Why Strain?

Engineers are interested in the effects of applied stresses and forces on structures. Most of the time these stresses are predicted via mathematical methods such as Finite Element Analysis (FEA), however, to verify these predictions it is essential to obtain stress data by measurement.

Stress (σ) cannot be measured directly, so typically the surface strains (ε) of a component are measured and the stresses then calculated using the stress-strain relations.

Understanding the Basics

At the most basic level, stress and strain are related by the Young’s modulus (E), such that:

A material under tension will extend in the longitudinal axis and contract in the transverse axis, by an amount equal to the materials Poisson ratio, ν. Any measurement of strain must also take this into account.

For plane stress (σZ = 0), the relations are:

Where E=Youngs Modulus and v=Poisson ratio.

Once the strains are converted to stresses, force can be obtained easily from knowledge of the cross-sectional area:

How they work

The most common type of strain gauge is the thin metal foil resistance strain gauge. A thin metal foil is mounted to a backing material, which is typically adhesively bonded to a surface. When the surface and by association the gauge undergo strain, a measurable change in resistance (ΔR/R) is seen relative to the sensor’s gauge factor (K):

For a general-purpose Constantan metal foil gauge, the gauge factor is usually around 2.

Wheatstone Bridge Configuration:

To convert the change in resistance into a measurable electrical signal, strain gauges are often configured in a Wheatstone bridge circuit. This circuit balances the electrical resistances, and any change in resistance caused by strain creates an imbalance in the bridge. This imbalance is then converted into an electrical voltage signal that can be easily measured and interpreted.

Gauge arrangements

There are a range of gauge arrangements, which when wired up in the correct configuration can provide gain in a particular direction, whilst reducing the effects of other strain components and other erroneous effects such as temperature drift and Poisson’s strain. The table below shows a range of gauge arrangements and configurations:

Range of gauge arrangements and configurations | Xi Engineering Consultants

Benefits of using strain gauges
  • Measure static and dynamic stresses of components in-situ.
  • High accuracy of up to 0.5 % possible for properly installed strain gauges.
  • Can be applied to a wide range of materials such as metals, composites and polymers.
  • Cost effective.
Client benefits

Xi Engineering Consultants have the expertise, experience, and equipment to carry out strain gauge installations and collect and analyse data from any type of component in the harshest of environments. We can also model the expected strains beforehand in COMSOL to help in specifying the correct sensor for the task. Please contact us to discuss your project’s requirements and how Xi can help.