Resistive Strain Gages Overview

Resistive Strain Gages

• Bonded gages: Fine wire or conducting film cemented to a structural member to measure strain.
• Unbonded gages: Wires not bonded to any structure, but might have initial tension. They change resistance in response to mechanical loading.
• Changes in resistance are measured using a Wheatstone bridge circuit.
• Elongation or compression of the gage wires leads to a change in the resistance of the gage conductor (wire), which can be accurately sensed.

Gage Construction Details

• Composition: Thin metal (e.g., constantan) wire or film (approximately 0.025 mm thick), deposited on a base (e.g., polyimide), for protection and electrical insulation.
• Arrangement: The strain gauge wire or film might be sandwiched between protective transparent layers, with solder tabs exposed. The package is bonded to the surface with an adhesive.
• Connection: Strain gauges are wired and soldered using a connector (bondable) terminal.

Resistance of a Wire

• General Formula: Resistance (R) = (p * L) / A, where p = resistivity, L = length, and A = cross-sectional area.
• Stress: Stress (s) = (Force (W)) / Area (A).
• Strain: Strain (ε) = (Change in length (ΔL)) / Length (L) = (stress / Young's modulus).
• Young's Modulus (Y/E): The material's elasticity.

Gage Factor

• Definition: A measure of how much the resistance changes for a given amount of strain. The formula varies but often involves changes in resistance (ΔR/R), resistivity (Δp/p), and length (ΔL/L)
• Constant: For most gages, the gauge factor (GF) is constant over a wide range of strains.

Unbonded Strain Gage

• Initial Condition: Two pairs of wires initially under tension.
• Strain Application: When force is applied, one pair is strained more and the other less in equal amounts. This is helpful, in part, because it gives more consistent and reliable measurements.
• Filaments: Fine-wire filaments are stretched around the mounting pins.

Strain Gage Mounting Considerations

• Surface Preparation: Surfaces must be clean, e.g., using emery cloth and acetone.
• Drying: Adequate time is required for the cement to dry and harden.
• Bond Strength: The resistance element should be securely bonded to its mounting (strong bond). Any slip causes errors.

Types of Bonded Strain Gages

• Wire Gage
• Foil Gage
• Semiconductor Gage (high GF)

Strain Gage Bridge Output Considerations

• Amplification: Strain gage bridge outputs usually need amplification (e.g., using op-amps).
• Temperature Compensation: Using two matched gauges on one side of the bridge can reduce temperature sensitivity. At least one gauge is unstrained while the other gauges are actively measuring. Both gauges must be at the same temperature.

Strain Gage Rosettes

• Types: Single element, two element rosette, three element rosette, special purpose rosettes.
• Applications: For complex loading or geometry. Strain gage rosettes are used to obtain the state of stress when one gage is not enough, i.e., in biaxial or more complex situations.

Other Important Considerations

• Frequency Response: The frequency response is dependent on the structure to which it's bonded.
• Power Excitation: Power excitation voltage is limited by dissipation in the wire and heating of resistors.
• Strain Sensitivity: The minimum deformation that can be indicated by the gage per unit base length.
• Pressure Vessels: Formulas are given to calculate pressure in cylindrical thin-walled pressure vessels based on strain measurements.

Different States of Stress

• Uniaxial Loading: Tension or compression, a single gauge can determine the state of stress.
• Biaxial Loading: Stress is applied in orthogonal directions, more than one gauge and complex calculations are required to determine the state of stress.
• Types of Rosettes: Rectangular strain gage rosettes and equiangular (delta) and T-delta strain gauge rosettes determine the state of stress under complex loading.

This quiz covers the fundamentals of resistive strain gages, including the differences between bonded and unbonded gages, and their construction details. Learn how changes in resistance are measured using a Wheatstone bridge and discover the materials used in gage construction. Perfect for students studying electronics or materials science.