Development of new footwear sole surface pattern for prevention of slip-related falls
Highlights
► A new footwear sole surface pattern was developed to prevent slip-related falls. ► Hybrid rubber block combining rough and smooth surfaces was prepared. ► Friction tests of the rubber blocks were conducted on liquid-contaminated surfaces. ► The hybrid rubber block provides high SCOF and DCOF values. ► The hybrid rubber block maintains sufficiently large contact area at slip initiation and during sliding.
Introduction
The number of slip and fall incidents in occupational accidents has been increasing in Japan as well as in other industrialized countries. (Ministry of Health, Labor and Welfare, 2006, Courtney et al., 2001, Courtney and Webster, 2001). Most slip and fall accidents in the workplace occur on liquid contaminated floor surfaces (Strandberg, 1985, Proctor and Coleman, 1988, Grönqvist, 1995, Leclercq et al., 1995, Manning and Jones, 2001). Such smooth floor surface is slippery when contaminated with water or oil due to the formation of a fluid film in the contact interface between the footwear sole and the floor. Thus, a footwear sole pattern with a high slip-resistance, even on such slippery surface, is required to prevent slip-related falls.
The coefficient of friction is often used for the evaluation of the slip resistance of a footwear sole. There have been controversies in selecting either static friction or dynamic friction as the critical frictional parameter at the contact interface between the footwear sole and the floor for the prevention of slip-related falls (Ekkubus and Killey, 1973, Tisserand, 1985, Pilla, 2003, Yamaguchi and Hokkirigawa, 2008). As Perkins (1978) pointed out, slip velocity and slip distance, which both have a strong correlation with a fall due to induced slip, increase with the difference of the values of static coefficient of friction (SCOF) and dynamic coefficient of friction (DCOF). In particular, if the SCOF is high but the DCOF was very low, slippage may not be stopped when the required coefficient of friction (RCOF) reaches the SCOF, resulting in slip initiation. On the other hand, if the SCOF is small enough for slip to occur but the DCOF is high, slippage stops and a fall will be avoided. Therefore, it has recently been considered that the DCOF is a more relevant measurement from slip biomechanics studies (Strandberg and Lanshammar, 1981, Perkins and Wilson, 1983, Strandberg, 1983, Grönqvist et al., 1989), and it is insufficient to evaluate the slip-resistance of shoe soles and floors only with the value of SCOF. However, if we have a shoe sole pattern which provides sufficiently high SCOF and DCOF, it would be safer because it helps to prevent slip initiation and to stop slip even if it occurs. The safe limit of DCOF for walking on level floor was suggested to be 0.20–0.40 by various studies (Grönqvist et al., 1989, Grönqvist et al., 2003, Redfern and Bidanda, 1994, Strandberg, 1983). Fong et al. (2009) reported that humans walk carefully to avoid slipping when the DCOF drops below 0.41. Grönqvist et al. (2003) suggested that the limit for preventing a slip was in the range 0.3–0.35, and if the DCOF was below this limit, a person would change their gait to adapt to the slippery surface. Nagata et al. (2009) also suggested that the fall risk due to induced slip increased when the coefficient of friction was below 0.4 based on a ramp test. Therefore, values of SCOF and DCOF greater than 0.4 would be required for the shoe sole/floor interface. However, too high friction could introduce tripping, and it is difficult to determine the upper limits of DCOF and SCOF to prevent this occurring.
The velocity of the slipping foot, which is also believed to be one of the determinants of whether an induced slip will result in the fall or the postural recovery (Perkins, 1978, Perkins and Wilson, 1983, James, 1990), is a function of the difference between the values of SCOF and DCOF (Tisserand, 1985). Thus, reduction in the difference between these values would also be one of the critical frictional properties between the footwear sole and the floor.
According to Bowen and Tabor (1950), friction force is a sum of an adhesive friction term (Fadh) and a deformation friction term (Fdef), as given byAdhesive friction results from the contact and subsequent shearing of individual surface asperities; and the deformation component is due to the ploughing or other forms of deformation caused by the harder surface on the softer surface. When a rubber slides on a smooth harder surface, the ploughing effect can be neglected. Hence the adhesive friction is directly proportional to the real area of contact and is given bywhere τ is the interfacial shear strength of the contact and Ar is the real area of contact.
