Strain Measurement in Mechanical Engineering

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Questions and Answers

What is the primary application of piezoresistive strain gauges?

Temperature measurement

Experimental stress analysis

Pressure sensors (correct)

Structural health monitoring

Which type of strain gauge is most susceptible to electromagnetic interference?

Metallic foil strain gauge

Semiconductor strain gauge (correct)

Piezoresistive strain gauge

Optical fiber strain gauge

What is the primary advantage of optical fiber strain gauges?

High sensitivity

Low cost

Ease of installation

Immunity to electromagnetic interference (correct)

What is the purpose of temperature compensation in strain measurement?

To reduce the effect of temperature on the strain measurement Signup and view all the answers

What type of strain gauge circuit is commonly used in experimental stress analysis?

Quarter-bridge circuit Signup and view all the answers

What is the primary advantage of bonded strain gauges?

Easier installation compared to unbonded strain gauges Signup and view all the answers

What type of material is commonly used in metallic foil strain gauges?

Constantan Signup and view all the answers

What is the primary application of strain gauges in mechanical engineering?

All of the above Signup and view all the answers

What is the primary advantage of semiconductor strain gauges?

Higher sensitivity than metallic foil strain gauges Signup and view all the answers

What is the purpose of calibration in strain measurement?

To ensure accurate readings Signup and view all the answers

What is the primary advantage of a Full-Bridge Circuit over a Half-Bridge Circuit?

It provides maximum sensitivity and temperature compensation Signup and view all the answers

What is the purpose of generating a calibration curve in strain gauge calibration?

To plot the known loads against the electrical output Signup and view all the answers

Which of the following is a method for temperature compensation in strain gauge measurements?

Using a dummy gauge method Signup and view all the answers

What is the key consideration when using strain gauges to measure torque and stress on rotating shafts?

Careful installation of strain gauges Signup and view all the answers

What is the primary criterion for selecting a strain gauge for a particular application?

Strain range Signup and view all the answers

What is the purpose of surface preparation in the installation of strain gauges?

To ensure strong adhesion of the strain gauge Signup and view all the answers

What type of pressure measurement is typically used for fluid dynamics applications?

Strain gauge pressure cell Signup and view all the answers

What is the principle of operation of a strain gauge pressure cell?

Deformation of a diaphragm under pressure Signup and view all the answers

Which of the following is NOT a consideration for selecting a strain gauge?

Color of the gauge Signup and view all the answers

What is the purpose of applying protective coatings or coverings to strain gauges?

To shield the gauge from environmental damage Signup and view all the answers

Which of the following is a common application of strain gauges?

Industrial processes and hydraulic systems Signup and view all the answers

What is the principle of operation of a Wheatstone bridge circuit?

Converting resistance changes into a measurable voltage output Signup and view all the answers

What is the primary component of a Bridgeman gauge?

Wire or material under pressure Signup and view all the answers

Which of the following is NOT a type of thermal conductivity gauge?

Knudsen gauge Signup and view all the answers

What is the principle of operation of a McLeod gauge?

Compressing a known volume of gas and measuring the resulting pressure increase Signup and view all the answers

Which of the following gauges is suitable for measuring extremely high pressures in scientific research?

Bridgeman gauge Signup and view all the answers

What is the principle of operation of an Ionization gauge?

Ionizing the gas and measuring the resulting ion current Signup and view all the answers

Which of the following gauges is used in vacuum systems and laboratories for very low pressure measurements?

McLeod gauge Signup and view all the answers

What is the principle of operation of a Knudsen gauge?

Measuring pressure based on the thermal transpiration effect Signup and view all the answers

Which of the following is a factor to consider when selecting a pressure measurement method?

The range and accuracy required Signup and view all the answers

What is the primary limitation of using semiconductor strain gauges in mechanical engineering applications?

Higher sensitivity to temperature changes Signup and view all the answers

Which of the following is a key consideration when using strain gauges on rotating shafts?

The effect of centrifugal force on the gauge Signup and view all the answers

What is the primary advantage of using a Quarter-Bridge Circuit in strain measurement?

