- Practice · 4+4 marks
Bonded and unbonded gauges, selection, installation
Short answer
Bonded gauges are cemented to the specimen; unbonded gauges are stretched wires in a frame. Common bonded types are wire, foil, semiconductor and rosettes; materials include constantan, karma, nichrome, isoelastic and platinum-tungsten. Select by gauge length, resistance, gauge factor, temperature range and environment; install by surface preparation, alignment, adhesive, curing, soldering and insulation check.
(a) Differentiate between bonded and unbonded resistance strain gauges. Name the common bonded types and gauge materials.
(b) List the factors for selecting a metallic strain gauge and describe the procedure for installing a bonded gauge.
(a) Bonded and unbonded gauges
| Point | Bonded gauge | Unbonded gauge |
|---|---|---|
| Construction | Grid of thin wire/foil cemented to a backing and bonded to the specimen surface | Wire stretched between insulated pins under pretension in a frame |
| Strain transfer | Through adhesive | Through relative movement of frame and armature |
| Use | Direct surface strain, stress analysis | Inside transducers (force, pressure, acceleration) |
| Size, cost | Small, cheap, used once | Larger, reusable |
| Hysteresis, creep | Depends on adhesive | Very low |
| Strain range | Up to a few percent | Small |
Common bonded types:
- Wire gauge: a grid of fine wire (about 0.025 mm) on paper or epoxy; the older type.
- Foil gauge: an etched metal foil grid on a polyimide backing; the most used type, it has better heat dissipation, stability and shape flexibility.
- Semiconductor gauge: silicon, high gauge factor (100-200), strongly temperature-dependent.
- Rosettes: two or three gauges at fixed angles for biaxial strain.
Gauge materials: constantan (Cu-Ni, , low temperature coefficient), Karma (Ni-Cr), nichrome, isoelastic (), platinum-tungsten (high temperature).
(b) Selection factors
- Gauge length (short for stress peaks, long for averaging on heterogeneous material such as concrete).
- Gauge resistance (120 or 350 ), gauge factor and its tolerance.
- Strain range, fatigue life and elongation limit.
- Temperature range, and self-temperature compensation matched to the specimen material.
- Backing, adhesive and environment (moisture, chemicals).
- Grid pattern: uniaxial, rosette, diaphragm, or shear pattern.
Installation of a bonded gauge
- Choose the position and direction of principal strain.
- Prepare the surface: degrease with solvent, abrade with fine emery paper, and clean again with a conditioner and neutralizer.
- Mark gauge alignment lines with a pencil (not a scriber).
- Apply adhesive (cyanoacrylate or epoxy), place the gauge using tape, and press with a thumb for about a minute under steady pressure.
- Cure as the adhesive requires (room temperature or heat).
- Solder lead wires to the terminals; use a three-wire arrangement for lead compensation.
- Check insulation resistance (above 500 M) and gauge resistance, and apply a moisture-proof coating.
✓ Answer checked by Claude Opus 5.5 (high effort)
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