Flexing endurance of insoles

Comparing the SATRA TM3 method for assessing flex resistance of insole boards with an alternative approach.

by Mark Southam

One of the most basic requirements for a forepart insole board is the ability for it to be able to flex with the foot during walking and running, without becoming damaged.

The most widely used method for assessing the flex endurance of an insole board is the SATRA TM3 Flexing Index (figure 1). In this test, a narrow strip of the board is suspended by one end from a clamp with a 2kg weight hung from its lower end. The upper clamp is rotated back and forth through 180° until failure (usually the test piece snaps at the point where it flexes). The number of flexing cycles to failure is used to calculate the flexing index (the base 10 logarithm of the number of flexes). This test does not attempt to replicate what happens in wear. Rather, it is an accelerated test which is used as an indicator of how a material will perform in the shoe.

This method has been used by SATRA for over 45 years and has proven to be a good indicator of wear performance. It also provides a general guide to the overall quality of the board. SATRA TM3 is normally used for fibreboard types – cellulose, nonwoven and leatherboard – but it is not generally suitable for fabric insoles.

“The type of failures which might occur are cracking and/or delamination of the surface”

Other methods of assessing flex performance of insoles have been used. In particular, we have seen specifications which include the use of a modified version of the Ross Flex Test method (SATRA TM60). This is a method specifically developed to assess the flex performance of soling materials. When used for solings, a strip of the material is flexed by 90° over a 10mm diameter mandrel. Prior to testing, a 2mm-wide cut is made through the sample at the flexing point. The length of this cut is measured as the test progresses and the amount of cut growth is assessed. Samples are tested for up to 150,000 flexes.

In this modified version of the method for insoles, the footside of the board is held against the mandrel so that the board flexes in the same mode as it might in the finished footwear. In one specification we have seen, the number of flexes used depends on the board’s thickness – for instance, a 1.5mm thick board is assessed after 25,000 flexes. Cut growth is not usually the critical factor here, although we understand some laboratories do put cuts in the sample. Rather, the assessment is a visual one, with a subjective assessment of the condition of the surface layers of the board. The type of failures which might occur are cracking and/or delamination of the surface. No surface damage after the appropriate number of flexes is usually specified.

An accurate assessment

SATRA has carried out comparative testing of the two methods on a range of cellulose insole boards. Using the SATRA TM3 flexing method and comparing the results against SATRA’s guideline recommendations, most of the boards were found to be suitable for what we term ‘medium use’ – such as for men’s and women’s fashion and everyday footwear. One board was found to be suitable for lighter, occasion footwear. However, using the Ross Flex Test method all but one of the samples tested showed surface damage (cracking) below 25,000 flexes. Furthermore, the one sample which did exceed 25,000 flexes without damage was not the one which gave the highest performance in the SATRA TM3 test.

These results confirm SATRA’s previous experience that the modified Ross Flex Test method used in this way for insole materials tends to give variable results, which not only correlate poorly with results from the SATRA TM3 test, but also provides misleading indicators of suitability. There is evidence that perfectly adequate materials are likely to be rejected unnecessarily.

The SATRA TM3 method has been in use for many years, and the guideline recommendations we use to assess the results are both well established and a good predictor of performance. For this reason, SATRA continues to recommend the use of SATRA TM3 for assessing the flex resistance of insoles. We are not currently aware of any supporting evidence to demonstrate correlation between the insoles Ross Flex Test method and wear performance. However, we would be happy to assist any members who are interested in establishing this.

Publishing Data

This article was originally published on page 10 of the February 2012 issue of SATRA Bulletin.

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