Simplified circuit design and implementation Signup and view all the answers

Which of the following pressure measurement methods is typically used for measuring extremely low pressures?

Ionization gauge Signup and view all the answers

What is the primary purpose of surface preparation in the installation of strain gauges?

To ensure a strong bond between the gauge and the surface Signup and view all the answers

Which of the following is a common application of strain gauges in mechanical engineering?

Measuring torque and stress on rotating shafts Signup and view all the answers

What is the primary advantage of using optical fiber strain gauges in harsh environments?

Immunity to electromagnetic interference Signup and view all the answers

Which of the following is a key consideration when selecting a strain gauge for a particular application?

The operating temperature range of the application Signup and view all the answers

What is the primary purpose of calibration in strain measurement?

To improve the accuracy of the strain measurement Signup and view all the answers

Which of the following is a limitation of using bonded strain gauges in mechanical engineering applications?

Limited flexibility in installation Signup and view all the answers

What occurs when pressure is applied to a diaphragm with bonded strain gauges?

The diaphragm deformation causes a change in electrical resistance of the strain gauges. Signup and view all the answers

In a Bridgeman gauge, what is the purpose of the pressure vessel?

To contain the wire or material subjected to high pressure. Signup and view all the answers

What is the principle of operation of a McLeod gauge?

The compression of a known volume of gas and measuring the resulting increase in pressure. Signup and view all the answers

What is the purpose of a thermocouple in a Knudsen gauge?

To measure the temperature difference between the hot and cold sides. Signup and view all the answers

What is the principle of operation of a Pirani gauge?

The heat loss from a heated wire, which depends on the gas pressure. Signup and view all the answers

What is the primary advantage of using a Wheatstone bridge circuit in strain gauge measurements?

It provides a voltage output proportional to the pressure. Signup and view all the answers

What is the principle of operation of an Ionization gauge?

The ionization of the gas and measuring the resulting ion current. Signup and view all the answers

What is the purpose of a manometer in a McLeod gauge?

To measure the pressure increase in the capillary tube. Signup and view all the answers

What is the principle of operation of a Bridgeman gauge?

The change in electrical resistance of a wire or material under pressure. Signup and view all the answers

What is the primary application of Thermal Conductivity Gauges?

Industrial processes, hydraulic systems, and fluid mechanics experiments. Signup and view all the answers

What is the primary advantage of using a Half-Bridge Circuit over a Quarter-Bridge Circuit?

Improved temperature compensation Signup and view all the answers

What is the purpose of using a dummy gauge in the Dummy Gauge Method?

To compensate for temperature-induced changes Signup and view all the answers

What is the primary consideration when selecting a strain gauge for measuring torque and stress on rotating shafts?

Wireless data transmission Signup and view all the answers

What is the primary advantage of using a strain gauge pressure cell over a Bridgeman gauge?

Lower cost Signup and view all the answers

What is the primary purpose of bonding a strain gauge to a test specimen?

To ensure accurate readings Signup and view all the answers

What is the primary advantage of using a Full-Bridge Circuit over a Half-Bridge Circuit?

Greater sensitivity to strain measurements Signup and view all the answers

What is the primary purpose of generating a calibration curve in strain gauge calibration?

To relate the electrical output to the actual strain Signup and view all the answers

What is the primary consideration when selecting a strain gauge for a particular application?

Material compatibility Signup and view all the answers

What is the primary purpose of applying known loads to a test specimen in strain gauge calibration?

To generate a calibration curve Signup and view all the answers

What is the primary advantage of using a strain gauge over a Bridgeman gauge in pressure measurement?

Lower cost Signup and view all the answers

Study Notes

Strain Measurement

Strain gauges are essential tools in mechanical engineering for measuring strain on various structures.
Types of strain gauges:
- Metallic Foil Strain Gauges: consist of a grid of thin metallic foil bonded to an insulating backing material.
- Semiconductor Strain Gauges: made from silicon or germanium, with higher sensitivity but more temperature-sensitive.
- Bonded and Unbonded Strain Gauges: bonded gauges are fixed to the surface of the test specimen, while unbonded gauges are mounted on a frame.
- Optical Fiber Strain Gauges: use changes in light transmission to measure strain, immune to electromagnetic interference.
- Piezoresistive Strain Gauges: primarily used in pressure sensors, utilizing the change in electrical resistance with applied strain.

