The same strain acting on a gauge can add to or subtract from the output depending on which bridge arm it is connected to. Explore the relationships introduced in Strain Gauge Principles and Strain Measurement.
Before experimenting
As in the earlier article, attach the gauges in order counterclockwise from the upper left: G1 → G2 → G3 → G4, and define the output as Vout = VR − VL, the voltage at the right midpoint minus the voltage at the left midpoint. Adjacent arms have opposite contribution signs, so the sequence is + / − / + / −. The important part is not the number itself, but the consistent polarity of the actual arm position.
A gauge grid parallel to the specimen length measures longitudinal strain, while a perpendicular grid measures transverse strain. In this uniaxial-stress teaching model, the transverse strain is calculated using the Poisson relationship εT = −ν εL, with ν = 0.30. In bending, the reference direction is still the beam length, not the transverse load arrow.
Tension sets both faces to εL = +1000 µε. Pure bending sets the top surface to −1000 µε (compression) and the bottom to +1000 µε (tension). This differs from the downward-loaded cantilever example in the earlier article; the tension and compression faces can change depending on the load and constraints.
How to set it up
- Use the Load button to switch between tension and bending, and the Bridge button to cycle through Quarter, Half, and Full repeatedly.
- Once the Load and Bridge configuration is fixed, the gauge number and attachment position appear directly below. The number of available gauges depends on the bridge type. (Theoretically, one bridge box can contain four quarter-bridge circuits, but that is omitted in this experiment.)
- In Advanced settings, each gauge can be changed to an unloaded dummy off the specimen. Turning the dummy off restores the saved attachment face and direction.
- As you change the settings, verify that even when the same gauge is attached at the same location and in the same direction, the output changes if the gauge number changes.
Example configuration · ν = 0.30 · ε₀ = 1000 µε
Gauge Configuration
Selecting an occupied G number swaps only the two arm assignments. Surfaces and orientations stay unchanged.
TopBottom (hidden gauges)
G1–G4 identify gauge circuit arms. Top and bottom are mounting surfaces; grid orientation determines the measured direction. Spacing within the central region prevents overlap without affecting calculations. Dark ochre gauges are on top, and pale yellow gauges are on the bottom. Dashed lines show hidden bottom edges. Bending assumes uniform curvature, with compression on top and tension below; deformation is exaggerated. In tension, both surfaces have equal strain in the same direction.
Advanced settings
A dummy is an unloaded gauge off the specimen (ε = 0). Uncheck it to restore its saved surface and orientation. Hidden gauges are excluded from calculations and their arms are shown as fixed resistors.
Counterclockwise from upper left: G1 (+) → G2 (−) → G3 (+) → G4 (−).
Vout = VR − VL
S = ε₁ − ε₂ + ε₃ − ε₄
Circuit legend — solid: active · dashed: dummy · gray: unused (fixed resistor). Quarter uses one gauge and three fixed resistors; Half uses two of each; Full uses four gauges.
Combinations to explore
- Show the default bending result: four contributions of +1000 µε give S = +4000 µε.
- Select Load: Tension and Bridge: Half. Set Gauge A to G1 / Top · longitudinal and Gauge B to G2 / Top · transverse. Their strains are +1000 µε and −300 µε, giving S = +1000 − (−300) = +1300 µε.
- Change Gauge A’s connection from G1 to G2. The two gauges swap arm numbers. The longitudinal gauge originally labeled G1 retains its mechanical strain but now contributes −1000 µε.
- Quarter Bridge configures only Gauge A, with three fixed completion resistors. Making this gauge a dummy in Advanced settings gives zero mechanical response.
Scope of the model
The result S is the signed sum of strains. For small deformation, identical nominal resistance, and equal gauge factor (GF), Vout/Vex ≈ (GF/4) × S × 10⁻⁶ (with S in µε). S should not be interpreted as the actual strain at a single specimen point or as the calibrated reading of the instrument.
Half/Full indicates the number of gauge arms being combined. If a selected arm is changed to a dummy, the number of mechanically active arms decreases. This is not a standard experiment specifying an actual instrument naming convention or wiring scheme. The actual output also depends on GF, excitation voltage, wiring, and temperature. Dummies are treated as unloaded reference gauges, and temperature changes are not calculated. This is an educational model for small strains in isotropic linear-elastic materials.
The Poisson effect and the basic concept of active and dummy gauges can be reviewed in NI’s strain measurement guide. Since numbering and output polarity can vary between references, compare the circuit diagrams together.