Birth of the Foil Strain Gauge
The birth of the foil strain gauge was a direct result of necessity, the Mother of Invention.
In the early 1950s Peter Scott Jackson of the Saunders-Roe Aircraft Company, East Cowes, Isle of Wight, UK, was tasked with finding an alternative to the wire-wound strain gauge invented by Simmonds & Ruge, MIT & CIT USA.
Saunders-Roe was constructing the Princess Flying Boat, and wanted to conduct stress analysis tests on the air frame. They required strain gauges that were more sensitive, had better fatique capabilities and were less likely to self-heat with the higher excitation voltages.
The Princess was a real beast, coming in at 42.1 m (138 ft) in length, with a staggering 66.9 m (219 ft) wingspan and measuring 17 m (56 ft) tall. Empty, she weighed 86,184 kg (190,000 lbs) and had a maximum take-off weight of 156,500 kg (345,025lbs) – give or take, that’s the same as a Boeing 767-300.
Jackson had observed that printed circuit boards for the aircraft electronic systems used a copper laminating process, which involved a photo-chemical etching technique. In a lightbulb moment he realised that a bonded strain gauge could be designed in exactly the same way, using copper/ nickel foil laminated to a thin epoxy carrier. The rest, as they say, is history.
Saunders-Roe used these first foil strain gauges on the airframe of the Princess Flying Boat, with great success. The flat foil strain gauges bonded to a flat surface provided the maximum transfer of thermal and strain sensing properties, whereas the wire-wound strain gauges made minimal contact with the surface that they were bonded to, giving poor heat sink properties in response to the applied excitation currents.