Extensometer

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Extensometer

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A typical extensometer used in material testing

Other names
Classification
Hornbostel–Sachs classification
Inventor(s)
Developed

An extensometer is a device used to measure the change in length of an object. It is commonly used in the field of materials science and engineering to determine the strain of a material under stress. Extensometers are crucial in testing the mechanical properties of materials, such as Young's modulus, tensile strength, and Poisson's ratio.

History

The concept of the extensometer was first introduced by Charles Wheatstone in 1821. Wheatstone's device was designed to measure the elongation of materials under tension. Over the years, extensometers have evolved significantly, incorporating advanced technologies to improve accuracy and ease of use.

Types of Extensometers

Extensometers can be classified into several types based on their working principles and applications:

Contact Extensometers

These extensometers physically attach to the specimen being tested. They are further divided into:

• Clip-on Extensometers: These are the most common type, where the device is clipped onto the specimen. They are suitable for testing metals and other rigid materials.
• Averaging Extensometers: Used for measuring the average strain over a specific gauge length, often used in testing concrete and composites.

Non-contact Extensometers

These devices measure strain without physically touching the specimen. They include:

• Laser Extensometers: Use laser beams to measure the change in distance between two points on the specimen.
• Video Extensometers: Utilize high-resolution cameras and image processing software to track the deformation of the specimen.

Working Principle

The basic principle of an extensometer is to measure the change in length (ΔL) of a specimen when subjected to a force. This change in length is then used to calculate the strain (ε) using the formula:

ε = ΔL / L₀

where L₀ is the original length of the specimen.

Applications

Extensometers are widely used in various fields, including:

• Materials testing: To determine the mechanical properties of materials.
• Structural engineering: To monitor the deformation of structures under load.
• Biomechanics: To study the mechanical properties of biological tissues.

Advantages and Limitations

Advantages

• High accuracy in measuring small changes in length.
• Versatility in testing different types of materials.
• Availability in both contact and non-contact forms.

Limitations

• Contact extensometers can affect the specimen's behavior due to their weight.
• Non-contact extensometers can be expensive and require complex setup.

Also see

• Strain gauge
• Tensile testing
• Young's modulus
• Poisson's ratio