Available Coefficient Of Friction Research Articles

Barefoot Slip Risk in Indian Bathrooms: A Pilot Study

Barefoot slips and falls are commonly reported in bathrooms in Indian households and apartments. Due to lack of awareness on how to prevent such incidents, they are often neglected, and may lead to recurrent and severe traumatic injuries. To date, there is no way to assess the barefoot slip risk of the bathroom flooring tiles under realistic contaminated conditions such as with water and shampoo. In this work, a novel human heel surrogate was developed for barefoot slip testing, to address this gap. This surrogate was tuned to precisely mimic the frictional and biomechanical properties of the human heel, through three-dimensional (3D) scanning and printing. A mechanical slip testing device was employed with the heel surrogate to test 15 different bathroom floorings in residential apartments in dry and slippery conditions (i.e., wet and with shampoo spill). The barefoot available coefficient of friction (ACOF) was determined for the different floor conditions and tested for repeatability with two modeled heel surrogate samples. Also, the correlation of ACOF with surface roughness and between floorings was studied, across contaminant conditions. High repeatability (r 2 > 0.92) was confirmed in the dry condition, and the ACOF values decreased significantly when going from dry to wet and wet to shampoo spill conditions. The correlation of ACOF with surface roughness was found to be high (r 2 = 0.72), medium (r 2 = 0.62), and low (r 2 = 0.38) for the dry, wet, and shampoo spill conditions, respectively. Across contaminant conditions, several tiles exhibited high correlations in ACOF due to shampoo spills, indicating generalizability of barefoot slip risk. The findings of low barefoot frictional resistance on Indian bathroom floorings in wet and shampoo spill conditions, lack of correlations with surface roughness, and high generalizability of slip risk across floorings in the shampoo spill condition have not been reported to date, and shed light on the significant barefoot slip risks.

Evaluation of an actuated force plate-based robotic test setup to assess the slip resistance of footwear

Slipping is a frequent cause of occupational accidents. This is often due to an insufficient available coefficient of friction (ACOF) of footwear. The aim of this study was to present a footwear ACOF test setup, and to evaluate it based on device requirements presented in the ISO 13287 test standard. One left Airtox TX2 shoe underwent slip resistance measurements under three test modes (Forward flat slip, backward slip on the forefoot at angled contact and forward heel slip at angled contact), with six contaminant conditions (dry steel, dry tile, glycerine on steel, glycerine on tile, canola oil on steel and canola oil on tile). The test setup was successfully able to measure ACOF in close accordance to the ISO 13287 test standard with a good repeatability. Furthermore, the test setup can alter biomechanical and tribophysical testing conditions, which may provide more valid footwear ACOF measurements in the future. Relevance to industryThe setup can accommodate biomechanical and tribophysical testing conditions, hence the setup can be a tool for accessing more valid ACOF measurements - closer to real world slip events. Footwear manufactures or researchers, with the goal of improving footwear slip resistance, can implement the setup.

Barefoot slip risk assessment of Indian manufactured ceramic flooring tiles

Barefoot slips and falls are commonly reported across different floorings in Indian households and apartments. Due to lack of awareness on how to prevent such incidents, they are often neglected, and may lead to recurrent and severe traumatic injuries. To date, footwear based slip testing methods were widely employed for slip risk assessment, however there have been limited studies on characterization of barefoot slips. In this work, a novel human barefoot surrogate was fabricated to study barefoot slip safety of common ceramic flooring tiles. The available coefficient of friction (ACOF) under barefoot condition was characterized across the flooring tiles in dry and wet conditions and also in the presence of Lizol floor cleaner as a contaminant. The ACOF measured with the novel human barefoot surrogate was found to be repeatable across all slippery conditions. Barefoot ACOF was reported to be the lowest for the Lizol contamination. Generalizable trends of ACOF were identified for a few floor-contaminant combinations. The results would be indispensable for understanding the barefoot slip risk on contaminated common Indian floorings, and also provide important guidelines for selecting flooring as an intervention against barefoot slips and falls.

