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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
S h e a r i n g S t r a i n (  ) i s a
m e a s u r e o f t h e a n g u l a r
distortion of a body. It is
approximated by the tangent
of the angle formed by the
angular change between two
l i n e s e g m e n t s t h a t w e r e
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
w h e n a b o d y h a s 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
F F
F
Force
(F)
Force
(F)
L +  L
L +  L
34970A Refresh UG.book Page 295 Wednesday, February 17, 2010 12:34 PM