SATRA flexing machines for specific tasks

The value of testing specific footwear materials and components for flex resistance.

by Peter Allen

SATRA produces a wide range of test equipment applicable to the footwear industry. This article outlines three pieces of SATRA test equipment which are used to conduct flexing endurance tests on key structural materials used in the bottom constructions of footwear. The SATRA STM 129 Fibreboard Flexing machine is used to test the flex endurance of insole boards, the Midsole Flexing machine (SATRA STM 441) assesses the flexing endurance of penetration-resistant inserts used in safety footwear, and the SATRA STM 141 Ross Flexing machine is used to test the resistance of a polymeric material to cut growth. The tests conducted with the STM 141 are applicable to all types of flexible sheet material – especially those used in footwear solings.

Insole boards

The contribution that the insole board makes to the stability, structural integrity and performance of a shoe overall will depend on the type of footwear construction being employed. For example, in a conventional stuck-on sole, the insole adds strength and shape, and provides a backbone for the shoe onto which the upper is secured. In addition to these functional aspects, the insole board needs to provide sufficient flexibility for the walking step while having the ability to withstand repeated flex cycles without damage.

The SATRA TM3 test method is widely used as a method for assessing the flex endurance of insole boards and can be conducted using SATRA’s STM 129 test machine. In SATRA TM3, a narrow strip of the board is suspended by one end from a clamp and a 2kg weight is hung from its lower end. During the test, the upper clamp is repeatedly rotated through 180° (+/-90° to vertical) until a failure of the material at the flexing point occurs. The number of flexing cycles to failure is used to calculate the flexing index. This test does not set out to reproduce the actual flex angles encountered in footwear use, but the method does allow an accelerated test to be conducted which can then be correlated against actual footwear use. The SATRA STM 129 test machine has six test stations, allowing this number of samples to be tested at the same time – each with its own cycle counter which stops automatically when the sample breaks on that particular station.

The machine can also be used to determine the effect heat can have on insoles, which can be caused by direct-moulded construction or vulcanisation. Heat applied in the manufacturing process can cause some materials to become more brittle and, hence, more vulnerable to a flex endurance failure. To assess this effect, the samples tested should have previously been subject to the temperatures encountered in the appropriate manufacturing process. Comparison can be made to tested samples produced from the same material, but which have not been subjected to heat.

Flexing nail penetration-resistant inserts

Safety footwear can include requirements for penetration-resistant soles to protect the user from piercing injuries through the underside of the sole. These inserts are commonly made from thin metal, but can also be made from non-metallic materials. In addition to providing the required resistance to nail penetration, the inserts must be capable of avoiding failure during the repeated flexing which occurs when walking. European safety footwear (in which penetration-resistant inserts are required, conforming to the requirements of EN 12568) need to pass a test to confirm their resistance to cyclic flexing.

SATRA’s STM 441 Midsole Flexing machine is a six-station machine capable of conducting tests conforming to these requirements. The penetration-resistant insert is clamped at a prescribed position, and the toe of the insert is flexed through a defined amount at a prescribed distance from the clamp. The flexing cycle is applied at a rate of 16 cycles per second. EN 12568 prescribes a requirement for 1 million cycles to be applied, after which the specimen is inspected for any sign of disintegration, cracking or delamination. The machine also includes the ability for the amount of deflection to be varied allowing other tests to be performed – for example, in accordance with CSA or ASTM requirements.

Resistance to crack growth in non-leather materials

Another SATRA test machine – the STM 141 Ross Flexing machine – is designed to test non-leather materials used for shoe soles and other applications at room temperature. The machine allows tests to be conducted in accordance with SATRA TM60 (Resistance to cut growth on flexing). This method is intended to determine the resistance of polymeric material to cut growth during repeated flexing cycles. When the test is conducted on sheet materials, samples are cut to the prescribed size, and one end is clamped to a flexing arm. The free end of the sample is supported on rollers and between pinch rollers. The clamped end flexes through an angle of 90°, bending the material around a flexing mandrel while the rollers allow the free end of the material to move. This permits the specimen to bend, but not stretch around the mandrel.

The test is conducted at a flexing rate of 60 flexes a minute, with the sample being flexed from the flat state to an angle of 90°. Before conducting the test, a cut of defined dimensions is made through the centre of the test specimen. The test is designed to see how resistant the material is to propagation of this induced ‘crack’, and is stopped after the crack length has grown by 6mm or after 150,000 cycles – whichever is the sooner. Detailed measurements of the crack allows an average crack growth rate to be determined per thousand flexing cycles. SATRA TM60 also describes methods detailing how to conduct the test using larger test specimens, such as forepart soles.

Most materials become more brittle at low temperatures, in which they are more vulnerable to an increased rate of crack growth. One notable exception to this rule is thermoplastic rubber (TR), which can show increased resistance to crack growth at low temperatures, and so should be tested at room temperature. SATRA STM 141F, which contains the operating mechanism within a freezer cabinet, allows tests to be conducted at room temperature, as well as at -5°C (the temperature at which most materials – except TR – should be assessed). Another variant of the machine allows a higher flexing cycle rate of 100 cycles a minute, which is applicable to ASTM standards. Both the STM 141 and STM 141F machines are 12 station units, allowing this number of specimens to be tested simultaneously. SATRA can also provide a chisel holding jig and the associated chisels used for producing the pre-test cut. Separate chisels are required for the SATRA Ross test and the ASTM Ross test.

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Publishing Data

This article was originally published on page 30 of the September 2012 issue of SATRA Bulletin.

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