Using tensile testers in the footwear industry – part 1
How this equipment can be used to effectively assess footwear materials.
by Peter Allen
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, and can be used to carry out a wide range of tests. The different categories of tests which 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. The organisation has produced over 50 test methods which require the use of a tensile tester and that relate to testing footwear and leather products. SATRA sells two tensile tester models: the SATRA 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 for using compression cages and aids with the speed of movement for setting up the jaw separation.
This first article describes a number of SATRA tests which can be used to determine fundamental material properties for materials used in footwear and leathergoods and are representative of the generic types of tests which can be conducted on a tensile tester. Future issues of SATRA Bulletin will consider SATRA tensile test methods which can be used to assess some characteristics specific to footwear construction. In addition to SATRA test methods, the SATRA STM 566 and STM 766 tensile testing machines can also be utilised to conduct tests conforming to a wide range of national, international or customer-specific methods.
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 (figure 1). 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 dumb-bell 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.
Tear properties of footwear materials are important characteristic 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 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 test is conducted in two principle directions of the sheet with three test specimens cut parallel to the principal direction of the sheet and three specimens cut at 90º to this direction. During the test, the jaws are separated at a specified rate and the maximum force needed to propagate the split is recorded. Insufficient split tear strength in a midsole can result in a sole separation failure in use.
SATRA TM37:1999 – ‘Nail penetration – shoe bottoms and protective midsoles’ (figure 2) is an example of a puncture test. This is a variation on a compression test in which the load is applied through one of the jaws via a small focused point of prescribed geometry – typically defined by diameter, point chamfer angle and terminal point geometry. SATRA TM37 is used to assess the nail penetration resistance of shoe bottoms and protective midsoles. It determines the force required to drive a hardened steel nail through a boot sole or shoe bottom to assess the protection given to the wearer, where there is a risk of treading on spikes or other sharp objects.
The method is applicable to safety footwear incorporating protective metal inserts or midsoles and can be used to test either the complete shoe bottom or the protective insert alone. When conducting a SATRA TM37 test, a blunt-ended steel spike of specified dimensions is forced through the test specimen. The force required is recorded, along with the type of damage produced.
The SATRA TM401:2000 – ‘Peel strength of adhesive bonds’ test determines the peel strength of an adhesive bond. The method 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 for solvent or water borne adhesives, or SATRA TM414 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 assessed. 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 would compare the strength at room temperature against specimens conditioned by wetting and drying, heat ageing and moist ageing.
SATRA TM440:2011 – ‘Edgewise compression strength of corrugated fibreboard’ (figure 3) is an example of an assessment which uses a tensile tester to conduct a compression test. The test assesses the resistance of corrugated cardboard to crushing under load applied vertically to the edge. This method is applicable to all types of corrugated fibreboards used for packaging, such as shoe boxes and shipping cartons.
In order to conduct a test to SATRA TM440, accurately cut specimens of the material are placed between two rigid platens in the tensile tester, with their faces perpendicular to the direction of applied load. Two guide blocks are used to support each side of the specimen perpendicular to the platens, while the initial load is applied across the specimen edges. As the load continues to be applied, the guide blocks are pulled away from the specimen, thus allowing the specimen to bow or buckle. The test continues until the specimen buckles and the maximum load before buckling is recorded. The failure mode is also recorded.
SATRA TM123:1992 – ‘Closure strength of touch-and-close fasteners’ determines both the peel strength and the shear strength of touch-and-close fasteners. The loss of strength after repeated use is also assessed. The method is applicable to all types of touch-and-close fasteners, including those forming part of completed footwear. SATRA TM123 can be used to assess both the peel strength and the shear strength of touch-and-close fasteners, as well as those properties after subjecting the fastener to repeated opening and closing.
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 180º. 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.
Using the right equipment
The SATRA test methods referenced in this first 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 – examples of which will be described in future articles.
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.
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This article was originally published on page 38 of the June 2015 issue of SATRA Bulletin.