Extensometer Working Principle: Electrical Resistance Variation

An extensometer is a precision instrument used to measure strain, which is the deformation or change in shape of a material in response to applied stress. Understanding how different types of extensometers work is crucial in material science and engineering.

Understanding Strain and Electrical Resistance

Strain is defined as the ratio of the change in length to the original length ($\epsilon = \frac{\Delta L}{L_0}$). Several methods can be used to measure this change. One common principle involves the variation in electrical resistance of a material when it is deformed (strained). This phenomenon is the basis for certain types of sensors.

Analyzing Extensometer Types

Let's examine the options provided to see which one operates based on the variation in electrical resistance due to strain:

• Optical Extensometer: These devices use optical principles, such as interferometry or light beams, to measure displacement or strain. They do not primarily rely on changes in electrical resistance.
• Strain Gauge Extensometer: This type utilizes electrical strain gauges. A strain gauge is a sensor whose electrical resistance changes proportionally to the amount of strain it undergoes. When bonded to a specimen, the strain gauge deforms with the specimen, and its resistance changes. This change in resistance can be measured using a Wheatstone bridge circuit and converted into a strain value. This directly matches the principle mentioned in the question.
• Magnetic Extensometer: These extensometers typically use magnetic principles, like changes in magnetic field strength or inductance, to measure displacement. They do not rely on the variation of resistance due to strain.
• Clip-on Extensometer: This term often describes how an extensometer is attached to a specimen (clipped on). While a clip-on extensometer could potentially use strain gauges or other principles, the term itself doesn't define the working mechanism based on electrical resistance variation. Many clip-on devices use strain gauges.

Conclusion on Working Principle

Based on the analysis, the strain gauge extensometer is the type that specifically works on the principle of variation in electrical resistance due to strain. The deformation of the metal foil or wire within the strain gauge changes its resistance, allowing for precise strain measurement.