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Steel Wire Strand Extensometer

Steel Wire Strand Extensometer

Model: UT1022-GJX

Steel Strand Extensometer is a high-precision electronic instrument specifically designed for measuring the deformation (strain) of steel strands, prestressed tendons, and similar elongated specimens during tensile tests. It is essential for determining key mechanical properties like Elastic Modulus and proof strength. 


Working Principle: 

The core of a typical contact-type steel strand extensometer is a resistance strain gauge configured in a full Wheatstone bridge circuit. 

General introduction

Steel Strand Extensometer is a high-precision electronic instrument specifically designed for measuring the deformation (strain) of steel strands, prestressed tendons, and similar elongated specimens during tensile tests. It is essential for determining key mechanical properties like Elastic Modulus and proof strength. 


Working Principle: 

The core of a typical contact-type steel strand extensometer is a resistance strain gauge configured in a full Wheatstone bridge circuit. 

Steel Wire Strand Extensometer

Deformation Transfer: As the steel strand stretches, the extensometer's knife edges, which are in contact with the specimen, undergo relative displacement.

Signal Generation: This displacement causes a corresponding deformation in an internal elastic element. Strain gauges bonded to this element change resistance proportionally.

Signal Output: The Wheatstone bridge converts these tiny resistance changes into a measurable millivolt-level voltage output, which is sent to a measuring amplifier or data acquisition system.


Operation Procedure: 

Proper operation is critical for accurate results and extensometer protection .

  1. Selection and Preparation: Choose a gauge length exceeding the strand's lay length (e.g., ≥200mm) to ensure accurate deformation capture . Ensure the contact area on the strand is clean.

  2. Installation: Gently clamp the extensometer onto the specimen, aligning the knife edges with marked points. Use rubber bands or springs to secure it firmly but gently. Proper alignment of the knife edges with the specimen axis is crucial to prevent slippage .

  3. Connection and Zeroing: Connect the cable to the measuring system. Perform a zeroing operation before applying load.

  4. Testing and Data Acquisition: Start the tensile test. Monitor the force-strain curve in real-time.

  5. Timely Removal (Critical for contact types): To prevent damage from the shock of specimen fracture, carefully remove the extensometer before final fracture, typically after the yield stage.

  6. Post-Test Care: After removal, re-insert the locating pin (if present) to protect the mechanism and store the extensometer properly. 


Key Features

  • Key Considerations
  • Operation notice
  • Application Summary
  • Main standard

Preventing Slippage: The smooth surface of steel strands makes the extensometer prone to slipping. Adequate clamping force and techniques like using medical tape on the contact points can improve grip .


Avoiding Overload: Do not exceed the extensometer's maximum range.


Environmental Conditions: Operate within specified temperature and humidity ranges, away from strong vibrations and electromagnetic interference 

When testing specimens such as prestressed steel strands, rebar, and steel wires, the gauge length of the extensometers used by various testing units generally ranges from 200mm to 600mm. However, there are two situations that need special attention:

a. Steel strands have a special structure, with a typical lay length of about 170mm, so the gauge length of the extensometer generally needs to be greater than 200mm for testing steel strands;
b. Some special specimens may be made smaller or larger and are not limited to the 200-600mm range.

When testing steel strands, rebar, or steel wires, special attention should be paid to the method of fixing the extensometer. 

We know that the jaws of a typical extensometer are flat. If we use rubber bands to secure it, we can only clamp a few wires of the steel strand or a couple of points on the rebar. Due to the relatively heavy weight of the extensometer or unstable clamping, slippage between the extensometer and the clamping points can easily occur, affecting measurement accuracy.

a. Apply medical tape at the clamping points of the extensometer and the rebar or steel strand to increase friction.
b. Increase the contact area and use a ring-type extensometer with threaded fastening to prevent slippage.

The steel strand extensometer is vital for obtaining accurate mechanical property data. Correct gauge length selection, proper installation, and adherence to operating procedures are key to reliable results. 


For labs with diverse testing needs or those requiring full fracture data, non-contact video extensometers are a powerful modern alternative

ASTM E83: Standard Practice for Calibration, Verification, and Classification of Extensometer Systems

ISO 9513: Metallic materials — Calibration of extensometer systems used in uniaxial testing

GB 12160: Metallic materials-Calibration of extensometers systems used in uniaxial testing

Main technical specification

Gauge Length

200mm, 250mm, 500mm, 600mm
Deformation range0-25mm
Strain gauge resistance350Ω
Voltage≤6V (DC or AC)
Output sensitivity2.3mV/V
Output connectorselected as required
Clamping methodrubber band or spring clamp


Contact vs. Non-Contact Extensometers

While traditional contact types are widely used, non-contact video extensometers offer an alternative.


FeatureContact ExtensometerNon-Contact Video Extensometer
Pros

Established technology,

Lower cost, 

High precision for standard tests.

No physical contact, eliminates slippage risk and specimen damage, can record the entire test until fracture

Gauge length is software-defined.

Cons

Risk of slippage, must be removed before fracture,

Requires different instruments for different gauge lengths.

Higher initial cost, 

Requires good lighting and surface preparation (e.g., markers).

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Beijing United Test Co., Ltd.