Strain Gage Principles: Measuring Mechanical Deformation

Understanding Strain Gages

Strain gages are sensors that exhibit a change in electrical resistance when subjected to deformation. The standard procedure involves bonding these sensors to mechanical structures to perform load testing. They provide data on deformations occurring in materials under mechanical stress or tension.

Principles of Operation

The operation of a strain gage is based on the properties of elasticity and Hooke's Law.

Types of Strain Gages

• Metal Gages: Resistance is sensitive to changes in length and cross-section. These can be filament or pellicular:
  • Filament: Constructed from a thread, usually a Copper alloy with Nickel or Chromium, bonded with strong adhesive to an insulating base.
  • Pellicular: Produced via screen printing, using chemical etching to define the layout of the conductive material.
• Semiconductor Gages: These offer significantly higher sensitivity to length changes caused by deformation.

Mathematical Foundation

As linear distortion is proportional to tension, the variation of the wire section and mechanical stress provokes a volume deformation proportional to the effort. For longitudinal deformation, the relationship is defined as:

ΔR / R = K • ΔL / L

The change in resistivity is proportional to strain, expressed as:

R = R₀(1 + K • ε)

Signal Processing and Compensation

Because these are resistive elements, they are best utilized with Wheatstone bridge circuits for signal processing. Since these components are sensitive to temperature fluctuations, bridge topologies are used to eliminate errors caused by environmental changes.

A common method for compensation involves a resistive bridge measurement where external conditions affect two gages equally, ensuring the electrical voltage at the junction remains stable. This accuracy is only achieved when one gage is subjected to mechanical stress while the other acts as a reference.

Best Practices for Installation

Proper placement of the probes is the primary factor in obtaining accurate measurements:

1. Identify critical points on the material surface that exhibit the principal strains of the fibers.
2. Ensure the appropriate gage is selected and correctly bonded to the structure.