texas 77005 (Received 5 September 1995; accepted 10 November 1995)
The influence of applied stress on the measurement of hardness and elastic modulus using nanoindentation methods has been experimentally investigated using special specimens of aluminum alloy 8009 to which controlled stresses could be applied by bending. When analyzed according to standard methods, the nanoindentation data reveal changes in hardness with stress similar to those observed in conventional hardness tests. However, the same analysis shows that the elastic modulus changes with stress by as much as 10%, thus suggesting that the analysis procedure is somehow deficient. Comparison of the real indentation contact areas measured optically to those determined from the nanoindentation data shows that the apparent stress dependence of the modulus results from an underestimation of the contact area by the nanoindentation analysis procedures.
I. INTRODUCTION
It has long been recognized that the hardness of metals and alloys measured by conventional hardness techniques is dependent on the stress state of the material. This was demonstrated as early as 1932, when Kokubo documented the influence of applied bending stress on Vickers hardness measurement in a wide variety of commercial metals and alloys.1 Since then, a large number of studies have been conducted to further explore the phenomenon using both uniaxial and biaxial stress states and measuring the hardness by Brinell, Rockwell, and Vickers methods.2–6 In addition to the obvious implications concerning the nature of hardness as a fundamental material parameter, the results of these studies suggest that hardness measurement may be useful for characterizing residual stresses in materials.2 A typical observation of the influence of stress on hardness is shown in Fig. 1, which presents data obtained by Sines and Carlson for the Rockwell B hardness of a high carbon steel bar loaded in 4-point bending.2 Both tensile and compressive stresses were applied, with care being taken to assure that the stresses never exceeded the elastic limit. The data in the figure show three features commonly observed in such tests. First, the effects of stress are small; rarely is it observed that applying a stress produces a hardness change of more than 10%. Second, the measured hardness decreases with applied tensile stress and increases with applied compressive 752
http://journals.cambridge.org
J. Mater. Res., Vol. 11, No. 3, Mar 1996
Downloaded: 26 Aug 2014
stress. Third, when the stress is predominantly uniaxial, the influence of stress is greater for specimen
Data Loading...
