Tools for Selecting the Right Strain Gauges

Before the right strain gauge can be selected, the measurement goal needs to be clearly set. The main question to be considered is whether the strain gauge will be used for experimental tests or for transducers' manufacturing.

Experimental Tests

Transducer manufacturing

 
  • Experimental stress analysis
  • Residual stress analysis
  • Analysis of loading
  • Life-time analysis

  • Determining thermal stress
 
 
  • Force
  • Mass
  • Torque
  • Pressure
  • Strain
 

 The strain gauge selection for experimental test will be done regarding the following selection criteria:

Geometry: Number and position of grids (pattern)
Strain gauge series: Construction of strain gauge
Connections: Type and position
Temperature response adaptation: Material to which strain gauge temperature response is matched
Active grid length: in mm
Electrical resistance: in Ohm

Geometry of the Strain Gauge

  Linear strain gauges (e.g. LY4) have one measuring grid and measure the strain in one direction.
  T rosettes (e.g. XY3) have 2 measuring grids arranged at a 90° degree offset from each other. Typical applications for this strain gauge type include analysis of a biaxial stress state with known principal directions as well as measurements on tension and compression bars.
  V shaped strain gauges (e.g. XY4) have 2 measuring grids arranged at a 90° degree offset from each other. Typical applications for these strain gauges include measurements on torsion bars and determining shear stresses as they occur in shear beams in the area of neutral fibers.
  Double linear strain gauges (e.g. DY4) have two measuring grids arranged parallel to each other. Typical applications for these strain gauges include measurements on bending beams.
   Rosettes with 3 measuring grids (e.g. RY8) arranged at an angle of 0°/45°/90° or 0°/60°/120° are an appropriate choice for analyzing the biaxial stress state with unknown principal directions.
  Strain gauge chains (e.g. KY8) consist of 10 or 15 very small measuring grids that are placed on a common carrier at a constant spacing (equidistant apart) plus one compensation SG. Strain gauge chains are especially suitable for determining strain gradients.
  Full bridge strain gauges (e.g. VY4) have 4 measuring grids, which are arranged so that each is offset to the next at an angle of 90°. Typical applications for full bridge strain gauges include measurements on tension/compression bars and determining shear stresses as they occur in shear beams.

Strain Gauge Series

HBM offers different strain gauges series for strain measurement. Strain gauge series are defined by the combination of strain gauge carrier (e.g. Polyimide) and the measuring grid foil (e.g. Constantan). All strain gauges within a strain gauge series offer the same carrier and measuring grid foil material. Therefore many specifications are identical for one strain gauge series.

For experimental tests robust and flexible strain gauges, which can be used under arduous conditions, have distinct advantages. Strain gauges having the synthetic, polyimide, as carrier material for the measuring grid and with the series identification “Y” are in this category. This series contains a large number of different types of strain gauges used for a variety of tasks in experimental tests. There are also numerous special types, e.g. hole-drilling and ring core rosettes for the determination of residual stresses in structural parts; strain gauge chains for the investigation of stress distribution on complex structures.

Connections

 HBM offers strain gauges with different connection configurations.

 

Integrated solder tabs (e.g. LY4)

  • allow direct soldering on the strain gauge
 
 

Big solder tabs with strain relief (e.g. LY6)

  • allow direct soldering on the strain gauge, at the same time providing nearly full mechanically decoupling of solder tabs and strain gauge carrier
 
 

Ni-plated copper leads, uninsulated, approx. 30 mm (1.18 inch) long (e.g. LY1)

  • no direct soldering at the strain gauge
  • for full mechanical decoupling of cables and strain gauge
  • use of separate solder terminals directly on the strain gauge required
 
 

Fluoropolymer-insulated connection wires, approx. 50 mm (1.97 inch) long (e.g. K-C LY4)

  • no direct soldering at the strain gauge
  • Fluoropolymer insulation prevents the cable from sticking during installation
  • use of separate solder terminals near the strain gauge required
 
 

Fluoropolymer-insulated connection wires, approx. 50 mm (1.97 inch) long (e.g. K-C LY4)

  • cable length as required from 0.5 to 10 m (1.64 to 32.81 ft) in 2-wire, 3-wire and 4-wire option available
  • no direct soldering at the measuring point at all
  • Fluoropolymer insulation prevents the cable from sticking during installation
 

Temperature Response Adaptation

Strain gauges that are connected individually in a Wheatstone quarter bridge circuit will show an output signal if the temperature changes. This signal is called “apparent strain” or “thermal output” and is independent of the mechanical load on the test object.

However it is possible to adjust a strain gauge to the thermal expansion coefficient of a specific material such that the output signal is very small in case of a temperature change. Such strain gauges are called strain gauges with “matched temperature response” or “self-compensated” strain gauges.

To benefit from their matching to the temperature response, strain gauges must be selected to the thermal expansion coefficient a of the test material.

Code Material Thermal expansion coefficient ∝
1 Ferritic steel 10.8 ⋅ 10-6/K (6 ⋅ 10-6/°F)
3 Aluminum 23 ⋅ 10-6/K (12.8 ⋅ 10-6/°F)
5 Austenitic steel 16 ⋅ 10-6/K (8.9 ⋅ 10-6/°F)
6 Quartz glass / composite 0.5 ⋅ 10-6/K (0.3 ⋅ 10-6/°F)
7 Titanium / gray cast iron 9 ⋅ 10-6/K (5 ⋅ 10-6/°F)
8 Plastic 65 ⋅ 10-6/K (36.1 ⋅ 10-6/°F)
9 Molybdenum 5.4 ⋅ 10-6/K (3 ⋅ 10-6/°F)

Active Grid Length

The strain gauge measuring grid length depends on aim of measurements, as the result of a measurement using strain gauges will be the mean strain underneath the measuring grid. In general, measuring grid lengths of 3 or 6 mm (0.118 or 0.236 inch) represent a good solution.

Long measuring grids are recommended where there is an inhomogeneous material such as e.g. concrete or wood. A long strain gauge will bridge the inhomogeneities of the work piece and, as a measurement result, will supply the strain underneath the measuring grid.

Short measuring grids are suitable for detecting a local strain state. They are therefore suitable for determining strain gradients, the maximum point of notch stresses and similar stresses.

Electrical Resistance

 

HBM strain gauges are available in 120, 350, 700 and 1,000 Ohm versions. The selection of the resistance depends on the constraints of the measurement task. Other resistances on request.

Low Ohm strain gauges

High Ohm strain gauges

+ Lower influence of electromagnetic intreferences + Lower influence of electrical resistance in connection paths (slip-rings, cables,...)
+ Lower influence of a change in isolation resistance - „Better“ antennas in the case of interferences
- Higher power requirement - Higher influence of a change in isolation resistance
- More self-heating due to higher current flow compared to high ohm strain gauges