Using tensile testers – part 1

Explaining how this equipment can be used to effectively assess footwear materials.

by David Billington

Tensile testing machines (often also called ‘universal testing machines’, as many also operate in compressive mode) are fundamental pieces of test equipment in many types of physical testing laboratories which can be used to carry out a wide range of tests. The different categories of tests that can be conducted include tensile strength, as well as resistance to tearing, peeling, puncturing, compression, shearing and three-point bending, and the assessment of specific characteristics of a product component or assembly.

SATRA has many years’ experience in the development of tensile testers, associated jaws and test methods. We have produced over 50 test methods which require the use of a tensile tester and that relate to testing footwear and leather products. The two best-selling tensile testers within SATRA’s range of equipment are the STM 566 (5kN) and the STM 766 (20kN). Other lower-capacity load cells can be fitted to the STM 566 to improve the sensitivity when testing at lower loads. The STM 566 and STM 766 tensile testers can directly apply both tension and compression loading. This avoids the necessity of using compression cages and aids with the speed of movement for setting up the jaw separation.

This article and others to be published in future issues of SATRA Bulletin will describe a number of SATRA tests to determine fundamental material properties for materials to be used in footwear and leathergoods, and are representative of the generic types of evaluations which can be conducted on a tensile tester. In addition to SATRA test methods, the SATRA STM 566 and STM 766 machines can also be utilised to conduct tests conforming to a wide range of national, international or customer-specific methods.

SATRA TM43

Tensile (or breaking) strength and extension are not direct indicators of the performance level of a leather. Nevertheless, they may be used as an indicator of general quality and are included in many specifications. For leather, SATRA TM43:2000 – ‘Tensile strength and extension at break of leather’, allows an assessment to be made of the tensile strength of leather, as well as the elongation at break and the extension at a predefined tensile stress. This latter property gives an indication of the ease with which the leather may be stretched. In addition, both the tensile stress and the elongation can be determined at the onset of grain crack.

When conducting a test to SATRA TM43, accurate dumbbell-shaped specimens are prepared. This shape produces a lower stress region where the specimen is held in the jaws. During the test, therefore, the most likely result is a break occurring in the narrower region, away from the jaws. The tensile strength can be determined from the maximum force recorded during the test. The tensile strength is the maximum load divided by the cross-sectional area (width x thickness) of the specimen.

The percentage extension at break is given by the extension multiplied by 100 and divided by the original jaw separation. The test method sets out the details for the test, including the number of specimens to be taken, their orientation and geometry, specimen conditioning and how thickness measurements are to be taken, as well as how to determine the extension at a predefined stress and tensile stress and elongation at grain crack.

SATRA TM65

Tear properties of footwear materials are important characteristics to be considered in the manufacture of footwear and leathergoods. For example, SATRA TM65:2015 – ‘Split tear strength’ is a test used to assess the split tear strength of cellular materials. It is mainly applicable to sheet soling materials, but can also be used for specimens prepared from moulded soles. An important application of this test is the assessment of the split tear strength of ethylene vinyl acetate (EVA) solings. When conducting a test, rectangular specimens are split for part of their length, parallel to the top and bottom faces and through mid-thickness. This produces two tongues which are clamped into the jaws of the tensile tester.

When testing sheet material, the evaluation is conducted in two directions on the sheet, with three test specimens being cut parallel to the principal direction of the sheet and three specimens cut at 90 degrees to this direction. During this assessment, the jaws of the tensile tester are separated at a specified rate and the maximum force that is needed to propagate the split is recorded. Insufficient split tear strength in a midsole can result in a sole bond failure due to the material tearing.

SATRA TM401

The SATRA TM401:2000 – ‘Peel strength of adhesive bonds’ test is applicable to all types of bonded joint where at least one of the adherends is flexible. Test specimens are cut from a bonded assembly which has been previously prepared – for instance, using the preparation procedures set out in SATRA TM402 parts A, B, C, D and E (for solvent-based or water-based adhesives), or SATRA TM414:1992 – ‘Preparation of hot melt bonded assemblies for peel tests’ for hot-melt adhesives. The test specimen is then peeled apart using a tensile testing machine, while the force required to separate the adherends is measured and the type of bond failure is evaluated. Important information can be gained from an examination of the failure mode.

Optional pre-treatments can be applied to the test specimens prior to testing, which allow the effect of these conditions on the peel strength to be assessed. Typical tests compare the strength at room temperature against specimens conditioned by wetting and drying, heat ageing and moist ageing.

SATRA TM123

SATRA TM123:2018 – ‘Closure strength of touch and close fasteners’ determines both the peel strength and the shear strength of new, unused touch and close fasteners, as well as those properties after subjecting the fastener to repeated opening and closing. This method is applicable to all types of touch and close fasteners, including those forming part of completed footwear.

In this article, SATRA TM123 is referenced as an example of how the tensile tester is used to conduct an assessment of shear strength.

Because touch and close fasteners are made of two parts (hook and loop), specimens are prepared to represent the different configurations of how the two parts can be combined. Both parts of the touch and close fastener are pressed together under controlled conditions, with one of the parts offset to the other so that there is a standard contact area between the two parts. The maximum shear load required to pull them apart along their length in both directions is measured with a tensile testing machine. This procedure is then repeated with one of the parts of the fastener turned though 180 degrees. The shear strength is then calculated from the maximum load divided by the contact area (area of overlap) between the two parts of the fastener.

SATRA TM162

For many applications, the tear strength of leather is an important parameter to be determined. The ‘Baumann tear test’ was specifically developed for the assessment of the tear strength of leather. This test is covered by SATRA TM162:2017 – ‘Tear strength – Baumann method’. When conducting this evaluation, a defined shape is punched out of the leather material (SATRA can supply a cutting knife for use in preparing specimens). The test can be conducted using SATRA’s STM 566 tensile test machine fitted with specialist STM 566ST jaws, which allows a load to be applied pulling across the cut-out in the specimen. Typically, the tearing pattern results in two tears – one from each end of the cut-out. The maximum load achieved is recorded, and an average taken from three specimens in both of the 'along' (parallel to the backbone) and the ‘across’ (perpendicular to the backbone) directions.

Using the right equipment

The SATRA test methods referred to in this article are only indicative of the range of types of standard assessments that can be carried out using a tensile tester. As previously mentioned, other tests using a tensile tester can be used to assess specific aspects of footwear construction­.

To obtain good testing outcomes using a tensile tester, it is important to have an effective machine fitted with jaws appropriate for the test to be conducted, in addition to taking care when preparing test specimens. A number of tests are also sensitive to the conditioning of the specimens. It is also important to select test methods appropriate to the characteristic to be evaluated.

Publishing Data

This article was originally published on page 28 of the December 2020 issue of SATRA Bulletin.

Other articles from this issue »

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