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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, HP 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 ( ). 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.
Force
Force
(F)
(F)
Normal Strain ( ) is a measure
of the deformation along the axis
L + L
of the applied force.
= L / L
Shearing Strain ( ) is a measure
F
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
F
F
negative ratio of transverse and
longitudinal normal strain
D - D
when a body has a longitudinal
L + L
tensile force applied.
= -
/ ,
t
where
= D / D and = L / L
t
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.
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