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The effects of slip criterion and time on friction measurements
Safety Science 40 (2002) 593–611 www.elsevier.com/locate/ssci
The effects of slip criterion and time on friction measurements Wen-Ruey Chang* Liberty Mutual Research Center for Safety and Health, 71 Frankland Road, Hopkinton, MA 01748, USA
Abstract Different institutions reported different results for friction measurements of identical material combination and surface conditions using identical slipmeters. The objective of this study was to evaluate three factors, slip criterion, sample, and time, that could contribute to such differences with two commonly used slipmeters, the Brungraber Mark II and the English XL. Friction between 16 commonly used footwear materials and three floor materials was measured under four surface conditions with two slip criteria at the interface. Some of the measurements were repeated at a different time. The results indicated that variations due to different samples were probably the smallest statistically among the three factors evaluated. The effect of slip criterion on friction coefficient could be quite significant compared with the effect of time for some material combinations and surface conditions. A more consistent slip criterion could potentially reduce the differences significantly in the results reported among different institutions. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Slip criterion; Time; Sample variation; Friction
1. Introduction Slips and falls are a serious problem. The annual direct cost of occupational injuries due to slips and falls in the USA may be as high as seven billion dollars based on the information provided by an earlier study (Leamon and Murphy, 1995). Falls on the same level accounted for 65% of claim cases and, consequently, 55% of claim costs in the total direct workers’ compensation for occupational injuries due to slips and falls (Leamon and Murphy, 1995).
* Tel.: +1-508-435-9061, ext. 219; fax: +1-508-435-8136. E-mail address: [email protected] (W.-R. Chang). 0925-7535/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0925-7535(01)00061-3
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Many slipmeters have been used to assess the slipperiness between footwear and floor surfaces which is a major factor in slips and falls accidents on the same level. These slipmeters can be quite different in their measurement characteristics. Friction coefficient, also called slip resistance, is the typical output of a slipmeter measurement. The Brungraber Mark II and the English XL as shown in Figs. 1 and 2, respectively, are widely used in the USA and were used in this experiment. Therefore, the following literature review is limited to the relevant publications related to these slipmeters. The operating principle of these slipmeters is to simultaneously apply forces parallel and normal to a floor surface with an impact of a footwear sample on the floor at an inclined angle in order to eliminate the dwell time problem with the static friction measurement. The Brungraber Mark II and the English XL are also known as a portable inclinable articulated strut slip tester (PIAST) and a variable incidence tribometer (VIT), respectively. Standard test methods for using these slipmeters are published by the American Society for Testing and Materials (ASTM; ASTM F-1677-96, 1996; ASTM F1679-96, 1996). The Brungraber Mark II is an inclined-strut slipmeter driven by gravity. The footwear pad impacts floor surfaces at an inclined angle with the vertical direction. If a non-slip occurs at the interface, the inclined angle is increased. Conversely, the angle is decreased if a slip occurs. According to the standard (ASTM F-1677-96, 1996), the starting angle is supposed to be smaller than the angle that a slip is
Fig. 1. The Brungraber Mark II, a portable inclinable articulated strut slip tester (PIAST).
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anticipated and it should be increased until a slip occurs. The tangent of the angle is the friction coefficient shown on the slipmeter. As indicated in the standard, it might be necessary to use the average of the maximum friction coefficient that a non-slip occurs and the minimum friction coefficient that a slip happens as the result of the measurement. The English XL is an inclined-strut slipmeter driven by a pneumatic pressure of 172 kPa (25 psi). According to its standard (ASTM F-1679-96, 1996), the contact force should not be applied to the footwear pad for more than 1 s and a slip occurs when the strut kicks out in an arc with the pneumatic cylinder extending to its full stroke. The operating principle of the English XL is very similar to that of the Brungraber Mark II. A measurement should start from a small angle with a non-slip to a larger angle until a slip occurs and the friction coefficient obtained from the angle at which a non-slip is changed to a slip should be recorded. The friction coefficient obtained directly with the Brungraber Mark II was compared with that calculated from the ground reaction forces, obtained with a force plate, produced by this slipmeter (Marpet, 1996; Marpet and Fleischer, 1997; Gro¨nqvist et al., 1999; Powers et al., 1999). The results indicated that the friction coefficients obtained from these two methods were in good agreement over different floor surfaces with different surface contaminants for non-slip and barely slip conditions. The friction coefficient measured with the Brungraber Mark II was also
Fig. 2. The English XL, a variable incidence tribometer (VIT).
