New backpart fatigue tester

Describing the benefits of the new STM 437 test machine.

by Peter Allen

The failure of shoe heels while in use can be a safety risk and very costly – both in terms of lost reputation and costs incurred for compensation or replacements. Whether you are a brand owner or a manufacturer, failing to apply appropriate tests to heels and their attachment can be a false economy.

To meet the need for such assessment, SATRA has launched a new test machine – the STM 437 backpart fatigue tester – which can be used to test the fatigue resistance of whole shoe backparts. This new equipment has been designed as a result of SATRA’s research into what makes a heel and backpart combination work. One outcome of this investigation has been the development of more than 12 separate SATRA test methods which allow for the assessment of a number of important footwear characteristics relating to heels and their attachment. Most of these test methods can be used to assess a component of the construction or the interface between two components.

While it is important to ensure that suitable components are used, it is also vital to make an overall assessment of the completed construction. Two important tests which allow assessments to be made of the completed back part construction are SATRA TM113:1996 – ‘Measurement of the strength of attachment of heels to footwear and the backpart rigidity of such footwear’ and SATRA TM104:2010 – ‘Fatigue resistance of whole shoe backparts’. Testing the completed backpart with the attached heel is a very good way to allow an assessment to be made of how the selected materials, the overall design and quality of manufacture are working together to produce footwear that is fit for purpose.

Associated test methods

There are a number of other SATRA test methods which can be used to assess characteristics of components of construction for backparts and heel attachments. These include:

• SATRA TM11:1993 – ‘Pin holding strength of insole materials’
• SATRA TM58:1999 – ‘Stiffness of steel shanks’
• SATRA TM59:2000 – ‘Longitudinal stiffness of insole backparts’
• SATRA TM96:1995 (2017) – ‘Security of heel pins
• SATRA TM97:1999 – ‘Fatigue test for steel shanks’
• SATRA TM112:1995 – ‘Fatigue resistance of insole backparts and backpart components’.

Having selected materials and components with suitable properties, it is also important that good design is applied – for example, using the correct number and positioning of heel pins or the correct positioning of the shank. Good quality of manufacture is also important to achieve the accurate alignment of components and their integration.

When conducting a test to SATRA TM113, a tensile tester (or universal testing machine – ‘UTM’) is used to pull the heel from the backpart of the shoe. This test allows the following to be reported: i) the onset and nature of damage, ii) deformation under load, iii) permanent set under load, and iv) the ultimate load to produce heel separation. This test can be conducted using SATRA’s STM 566 tensile tester fitted with specialist jaws (STM 566HA in conjunction with flat jaws STM 566F).

The new SATRA STM 437 backpart fatigue tester has been developed to conduct methods 1 and 2 of SATRA TM104. Method 1 allows for the assessment of chunky or low, slender heels. For this test, the heel is clamped and cyclic loads are applied via the forepart. Method 2 permits slender heels, with heights above those specified in method 1, to be assessed. In this method, the forepart is clamped and alternating forces are applied via the heel.

SATRA TM104 sets out the geometry required for preparing, clamping and supporting the samples, depending on measurements taken from the footwear. The different loads which are applied in forwards and backwards directions to the heel or forepart (depending on whether method 1 or 2 is being used) are also determined by a calculation based on measurements taken from the footwear. The test method sets out inspection intervals and when deflection measurements should be taken. Any signs of damage are reported at inspection intervals. The test stops when the set number of cycles have been completed or when excessive movement of the sample occurs (which triggers the machine to stop automatically). On completion of the test, the footwear sample can be dismantled to allow damage within the construction to be recorded.

While testing to SATRA TM113 provides key information about heel attachment in a short-term test, testing to SATRA TM104 (which is a longer assessment), conducted using the STM 437 tester, allows for assessment of the performance of the heel under repeated application of loads, typical of the loads applied in walking. This longer test has been found to be a good assessment for the diagnosis of faults in footwear backparts.

Because SATRA TM113 and SATRA TM104 are tests of the whole backpart and heel attachment, they are suitable for assessing footwear using novel materials or novel methods of construction. Some of the individual component tests may not be applicable in these cases, as the components would have been combined or replaced with other structures. By contrast, testing the completed construction of new designs can help to verify their durability, along with their ultimate failure modes, and hence determine that they are fit for purpose. 

 In addition to ensuring the heel attachment to the backpart is secure, it is also important to consider some other failure modes for heels. These include insufficient slip resistance of top pieces and impact and fatigue cracking of high slender heels.

Publishing Data

This article was originally published on page 14 of the May 2018 issue of SATRA Bulletin.

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