Assessing the flexing capability of footwear fibreboard

Explaining the use of SATRA’s STM 129 test machine and the value of assessing this important component.

by Peter Allen

The level of contribution that insole board makes to the overall stability, structural integrity and performance of a shoe 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 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. SATRA’s TM3:1999 – ‘Flexing Index’ test method is widely used to assess the flex endurance of insole boards.

Insoles are made from a variety of materials, including cellulose boards and nonwoven materials. There are a number of tests which can be used to assess insole boards, many are applicable to both of these materials.

The SATRA TM3 test method is applicable to most types of semi-rigid sheet material – particularly fibreboards and is conducted on the SATRA STM 129 machine. This unit contains six stations, which allows three specimens cut parallel to the direction of material manufacture and three cut at 90º to this direction to be tested at the same time (as specified in SATRA TM3).

The machine in action

When conducting a test, a specimen is held in tension and repeatedly flexed through 180º until failure occurs (figure 1). The 70x10mm test specimens are clamped in wedge-shaped top clamps. The lower end of the specimen is clamped to a mass, thereby applying a total load of 19.6N to the specimen. The top jaws rotate cyclically under simple harmonic motion between two positions, 90º either side of the vertical. This flexes the specimen below the nose of the wedge clamps, which have a prescribed profile and the test continues until the specimen breaks. A counter for each station records the number of flexes achieved. The breakage of the specimen triggers the counter to stop for that particular test station.

Once all the specimens have failed, the number of cycles achieved for each specimen is used to calculate the flexing index of the material. The base 10 logarithm of the number of flexing cycles to failure is calculated for each specimen. The arithmetic mean of the log values for each of the test directions is calculated, and these figures form the flexing indices for the material. This accelerated flexing test has proved to be a valuable means of predicting how materials will perform in wear.

If insoles are to be used in constructions where they are likely to encounter high shoemaking temperatures (such as can occur in the direct moulded-on vulcanised construction), it is important to check whether or not the heat will cause the material to become brittle and so reduce the flexing performance. SATRA test method TM4:1993 – ‘Change in flexing index due to exposure to heat’, sets out an assessment of how this characteristic can be evaluated. The test method defines a procedure in which rectangular pieces of the test material are subjected to specified levels of temperature and pressure for controlled periods of time in a platen press. Tests are conducted using materials cut parallel to the direction of manufacture, as well as at 90º to this direction. Test specimens are cut from these heated treated rectangles, which are then tested, after a period of conditioning, on the SATRA STM 129 machine as per SATRA TM3. Unheated samples from the same material are also tested. This allows a comparison to be made in the Flexing Index between non-heat treated materials and heat-treated items.

Click here for comprehensive information on SATRA's range of test equipment.

Publishing Data

This article was originally published on page 44 of the June 2014 issue of SATRA Bulletin.

Other articles from this issue »

---