HP 34970A Stress, 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, 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 (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 (με). There are three common types of strain measurements as shown below.

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