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Fibre strength measurement - Bundle, Pressley and stelometer instrument

Fibre strength measurement

Bundle strength

A bundle fibre strength test is more suitable for appropriate results as well as ease of test.

Practically fibres are always in the group like in tuft, yarn, or fabric. Therefore, bundles or groups of fibres come into play during the sampling or testing. 

Bundle strength is actually real test output that will apply to yarn or fabric properties.

Bundle fibre strength is measured by various instruments among them 2 are more popular. One is the Pressley strength tester and another is Stelometer.

Pressley fibre bundle strength tester

Pressley fiber bundle strength tester
Pressley fibre bundle strength tester

Pressley fibre tester is used to measure the bundle tensile strength of natural fibres.

Pressley strength tester is measuring the bundle strength of fibres with the help of the inclined plane apparatus principle.

This instrument includes 1 pair of clamps, a spacer, a torque vice, a wrench, a cutter, a comb, and tweezers. 

A 5 mg x 0.01 mg capacity balance is required to measure weight.

In modern days, due to some limitations like lesser efficiency and results variation Or due to servicing factors, this instrument is rarely used.  


Stelometer instrument is used to measure the bundle strength of fibres with the help of the pendulum lever principle.

Stelometer is almost similar to a pendulum-type instrument, with a difference in movable parts. 
In this instrument, the pendulum weight remains stable while the axis moves around the arc of numbering which shows the final result.

In this instrument, there is a special dashpot device, which is measured the rotation of the pendulum axis or beam. 

Dashpot is arranged and moves in such a way that the rate of loading is approximately constant during its movement. (That will allow one parameter of strength testing constantly throughout the test.)

This rate of loading is adjusted at 1 kg/sec by special arrangement. (It is standard for this test.)

A pointer, freely mounted on the axis and driven by a sensing pin that is mounted on the pendulum, moves over a scale graduated from 2 to 7 kg. 

This numbering shows the breaking load. (For more accuracy, a smaller pointer is dedicated to the percentage of elongation on a secondary scale.)

The indicators can be read to the nearest 0.01 kg for breaking load and 0.1 per cent for elongation of specimen. 

The clamps, after removal of the protruding fibres, are loaded in the slots on the top of the Stelometer and the beam is released. 

Stealometer diagram
Stealometer working

One part of the Pressley clamps is held in the adjustable holder carried by the beam while the other is held in a slot on the top end of the pendulum. 

The movement of the beam applies tension to the bundle by pulling apart the two parts of the clamps. 

As soon as the bundle breaks, the sensing pinfalls away and the pointer stops immediately. 

The breaking load can be read on the scale. The broken fibres are collected from the clamps and the weight in milligrams is determined accurately. 

Six bundles are tested by two operators, on every sample. 

The bundle tenacity is obtained by dividing the breaking load by the weight of the sample. 

Stelometer value at zero gauge length
Stelometer value = Breaking strength of bundle (in kg.) * 11.81 / Weight of bundle (in mg.)
Stelometer value at 1/8 inch gauge length
Stelometer value = Breaking strength of bundle (in kg.) * 15 / Weight of bundle (in mg.)

Questions -
  1. How strength is measured?
  2. What is a single fibre test?
  3. What is the universal test?
  4. How universal tensile tester works?


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Elsevier. (2016). Performance testing of textiles (1st ed.). Woodhead Publishing.

Houck, M. M. (2009). Identification of Textile Fibers (M. M. Houck, Ed.; 1st ed.). Woodhead Publishing.

Textile School. (2010, October 27). Fibre Identification - tests to identify a fibre. Textile School

Trivedi, Y. (2020, April 10). Identification of Textile Fibers. Textilesphere.com

(N.d.). from

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