Strain Gauge - how does it work?

Strain Gauge, also known as Foil Gauge is a device whose electrical resistance varies in proportion to the amount of strain in the device. The most commonly used strain gauge is the bonded metallic strain gage. It consists of a fine wire or metallic foil arranged in a grid pattern as shown below.

The white part in the above image is a high quality metallic wire of uniform cross-section which is bonded to a thin backing, called the carrier, which is attached directly to the test specimen or the transducer element. The strain experienced by the specimen/element is transferred to the strain gage, which responds with a linear change in electrical resistance.

Strain gauges are used for Stress Analysis and Manufacture of Transducers. Two distinct classes of strain gauges are available for Stress Analysis and Transducers. Further, within the transducer class, strain gauges for steel and aluminium are available. Strain gages are available commercially with nominal resistance values of 120Ω, 350Ω and 1000Ω.

A fundamental parameter of the strain gage which expresses it's sensitivity to strain is the gauge factor. Gage factor is defined as the ratio of change in resistance to the change in length (strain). The gage factor for metallic strain gages is typically around 2.

Strain gauges must be properly mounted onto the element/test specimen so that the strain is accurately transferred from the test specimen > adhesive > strain gage > foil.

Below are types of transducer class strain gauges-

• Linear strain gages
• Double linear strain gages
• Single shear strain gages
• Double shear strain gages
• Columnar strain gages
• T rosettes
• Half bridge strain gages
• Full bridge strain gages
• Membrane rosettes

HBM, Omega and Micro Measurements are few well known strain gauge manufacturers.

The article on Sensorland explains the working of a strain gauge.

Measuring Strain with Strain Gages - This tutorial is part of the National Instruments Measurement Fundamentals series. Each tutorial in this series will teach you a specific topic of common measurement applications by explaining theoretical concepts and providing practical examples.This tutorial introduces and explains the concepts and techniques of measuring strain with strain gages.

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