Technical Tips

1) The Gage Length, the original length of that portion of the specimen over which the strain or change of length is determined, needs to be consistent. Most ASTM, ISO, and TAPPI standards specify the appropriate gauge length for a particular test or material...

Studies have shown that smaller gage lengths will yield higher test results. The observed difference increased as the force required to break the sample increased. Variability in the gage length will adversely affect your results.

2) The Extensometer is used for precision measurements...

Customers testing metals, plastic film, or rubber, sometimes use an extensometer, an instrument specifically designed for the highly accurate measurement of strain and deflection. Strain is calculated as the change in length of a specimen divided by the original length of the specimen.

3) Selecting the correct grip and grip faces is critical for testing success. When selecting tensile grips, many factors have to be given consideration including: load capacity of the grips, the opening distance , pneumatic grips or mechanical grips, and grip face selection...

Thwing-Albert recommends pneumatic grips to eliminate one more variable as the grips will always close at the same pressure, as opposed to the mechanical grips where individuals may tighten the grips differently. Grip inserts are available as smooth, serrated, line contact, and rubber covered. Proper insert selection is important because serrated inserts work well for fiberglass mat, but are too aggressive for plastic films and often result in sample breaks at the grip. Thwing-Albert would recommend rubber covered inserts to prevent slippage for flexible packaging materials. Make sure your grips are rated high enough to hold your sample and avoid slippage. Also, if using pneumatic grips, check that your incoming air line pressure is set according to the manufacturer's recommendation.

4) Sample Preparation - Preparing a sample WITHOUT jagged edges or nicks is vital to ensure accurate tensile results. Those imperfections will affect your results and your ability to provide consistent results. Use of a precision cutter, such as the JDC Sample Cutter, is ideal for sheet materials such as paper, plastic films, thin foils, and nonwoven materials.

5) Load Cell Selection - Equipping your tensile tester with an appropriate load cell is a key factor in tensile testing. As a general rule, testing should be performed at about 50% of the capacity of the load cell. If testing goes below 10% of the capacity of the load cell, the accuracy is not as great. For example, between 10% and 100% of the capacity of the load cell, Thwing-Albert load cells have an accuracy of .25% of the MEASURED VALUE. Below 10% of the capacity of the load cell, the accuracy changes to .025% of the CAPACITY OF THE LOAD CELL. If you test to close to the capacity of your load cell, you run the risk of overloading the load cell.

6) Load Cell Care - It is a good practice to warm up your load cell for 15 minutes before starting your testing. This provides enough time for the load cell to stabilize and provide consistent results.

7) Consistency - The key to accurate and repeatable test results is consistency in the test setup, procedure and control of environmental conditions that may affect the physical properties of the material. The test setup should insure that the appropriate gage length, test speed, type of grips and results selected are the same. Insure that the specimen is inserted in the grips correctly and clamped securely. Control, if possible, the environmental conditions (i.e. temperature and humidity) to standard laboratory conditions or record the actual conditions when a test is performed. Variations in testing can result in poor data and data correlation issues.

Industry Resources

ASTM International (The American Society for Testing and Materials)
ASTM International develops and publishes technical standards for a wide range of materials.  The organization defines the test methods and the precision of the results that are used around the world to meet specific standards. 

TAPPI (Technical Association of the Pulp and Paper Industry)
TAPPI is the leading association globally for the pulp, paper, packaging, and converting industries.  The test methods established by TAPPI provide a comprehensive collection of reliable technical information for the pulp and paper industry.

Please contact Thwing-Albert at 856-767-1000 or email us at info@thwingalbert.com if you have any testing questions or test equipment needs.