Mechanical measurements at the micron and nanometer scales

Experimentation at the micron level requires specific tools and methods. It will be illustrated how some of these tools have to be combined to achieve this goal. Because the determination of strains at the micron and nanoscales has been explored with the aid of probe microscopy. attention needs to be devoted to the limitations of digital image correlation. In this context it is illustrated how the results of the correlation method are affected by several process parameters. such as subset size, out-of-plane deformation. displacement gradients and scanning noise introduced in measurements. We also present measurements of material fracture on small, elliptically perforated (MEMS) specimens under tension via specially constructed equipment. Of particular interest is how the failure strength of polycrystalline silicon (grain size similar to0.3 mum) is influenced by the magnitude of the notch radius (1-8 km) and the stress concentration factor (3-10). It is demonstrated that when the notch radius falls below 3 mum, the strength of the material is no longer governed by the critical stress criterion that controls failure initiation for larger radii (e.g. 8 mum or larger). In fact, the stress gradient plays a significant role in the failure process. which is explained in terms of the statistical spatial distribution of small flaws or cracks and the size of the zone at the notch tip under high stress. Failure stresses increase by a factor of two or more at a characteristic size of 1 mum. (C) 2002 Elsevier Science Ltd. All rights reserved.