Using tensile testers in the footwear industry – part 2
Continuing our explanation of how tensile testing equipment can be used to assess footwear materials.
by Peter Allen
Although most tensile tests for footwear, leathergoods or textiles are neither expensive nor time consuming to conduct, they can provide very valuable data to confirm the properties of a wide range of materials, components or completed products for product development. In addition, when used as part of a quality control process, these assessments contribute to providing protection against faulty products going into the market.
This second part of this article (click here for part one) examines SATRA test methods which can be conducted with a tensile tester. Following on from part one, further material tests are described, as well as assessments for stitched seam strength and tests applicable to the assessment of slide fasteners (typically referred to as 'zips' or 'zippers').
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 TM162:1992 – 'Tear strength – Baumann method' (figure 1). When conducting this test, a defined shape is punched out of the leather material. SATRA supplies 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.
SATRA TM30:1995 – 'Tear strength – trouser leg method' (figure 2) is intended to determine the force required to tear a non-leather material – for example, coated fabric footwear uppers and lining materials – but can also be used with all types of thin, flexible materials. The test allows for two specimen sizes to be used with the smaller of the two chosen if insufficient material is available to prepare the larger specimens. The name of the test is derived from the shape of the test specimens, which have the appearance of a pair of trousers.
Six specimens are used for each material test – three cut in the along direction and three cut in the across direction. In the case of a woven fabric, the 'warp' and 'weft' directions are taken as the along and across directions respectively. The method also sets out the analysis of results required to give the average and initial tear strength results for the principle directions, based on averages from the specimens cut for each direction. Any abnormality in tearing is reported, alongside the tear values. Tensile jaws with a minimum clamp width of 50mm are required for this test.
SATRA TM137:1995 – 'Tensile properties of plastics and rubbers' was written specifically to allow the tensile strength and elongation of soling materials to be assessed. Tensile tests on solings are usually conducted on specimens taken from sheet material, but specimens can also be taken from finished footwear. The test method describes the requirements for the dumb-bell shaped test specimen. These need to be produced with care to achieve the specified shape and thickness. They should be free from cuts or other small incisions – particularly on the edges of the material in the neck region of the specimens – as they could lead to a premature failure during the test and give misleadingly low results.
Good quality press knives (which need to be inspected regularly for damage) should be used to prepare the specimens. The test method allows for two dumb-bell-shaped pieces – 115mm long ('type 1') and 75mm long ('type 2'). While the type 1 specimen is preferred, a type 2 specimen may be used where there is insufficient material to produce the larger piece. The thickness of the test specimens also have to be taken accurately – for instance, using the SATRA STD 495 thickness gauge for rubber.
The SATRA TM137 test method sets out the number of specimens required, depending on whether the test specimens are cut from sheet material or cut from a sole. The test method also describes how to calculate and present the tensile strength results and the means of determining the elongation at break for the material. This is determined with the aid of an 'extensometer' device, which is available as an option for the SATRA STM 566 tensile test machine. Extension results are expressed as a percentage of the original distance (marked on the specimen and tracked by the extensometer). The modulus may also be calculated and used as a measure of the 'stretchiness' of the material.
Stitched seams play an important role in footwear, as they affect both the appearance and the ability of individual components and materials to function as a unit, so allowing load transfer between them. Reinforcements can be applied locally at seams (to provide support at positions of high stress) or to attach reinforcements required to support an inherently weak upper material – often selected for fashion considerations.
SATRA TM180:1995 – 'Measurement of the strength of stitched seams in upper and lining materials' has been developed to allow the breaking strength of stitched seams in shoe upper and lining materials to be assessed. The method is applicable to seams cut from shoes or made up to simulate footwear constructions, but can also be used with seams cut from garments. A test specimen containing a stitched seam is gradually stretched by a tensile test machine in a direction perpendicular to the seam, until failure occurs. The breaking strength and type of breakdown are thereafter determined.
The test specimens are prepared in a manner which allows the seam to continue beyond the area which is subjected to load during the test. This avoids the results being influenced by the weakness introduced by cutting through the seam and stitching at the edge of the test specimen. The test can be conducted using the SATRA STM 566 tensile test machine.
The test method sets out the number of test specimens. It also details the preparation of specimens cut from shoes, as well as specimens prepared from materials to which a seam is added. The test method also sets out the reporting information required, which includes both the seam strength (determined from the test), and a description of the failure mode. If a poor seam strength is reported, it may be possible to recommend improvements in construction by examining how the seam failed – for example, by evaluating thread density, size and type of thread, needle type/size, and seam allowance.
