Original publication: 2015 | Peer-reviewed journal article
Source authors: J. D. Clarke, K. Hallas, R. Lewis, S. Thorpe, G. Hunwin and M. J. Carré
About this research
Why can different slip-resistance instruments produce substantially different friction measurements on exactly the same wet flooring?
Friction between rubber and flooring depends on the materials, surface roughness, applied force, sliding speed and contamination. Standardised testing methods aim to produce repeatable measurements, but different instruments reproduce different contact conditions.
This collaborative research investigated the friction mechanisms underlying two established mechanical test methods: the pendulum and the SATRA STM603 footwear friction tester.
How the research was conducted
Six commercially available flooring materials, including ceramic, PVC, nylon, laminate and two deformable vinyl surfaces, were tested under water-wet conditions.
The pendulum used Slider 96 rubber, following BS 7976-2. The STM603 used a smooth rubber footwear heel under conditions based on EN ISO 13287:2007, with adaptations enabling comparison of flooring surfaces rather than different shoes.
Using the same rubber compound helped separate differences caused by material choice from those caused by the mechanical testing conditions. Surface roughness and stiffness were examined, and additional STM603 testing varied applied normal force between 150 N and 600 N.
Key findings
The two mechanical test methods produced different friction measurements.
Across the six flooring materials, there was no statistically significant linear relationship between the friction results from the pendulum and STM603 (p = 0.383). On several smooth, rigid surfaces, the STM603 produced considerably higher friction measurements.
Sliding speed and liquid-film behaviour helped explain the differences.
The pendulum's sliding speed was approximately 2.8 m/s, compared with 0.3 m/s on the STM603. At higher sliding speeds, water is less readily displaced and may continue lubricating the interface. Slower sliding and prior loading can give water more opportunity to escape, allowing more direct rubber–floor contact.
Flooring stiffness changed the measured friction mechanisms.
The deformable vinyl surfaces dissipated additional energy through compression and recovery during pendulum contact. This could increase the measured apparent resistance even where surface roughness alone would not predict it.
The effect of force depended on the surface.
On one deformable vinyl, changing the normal force from 150 N to 600 N reduced the STM603 coefficient of friction from approximately 0.60 to 0.33. The four comparatively stiff surfaces did not show the same pronounced relationship.
Roughness alone did not explain the full set of results.
Across the six surfaces, the measured Rz roughness did not display a significant simple linear relationship with the results of either test. Adhesion, deformation and lubrication all mattered.
Understanding the friction mechanisms
Rubber–surface friction may involve adhesion where the materials make direct contact; hysteresis where energy is dissipated through deformation; and liquid-film lubrication where water separates the surfaces. Different test conditions can change which mechanisms dominate.
The key scientific message is that two tests can both be repeatable while still measuring different combinations of these mechanisms.
Interpreting the findings
The work involved six water-contaminated surfaces and a consistent rubber contact material. It does not show that either test is always wrong or always right. The pendulum's higher contact speed may be relevant to some severe wet-slip conditions, while its impact on compliant surfaces can introduce other effects.
The explanations about water-film thickness were informed by physical theory rather than direct film-thickness measurements during testing.
Why it matters
A friction coefficient is not necessarily a fixed property of a flooring material. It is a response of a complete contact system under particular test conditions. Meaningful slip-resistance assessment requires understanding the physical mechanisms being measured, not simply obtaining a repeatable numerical value.
Original research
Clarke, J. D., Hallas, K., Lewis, R., Thorpe, S., Hunwin, G. and Carré, M. J. (2015). Understanding the Friction Measured by Standardised Test Methodologies Used to Assess Shoe-Surface Slip Risk. Journal of Testing and Evaluation, 43(4), 723–734. DOI: 10.1520/JTE20120334.