Structure and Materials of Metal Resistance Strain Gauges
Structure and Materials of Metal Resistance Strain Gauges
Metal resistance strain gauges are divided into two types: wire-type and foil-type. The wire-type strain gauge consists of four basic components: the sensitive grid, the base and cover layers, the adhesive, and the lead wires.
Sensitive Grid: This is the most important part of the strain gauge, typically made of wires with a diameter of 0.015 to 0.05 mm. The longitudinal axis of the sensitive grid is referred to as the strain gauge axis. Depending on the application, the grid length can range from 0.2 to 200 mm.
Base and Cover Layers: The base layer maintains the geometric shape and relative position of the sensitive grid and lead wires, ensuring that strain from the measured object is quickly and accurately transferred to the sensitive grid. Therefore, the base layer is made very thin, usually between 0.02 and 0.4 mm. The cover layer protects the sensitive grid. If both the base and cover layers are made of special thin paper, the gauge is called a paper-based strain gauge. If made from various adhesives and thin films of organic resin, it is called a glue-based strain gauge, which is more commonly used today.
Adhesive: The adhesive bonds the sensitive grid, base, and cover layers together. When using strain gauges, adhesives are also used to firmly attach the strain gauge to the measured object.
Lead Wires: The lead wires are typically made of tinned copper with a diameter of 0.10 to 0.15 mm, soldered to the two output ends of the sensitive grid.
The foil-type strain gauge features a sensitive grid made from very thin metal foil, with a thickness of only 0.003 to 0.10 mm, manufactured using photolithography. Compared to the wire-type strain gauge, the foil-type has the following advantages:
Photolithography allows for the creation of sensitive grids in various complex shapes.
It exhibits minimal transverse effects.
It permits larger currents and has good heat dissipation, allowing higher matching bridge voltages, thereby increasing output sensitivity.
It has a long fatigue life and minimal creep.
It is highly efficient to produce.
However, the resistance values of foil-type strain gauges tend to vary more than those of wire-type strain gauges, sometimes differing by tens of ohms, requiring resistance adjustment. Due to these advantages, foil-type strain gauges are gradually replacing wire-type strain gauges.
Basic Requirements for Sensitive Grid Materials in Metal Resistance Strain Gauges:
High Gauge Factor: The gauge factor should be large and remain constant over a wide strain range.
Low Temperature Coefficient of Resistance: The resistance temperature coefficient should be small.
High Resistivity: The material should have high resistivity.
High Mechanical Strength: It should have high mechanical strength and be easy to draw into wires or roll into thin sheets.
Good Solderability with Copper Wires: It should have good solderability with copper wires and low thermoelectric contact potential with other metals.
Common materials for strain gauge sensitive grids include Constantan (copper-nickel alloy), Nichrome (nickel-chromium alloy), aluminum-magnesium alloys, aluminum-iron alloys, Manganin (copper-manganese alloy), and Phosphor bronze alloys. Foil-type strain gauges typically use materials like Constantan and Nichrome.