Strain Gauge Measurement – A Tutorial（应变仪测量u2014u2014一个教程）.pdf

Application Note 078 Strain Gauge Measurement – A Tutorial What is Strain? Strain is the amount of deformation of a body due to an applied force. More specifically, strain (ε) is defined as the fractional change in length, as shown in Figure 1 below. Force Force D L ∆L ε = L Figure 1. 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 –6 expressed as microstrain (µε), which is ε × 10 . When a bar is strained with a uniaxial force, as in Figure 1, 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 Poissons Ratio. The Poissons Ratio ν 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 ν = – ε /ε. Poissons Ratio for steel, for example, ranges from 0.25 to 0.3. T The Strain Gauge While there are several methods of measuring strain, the most common is with a strain gauge, a device whose electrical resistance varies in proportion to the amount of st