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shown to have a good correlation (r>0.954) with that measured with a dynamic apparatus to simulate a slip although the absolute values of friction coefficients from these two devices could be quite different (Gro¨nqvist et al., 1999). Fendley et al. (1999) investigated the effect of sanding on the repeatability of friction measurement with the Brungraber Mark II. Their results indicated that the repeatability was better with sand paper of 180 grit than with that of 400 grit over three test surfaces. The friction coefficient directly obtained with the English XL was shown to be in good agreement with that obtained with a force plate for non-slip conditions (Powers et al., 1999). Chang (1999) investigated the effect of floor tile surface roughness on the friction measured with five commonly used slipmeters. The friction coefficient measured with the English XL was shown to have the best correlation with certain surface roughness parameters under dry and wet surface conditions among the slipmeters evaluated, while the friction coefficient measured with the Brungraber Mark II was the second best. Although the guidelines for these slipmeters are specified in their respective standards, the details of how these slipmeters ought to be used are somewhat ambiguous. One of the most critical elements in using these slipmeters is to determine whether a slip happens at the interface between footwear sample and floor surfaces. The manufacturers’ user manuals for these slipmeters are not clear regarding the slip criterion and operators are left to develop their own criteria. Any movement at the interface can potentially be interpreted as a slip. However, experiments in biomechanics suggested that slips without any fall or injury occur quite often (Leamon and Li, 1990) and a slip will lead to a fall if the sliding speed exceeds 0.5 m/s (Strandberg, 1983). Therefore, some movements can be allowed at the footwear/floor interface without leading to an accident. This concept can be applied analogously to the slip criterion used in friction measurements between footwear and floor samples. Therefore, some movements at the interface can be considered as a non-slip from an accident prevention viewpoint. Different slip criteria could potentially lead to very different results. Different results of friction measurements from identical material combination and surface conditions using identical slipmeters were reported from different institutions. The possible sources of discrepancy could include different slip criteria, different samples, measurements taken at different times, sample preparation, humidity, and temperature. It is not clear which factors contribute the most. While some of the factors such as measurements taken at different times and different samples are somewhat uncontrollable, slip criterion for the English XL and the Brungraber Mark II can be controlled through a more detailed and consistent measurement protocol. In this experiment, two slip criteria were used to determine the friction coefficients obtained with these two slipmeters. These two slip criteria are two extreme conditions and could cover all possible criteria that the operators could use. Multiple samples were used for the identical material combinations and identical surface conditions to ensure consistent results across different samples. Some of the measurements with identical samples were taken at a different time. An analysis of
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variance (ANOVA) was performed to determine if the differences among different slip criteria, different samples and different times were statistically significant.