The elastic modulus of rubber, usually used as footwear sole material, is low compared to other engineering materials. Hence, a high real area of contact between mating surfaces provides high values of static and dynamic friction under dry conditions. However, when sliding against a smooth surface contaminated with water or oil, a fluid film may be formed at the contact interface. The interfacial shear strength is determined by the fluid film, which results in low values of static and dynamic friction. Therefore, increasing the amount of contact area between the rubber and the mating surface by removing the fluid film is important to increase the coefficient of friction under lubricated conditions.
The slip resistance, i.e. coefficient of friction, of shoe soles with various tread pattern (macroscopic pattern) and surface roughness (microscopic pattern) characteristics has been measured on contaminated floors (Grönqvist, 1995, Grönqvist et al., 1999, Wilson, 1990, Chang et al., 2001a, Li and Chen, 2004, Li and Chen, 2005). These studies indicated that the surface roughness and tread pattern of the shoe sole are helpful for liquid drainage to increase the coefficient of friction. Hence the surface pattern design of a rubber sole, including tread pattern and surface roughness, is of great importance in improving slip-resistance. However, adequate design criteria (guidelines) for a shoe sole pattern with sufficient slip resistance on contaminated surfaces have not been fully understood. Therefore, the surface pattern design of the footwear sole required to increase SCOF and DCOF on contaminated surfaces is unclear.
In this study, a new rubber surface pattern for footwear soles using a hybrid rubber block combining smooth and rough surfaces, which showed sufficiently high SCOF and DCOF when slid against a liquid contaminated surface, was developed. The mechanisms of the increased SCOF and DCOF of the hybrid rubber block were also investigated based on the contact area measurement between the rubber block surface and the counterpart material surface by use of total reflection of light.
Section snippets
Sample preparation
NBR (acrylonitrile butadiene rubber) was formed into a rectangular block geometry (25 mm × 25 mm × 5 mm) using two kinds of metallic molds with different surface roughness. The tensile strength of the NBR was 9.52 MPa, the elongation was 875%, the 300% modulus was 1.12 MPa, and the shore hardness (A/15) was 45. The surface roughness Ra of each rubber block was 0.98 μm (smooth surface) or 30.4 μm (rough surface). The hybrid rubber blocks, in which a rubber block with a rough surface was sandwiched between
Effect of the rough surface area ratio on the static and dynamic coefficients of friction of the rubber blocks on the smooth stainless steel plate contaminated with glycerol solution
Fig. 3 shows the variation of the coefficient of friction with time for the rectangular rubber block with different rough surface area ratios. For the rubber block with a fully smooth surface, as shown in Fig. 3a, the coefficient of friction at slip initiation (SCOF) was a small value around 0.1. During the stage acceleration period after the slip was induced, the DCOF increased with time and reached around 1.4, then slightly decreased and resulted in a steady state value greater than 1.0.
Conclusions
In order to prevent slip initiation during walking, sufficient static friction is needed in the contact interface between the footwear sole and the floor. Sufficiently high dynamic friction is also required to help postural recovery after slipping. In this study, a new footwear sole surface pattern was developed to increase the static friction and the dynamic friction when sliding against a liquid-contaminated surface. Hybrid rubber blocks, in which a rubber block with a rough surface (Ra = 30.4
Acknowledgment
The authors would like to thank Dr. Hisao Nagata for valuable comments and suggestions that have strengthened this paper.
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2022, Tribology InternationalCitation Excerpt :Hence, the development of shoe-sole designs with high slip resistance is required to reduce the number of falling accidents due to induced slips. Rubber materials are often used in slip-resistant soles, and the slip resistance of shoes is often evaluated using the friction coefficient between the footwear and road/floor surfaces [4–16]. Several shoe outsole parameters have been found to influence friction performance, including the geometry of the tread grooves [4–6] and tread blocks [7–9], the sole-surface topography (texture) [10–12], and outsole material properties [13–16].