User`s guide

295
Chapter 7 Tutorial
Measurement Fundamentals
4
7
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 3 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 (). 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 (). There are three common types of strain
measurements as shown below.
Normal Strain () is a measure
of the deformation along the
axis of the applied force.  =
L/L
Shearing Strain ( ) 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 () 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.  = -
t
/, where

t
= D/D and  = L/L
Stress Stress is a term used to compare the loading applied to a
material with its ability to carry the load. Stress () in a material
can not be measured directly; it must be computed from material
properties and measurable quantities such as strain and force.

D -D
FF
F
Force
(F)
Force
(F)
L + L
L + L
34970A Refresh UG.book Page 295 Wednesday, February 17, 2010 12:34 PM