Selection and models of strain gages (SGs)
SGs are selected depending on the measurement task by model, series, connection type, temperature response adaptation, active measuring grid length, and electrical resistance. In terms of the application, the model of SG that is chosen is one of the first selection criteria:
Linear SGs...
...record strain in the direction in which their measuring grids are aligned. One possible application is determining the modulus of elasticity on tension and compression bars. The E-modulus for the material can be determined from the measured linear expansion in the primary load direction and calculated mechanical stress according to Hooke’s Law.
Double SGs...
...are used when 2 linear measuring grids must be laid parallel to each other – for example to record the bending load of a bending beam. To compensate for overlaid tensile/compression loads and thermal expansion caused by temperature, 4 active SGs are normally used in the Wheatstone bridge circuit. Another advantage of this type of circuit is the higher output signal compared to a quarter SG or half SG bridge.
T rosettes...
...have two measuring grids arranged on a carrier at a 90° angle to each other. They are used in experimental strain analysis to determine the biaxial stress state with known principle directions and to construct transducers with a design corresponding to a tension/compression bar. T rosettes are also used to determine the modulus of elasticity and transverse contraction index (also called the transverse strain index or transverse index).
V-shaped SGs...
...also have two measuring grids arranged at an angle of 90° to each other. They are also referred to informally as “shear strain gages.” They are typically used in the construction of torque transducers, also called shear force transducers.
Rosettes with 3 measuring grids...
...on a shared carrier are available in two basic shapes, which are identified by the angular spacing of their measuring grids. The two designs are 0°/45°/90° rosettes and 0°/60°/120° rosettes. The angle specifications refer to the directions of the 3 measuring grids. They are used to determine the biaxial stress state with unknown principle directions according to magnitude and direction.
Full bridge SGs...
...have four measuring grids that are already switched to a Wheatstone full bridge. They have numerous uses, for example to determine shear load on shearing columns or to determine a torsion moment of a shaft if it is only accessible on one side for SG installation.
Membrane rosettes...
...are used to construct membrane pressure transducers. The constructional design of the transducer membrane (or membrane rosette) requires that the four measuring grids of the rosette all be located precisely in the expanded or compressed zones when load is placed on the membrane – including warping of the membrane. Positive strain proportions and negative ones of equal magnitude are added together within the Wheatstone bridge circuit.
SG chains...
...combine a series of similar or evenly spaced measuring grids which share the same carrier. Typical designs of SG chains are: Measuring grid direction parallel or perpendicular to the longitudinal chain axis, measuring grid direction alternating between parallel and perpendicular to the longitudinal chain axis, and SG rosette chain (for example 5 rosettes in the design 0°/60°/120°). SG chains are used to determine the stress curve (strain or the stress curve resulting from it) over a specific section or the shift in the peak value due to the effect of loading being introduced at a moving point.
