Experimental mechanics is about measuring the behaviour of materials and structures when they are subjected to a load. The picture shows the photoelastic fringe pattern in a plastic model of a crane hook, from which a load has been suspended. The fringes are contours of stress (force per unit area) and where the fringes or contours congregate are sites of likely failure. Using polarised light, photoelastic fringes can be seen in any stressed transparent object, for example make a sandwich of two pairs of polarised sunglasses and a transparent freezer bag then stretch the freezer bag while holding the sandwich to the light. Yes, ok you will need two pairs of hands.

Photoelasticity has fallen out of fashion in experimental mechanics because, except for objects made from glass or transparent plastic, it cannot be used directly on engineering components. However, this photograph is an old favourite that has been used to illustrate several mechanics books and inspired an art installation in the Engineering Library at the University of Sheffield. It was taken in the 1980s by John A Driver, a technician in the Department of Mechanical Engineering at the University of Sheffield.

For more a little more on photoelasticity see http://www.experimentalstress.com/basic_experimental_mechanics/photoelasticity.htm

Experimental mechanics is rather vague, or all embracing term, given to the area of engineering in which I have specialised for the last 25 years.  ‘Mechanics’ is study of the behaviour of a body when it experiences forces or displacements.  The ‘body’ could be anything from the human body, a planet, an aeroplane or a part of an engine.  ‘Experimental mechanics’ is about measuring the behaviour of the body under the action of loads, i.e. forces or displacements, so that we can develop a better understanding of  the performance of the ‘body’ and predict its future behaviour.   The behaviour we measure is usually some type of deformation, for instance if you have the misfortune to reverse (displace) your car (the body)  into an embarkment retaining wall while parking then the car will be bent (deformed).  We might do something similar in a laboratory and then predict the performance of your car in a crash.

The challenge is to measure deformation over the complete surface of a ‘body’ in real-time for events that might be happening quite fast, as in the example above, and in extreme conditions.  ‘Extreme conditions’ is short-hand for a collection of difficult situations including very high temperatures found in an engine;  hazardous environments created by chemical or nuclear activity; as well as very large structures, such as the wings of passenger aircraft, and very small things, such as human cells.  My specialist area is using light-based techniques to make such measurements.  I am also interested in how to use the measurements to provide credibility for models that predict the behaviour of the ‘body’.

If  you would like to find out more, or if you found your way here expecting to find out about experimental mechanics in more detail then, you could try the website of my research group: experimentalstress.com.  If you are really keen, then I am helping to organise a workshop on ‘Strain Measurement in Extreme Environments’ in Glasgow at the end of August 2012, see http://mpsva2012.iopconfs.org/107471.