Strain gauges

Strain gauges are used to detect linear deformation, or strain, of a material. When bonded to the surface of a structure such as a bone, they can detect even the slightest bending (that is, change in outside diameter length) in that structure. They function on a principle first described by Prof. William Thomson (Lord Kelvin), who found that the resistance of metal wires changed when then were subjected to tensile strain. By fixing such a wire - in the form a strain gauge - to the surface of a structure, the wires will be stretched as the material is stretched. By measuring and calibrating the change in voltage resulting from this change in resistance, the strain gauge can determine the degree of deformation of a material. In turn, the degree of deformation can then be calibrated to the force causing the deformation.

Using strain gauges bonded to the site of the pectoral muscle's insertion on the humerus, (delto-pectoral crest, or DPC) researchers in the Flight Laboratory have measured the force this muscle produces during the downstroke in a flying bird. As the pectoral muscle contracts, it exerts a force on the DPC, bending it and the associated strain gauge.

Below are data collected from a magpie flying in the windtunnel at 6 ms-1 . The top series is the voltage change from the bending strain gauge, the bottom the electromyogram from the pectoral muscles. Note (on three successive wingbeats) (1) the rapidly increasing strain and high intensity of the EMG as the wing's upstroke inertia is arrested; (2) the momentary leveling of strain at the upstroke/downstroke turnaround; (3) and the rapid increase in EMG activity and DPC strain as the aerodynamic forces build to a peak at mid-downstroke.

Strain gauges attached to a streamlined cross-section aluminum model airplane fairing.

The following are links to .avi movies illustrating the change in voltage caused by the bending strain gauge as the metal fairing is bent along its thin and thick cross-section, and as it would bend if it were a humerus subjected to the forces of flapping flight.

• Strain thick (1 mb Quick Time movie)
• Strain thin (1 mb Quick Time movie)
• Low-amplitude strain (1 mb Quick Time movie)
• High-amplitude strain (1 mb Quick Time movie)