Development of a mud simulant slip surface

Explaining the design and application of this valuable new test surface.

Image © Lorraine Boogich

SATRA has considerable experience of slip resistance testing of footwear and floorings, ranging from consumer complaints in the retail sector to meeting the requirements of global safety standards. Alongside a vast range of footwear styles is the complexity of underfoot surfaces and shoe-surface interactions. Accordingly, SATRA can assess slip resistance against numerous surface types and common lubricating substances. The SATRA TM144:2011 test method standardises testing to a surface of known frictional properties in both dry and wet conditions. This provides a performance criterion and a means of footwear comparison. Having an established range of test flooring and lubricants has led to an understanding of the factors of sole design relating to slip resistance. Materials selection and tread pattern are two key variables in coefficient of friction (CoF) behaviour.

Surface requirements

The growing demands of industry often call for the development of novel testing approaches. In this context, combinations of solid surfaces and lubricants can help to represent real-world situations. However, there is currently no standardised deformable slip material capable of simulating unpaved or off-road surfaces (for example, mud) in a slip event. The properties of mud depend on environmental factors – such as soil composition and the hydrological cycle. Therefore, it is important to consider a given shoe’s potential end use and associated outdoor situations. For the wearer, underfoot mud can be highly unpredictable. Translating this to a standard test surface is experimentally challenging, and overcoming this difficulty has been the aim of recent SATRA research.

An ideal mud simulant must be able to undergo repeated slip cycles as one sample, withstand total fracturing and have Newtonian behaviour. The ‘Mud Simulant’ project has enabled SATRA to develop a material meeting these requirements, and promising results have been obtained. Formulating an artificial surface allows both composition and properties to be precisely tailored.

Unlike externally sourced test floors, this gives SATRA full control over how the material is prepared. In keeping with current standards, the surface: i) can be calibrated, ii) it is consistent across equivalent samples and iii) it can be used to distinguish footwear style by slip resistance. Forming the mud simulant is inexpensive, fast and simple. This is also the case for calibration, which provides data in a form closely related to slip behaviour.

Experimental factors

Work has been undertaken to establish a test protocol to accommodate compressible and deformable slip resistance surfaces. Calibration of the mud simulant involves a standard compression test, partly reflecting surface deformation in slip events (see figures 1 and 2). Minor modifications have allowed the simulant to be tested with a current test method as if it were a conventional surface. Moreover, the material generates reproducible slip resistance results across multiple shoe and surface combinations.

Figure 1: Footwear under test on mud simulant

Figure 2: Post-slip state of simulant surface

The standard quarry tile is characterised by a CoF ranking of various materials, and calibration involves dry and wet slip runs using a particular rubber specimen. On a mud-like surface, material alone is unlikely to affect slip resistance. The compressible and deformable nature of the simulant serves as a greater test of tread pattern than soling material in terms of slip resistance. Loose material can be displaced and accumulate ahead of footwear in a mud slip event, unlike the fixed dimensions of a ceramic tile. This places unique slip resistance demands on footwear. Shoes with good slip resistance on paved surfaces may be prone to slipping on mud and similar materials. For example, a flat sole with large contact area would not provide traction against a loose, free-flowing surface. Conversely, certain sole features act to oppose slip. The studs of a football boot will stabilise the wearer against loose underfoot mud, with the opposite effect likely on pavement. Assuming the mud simulant is a viscous solid, slip resistance can be considered a function of sole geometry more so than materials selection.

Having achieved a mud simulant surface, SATRA can help design for unpredictable slip situations. A key focus of the project was to ensure that the mud simulant could represent the potential range of underfoot properties, yet remain reproducible for repeated testing. In particular, being able to assess soling pattern as distinct from materials may aid the design process regarding slip resistance. By isolating tread pattern as a variable, the slip resistance between footwear styles tested on mud will differ to that on paved surfaces. Preparing surfaces to a wide range of flow properties can further distinguish footwear by slip resistance.

Across the range of simulant properties achieved in testing, there are points of crossover in CoF between different shoe styles. Figure 3 highlights the effect of increasing surface compressive strength (load in newtons) on the slip resistance of various items of footwear. Beyond this range, similar patterns may exist. Considering a mud surface of near-solid consistency or a tiled surface, the extent of deformation is either minimal or non-existent. Here, frictional properties of the sole material are predominant in slip resistance. Meanwhile, free-flowing or liquid substances rely on geometric tread features for displacement, which prevents ‘hydroplaning’.

Figure 3: Coefficient of friction (forepart mode) against compressive resistance of simulant in N

Applications

Designing footwear towards a specific end use will help in testing for slip resistance in terms of potential outdoor environments. This may range from recreational activities to meeting the demands of emergency services and military roles. Extremes of weather and terrain will worsen muddy underfoot conditions. Mud easily absorbs water, to the point of becoming saturated. Therefore, mud will realistically be inhomogeneous (not uniform in character or content) throughout its depth. Equally, water can rapidly evaporate from mud’s structure given the right conditions.

The introduction of water to mud will increase its tendency to flow and, in some cases, will promote adhesive behaviour. In this case, material occupying recessed regions of the sole can hinder slip resistance. This has been reflected by the mud simulant during testing, an example of which can be seen in figure 1. Experimentally, testing compressible surfaces is challenging, due to the vertical force component acting against a fixed machine table height. With some modifications to setup, this can be managed.

Achieving a realistic mud simulant can help to identify how to ‘anchor’ footwear at critical slip points of the step. Naturally, exposed mud often coexists with grassland. Against the latter surface, slip resistance can be represented experimentally by artificial turf. This is a common test floor, and testing can be against ‘SATRATurff’ (which was developed several years ago), or may be conducted to customer specification. While grass can vary by length and density, mud is a continuous material of infinitely variable consistency. Given that mud and grassland frequently coexist, the ability to simulate both in testing is advantageous. For example, a given sole feature may generate high friction against turf but underperform against the mud simulant. Other sole patterns may give similar slip resistance across the two surfaces. Similarly, alternative SATRA slip resistance test methods may offer further scope for comparative testing – such as rotational slip resistance.

Mud and grass frequently coexist, so the ability to simulate both in testing is advantageous – for example, by using the mud simulant and then SATRATurff artificial grass

In summary

As a first attempt to simulate a loose, deformable surface, the requirements of repeatability and differentiating footwear by CoF were prioritised in this work. The intrinsic variability of mud makes realistic testing difficult. However, this project has provided a good foundation for development. Real-world characterisation and correlation testing of mud in terms of slip will contribute greatly to simulation as an experimental surface. Human factors are also an important consideration, given the higher probability of slip occurring on mud compared to a paved surface. For instance, modifying foot placement and contact angle might affect the ability of a sole to provide traction to the wearer. SATRA welcomes any members interested in the novel aspects of slip resistance testing to tell us of their experiences with mud-related slip.

How can we help?

Please email research@satra.com for assistance with the application of SATRA’s mud simulant slip surface.

Publishing Data

This article was originally published on page 42 of the October 2017 issue of SATRA Bulletin.