Effect of footwear material wear on slips and falls

Recurrent injuries related to slips and falls are common in workplaces, caused due to reduction of friction between the floor and footwear, due to material wear. However, due to lack of public awareness, such events are usually neglected and the role of footwear wear is underestimated. To date, a plethora of studies have tested footwear traction across floorings and contaminants, however the effect of shoe material wear on the friction has not been studied much. In this study, the outsole features of four common formal shoes were modelled and tested in their new and progressively worn conditions. The available coefficient of friction (ACOF) of the outsoles were estimated using a mechanical slip testing device across three common floorings (i.e., gloss, matt, and anti-skid), and in dry and slippery conditions such as with floor cleaner and canola oil. ACOF reductions in the range of 25–80% were observed in progressively worn outsoles. Outsoles with vertical tread orientation were found to generate higher ACOF in fluid contaminant conditions. Outsoles which had larger treads at the heel region showed consistent trend in the traction performance across all the conditions. The outsoles were found to produce generalizable results in the presence of canola oil as a contaminant. The results are anticipated to provide important guidelines for selection and design of footwear outsole features, and also in determining the replacement threshold of formal shoes.

Effects of natural shoe wear on traction performance: a longitudinal study

Footwear outsole design is an important factor for shoe-floor friction and for preventing slipping. Shoes with small, uniformly-separated tread blocks (often included on slip-resistant shoes) have decreased slip risk due to their increased friction and better under-shoe fluid drainage. However, these traction performance metrics (friction and fluid drainage) diminish with wear. This study quantifies shoe traction performance in response to natural wear and compares the relationship between common wear metrics: time, distance walked, and worn region size (WRS). Participants wore two pairs of shoes in the workplace for up to 11 months and the distance walked was tracked with a pedometer. After each month of wear, traction performance and WRS of each shoe were measured. Traction performance was quantified by the under-shoe available coefficient of friction and fluid force during a simulated slip test. Increased wear (months worn, distance walked, and WRS) was associated with decreased traction performance. A WRS of 800 mm2 was associated with reductions in friction of 16–38% and increases in fluid force by 286–528%. Three and six months of wear were associated with WRS values of 251 mm2 and 462 mm2 and distances of 203 km and 519 km, respectively. A walking distance of 500 km was associated with a WRS of 406 mm2. This study showed that all of these wear metrics are good indicators of shoe traction performance loss. Thus, the most practical metric in a particular application can be selected. We argue that WRS may be the best indicator due to variations in wear rate from the user and environment. Therefore, tracking footwear usage and monitoring outsole wear can aid in shoe replacement recommendations to reduce slips and falls.

Effect of tread design and hardness on interfacial fluid force and friction in artificially worn shoes

Shoe wear is a major contributor to slip risk. Knowledge of the impact of shoe design features on wear progression and its impact on friction performance is emerging. This study investigated the effect of tread geometric design and shoe hardness on the shoe’s friction response to wear. Three tread designs (two designs using large tread lugs and one tread design using small-patterned tread features) each at three different hardness levels underwent available coefficient of friction (ACOF) and under-shoe fluid pressure testing on a vinyl composite tile contaminated with 90% glycerol solution. These tests occurred at baseline and after each of six iterations of an accelerated wear procedure. The results of this study revealed that tread design influenced under-shoe fluid force, friction, and their response to wear. One of the lug designs experienced high fluid pressures and low friction performance even in its new condition although its performance improved with wear. The other lug design exhibited good drainage, good friction performance, and resilience to wear. The small-patterned tread experienced high ACOF and low fluid pressures at baseline and declined in performance with wear. Thus, lug designs offer a wide range of friction performance and resilience. No effects were observed due to hardness. This study determined that tread geometry is an important factor for designing shoes with durable traction performance.

Worn region size of shoe outsole impacts human slips: Testing a mechanistic model

Shoe outsole tread wear has been shown to increase slip risk by reducing the tread’s ability to channel fluid away from the shoe-floor interface. This study establishes a connection between geometric features of the worn region size and slipping. A mechanistic pathway that describes the relationship between the worn region size and slip risk is assessed. Specifically, it is hypothesized that an increased worn region size leads to an increase in under-shoe fluid pressure, which reduces friction, and subsequently increases slipping. The worn region size, fluid pressure, and slip outcome were recorded for 57 participants, who were exposed to an unexpected slip condition. Shoes were collected from each participant and the available coefficient of friction (ACOF) was measured using a tribometer. A greater shoe worn region size was associated with increased slip occurrence. Specifically, a 1 mm increase in the characteristic length of the worn region (geometric mean of its width and length) was associated with an increase in slip risk of ~10%. Fluid pressure and ACOF results supported the mechanistic model: an increase in worn region size correlated with an increase in peak fluid pressure; peak fluid pressures negatively correlated with ACOF; and increased ACOF correlated with decreased slip risk. This finding supports the use of worn region size as a metric to assess the risk of slipping.

