Increasing Initial Yield Stress at Small Length Scales
- PDF / 154,011 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 52 Downloads / 240 Views
U4.9.1
Increasing Initial Yield Stress at Small Length Scales I.Spary1,2, A.J.Bushby1, N.M.Jennett2 and G.M.Pharr3,4 1
Centre for Materials Research, Queen Mary, University of London, E1 4NS, UK Materials Centre, National Physical Laboratory, Teddington, TW11 0LW, UK 3 Oak Ridge National Laboratory, Oak Ridge,TN 37831 4 Materials Science and Engineering Department, University of Tennessee, TN 37996 2
ABSTRACT Plasticity size effects are well known in a wide variety of situations where either the material microstructure or a strain gradient exist at small length scales. Several theories have been developed to describe changes in the work hardening behaviour under these conditions but none that predict a change in the initial yield stress. Careful studies by Chaudhri et al and Pharr et al have unambiguously demonstrated plasticity size effects in ductile metals. In those experiments indentation stress-strain curves were generated using spherical indenters with radii ranging from a few micrometres to several hundred micrometres and these were compared to data from conventional compression tests. Large radius indenters produced a single indentation stress-strain curve independent of indenter radius with a power law hardening coefficient equivalent to that in the compression tests. However, the indentation stress-strain curves appeared at progressively higher pressures for smaller radius indenters. In this paper we model those experiments using finite element analysis methods. By inputting the uniaxial stress-strain data to the model (effectively, using von Mises criterion) the indentation stress-strain curves for the macro size indenters are reproduced. However, the model shows no length scale dependence for any size of indenter. We show that by off-setting the compression stress-strain curve by increasing the initial yield stress and inputting this data to the model, the indentation behaviour of the smaller radius indenters can be modelled. The increase in yield stress with decreasing indenter radius is demonstrated for Cu, W and Ir and is shown to be consistent with the initiation of yielding over a finite volume.
INTRODUCTION Size effects in indentation, and other situations where steep strain gradients exist, are well known (see ref. [1]). Gane and Bowden demonstrated that gold surfaces remained in the elastic regime during indentation under loads that would normally be expected to cause permanent deformation [2]. It has recently been shown from experiments on the indentation of semiconductor superlattice materials that yielding initiates over a finite volume [3,4]. As a consequence of this an indentation size effect was observed in the initial departure from elasticity. For smaller radius indenters, the initial yield pressure increased by as much as a factor of two [5]. In this paper we show that the same phenomenon occurs in ductile metals by comparing experimental data obtained with spherical indenters, uniaxial stress-strain data from the same materials and simulations of indentation experiments obtained fro
Data Loading...
