Agilent Technologies 34970A manual Strain Gage Measurements

Chapter 8 Tutorial

Measurement Fundamentals

Strain Gage Measurements

Although the instrument does not directly support strain measurements, you can measure a strain gage using a 4-wire resistance measurement with scaling. However, BenchLink Data Logger software has built-in strain gage measurement capability.

When a force is applied to a body, the body deforms. The deformation per unit length is called strain (ε ). Strain may be either tensile (+) or compressive (-). Practical strain values are usually quite small (typically less than 0.005 inch/inch for most metals) and are often expressed in micro-strains (∝ε). There are three common types of strain measurements as shown below.

Normal Strain (ε ) is a measure of the deformation along the axis of the applied force. ε = ∆ L/L

Shearing Strain (γ ) is a measure of the angular distortion of a body. It is approximated by the tangent of the angle formed by the angular change between two line segments that were parallel in the undeformed state.

Poisson Strain (ν ) measures a property of materials known as the Poisson Ratio. It is the negative ratio of transverse and longitudinal normal strain when a body has a longitudinal tensile force applied. ν = -εt / ε , where ε t = ∆ D/D and ε = ∆ L/L

Force

(F)

F

D-∆ D

Force

(F)

L + ∆ L

Fγ

F

L + ∆ L

Stress Stress is a term used to compare the loading applied to a material with its ability to carry the load. Stress (σ ) in a material can not be measured directly; it must be computed from material properties and measurable quantities such as strain and force.

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