STRAIN GAUGES

RESISTANCE FOIL STRAIN GAGES

In 1856 Lord Kelvin reported that the electrical resistance of copper and iron wires increased when subjected to tensile stresses.  This observation ultimately led to the development of the modern "strain gage" independently at California Institute of Technology and Massachusetts Institute of Technology in 1939.  The underlying concept of the strain gage is very simple.  In essence, an electrically-conductive wire or foil (i.e. the strain gage) is bonded to the structure of interest and the resistance of the wire or foil ismeasured before and after the structure is loaded.  Since the strain gage is firmly bondedto the structure, any strain induced in the structure by the loading is also induced in the strain gage.  This causes a change in the strain gage esistance thus serving as an indirect measure of the strain induced in the structure.

Originally, strain gages were made of wire and, in fact, wire strain gages are still in use under special circumstances.  However, today foil strain gages are most widely used.  A typical strain gage is shown in the sketch below.  The strain sensing region of the strain gage is called the "gage grid."  The grid is etched from a thin metallic foil.  The orientation of the grid defines the strain sensing axis of the strain gage.  Electrical connections are made by soldering lead wires to the strain gage "solder tabs."  The entire strain gage is bonded to a thin polymeric backing which helps protect and support the delicate metal foil.

Foil strain gages are available in literally hundreds of shapes and sizes.  The strain gage shown is called a "uniaxial strain gage."  Other common strain gage configurations are: 

Biaxial strain gages which consist of two individual strain gage elements oriented precisely 90° apart, allowing strain measurements in two orthogonal directions. 

Rectangular, three-element strain gage rosettes which consist of three individual strain gage elements oriented precisely 45° apart, allowing the resolution of principal strains and principal directions regardless of the orientation of the rosette or the applied stress/strain.

Delta, three-element strain gage rosettes which consist of three individual strain gage elements oriented precisely 60° apart, allowing the resolution of principal strains and principal directions regardless of the orientation of the rosette or the applied stress/strain.