Taking a further step
A review of new test methods and test protocols for use with SATRA’s STM 528 Pedatron test machine.
by Sam Huzzard
Being a research organisation, SATRA is constantly endeavouring to not only improve equipment but also to find new ways of using it. The Pedatron STM 528 is one of SATRA’s flagship machines for the testing of whole footwear constructions. The primary test method for this equipment is SATRA TM362:2014 – ‘Abrasion resistance of soles – Biomechanical method’. As the name suggests, this is designed primarily as a sole abrasion test for whole footwear.
However, footwear tested on this equipment is not only abraded at the sole, but also receives several other types of wear. Effects such as damage due to flexing in the upper, and even compression set of the insole can be observed. One of the key properties of the STM 528 Pedatron that makes it so effective at simulating walking is its ability to react to the floor surface. If the footwear being tested has particularly poor slip resistance, it is likely to slip on the floor surface during the test, creating sole abrasion patterns characteristic of those expected in actual wear. If the footwear has higher shock absorption, this changes the underfoot forces as would be expected. This realistic walking action is due to the momentum in the system, resulting in a natural degradation of the footwear.
For some time now, SATRA Pedatron has also been used in the assessment of flooring and stair nosing in accordance with SATRA TM391:2016 – ‘Flooring – Biomechanical abrasion method’. However, SATRA has been expanding the types of wear that can be simulated using this equipment. The next revision of SATRA TM362 is scheduled to include the option to test footwear with the use of angled and contoured concrete slabs as floor surfaces, as described in the article ‘Assessing wear from angled surfaces’. This provides the opportunity to achieve different kinds of wear patterns – for example, to simulate going up and down hill or pronation and supination. Such assessments can easily be conducted with STM 528 Pedatron machines located in member laboratories.
Another useful test that can be conducted using STM 528 with the aid of a force platform is done in accordance with SATRA TM449:2018 – ‘Rotational slip’, to determine rotational slip (torque) of footwear and flooring combinations. This is an important metric to measure to account for the occasions when torsional grip is too high, as such high torsional forces can cause serious injury in many sports and occupations.
Alternative testing on the Pedatron
As discussed in the article ‘Using the STM 528 Pedatron to simulate running’ published in the May 2016 issue of SATRA Bulletin, the gait of the Pedatron can be adjusted to produce higher ground reaction forces, which mimic the higher underfoot forces generated by a running subject weighing around 70kg. This is mostly achieved by angling the foot forwards and increasing the impact speed of the footwear with the floor. While this is not a perfect simulation of the running gait due to the speeds required, this mode will still assess whether the footwear can withstand the high forces demanded by running. This heavy loaded testing is only performed at SATRA, where we have the resources on hand to perform the required modifications to the equipment.
For some time now, products have been tested at SATRA to the ‘TP15’ protocol called ‘accelerated ageing’. In this test, SATRA TM362 and the ‘sweating foot’ equipment used in SATRA TM376:2009 – ‘Advanced moisture management test’ (AMMT) are combined. By performing these two tests alternately, the footwear is aged as if it has been worn by a sweaty foot. The addition of the moisture in the footwear, when replaced on the STM 528 Pedatron machine, is to affect the abrasion between surfaces. This procedure has been used to great effect to accelerate ageing in a variety of footwear types, but has proven particularly effective at assessing footwear containing electronics, where the introduction of moisture presents a challenge to their durability.
Of course, it is worth considering that most footwear properties change as the footwear ages. This can include thermal properties, water resistance, flex resistance and shock absorption. Sometimes this ageing effect can have positive effects, as the Pedatron can effectively ‘break in’ an item of footwear, decreasing the flex resistance which should make it more comfortable in which to walk.
However, compression set (a permanent compression of materials due to wear), is one type of wear seen in footwear tested on the Pedatron which can negatively affect underfoot comfort and insulative properties. Just like squeezing a sponge, it can reduce the absorption potential of materials.
Research has been conducted into alternative methods of introducing moisture into footwear aged on the Pedatron, by eliminating the need to transfer the footwear to a second piece of equipment. A Pedatron AMMT hybrid test has been successfully conducted and shown to be effective. This has involved attaching pipes to the Pedatron’s prosthetic foot in a number of specific locations, which allows the simulation of sweat while walking, or moisture introduced by rainy or damp conditions.
The pipes are attached to the same pump used to control the flow rate in the AMMT test so, as a result, the flow rate can be finely controlled. One pipe is attached on the outer side of the foot in line with the ankle bone, but with the end of the pipe touching the footbed, and a flow rate of moisture at 20ml/hour is used to simulate sweat. Alternately, three separate pipes can be attached on the foot – one in the same position as used for the sweat protocol, one in the same position but on the inner part of the foot, and one on the top of the foot, usually around half way up the tongue of the footwear. While the flow rate for this latter test is the same as for the single pipe arrangement (20ml/hour), the water comes out of each of the three pipes, meaning 60ml/hour is being pumped into the footwear to create the effects of running in the rain. As the footwear moves in the Pedatron machine, the inside of the footwear heats up to around 27°C. Although this is not as high as the 34°C used for a standard AMMT test, it still causes evaporation to occur during the test. This is an ongoing research project and the protocol still needs to be finalised.
How can we help?
Please email firstname.lastname@example.org for further information on the use of the SATRA STM 528 Pedatron test machine.
This article was originally published on page 46 of the April 2019 issue of SATRA Bulletin.