Document Type: Tutorial
NI Supported: Yes
Publish Date: Sep 6, 2006
Choosing the Right Strain-Gauge for Your
This article introduces strain, types of commonly measured of strain, and how to choose the strain gauge that best meets your needs, so you can take faster, more precise measurements.
Table of Contents
The Strain Gauge
Choosing the Right Type of Strain Gauge
Strain is the amount of deformation of a body due to an applied force. More specifically, strain (e) is defined as the fractional change in length, as shown in the figure defining strain gauge below.
Definition of Strain
Strain can be positive (tensile) or negative (compressive). Although dimensionless, strain is sometimes expressed in units such as in/in or mm/mm. In practice, the magnitude of measured strain is
very small. Therefore, strain is often expressed as microstrain (
), which is E x 10-6.
When you strain a bar with a uniaxial force, as depicted in the figure defining strain gauge above, a phenomenon known as Poisson strain causes the girth of the bar, D, to contract in the transverse,
or perpendicular, direction. The magnitude of this transverse contraction is a material property indicated by its Poisson's ratio. The Poisson's ratio (v) of a material is defined as the negative ratio of
the strain in the transverse direction (perpendicular to the force) to the strain in the axial direction (parallel to the force), or
. For example, Poisson's ratio for steel ranges from 0.25 to 0.3.
The Strain Gauge
While there are several methods of measuring strain, the most common is with a strain gauge. A strain gauge's electrical resistance varies in proportion to the amount of strain placed on it. The most
widely used gauge is the bonded metallic strain gauge.
The metallic strain gauge consists of a very fine wire or, more commonly, metallic foil arranged in a grid pattern. The grid pattern maximizes...