common types of strain measurements as shown below:
Normal strain (ε) is a measure of the deformation along the axis of the applied force, where ε=ΔL/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 (υ) 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, where υ = -
ε
t
/ε, where ε
t
= ΔD/D and ε = ΔL/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.
Strain Sensors The metal foil resistance strain gage is by far the most widely used strain measurement
sensor. It consists of a thin metallic foil grid bonded to a thin insulating, adhesive backing. The resistance
of the foil varies linearly with strain. Strain in the test body is simply the ratio of the foil’s strained to
unstrained resistance: ε = ΔR / R.
The Gage Factor (GF) indicates the sensitivity of a strain gage and is a measure of the fractional resistance
change per strain: GF = (ΔR / R) / ε. Devices with a higher gage factor will exhibit a greater resistance
change per strain applied.
Strain gages are available in various numbers and configurations of elements. The most common element
is the single gage form shown below. Multi-element strain gages, also called rosettes, are used to measure
Keysight DAQ970A User's Guide 207
4Measurement Tutorials
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