Prediction of coefficient of friction based on footwear outsole features

Traction testing of footwear is expensive, which may create barriers for certain users to assess footwear. This study aimed to develop a statistical model that predicts available coefficient of friction (ACOF) under boundary lubrication conditions based on inexpensive measurements of footwear outsole features. Geometric and material hardness parameters were measured from fifty-eight footwear designs labeled as slip-resistant. A robotic friction measurement device was used to quantify ACOF with canola oil as the contaminant. Stepwise regression methods were used to develop models based on the outsole parameters and floor type to predict ACOF. The predictive ability of the regression models was tested using the k-fold cross-validation method. Results indicated that 87% of ACOF variation was explained by three shoe outsole parameters (tread surface area, heel shape, hardness) and floor type. This approach may provide an assessment tool for safety practitioners to assess footwear traction and improve workers’ safety.

Influence of averaging time-interval on shoe-floor-contaminant available coefficient of friction measurements

Available coefficient of friction (ACOF) is a common metric of footwear traction performance. ACOF is the ratio of friction to normal force, often averaged over a time-interval. The time-interval needed to achieve repeatable and valid ACOF is unknown. A post-hoc analysis was performed on nine shoe-floor-contaminant combinations to assess the repeatability and bias of data averaged across 4 time-intervals (2 ms, 50 ms, 100 ms, 200 ms) after the target normal force was reached. The ability to predict human slips was assessed for ACOF across these intervals. Differences in repeatability and validity across the four intervals were small. However, statistically significant differences were observed for the shortest compared with the longest interval (lower repeatability yet modestly improved predictive ability). Given the limited impact of time-interval on the results, a shorter interval of 50 ms is recommended to enable testing of smaller floor samples.

Changes in under-shoe traction and fluid drainage for progressively worn shoe tread

Shoe wear is known to increase slipping risk, but few studies have systematically studied this relationship. This study investigated the impact of progressive shoe wear on the available coefficient of friction (ACOF) and under-shoe fluid dynamics. Five different slip-resistant shoes were progressively worn using an accelerated, abrasive, wear protocol. The ACOF and fluid forces (the load supported by the fluid) were measured as shoes were slipped across a surface contaminated with a diluted glycerol solution. As the shoes became worn, an initial increase in ACOF was followed by a steady decrease. Low fluid forces were observed prior to wear followed by increased fluid forces as the worn region became larger. Results suggest that traction performance decreases particularly when the heel region without tread exceeds a size of 800 mm2. This study supports the concept of developing shoe replacement guidelines based upon the size of the worn region to reduce occupational slips.

Vinyl Composite Tile Surrogate for Mechanical Slip Testing

OCCUPATIONAL APPLICATIONSThis study aimed to identify a durable laboratory-grade surrogate material for vinyl composite tile (VCT) suitable for footwear traction testing. To conduct this investigation, eight polymer tiles (cast nylon, polyethylene, polycarbonate, acetal, Delrin, PTFE, polypropylene, and nylon) were tested across different shoes and contaminants (water, sodium laurel sulfate, and oil), and the available coefficient of friction (ACOF) was measured and compared with that of two designs of VCT. Shoe ACOF performance on cast nylon tile with oil contamination was generally applicable to both VCTs in oil contaminated condition. Shoe ACOF performance for none of the eight candidate tiles were applicable to the VCTs for other contaminated conditions. Cast nylon is anticipated to serve as a durable VCT surrogate, for standard slip testing of footwear in oil-contaminated conditions.TECHNICAL ABSTRACT Background: Vinyl composite tile (VCT), which is a common flooring in workplaces, is sometimes utilized as the standard floor material for mechanical slip testing experiments. Unfortunately, VCT is a sub-optimal standard test material, since it changes over time and is difficult to manufacture consistently. Purpose: This study aimed to identify a durable laboratory-grade substitute flooring that could provide traction results that are representative of footwear performance on VCT. Methods: Eight polymer tiles (cast nylon, polyethylene, polycarbonate, acetal, Delrin, PTFE, polypropylene, and nylon) were tested and the available coefficient of friction (ACOF) was measured and compared with that of two VCT designs. First, a screening test was performed to identify good material candidates based on six shoes and two contaminants (water and oil). Two surrogate candidate tiles were then tested across 17 shoes and 3 contaminant conditions (water, sodium laurel sulfate, and oil). Results: Cast nylon tile was found to be the most generalizable VCT surrogate, exhibiting strong correlations with both VCTs for oil contamination. None of candidates were representative of the VCTs for other contaminants.Conclusions: Cast nylon may be a useful alternative for VCT for standard slip testing of footwear in oily conditions.

