Electric Resistance strain gauges

 A Strain gauge (sometimes refered to as a Strain gage) is a sensor whose resistance varies with applied force; It converts force, pressure, tension, weight, etc., into a change in electrical resistance which can then be measured. When external forces are applied to a stationary object, stress and strain are the result. Stress is defined as the object's internal resisting forces, and strain is defined as the displacement and deformation that occur.

The strain gauge is one of the most important sensor of the electrical measurement technique applied to the measurement of mechanical quantities. As their name indicates, they are used for the measurement of strain. As a technical term "strain" consists of tensile and compressive strain, distinguished by a positive or negative sign. Thus, strain gauges can be used to pick up expansion as well as contraction.

The strain of a body is always caused by an external influence or an internal effect. Strain might be caused by forces, pressures, moments, heat, structural changes of the material and the like. If certain conditions are fulfilled, the amount or the value of the influencing quantity can be derived from the measured strain value. In experimental stress analysis this feature is widely used. Experimental stress analysis uses the strain values measured on the surface of a specimen, or structural part, to state the stress in the material and also to predict its safety and endurance. Special transducers can be designed for the measurement of forces or other derived quantities, e.g., moments, pressures, accelerations, displacements, vibrations and others. The transducer generally contains a pressure sensitive diaphragm with strain

Semi Conductor or Piezo Resistive Strain Gauge Principle

These gauges are directly bonded (that is pasted) on the surface of the structure under study. Hence they are termed as bonded strain gauges.

The three types of bonded strain gauges are

1. Fine wire strain gauge
2. Metal foil strain gauge
3. Semi-conductor strain gauge

Semi Conductor or Piezo Resistive Strain Gauge Principle

The arrangement of a semi-conductor strain gauge is as follows:

The sensing element is rectangular filament made as a wafer from silicon or geranium crystals.

To these crystals, boron is added to get some desired properties and this process is called doping and the crystals are called doped crystals.

This sensing element is attched to a plastics or stainless steel backing. Leads made of gold are drawn out from the sensing element for electrically connecting the strain gauge to a measuring instrument (wheat stone bridge).

There are two types of sensing element namely:

• Negative or n-type (resistance decrease with respect to tensile strain).
• Positive or P-type ( resistance increase with respect to tensile strain).

Operation

With the help of an adhesive material, the strain gauge is pasted/bonded on the structure under study.

Now the structure is subjected to a force (tensile or compresive). Due to the force, the structure will change the dimension.

As the strain gauge is bonded to the structure, the stain gauge will also undergo change in both in length and cross-section (that is, it strained).

When the sensing element (crystal) of the semiconductor strain gauge is strained, its resistivity changes contributing to a change in the resistance of the strain gauge.

The change in the resistance of the strain gauge is measured using a wheat stone bridge.

This change in resistance of the strain gauge becomes a measure of the extent to which the structure is strained and a measure of the applied force when calibrated.

Advantages of semi-conductor Strain gauges

• These gauges have high gauge factor and hence they can measure very small strains.
• They can be manufactured to very small sizes.
• They have an accuracy of 2.3%
• They have excellent hysteresis characteristics.
• They have a good frequency of response.
• They have good fatigue life.

Limitation of semi-conductor Strain gauges

• These gauges are brittle and hence they cannot be used for measuring large strain.
• The gauge factor is not constant.
• These gauges have poor linearity.
• These gauges are very costly and are difficult to be bonded onto the structure under study.
• These gauges are sensitive to change in temperature.

Grid Method of Strain Analysis

The grid method is a technique suitable for the measurement of in-plane displacement and strain components on specimens undergoing a small deformation. It relies on a regular marking of the surfaces under investigation. Various techniques are proposed in the literature to retrieve these sought quantities from images of regular markings, but recent advances show that techniques developed initially to process fringe patterns lead to the best results. The grid method features a good compromise between measurement resolution and spatial resolution, thus making it an efficient tool to characterise strain gradients. Another advantage of this technique is the ability to establish closed-form expressions between its main metrological characteristics, thus enabling to predict them within certain limits. In this context, the objective of this paper is to give the state of the art in the grid method, the information being currently spread out in the literature. We propose first to recall various techniques that were used in the past to process grid images, to focus progressively on the one that is the most used in recent examples: the windowed Fourier transform. From a practical point of view, surfaces under investigation must be marked with grids, so the techniques available to mark specimens with grids are presented. Then we gather the information available in the recent literature to synthesise the connection between three important characteristics of full-field measurement techniques: the spatial resolution, the measurement resolution and the measurement bias. Some practical information is then offered to help the readers who discover this technique to start using it. In particular, programmes used here to process the grid images are offered to the readers on a dedicated website. We finally present some recent examples available in the literature to highlight the effectiveness of the grid method for in-plane displacement and strain measurement in real situations.