Testing soles for flex damage resistance

Describing the range of SATRA’s sole testing machines.

Poor design, the selection of inferior materials or inadequate methods of construction can result in soles that are vulnerable to fatigue cracking, due to the repeated flexing induced when walking. To assess the risk of such component breakdown, SATRA produces a number of test machines which play different roles in the assessment of the flexing resistance of soles, and each is also available in a low temperature version. These are the SATRA STM 141 Ross flexing machine/STM 141F low temperature variant), the STM 465 whole sole flexing machine/STM 465F low temperature model, and the STM 612 Bata belt test machine/STM 612F low temperature version. This article sets out a description of the machines and outlines their different uses.

STM 141

The SATRA STM 141 Ross flexing machine can be used to assess the resistance to crack growth on flexing. This can be achieved by testing to SATRA TM60:1992 – ‘Ross flex test – resistance to cut growth on flexing’, using the STM 141 machine. This test is designed to test non-leather materials used for shoe soles. It can also be used to test materials for other applications. The SATRA TM60 test 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, specimens of the material are cut to the prescribed size and one end is clamped to a flexing arm. The other free end of the specimen is supported on rollers and between rollers. The flexing arm moves through an angle of 90 degrees, bending the material around a flexing mandrel while the rollers allow the free end of the material to move. This allows the specimen to bend, but not to stretch around the mandrel.

Before carrying out the test, a cut of defined dimensions is made through the centre of the test specimen. The purpose of test is to see how resistant the material is to propagation of the cut. The test is stopped after the cut length has grown by 6mm or after 150,000 cycles (whichever is the sooner). Detailed measurements taken of the cut allow an average cut growth rate to be determined per thousand flexing cycles. SATRA TM60 also describes how to conduct the test using larger test specimens, such as sole foreparts.

The low temperature option permits assessment of materials which may be vulnerable to increased rate of cut growth due to increased brittleness of the material at low temperatures.

The Ross flex method is best suited to smooth soles or those with a very shallow tread pattern (for example, below 2mm). The SATRA TM60 test method specifies the maximum thickness which can be assessed between 2.5mm and 6mm, depending on the type of material being tested.

The Ross flex testing machine is also specified within European safety footwear requirements (EN ISO 20345:2011) as a test required for the assessment of oil-resistant soles and for the effects of hydrolysis on polyurethane (PU) soles. These tests are conducted at low temperature which would require the STM 141F test machine.

STM 465

The SATRA STM 465 whole sole flex testing machine allows for the determination of a sole’s resistance to the growth of a cut during repeated flexing. This machine, commonly referred to as a ‘Bennewart flexing machine’, is designed for the assessment of complete bottom constructions (including midsoles and insoles), and is used to comply with testing requirements specified in EU safety footwear legislation.

This machine comprises of three workstations, presented horizontally for ease of loading, inspection and measuring during the test. The test specimens are clamped into position, and as the machine operates they are subjected to repeated flexing cycles at the rate of 140 flexes per minute. The number of cycles completed is recorded on a counter, which also allows for a total number of flexes to be pre-set.

The machine can be used to conduct a number of tests at ambient temperatures, including SATRA TM161:2004 – ‘Bennewart flex test – resistance to cut growth on flexing’, EN ISO 20344:2011 8.4, and EN ISO 17707:2005.

Controlled cuts are introduced into the soling using a chisel (SATRA STD 465C). On completion of a set number of cycles, the growth of the cut is determined by inspection and measurement.

Although low temperature testing is not specified in these test methods as a requirement for CE-marked safety footwear when using the SATRA STM 465 machine, a wide variety of materials do become brittle at low temperatures. This is also often accompanied by an increased vulnerability to crack growth under repeated flexing – an important consideration when footwear is used in a cold natural or working environment. The SATRA STM 465F variant allows testing to be conducted at low temperatures down to -20°C. The flexing stations are contained in a temperature-controlled cabinet but, in other respects, STM 465F operates in the same manner as the standard SATRA STM 465 machine. Both machines are protected with a fully interlocked guard system to protect the operator from injury while performing the test.

The SATRA STM 465F whole sole flex testing machine allows for assessments at low temperature down to -20ºC

STM 612

SATRA’s STM 612 Bata belt flex test machine is seen as providing one of the best ways of assessing the flex crack resistance of shoe soles, and the machine is simple in both principle and operation.

When conducting a test using the STM 612 machine, the forepart and waist section of a sole unit is attached to an endless belt. This is then passed around two different-sized pulleys – a fixed pulley that is directly connected to a drive mechanism, and a smaller, interchangeable pulley which, when the belt and sole passes over it, produces a flexing action in the sole similar to that found in wear. All machines are supplied with three different-sized interchangeable pulleys, which control the severity of the flexing action (the smallest diameter pulley being the most severe).

Each belt can usually accommodate up to six large (adult size) sole units, subject to style. The sole unit is attached to the belt by strong adhesive, although for stiff sole units it may be necessary to stitch the toe and waist ends to the belt. Once the sole units are attached, the belt is rotated around the two pulleys for a pre-determined number of cycles.

SATRA’s STM 612 Bata flex test machine provides one of the best ways of evaluating the flex crack resistance of shoe soles

The control system used on the STM 612 allows for this pre-determined number of cycles to be programmed into the machine. Once this number of cycles has been reached, the machine will automatically stop. This allows for manual inspection of the sole units, including regular inspection at intervals throughout the duration of a complete test.

Another feature of the STM 612 operating and control system is the automated switch-off mechanism, which is particularly useful for detecting a broken or detached belt. Not only does this save testing time, but it also prevents possible damage to the machine.

The bench-mounted STM 612 model is fitted with polycarbonate panels so that the belt and sole units are visible to the operative throughout the test. Whenever inspection of the sole units is required, the fully interlocked access panel is lifted in a ‘gull wing’ arrangement, which is aided by the use of gas-filled lifting cylinders. This is also a safety device to prevent access while the belt is moving.

Temperature, particularly when sub-zero, can affect flex crack resistance, especially with thermoplastic materials such as polyvinyl chloride (PVC). To monitor this, SATRA has developed the STM 612F test machine, which operates in the same way as the bench-mounted unit, but at temperatures down to -20°C. Owing to the need to provide high levels of thermal insulation to ensure accurate temperature control, this machine does not have polycarbonate viewing panels.

Both the STM 612 and STM 612F machines have been designed to perform testing in accordance with SATRA TM133:2017 – ‘Resistance to crack initiation and growth – belt flex method’.

The SATRA TM133 test allows for the assessment of thicker soles or soles with deeper cleats than are tested on the Ross flexing machine. The test also allows an assessment of the flexing resistance over the full forepart of the sole unit. This is in contrast to the STM 465 whole sole flexing (Bennewart) test machine, which focuses the flex on only one position. The Bata belt test is the only one in which a cut is not introduced into the sole unit prior to the commencement of the test.

Specifying and supplying appropriate soling materials is a prerequisite to producing good quality footwear. It is also important to assess the quality of the sole when it is integrated within the completed footwear. As outlined in this article, there are a number of relevant SATRA test machines and test methods available for ascertaining a product’s fitness for purpose.

How can we help?

Please email test.equipment@satra.com for further information on these test machines and other testing equipment produced by SATRA.

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