Predicting slips based on the STM 603 whole-footwear tribometer under different coefficient of friction testing conditions

Assessing footwear slip-resistance is critical to preventing slip and fall accidents. The STM 603 (SATRA Technology) is commonly used to assess footwear friction but its ability to predict human slips while walking is unclear. This study assessed this apparatus’ ability to predict slips across footwear designs and to determine if modifying the test parameters alters predictions. The available coefficient of friction (ACOF) was measured with the device for nine different footwear designs using 12 testing conditions with varying vertical force, speed and shoe angle. The occurrence of slipping and the required coefficient of friction was quantified from human gait data including 124 exposures to liquid contaminants. ACOF values varied across the test conditions leading to different slip prediction models. Generally, a steeper shoe angle (13°) and higher vertical forces (400 or 500 N) modestly improved predictions of slipping. This study can potentially guide improvements in predictive test conditions for this device.Practitioner Summary: Frictional measures by the STM603 (SATRA Technology) were able to predict human slips under liquid contaminant conditions. Test parameters did have an influence on the measurements. An increased shoe-floor testing angle resulted in better slip predictions than test methods specified in the ASTM F2913 standard.

Influence of Natural Wear Progression on Shoe Floor Traction – A Pilot Study

Slips and falls in the workplace are a major concern for injuries. Worn shoes are a known risk factor for slips and falls. The purpose of this pilot study was to analyze changes in shoe traction performance under fluid contaminant conditions as the shoes were progressively worn. Four subjects wore two different shoes with varying tread patterns. Shoes were tested after each month of wear. The two types of shoes responded to wear differently; one shoe experienced a substantial decrease in available coefficient of friction (ACOF) while the other shoe showed no substantive change. Loads supported by the fluid during slipping increased with wear of the shoes. Furthermore, ACOF was influenced by the shoe type and the walking distance. This study suggests that the impact of wear on shoe performance is dependent on the shoe design. Thus, future studies are needed to understand specific effects of shoe design.

Kinematics and kinetics of the shoe during human slips

This paper quantified the heel kinematics and kinetics during human slips with the goal of guiding available coefficient of friction (ACOF) testing methods for footwear and flooring. These values were then compared to the testing parameters recommended for measuring shoe-floor ACOF. Kinematic and kinetic data of thirty-nine subjects who experienced a slip incident were pooled from four similar human slipping studies for this secondary analysis. Vertical ground reaction force (VGRF), center of pressure (COP), shoe-floor angle, side-slip angle, sliding speed and contact time were quantified at slip start (SS) and at the time of peak sliding speed (PSS). Statistical comparisons were used to test if any discrepancies exist between the state of slipping foot and current ACOF testing parameters. The main findings were that the VGRF (26.7 %BW, 179.4 N), shoe-floor angle (22.1°) and contact time (0.02 s) at SS were significantly different from the recommended ACOF testing parameters. Instead, the testing parameters are mostly consistent with the state of the shoe at PSS. We argue that changing the footwear testing parameters to conditions at SS is more appropriate for relating ACOF to conditions of actual slips, including lower vertical forces, larger shoe-floor angles and shorter contact duration.

Coefficient of friction testing parameters influence the prediction of human slips

Measuring the available coefficient of friction (ACOF) of a shoe-floor interface is influenced by the choice of normal force, shoe-floor angle and sliding speed. The purpose of this study was to quantify the quality of slip prediction models based on ACOF values measured across different testing conditions. A dynamic ACOF measurement device that tests entire footwear specimens (Portable Slip Simulator) was used. The ACOF was measured for nine different footwear-contaminant combinations with two levels of normal force, sliding speed and shoe-floor angle. These footwear-contaminant combinations were also used in human gait studies to quantify the required coefficient of friction (RCOF) and slip outcomes. The results showed that test conditions significantly influenced ACOF. The condition that best predicted slip risk during the gait studies was 250 N normal force, 17° shoe-floor angle, 0.5 m/s sliding speed. These findings can inform footwear slip-resistance measurement methods to improve design and prevent slips.

