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Strain Gauges

By applying a strain gauge to a material, we can measure when stresses occur in the material, shape changes or “strain.” It is helpful when measuring with strain gauges to know the relationship between stress, force and strain. We can measure the stresses in the material or the forces and/or pressures exerted externally on the material because the resistance of the conductor of the strain gauge changes when the material of this conductor is stretched. We can calculate the strain by measuring the change in resistance of the conductor. In many cases, the strain can also be read directly with a (strain) measuring instrument.


Possibilities of applying strain gauges

A strain gauge can be applied to almost all materials such as (metals, plastics, wood, concrete composite material, bone (living) and etc…), the applications are therefore limited only by the creativity of the inventor. The limitation is often also in the environmental influences during the gluing process such as: “temp, wind and moisture”. This prevents the adhesive from curing properly. An alternative can then be weldable stretch strips (micro spot welding).

Scope of application of Strain Gauges

The area of application for a strain gauge is therefore very wide to know:
Test bars / construction parts / load cells / bathroom-kitchen scale / industrial weighing / medicine / dentistry / vibration measurement -monitoring / ship shafts – propulsion – torsion monitoring – power monitoring / Europter main shaft monitoring ect..

j.j. bos b.v. represents for more than 50 years the brand vpgsensor / Micro Measurements before it was Vishay / Micro measurements.

Strain gauges, stress, strain, poisson ratio, principal strain direction and Wheatstone Bridge

What is a Strain Gauge?

There are many different types of strain gauges, but the most common is resistance film. This consists of a very thin film bonded to an insulating backing, and the film is etched to resemble a long piece of wire applied in a zigzag fashion (early strain gauges were actually made of thin wire). This combination of foil and backing is often thinner than a human hair.

In use, the strain gauge is attached to a surface. After proper installation and connection to a suitable instrument, the gauge responds to deformations of the surface,  s0-called strain.


What is Strain?

What is Mechanical STRAIN?

When a body (a solid object) is stressed (is subjected to a load), the body is deformed.

Strain is a measurement of the deformation, or change in shape of a solid body.

Here is an example illustrating the concept of strain: When a rubber band (solid object) is stretched (subjected to a load), the rubber band becomes longer and thinner (changed in shape).  The change in length, compared to the original length, is strain.

Although it has no dimensions, strain is usually expressed in units represented as ε, which is in/in or mm/mm. Since the magnitude of the measured strain is small, the standard practice is to use units of microstrain (µε), which is [ε x 10^-6].


Stress, Strain and how are they tied together?

Stress and Strains strongly tied together. In the world of Experimental Stress Analysis, we use these ties to calculate the applied stress on a body by measuring the strain and using Hooke’s Law.

Hooke’s Law states that tensile stress (σ) is linearly proportional to the strain (ε) by the modulus of elasticity (E) in the linear elastic region:

This illustrates the need to provide high accuracy strain measurement, in order to perform Experimental Stress Analysis studies to validate the strength of materials and structural designs, as well as to accurately design Load Cells, Transducers and Force sensors with Strain gages.

How Does a Strain Gage Work?

The strain gage is a component used in an electrical circuit.  All components in an electrical circuit exhibit some opposition to the flow of electrical current, or resistance, which can be measured by electronic instrumentation.   When the strain gage is stretched (or strained), its resistance changes in direct proportion to the strain.  By measuring the change in electrical resistance, the strain is also known.  It’s as simple as that.

Different Strain Gages for different Stresses

Load can be applied in many different directions, with each causing a distinctive stress reaction on the body:

To meet the requirements of accurately measuring stresses & strains in such elements, specific strain gage pattern have been developed to do so:

Poisson Ratio and Structural Stress Analysis:

The ratio referred to as Poisson ratio describes the ration between material deformation in perpendicular direction to the load. So simply put. When you pull a material, the material will get longer but also narrower/thinner in the transverse direction.

Poisson’s ration is usually between 0.28 and 0.33 and is opposite in sign

How to Measure Strain when its direction is unknown?

Measuring stress where the strain has a known direction (for example: bending beam) is pretty straight forward:

A uniaxial stress condition will be calculated directly from the Strain gage reading of strain multiplied with the Modulus of elasticity

But how to measure & calculate stress while the direction of principal strains is unknown?

In order to do so, a 3-element Rosette can be used:

Using the 3-element rosette will allow to calculate he principal strains and to correctly calculated the main stress applied on the part:


Strain Gage and the Whetstone Bridge connection

Since the invention of the electrical resistance strain gage more than a half century ago, the Wheatstone bridge has become the sensing circuit of choice in most commercially available strain gage instrumentation. This is due in large measure to its inherent ability to:

1. detect the small resistance changes produced in the strain gage as it follows even minute dimensional changes on the surface of a test part under load,

2. produce a zero output voltage when the test part is at rest, and

3. provide for compensation of temperature-induced resistance changes in the strain gage circuit.

To varying degrees, each of these factors is essential for accurate strain gage measurements. In the majority of strain gage applications for the determination of the state of stress on a test-part surface, individual strain gage elements, whether from uniaxial or rosette strain gage configurations, are connected independently to the Wheatstone bridge in a quarter-bridge arrangement. As discussed

in the following sections, the wiring scheme chosen to connect the strain gage to the bridge circuit has a significant effect on the accuracy of measured strain dat

The Wheatstone bridge circuit in its simplest form (Figure 1) consists of four resistive elements, or bridge arms (R1, R2,R3, R4), connected in a series-parallel arrangement, with an excitation voltage source (E). The connection points formed by (adjacent) pairs of bridge arms and the lead wires from the excitation voltage source are input corners of the bridge; and those formed by pairs of bridge arms and the signal (eo) measurement leads are output corners. It is worth noting for this discussion that each input corner is adjacent to each output corner, and each bridge arm is connected between two adjacent corners.

Also, if the bridge circuit is resistively symmetrical about an imaginary line drawn through both output corners, the output voltage eo will be exactly zero, regardless of the excitation voltage level, and the bridge will be “balanced”.

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