The correlation between the internal material length scale and the microstructure in nanoindentation experiments and sim
- PDF / 821,773 Bytes
- 11 Pages / 584.957 x 782.986 pts Page_size
- 29 Downloads / 178 Views
.Y. Huang Civil Engineering, Northwestern University of Chicago, Evanston, Illinois 60208-3109
M. Go¨ken and K. Durst General Material Properties, University of Erlangen–Nu¨rnberg, 91058 Erlangen, Germany (Received 11 August 2008; accepted 1 December 2008)
In the present work a new equation to determine the internal material length scale for strain gradient plasticity theories from two independent experiments (uniaxial and nanoindentation tests) is introduced. The applicability of the presented equation is verified for conventional grained as well as for ultrafine-grained copper and brass with different amounts of prestraining. A significant decrease of the experimentally determined internal material length scale is found for increasing dislocation densities due to prestraining. Conventional mechanism strain gradient plasticity theory is used for simulating the indentation response, using experimentally determined material input data as the yield stress, the work-hardening behavior and the internal material length scale. The work-hardening behavior and the yield stress were taken from the uniaxial experiments, whereas the internal material length scale was calculated using the yield stress from the uniaxial experiment, the macroscopic hardness H0 and the length scale parameter h* following from the nanoindentation experiment. A good agreement between the measured and calculated load–displacement curve and the hardness is found independent of the material and the microstructure.
I. INTRODUCTION
The mechanical properties of materials in small volumes [e.g., microelectromechanical system (MEMS), thin coatings, thin films, and precipitation in high temperature materials] are of special interest for analyzing and optimizing materials performance. The nanoindentation technique is already well established to measure the hardness and Young’s modulus in the nano- and micrometer range. It has been repeatedly reported that the hardness of metallic materials displays a strong dependence on the indentation depths in the micrometer range. A significant increase in hardness with decreasing penetration depths is referred to the so-called indentation size effect (ISE) (e.g., see Refs. 1–8). Size effects in metals have been observed in different nonuniform experiments at the micro- and nanoscale. Fleck et al.9 have reported that the strength in microtorsion experiments increase with decreasing wire diameter, whereas the yield stress in tension is not affected. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0123 J. Mater. Res., Vol. 24, No. 3, Mar 2009
http://journals.cambridge.org
Downloaded: 27 Mar 2015
Sto¨lken and Evans10 and Motz et al.11 have found an increase in the bending moment and the yield stress for thin films, respectively. Fleck et al.9 attributed these size effects to geometrically necessary dislocations (GNDs) associated with strain gradients. This concept follows from the work of Nye,12 Cottrell,13 and Ashby.14 They separated the total dislocation d
Data Loading...