Performance testing of work shoes labeled as slip resistant

The variability in friction and slip propensity across slip resistant (SR) shoes is poorly understood. This study aimed to quantify the impact of shoe design features on the available coefficient of friction (ACOF) across shoes labeled as SR. Differences in ACOF and the slipping rate across SR shoes were also quantified. Twelve shoes were tested across five types of flooring and three contaminant conditions using a whole shoe mechanical slip tester. Geometric and hardness parameters were measured to determine the effect of heel outsole design on ACOF. The rate of slipping was evaluated for three of the shoes on vinyl tile with canola oil using human subjects. Differences in ACOF were significant across shoe outsole designs (p < .001). ACOF was correlated with geometrical and hardness parameters. Rate of slipping was lower for the highest ACOF shoe (p < .001). This information can be used to guide SR shoe selection and design.

The Combined Benefits of Slip-Resistant Shoes and High Traction Flooring on Coefficient of Friction Exceeds Their Individual Contributions

High traction flooring and slip-resistant shoes are often used to reduce slip and fall accidents. However, the relative contribution and interactions across these parameters on available coefficient of friction (ACOF) are not well understood. The purpose of this study was to quantify the impact of flooring and slip-resistant shoes on ACOF. Seventeen shoes, five flooring tiles and three contaminants were tested using a robotic slip-tester, while ACOF was measured. ACOF was higher for slip-resistant shoes than not slip-resistant shoes. Larger effects were observed for shoe classification compared with flooring. Interaction effects indicated that the gap across shoe classifications was greater for the high ACOF floorings compared to the low ACOF floorings. This study suggests that the benefit of combining high traction flooring and slip-resistant shoes exceed the summed benefit of these factors and has the potential to reduce slipping events.

Vertical Ground Reaction Forces During Unexpected Human Slips

Falls due to slippery conditions are among the primary causes of disabling workplace injuries. Despite the extensive amount of human slip studies in the literature, only a handful of studies have reported ground reaction forces at the instant of slip initiation. The purpose of this study was to quantify the vertical ground reaction forces (VGRF) at slip initiation during unexpected human slips across different footwear-contaminant conditions. Forty-seven healthy subjects were unexpectedly exposed to a liquid–contaminant, while the vertical force was measured at the moment that the foot began to start slipping. The average VGRF were between 100 and 300 N and varied significantly across the footwear. These forces were significantly less than the typical forces (400-700 N) applied during slip-resistance measurements. This finding may suggest that available coefficient of friction (ACOF) measurements should use lower force levels in order to achieve higher relevance to the onset of slipping.

Modelling the stochastic nature of the available coefficient of friction at footwear-floor interfaces

The available coefficient of friction (ACOF) is a measure of the friction available between two surfaces, which for human gait would be the footwear–floor interface. It is often compared to the required coefficient of friction (RCOF) to determine the likelihood of a slip in gait. Both the ACOF and RCOF are stochastic by nature meaning that neither should be represented by a deterministic value, such as the sample mean. Previous research has determined that the RCOF can be modelled well by either the normal or lognormal distributions, but previous research aimed at determining an appropriate distribution for the ACOF was inconclusive. This study focuses on modelling the stochastic nature of the ACOF by fitting eight continuous probability distributions to ACOF data for six scenarios. In addition, the data were used to study the effect that a simple housekeeping action such as sweeping could have on the ACOF.Practitioner Summary: Previous research aimed at determining an appropriate distribution for the ACOF was inconclusive. The study addresses this issue as well as looking at the effect that an act such as sweeping has on the ACOF.

Required coefficient of friction during level walking is predictive of slipping

The required coefficient of friction (RCOF) is frequently reported in the literature as an indicator of slip propensity. This study aimed to further develop slip prediction models based on RCOF by examining slips under moderately slippery conditions where the RCOF was approximately equal to the available coefficient of friction. Baseline RCOFs were found for normal walking trials and then an unexpected slip was introduced with a moderately slippery boot-floor contaminant combination for thirty-one subjects. Slip outcomes (i.e., whether a subject experienced a slip) were assessed based on the displacement of a marker placed on the heel. A logistic regression analysis was used to model the impact of RCOF on slipping. Results showed that subjects who walked with a greater RCOF were found to have a higher probability of slipping. The predicted probability of a slip across the RCOF ranged from 3% to 95% and an increase of 0.01 in RCOF was associated with a slipping odds ratio of 1.7. Thus, modest differences in RCOF can have a dramatic impact on slip propensity. This study shows that RCOF can be a sensitive and valid predictor of slipping in realistic frictional environments.