Laces continue to remain widely used for shoe fastening. There are a number of tests which can be used to validate the performance of laces. In this article, the emphasis is on breaking strength testing. SATRA TM94:1993 – 'Breaking force and extension at break of shoe laces' (figure 3) allows a determination of the breaking force and breaking extension of shoe laces to be made. The test can be carried out with both wet and dry specimens. The method is applicable to all types of shoe lace and is conducted with a tensile test machine.
Laces are difficult to grip in 'conventional' vice-type jaws without damaging the laces, which leads to premature failure at the clamp. For this reason, ‘bollard’ jaws typically are used, as the load in the clamp area is reduced via the geometry of the load path over the bollard. The SATRA STM 566 tensile test machine, fitted with STM 566TB 'thread bollard' jaws is suitable to conduct this test.
The lace specimen is clamped around the bollard jaws, and steadily stretched until it fails. The force at which this occurs is recorded. A minimum of three test specimens are required for each condition (wet or dry). The average value for a set of specimens is calculated. For calculating the extension at break, the test method sets out how to determine the apparent test length when bollard-type clamps are used. The determination of elongation at break is possible by using a tensile test machine which continuously records jaw separation, such as the SATRA STM 566 machine.
Four tests for slide fasteners
SATRA has developed four test methods which use a tensile test machine for evaluating key characteristics of zips: SATRA TM51, SATRA TM52, SATRA TM53 and SATRA TM226. These tests allow assessment to be made of the strength of different aspects of a zip. In addition, other SATRA tests – using other types of test equipment – are available to assess the fatigue life of a zip under repeated opening and closing activities, as well as the resistance to damage due to closure under a lateral force.
SATRA TM51:1993 – 'Lateral strength of slide fasteners' (figure 4) allows an assessment to be made of the lateral strength of a closed slide fastener. The sides of a closed slide fastener are clamped in specialist jaws in the tensile test machine, applying a load in a direction perpendicular to the chain. The force required to cause the fastener to fail is recorded. The test method defines the preparation of the specimen, the associated calculations and the information required in the test report, including the failure mode. A minimum of three test specimens should be used.
Slide fastener pullers may fail or become detached in use. SATRA TM52:2002 – 'Strength of slide fastener pullers' provides two tests to cover the failure of the puller from both 'tension' (method 1) and 'torsion' (method 2). Method 1 is conducted using a tensile test machine. The test is carried out after removing the puller and slider assembly from the teeth and tape assemblies. The test is conducted with a specialist pair of jaws, which allow the test fastener slide to be clamped in one jaw and an increasing tension applied to the puller via the opposing jaw. The test concludes when the puller or slider breaks leading to separation.
The test is carried out on three test specimens, and the arithmetic mean of the breaking force is reported along with the failure mode. Method 2 is conducted with a torque measuring device, rather than a tensile test machine.
Another test method – SATRA TM53:1992 – 'Attachment strength of slide fastener end stops' allows an assessment to be made of the strength of top and bottom stops of a zip. These are important considerations when assessing the capability of a zip. The test method covers three methods, as follows.
Method 1 – 'Top stop attachment strength'. The slider of a closed fastener is clamped in one jaw of the tensile test machine and the bottom end of the fastener is clamped in the other jaw. During the test, the jaws are steadily moved apart and the force required to pull the stop off the fastener is recorded.
Method 2 – 'Bottom stop attachment strength'. The slider of an open fastener is clamped in one jaw of the tensile test machine and the two free ends are clamped in the other jaw. When conducting a test, the jaws are moved apart steadily until failure occurs. The force required to pull the bottom stop off the fastener is recorded.
Method 3 – 'Bottom stop attachment strength'. To conduct this test, free ends of the stringer (the tape and teeth combined) of an open fastener are fitted into the two jaws of the tensile test machine. The jaws are then moved apart and the force required to pull the bottom stop from the stringers is measured. The locking mechanism, if present, is jammed open when conducting this test.
Each test should be repeated on three test specimens and the arithmetic mean of the maximum forces recorded.
The final SATRA zip test conducted using a tensile test machine – SATRA TM226:1999 – 'Slider locking strength of slide fasteners', allows a determination of the locking strength of a slide fastener slider to be carried out. In this test, the two open ends of a zip, where the slider is locked into the chain about 30mm from the top stops, are held in opposite jaws of the tensile test machine. The jaws are steadily separated until the locking mechanism slips or the test specimen fails, and the force at failure is recorded. A minimum of three test specimens should be tested, using a new slide fastener assembly for each test. The arithmetic mean of the failure load for each type of failure observed should be reported, together with the number of test specimens exhibiting the type of failure.
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SATRA produces a wide range of tensile testing jaws together with two tensile test machine models: STM 566 (5kN capacity) and STM 766 (20kN capacity). Please contact email@example.com for further information on SATRA test equipment.
This article was originally published on page 46 of the September 2015 issue of SATRA Bulletin.