Test equipment used for assessing soles – part 2
Concluding an overview of SATRA equipment available for testing this vital footwear component.
by Peter Allen
Part 1 of this article considered several items of SATRA test equipment used to assess a range of standard properties applicable to a variety of soling materials. In part 2, we examine a number of key tests and associated machines which assess the performance of soles in the context of complete footwear.
The tests considered are: i) shock absorption, ii) sole adhesion, iii) repeated compression testing, iv) sole abrasion in the context of whole footwear, and v) slip resistance.
The sole and mid-sole in footwear largely determine the energy absorption and energy return of bottom units of whole shoes – characteristics that can be assessed using the SATRA STM 479 dynamic shock absorption test machine (figure 1). The SATRA TM142 test method – ‘Falling mass shock absorption test’ – is conducted using this test machine. Sheet materials can be tested using this method, in addition to whole bottom constructions.
Sole adhesion testing
A sole adhesion test can be conducted to SATRA test method TM404 – ‘Rapid sole adhesion test’, using the SATRA STD 185 sole adhesion tester (figure 2). This allows a determination to be made of the strength of the bond between the sole and uppers at the toe and heel areas of the lasted margin for completed footwear. When using the STD 185 device to assess the bond at the toe, the sole of the completed footwear is positioned on the rounded top of a T-shaped anvil. This acts as a fulcrum, and the curved top piece of the instrument is fitted in the featherline groove between the sole and upper. A gradually increasing downwards force is applied by hand to the backpart of the footwear. This creates a downward force at the toe piece, which has the effect of attempting to separate the sole from the upper. The force is shown on a dial gauge on the instrument, which also incorporates a maximum load pointer.
When used to monitor the toe adhesion of footwear in production, a minimum required load can be specified against which footwear from the production line can be checked. A failure to maintain a secure sole bond (when subjected to the specified check load) should trigger an investigation into the process or materials which have contributed to the sole bond failure. One key advantage of this test is that it is non-destructive if the bond holds at the specified minimum load. An attachment is available (STD 185H) which allows the heel bond strength to be assessed in a similar manner to the toe bond strength.
Repeated compression testing
In use, footwear is subjected to repeated impact loads due to heel strikes. Shown in figure 3, the SATRA STM 512 (repeated compression test machine) allows for an assessment to be made of the resistance of heels of sole units and heel materials to the effects of repeated compression. The SATRA TM156 test method is conducted using the STM 512 test machine. The test employs a domed striker representing the wearer’s heel, which repeatedly applies a set load for a set number of cycles and at a defined strike rate. Lower loads are set for children’s and women’s footwear, which are also conducted with different sized domed strikers. Faster strike rates can also be set for accelerated testing.
A combination of SATRA TM142 and SATRA TM156 allows for an assessment to be made of how much shock absorption and energy return capability is maintained by a heel construction after a period of repeated heel strikes. To do this, the shock absorption and energy return tests are conducted before and after the repeated compression test. This allows any fall off in performance to be assessed by comparing the SATRA TM142 results before and after repeated compression testing.
Soling abrasion using SATRA STM 528
As described in part 1 of this article, a test conducted to SATRA TM174 using the SATRA STM 602 soling materials abrasion tester allows for an assessment of the abrasive wear resistance of soling materials. The actual wear performance of the sole when in use depends not only on the soling material selected, but also a number of other constructional features of the sole and within the footwear overall. The SATRA STM 528 Pedatron test machine (shown above) allows a full shoe test to be conducted with loads applied, which have been determined from the study of human gait. In use, the machine not only applies heel strike, toe-off loads and shoe flexing, but also the longitudinal and torsional shear loads which are encountered when walking.
The SATRA TM362 test method – ‘Abrasion resistance of soles – biomechanical method’ is conducted using the Pedatron, and a plain surfaced slab. The assessment of wear is based on mass or thickness loss, in addition to an examination of the wear pattern. The machine can be run continuously, generating 48,000 steps in a 24-hour period. Pedatron has proved to be a very versatile test machine, and is now used to assess other aspects of whole shoe performance in addition to sole abrasion resistance. When used in conjunction with other SATRA test machines, assessments can be made of footwear over a simulated period of use. For example, conducting a slip resistance test (SATRA TM144 using the SATRA STM 603 slip resistance test machine) both before and after a period of simulated wear on the Pedatron allows wear-induced changes in slip resistance to be identified. Similarly, water resistance tests (such as SATRA TM230, using the SATRA STM 505 dynamic water resistance tester) conducted before and after a period of Pedatron simulated wear allows for an assessment to be made regarding the capability of the footwear to retain its water resistance properties over a period of use.
Slip resistance testing
Testing against industry-accepted slip resistance standards (for instance, SATRA TM144) is a proven method to validate products and to demonstrate due diligence. Slip resistance testing is now a requirement within European safety footwear legislation (EN ISO 20344/5 series). SATRA STM 603 (figure 4) is a globally adopted slip resistance test machine and allows tests to be conducted to SATRA TM144, EN ISO 13287 and ASTM F2913-11. Providing slip resistance to footwear poses a number of challenges to the footwear manufacturer. The selection of outsole material and detail design of the sole pattern or cleats make important contributions to the level of slip resistance provided by the footwear. Testing is conducted on surfaces specified in the appropriate method or, for development purposes, other surfaces can be used. For everyday footwear, testing is typically conducted in both wet and dry circumstances. However, tests can also be carried out with other forms of lubrication. As an example, SATRA can provide an accessory which is fitted to the STM 603 slip resistance tester to produce an ice surface.
Overall, soles contribute significantly to footwear performance. Testing the materials, the sole unit or the integration into the completed footwear is important to ensure that the required performance is achieved, together with the anticipated product durability.
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It has not been practical to cover all the tests appropriate to solings within this article. Please email email@example.com to discuss test requirements for a particular soling material or the additional methods available for the assessment of soles. Contact SATRA’s test equipment team (firstname.lastname@example.org) for further information on SATRA test equipment.
This article was originally published on page 40 of the February 2014 issue of SATRA Bulletin.