A Simple Guide to Tensile Testing: Recommendations for accurate and repeatable strength testing

29 November 2017

Tensile testing is one of the most widely used physical tests for textiles and other materials. By measuring the force required to elongate a specimen to breaking point, the textile properties can be determined that will allow designers and quality managers to predict how fibres, yarns and fabrics will behave in their intended end-use.  

A big part of our development process for any product we make involves our Textile Technologists. The Titan10 Universal Strength Tester was launched this week at the ShanghaiTex Exhibition, and to get to this point in the development of the product our Technologists have been doing months of strength testing work. 

With this in mind, we asked them for some do’s and don’ts for this field of testing. Using their years of laboratory experience, and looking through frequently asked questions to our Applications Support KnowledgeHub, they have a compiled a list of common mistakes and best practices for strength testing. 

  • Use the correct type of strength testing instrument

Nowadays, the most widely specified type is the Constant Rate of Extension (CRE) type.  Other types are constant rate of traverse (CRT) and constant rate of loading (CRL).  The standard (norm) will specify the type to be used.    

  • Ensure the Universal Strength Tester and Loadcells are calibrated by the manufacturer, usually on an annual basis. 

Our Universal Strength Tester calibrations test 5 things on the instrument – jaw parallelism, speed, extension, displacement and the computer timer, as well as 10 different force readings on each load cell. An in-depth calibration guarantees the accuracy and repeatability of results.

  • Carry out frequent verifications between calibrations using Check Weights and In-house Verification Fabric(s).

A verification procedure done at set intervals throughout the year, using a fabric you know is reliable, could be the difference between accurate and inaccurate results. Our technologist recommends doing these tests every week.  

  • Use a Loadcell which has the appropriate range for the application.  

Use a loadcell which will give results between 20% and 80% of the expected result.  

  • Utilise the correct grips and jaw faces for the application - the grips should not damage the specimen and hold it without slippage.  

The incorrect jaw face might mean the surface of the jaw won’t grip your material correctly, or a too high clamping pressure could cause the specimen to break at the jaws. A quick check before testing begins means this variable is controlled.

  • Ensure the Universal Strength Tester is setup with all the parameters required to carry out the test procedure according to the specified standard (norm). 

These may include the gauge length, test speed, preload (pretension), force and extension units.  

This might seem like an obvious one, but it is worth remembering when troubleshooting if you are not getting the results you would expect to see. Even one parameter being slightly out could cause variance in your results.

  • Prepare specimens from fabric which is free from folds, creases and other faults which may affect the results.  

Fabric with a fault or flaw would behave differently to one without, so there is no guarantee of consistency of results.

  • Ensure the dimensions of specimens are correct.

For strip tests ensure threads are removed to give the correct width, for grab tests ensure the same threads are tested in the top and bottom grips.   

A frequent issue we see through our Applications Support KnowledgeHub is specimens that are not cut and prepared correctly, often not accounting for distortion of the warp and weft of the fabric. This easy mistake to make can impact the accuracy of your results, so we have two downloadable guides available for marking and cutting specimens:

 - Direction of Seam Slippage Specimens for ISO 13936-1 and ISO 13936-2

 - Direction of Tear Specimens for ISO 13937-2

  • Condition specimens at the correct temperature and relative humidity, and for the time specified in the norm.  

The strength of many fibres can be affected by environmental conditions and this effect should not be under-estimated.  

Making sure your Universal Strength Tester is set up in the Laboratory is crucial. Peter Goodwin, Head of Technical’ explains “if you are carrying out physical testing, controlled environment equipment will ensure your results are consistent. If you are processing things like cotton, wool and viscose, these fibres absorb/desorb moisture, and your results will not be correct if you do not control the environment they are conditioned and tested in.”

  • Position the specimen in the grips so that it is aligned centrally, vertically and/or horizontally.

A specimen not being aligned correctly has the potential to change the direction that it is being pulled in, or that the same threads are not being tested at the top and the bottom.

For guidance on the correct clamping of two part specimens in ISO 13936-1 and similar seam slippage tests, download our instructions.

 - The Correct Mounting of 2-part Specimens

  • Pneumatic grips offer strong and consistent gripping, while simultaneously taking less time to open and close.

If you are buying a new Universal Strength Tester this feature is worth considering, as it can impact both your results and the time taken to set up the test.

  • When the test is in progress, make a note of any abnormal modes of failure such as “jaw break”.  

This is useful information to refer back to once your results have been analysed.  Any outliers which are also "jaw breaks" should be excluded.  

  • After testing is completed, ensure the results are analysed as specified in the norm and correct terminology used to avoid confusion when reading the test report.  

Get familiar with the standard you are testing and the terminology it uses – each standard body may explain things slightly differently, and will often have a different way of expressing the results. 

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If you have further questions about tensile testing, our Applications Support KnowledgeHub is the best place to start. You can search our FAQs or submit a ticket to our Textile Technologists.

To find out more about our Universal Strength Testers, including the new addition Titan10, click here.

Other articles that might be of interest - A Simple Guide to Light Fastness Testing | A Simple Guide to Colour Fastness Testing

Tags: knowledge, tensile testing, guidelines, strength testing