Strain Gauge Circuits

Quarter-Bridge Circuit: uses a single active strain gauge and three fixed resistors in a Wheatstone bridge configuration, simplest and least expensive.
Half-Bridge Circuit: uses two strain gauges in the Wheatstone bridge, can measure bending strains and temperature compensation to some extent.
Full-Bridge Circuit: uses four strain gauges in the Wheatstone bridge, provides maximum sensitivity and temperature compensation, can measure axial and bending strains accurately.

Calibration

Calibration is essential to ensure accurate strain measurements.
Process involves applying known loads, generating a calibration curve, and deriving a calibration factor.

Temperature Compensation

Temperature changes can affect strain gauge readings due to thermal expansion of the specimen and the gauge itself.
Methods for temperature compensation include:
- Using temperature-compensated strain gauges.
- Dummy gauge method.
- Quarter-Bridge temperature compensation.

Use of Strain Gauges on Rotating Shafts

Strain gauges can be used to measure torque and stress on rotating shafts.
Key considerations include:
- Wireless data transmission.
- Slip rings.
- Careful installation.

Selection and Installation of Strain Gauges

Selection criteria:
- Strain range.
- Gauge factor.
- Environmental conditions.
- Material compatibility.
Installation process:
- Surface preparation.
- Bonding.
- Wiring.
- Protection.

Pressure Measurements

Basic methods of pressure measurement:
- Strain gauge pressure cells.
- High pressure measurement using the Bridgeman type gauge.
- Low-pressure measurement techniques:
  - McLeod gauge.
  - Knudsen gauge.
  - Ionization gauge.
  - Thermal conductivity gauges.

Strain Gauge Pressure Cell

Principle: measures pressure using the deformation of a diaphragm or other elastic element under pressure.
Components:
- Diaphragm.
- Strain gauges.
- Wheatstone bridge circuit.
Operation:
- Diaphragm deforms under pressure, causing strain in the strain gauges.
- Resistance change in the strain gauges is measured using a Wheatstone bridge circuit.

High Pressure Measurement - Bridgeman Type Gauge

Principle: measures high pressures by using the change in electrical resistance of a wire or material under pressure.
Components:
- Pressure vessel.
- Electrical circuit.
Operation:
- Material's electrical resistance changes under pressure.
- Resistance change is measured and correlated with the pressure.

Low Pressure Measurement

McLeod Gauge:
- Principle: measures low pressures by compressing a known volume of gas and measuring the resulting increase in pressure.
- Components:
  - Bulb and capillary tube.
  - Manometer.
Operation:
- Gas is trapped in a bulb and compressed into a capillary tube.
- Increase in pressure is measured with a manometer.
Knudsen Gauge:
- Principle: measures pressure based on the thermal transpiration effect.
- Components:
  - Porous plug.
  - Thermocouples.
Operation:
- Gas flows through the porous plug from the hot side to the cold side.
- Pressure difference created by this flow is measured and related to the absolute pressure.
Ionization Gauge:
- Principle: measures very low pressures by ionizing the gas and measuring the resulting ion current.
- Components:
  - Ionization chamber.
  - Electron source.
  - Collector electrode.
Operation:
- Electrons from a hot filament or cold cathode ionize the gas molecules.
- Ions are collected, and the ion current is measured.
Thermal Conductivity Gauges:
- Principle: measures pressure based on the thermal conductivity of the gas, which varies with pressure.
- Types:
  - Pirani gauge.
  - Thermocouple gauge.
Operation:
- Pirani gauge: heat loss from a heated wire to the gas depends on the gas pressure.
- Thermocouple gauge: heat loss from a heated element to the gas depends on the gas pressure.

Summary

The selection of a pressure measurement method depends on the range and accuracy required, the environment, and the specific application.