STRAIN GAGE : MEASUREMENT OF STRAIN

Strain is a dimensionally unit-less quantity used to quantify the transverse and longitudinal deformations of a material due to an applied load. Depending on the experiment, strain can be reported as engineering strain, percent elongation, percent strain, or true strain. Engineering strain is the ratio of a material’s change in length to original length due to an applied load. Since strain has dimensionless units it is common industrial practice to convert engineering strain to percent elongation or percent strain by multiplying the decimal value of engineering strain by 100%. True strain assumes a material retains constant volume while being deformed allowing area or length to be used for strain calculations. Although true strain gives a more precise representation of a physical situation, it is not used for engineering designs because plastic deformation occurs when a material’s yield strength is exceeded. Engineering safety factors use a material’s yield point as an upper limit guaranteeing reliability. Therefore, the strain experiments performed were concerned with transverse and longitudinal engineering strains.

Strain measurements can be obtained via strain gages. Electrical strain gages consist of conducting wires in a zigzag pattern attached to insulating membrane. They are attached to specimens, usually with adhesives, in the direction of strain, and a current is passed through the strain gage conducting wires as a load is applied to a specimen. As the load deforms the specimen, the strain gage conducting wires will either reduce in length or elongate. Since electrical resistivity is dependent upon the length of a conductor, applying a load to a specimen while passing a current through the strain gage conducting wires will produce a measureable change in current. The change in electrical resistance due to an applied load is known as the piezoresistive effect. Changes in current can be measured by using a Wheatstone bridge, a four legged circuit with a single resistor on each leg connected by a junction after each resistor. This allows four separate gages to be used, each as a resistor, to measure longitudinal and transverse strain on the top and bottom of a specimen. The changes in electrical resistance and strain are related by the gage factor, which is the ratio of change in electrical resistance to strain. 

Applications of strain gages include measuring strain for tensile, compressive, torsional, bending, and shearing loads. Since strain is the result of applied load, or stress, strain gages can be utilized for pressure sensors. Furthermore, strain gages can be attached to strain sensitive designs for long duration monitoring allowing maintenance as a design’s strain reaches critical levels, often as a result of crack propagation or failure of a component.

The stress values at a specific location can be determined based on measured strain values on that location. On those cases that principle directions on a given location are not clearly defined rosette strain gauge should be used. Rosette strain gauge is made of three single foil strain gauge. Rosette strain gauges are manufactured in two patterns. One pattern places the single foil gauge in a 45 degree apart from each other. Second pattern places the single foil strain gauges in a 60 degree formation.