Residual stress measurement at the micro-scale

The problem

The aim of this project was to measure residual stresses within metal samples at the micro-scale. This can be achieved by cutting a slot in the metal’s surface thus relieving the residual stress. This relaxation results in deformation of the surface around the slot. Measurement of the surface deformation enables calculation of the residual stresses at the bottom of the slot. The problem is therefore how to measure displacments in the order of a few nanometers on the surface of the specimen.

The solution

A first image was taken of the metal surface using a Focused Ion Beam (FIB). The FIB is similar to a scanning electron microscope except ions are used instead of electrons. The image shows an area of the specimen 60 μm in width. A 20μm long slot was then cut in the surface using the Focused Ion Beam. This relieved the residual stresses resulting in deformation at the specimen's surface. A second image was then taken of the surface. The two images are shown below.

The Video Gauge was used to analyse the two images and measure displacements of the surface around the slot. Displacements were measured at a number of points along a line perpendicular to the slot.

The graph below shows the measured displacements plotted against distance from the slot. The displacements predicted by a simple Finite Element model are also plotted and agree with the Video Gauge measurements to within a few nanometers.

Advantages of the Video Gauge

The Video Gauge is a truly scale independent measurement instrument. In this project it was used to measure displacements of less than 10nm on a 60μm specimen. In other projects, the very same system has been used to measure displacements of 1m on a real suspension bridge. That is a measurement range spanning eight orders of magnitude.

One of the strengths of the Video Gauge is that it measures in real-time. However, it is equally at home post-processing images. The Video Gauge is able to record live images for later post-processing but in this case the images came from a scanning electron microscope.

With the Video Gauge, the user has full control over the exact positioning of the measurement points. This enabled the surface deformation to be measured right up close to the edge of the slot.

Acknowledgements

Images and finite element prediction are Copyright 2008 Nicholas Daynes, Graeme Horne, Department of Mechanical Engineering, University of Bristol. Reproduced by permission.