Tensile Tests
Tensile tests inform about a number of characteristics: stiffness, modulus, tensile strength, tearing length and energy absorbtion.
Wet Tensile Tests on Offset-Printing Papers
Wet tensile tests are useful for determining several different paper characteristics such as usage properties of tissues, printing quality of offset printing papers, and runability characterisics in the wet end of a paper machine.
When running paper through the printing units of a 4-color offset printing machine, the humidity of the paper increases steadily on the printed areas. This increase of humidity causes an elongation of the paper in the machine and cross directions. While the elongation in machine direction can be corrected by gage pins, it cannot be corrected or adjusted in the cross direction. In order to avoid „fan out" of the individual color prints, the elongation in cross direction must remain within specific limits.
The importance of not exceeding these limits increases with increasing recycling rate and decreasing basis weight of the paper. Testing the force-deformation behavior during incoming quality control, is a powerful costs saving tool. For a valid test, the paper must be preconditioned at a well-defined humidity and then tested within a very short time.
The slope of the elongation-force-curve can be used to determine the limits of high quality offset printing. For this type of test, Zwick has developed a testing machine system that can test a specimen within seconds after taking a specimen out of a conditioning box (containing a water-acetone-solution in an exact ratio). Thus, the degree of humidity does not change within this short test time.
Only the elongation between the grips is needed for determining the force-elongation behavior of the specimen. Zwick´s standard grips and jaw faces fulfill these requirements and prevent grip slippage and failures within the clamping area of the sample.
Helpful for paper optimizing is its Z-dimensional force-deformation behaviour. Using the videoXtens, extension and transverse strain can be measured simultaneously, when using a wider specimen.
This test is according to ISO 1924.
The web´s fibre strength is measured with the Zero Span Test. Using an initial gage length near zero, a tensile test is performed. Now, mainly the pulp fibres are loaded.
The Zero Span Test is usually performed with dedicated instruments, being accordingly complex and expensive. If Zero Span Test are not frequently used, Zwick´s modular system of grips and accessories offers an affordable solution: For an exact horizontal and angular grip alignment, Zwick offers a mechanical extension kit. The grip already must despond to some minimum accuracy requirements, which is achieved by pneumatic grips.
The Zero Span jaws protrude from the grips inner edge and allow a free span of approximately 0.1 mm. This can only be achieved with numerically controlled machine such as Zwick´s ProLine or zwicki-Line, which guarantee best reproduceability and reduce the risk of danger to a minimum.
For this test the TAPPI T 231 is the Standard.
Runability at the Wet End
The paper is very susceptible to web breaks when it leaves the paper former machine and runs without any support by a felt or a screen. Adjusting and controlling the web tension, as well as using wet paper with minimal initial tensile strength values, can avoid web breaks. The minimal tensile strength of the paper depends on the stock composition (ratio of long and short fibers, chemical additives). The optimal price-strength-ratio can be evaluat-ed through initial wet tensile tests. Special tensile grips assure correct clamping, draining within the clamping area of the specimen, and prevents specimen breaks within the clamping area.
The advantages:
- Correct specimen gripping
- Fast and easy handling
- High flexibility through the testXpert® II Master Test Program
Since only the tear strength or tearing length result is to be determined in many cases, other results can be hidden by software. The testXpert® II software from Zwick accomplishes this with two functional levels. The first level is the use of built-in Standard tests.
Standard tests (to EN ISO 1924 or TAPPI T 494) contain only test sequences and results which are required by test standards. The adavantage of using the Standard tests is that training can be accomplished in a short time. The second level is the Master program. This level will provide functionality to meet all current and future needs for maximum flexibility.
The advantage:
- New: With videoXtens for Poisson´s ratio determination
