Strain gauges measure deformation or strain in materials and structures caused by applied forces. There are different types of strain including axial, bending, shear, and torsional. Strain gauges work by measuring changes in electrical resistance caused by physical deformation. Common types include semiconductor, thin-film, and bonded resistance strain gauges. Strain gauges are widely used to monitor structures like bridges, buildings, and aircraft to detect deformation and prevent failures or accidents. They provide important safety and monitoring functions across many industries.
2. INTRODUCTION:
Strain: The amount of deformation a material experiences due to an applied force is called
strain. Strain is defined as the ratio of the change in length of a material to the original,
unaffected length. Strain can be positive (tensile), due to elongation, or negative (compressive),
due to contraction. When a material is compressed in one direction, the tendency to expand in the
other two directions perpendicular to this force is known as the Poisson effect. Poisson’s ratio
(v), is the measure of this effect and is defined as the negative ratio of strain in the transverse
direction to the strain in the axial direction. Although dimensionless, strain is sometimes
expressed in units such as in./in. or mm/mm. In practice, the magnitude of measured strain is
very small, so it is often expressed as micro strain (µε), which is ε x 10-6
.
The four different types of strain are axial, bending, shear, and torsional. Axial and bending
strain are the most common.
• Axial strain measures how a material stretches or compresses as a result of a linear force
in the horizontal direction.
• Bending strain measures a stretch on one side of a material and the contraction on the
opposite side due to the linear force applied in the vertical direction.
• Shear strain measures the amount of deformation that occurs from a linear force with
components in both the horizontal and vertical directions.
• Torsional strain measures a circular force with components in both the vertical and
horizontal directions.
Factors of strain occurrence:
• The effect of applied external force (mechanical force)
• The influence of heat and cold (thermal strain)
• Internal force from the non-uniform cooling of cast components, casting and welding
(residual strain)
3. Strain Measurement:
Most commonly, strain is measured to determine the level of stress on the material. The absolute
value and direction of the mechanical stress is determined from the measured strain and known
properties of the material (modulus of elasticity and Poisson’s ratio). These calculations are
based on Hooke’s law. In its simplest form, Hooke's Law determines the direct proportionality of
the strain ε [m/m] and the stress σ [N/mm2
] of a certain material using its elasticity or Young's
modulus E [N/mm2
].
σ = ε⋅E
A strain gauge depends on the electrical resistivity of any conductor. The resistance in any
conducting device is dependent on its length as well as the cross-section area.
Suppose L1 is the original length of wire and L2 is the new length after an external force is
applied on it, the strain (ε) is given by the formula:
ε = (L2-L1)/L1
Now, whenever an external force changes the physical parameters of an object, its electrical
resistivity also changes. A strain gauge measures this deformity by using the Gauge Factor
formula.
In the case of real-life monitoring, while constructing concrete structures or monuments, the load
is applied at the load application point of a load cell that consists of a strain gauge underlying it.
As soon as the force is exerted, the strain gauge is deformed and, this deformation causes a
change in its electrical resistance which ultimately changes the output voltage.
4. Following are the types of strain gauges
Semiconductor Strain Gauges
In the year 1970, the first semiconductor strain gauges were developed for the use in a
automotive industry. Semiconductor strain gauges exhibit following key features:
Unlike other strain gauges, semiconductor strain gages are based upon the piezoresistive
effects of silicon or germanium and measure the change in resistance with stress as opposed
to strain.
The semiconductor bonded strain gage is a wafer with the resistance element diffused into a
substrate of silicon.
No backing is provided for the wafer element and bonding it to the strained surface needs
extra care since only a thin layer of epoxy is used to attach it.
Size of a semiconductor strain gauge is much smaller and the cost much lower than for a
metallic foil sensor.
Advantages:
It includes higher unit resistance and sensitivity.
Disadvantages:
Greater sensitivity to temperature variations and tendency to drift as compared to metallic
foil sensors. Also the resistance-to-strain relationship is nonlinear, varying 10-20% from a
straight-line equation. However, by means of computer-controlled instrumentation, these
limitations can be overcome via software compensation.
Thin-film Strain Gauges
Thin-film strain gage is more advanced form of strain gauge as it doesn’t necessitate
adhesive bonding. A thin film strain gauge is constructed by first depositing an electrical
insulation, usually a ceramic onto the stressed metal surface, and then depositing the strain
gage onto this insulation layer. Techniques used to bond the materials molecularly are:
Vacuum deposition
Sputtering method
Advantages
• Since the thin-film gage is molecularly bonded to the specimen, the installation is
very stable and the resistance values experience less drift.
• The stressed force detector can be a metallic diaphragm or beam with a deposited
layer of ceramic insulation.
5. Diffused Semiconductor Strain Gauges
A further improvement in strain gage technology was introduced with the advent of diffused
semiconductor strain gages since they purge the need for bonding agents. Main features are
listed below:
1. By eliminating bonding agents, errors due to creep and hysteresis also are eliminated.
2. The diffused semiconductor strain gage employs photolithography masking techniques
and solid-state diffusion of boron to molecularly bond the resistance elements.
3. Diffused semiconductors are frequently used as sensing elements in pressure transducers.
4. Limitations include sensitivity to ambient temperature variations, which can be
compensated by intelligent transmitter designs.
Advantages
• Small size
• Inexpensive
• Accurate and repeatable
• Available wide pressure range
• Generate a strong output
Bonded Resistance Gauges
Following are the chief characteristics of bonded resistance strain gauges:
• They are reasonably inexpensive.
• They can pull off overall accuracy of better than +/-0.10%.
• They are available in a short gauge length and have small physical size.
• These strain gauges are only moderately affected by temperature changes.
• They are extremely sensitive and have low mass.
• Bonded resistance strain gages can be employed to measure both static and dynamic
strain.
• These types of strain gauges are appropriate for a wide variety of environmental
conditions. They can measure strain in jet engine turbines operating at very high
temperatures and in cryogenic fluid applications at temperatures as low as -452*F (-
269*C).
Construction of a Bonded resistance strain gauge
6. Characteristics of strain gauges
The characteristics of strain gauges are as follows:
1. They are highly precise and don’t get influenced due to temperature changes. However, if
they do get affected by temperature changes, a thermistor is available for temperature
corrections.
2. They are ideal for long distance communication as the output is an electrical signal.
3. Strain Gauges require easy maintenance and have a long operating life.
4. The production of strain gauges is easy because of the simple operating principle and a small
number of components.
5. The strain gauges are suitable for long-term installation. However, they require certain
precautions while installing.
6. All the strain gauges produced by Cardio-Rite are hermetically sealed and made up of
stainless steel thus, waterproof.
7. They are fully encapsulated for protection against handling and installation damage.
8. The remote digital readout for strain gauges is also possible.
Industry where strain gauges are used
Strain gauges are extensively used in the field of geotechnical monitoring to keep a constant
check on structures, dams, tunnels, and buildings so that the mishaps can be avoided well on time.
Aerospace: Strain gauges are fixed to the structural load-bearing components to measure
stresses along load paths for wing deflection or deformation in an aero plane. The strain gauges
are wired into the Wheatstone Bridge circuits and, its application areas include onboard signal
7. Cable Bridges: Instrumentation of bridges is done to verify design parameters, evaluate the
performance of new technologies used in the construction of bridges, to verify and control the
construction process and for subsequent performance monitoring.
Well-instrumented bridges can alert responsible authorities about approaching failure so as to
initiate preventive measures. Choosing proper sensor types, technology, a measurement range and
their location on the bridge is very important to optimize costs and to extract full benefits of
instrumentation. It becomes necessary to monitor the bridges regularly for any kind of
deformation as it might lead to fatal accidents. Strain gauge technology is used in the real-time
monitoring of huge bridges, making the inspections precise. For example, Yamuna Bridge in
Allahabad-Nalini is a 630-meter cable-stayed bridge across river Yamuna. The bridge is installed
with many measurement channels that sense wind speed and strain on its cables.
Rail Monitoring: Strain Gauges have a long history in the safety of rails. It is used to measure
stress and strain on rails. Strain gauges measure axial tension or compression with no impact on
the rails. In case of an emergency, the strain gauges can generate a warning so maintenance can be
done early to minimize the impact on rail traffic.
Why are strain gauges important?
Strain gauges are extensively used in the field of geotechnical monitoring and instrumentation to
constantly monitor dams, inner linings of tunnels, structures, buildings, cable-stayed bridges, and
nuclear power plants to avoid mishaps and accidents in case there’s any deformity in them.
Timely actions taken can avoid accidents and loss of life due to deformities. Hence, strain gauges
are important sensors in the geotechnical field.
Strain gauges are installed on these structures and then, the complete data from them is remotely
retrievable through data loggers and readout units. They are considered as significant measuring
equipment for ensuring productivity and safety.