2. Test apparatus and design of experiment All the friction measurements were performed at a temperature of 21 1.7 C (70 3 F) and a relative humidity of 50 5%. The 16 footwear sole materials used in this experiment, listed in Table 1, represent typical materials used as shoe soles by shoe industries. The abbreviations in parentheses following the full names of the materials shown in Table 1 will be used throughout this paper. For example, SBR solid with diamond tread will be referred to as SBRS. The full names of the material abbreviations shown in Table 1 are ethylene vinyl acetate (EVA), polyvinyl chloride (PVC), styrene butadiene rubber (SBR), thermal plastic rubber (TPR) and thermal plastic urethane (TPU). All the footwear materials except leather (L) and European Standard G2AE (G2AE) had a diamond pattern on the surfaces of measurement. Most of the diamond patterns had a dimension of approximately 21.1 mm (0.83 inches) in the longitudinal direction and approximately 11.9 mm (0.47 inches) in the latitudinal direction. The width of the groove was approximately 1.55 mm (0.061 inches) measured in the direction perpendicular to the groove. The depth of the groove was approximately 0.80 mm (0.032 inches). The diamond pattern on the footwear samples varied slightly in dimensions on different materials due to thermal processes involved in the manufacturing processes. Three floor materials of unglazed
Table 1 A list of all footwear sole materials used Material name and abbreviation
Skin surface shore hardness
SBR Solid—Diamond Design (SBRS) SBR Blown—Diamond Design (SBRB) Nitrile Solid—Diamond Design (NS) Nitrile Low Density—Diamond Design (NL) EVA—Diamond Design (EVA) Leather—Smooth (L) Natural Rubber—Diamond Design (NR) PVC Solid—Diamond Design (PVCS) PVC Blown—Diamond Design (PVCB) TPR—Diamond Design (TPR) TPU—Diamond Design (TPU) Polyurethane Elastomer—Diamond Design (PE) Polyurethane Blown—Diamond Design (PB) Thermoplastic Butadiene—Diamond Design (G) European Standard G2AE—Smooth (G2AE) TPU Toplift Laripur 5725—Diamond Design (L5725)
873 A 555 A 703 A 553 A 603 A As Sold 653 A 783 A 555 A 703 A 685 A 705 A 705 A 655 A 953 A 563 D
EVA, ethylene vinyl acetate; PVC, polyvinyl chloride; SBR, styrene butadiene rubber; TPR, thermal plastic rubber; TPU, thermal plastic urethane.
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quarry tiles, smooth stainless steel and vinyl composition tiles were used. Dry and oil free compressed air was used for cleaning and drying. Due to a concern that hoses for the compressed air might contaminate the air, new hoses and nozzles were used beyond the filters for oil and moisture. 2.1. Sample preparation All the floor materials were cut into tiles of approximately 15.2415.24 cm (66 inches). Diluted mild liquid detergent, 1% by volume mixed with water, was applied to a clean paper towel to wipe the specimen surfaces. The surface was rinsed thoroughly with tap water and compressed air was blown across the surface for drying. Then, 50% ethanol mixed with de-ionized water was applied to a clean paper towel to wipe specimen surfaces. The surface was dried with the compressed air. These cleaning procedures were applied to all the specimens, including footwear and floor tile samples. In addition, the washed footwear pads were sanded by hand with a fresh section of 400 grit silicon carbide abrasive paper in the longitudinal direction of the diamond pattern for a total of 10 complete movements (five strokes). The specimen was rotated 90 and the sanding process was repeated on a new area on the sand paper. One stroke consisted of one forward and one backward movement. The length of the stroke in each direction was approximately 15 cm. Test surfaces were cleaned with compressed air to remove excess particles generated by sanding. 2.2. Test conditions Surface conditions of dry, wet, oily and oily-wet were used. However, the oily and oily-wet conditions were limited to the measurements with the quarry tiles. For dry conditions, the floor and footwear specimen preparation procedures described earlier were used. For wet conditions, de-ionized water was applied to cover the test area on the floor tile surface. The amount of water was sufficient to flood the test area. Water was replenished throughout repeated impacts on the floor surface during the measurements. Vegetable oil was applied (2 ml) and spread across the entire test surface with a brush for oily conditions. For oily-wet conditions, the vegetable oil was applied as in the oily conditions and then de-ionized water was applied as in the wet conditions. Dry and wet testing was performed on all footwear and floor materials. However, oily and oily-wet testing was performed on five different footwear materials and quarry tiles. 2.3. Friction measurements Friction was measured in the longitudinal direction of the diamond pattern on the footwear specimen and perpendicular to the grooves in the back of the quarry tiles. For the stainless steel and vinyl composition tiles, one direction was chosen on each sample for friction measurements. There were three repeated measurements in the given direction for each surface condition and each sample pair with each slipmeter.
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No cleaning of the samples was required during three repeated measurements. For each material combination, there were three tile samples for dry and wet conditions and another three tile samples for oily and oily-wet conditions. For each material combination and slipmeter, there were three footwear pads for dry and wet testing and another three footwear pads for oily and oily-wet testing. The tests were performed in the sequence of dry then wet on one tile, and oily then oily-wet on another tile. The same tiles were used for different slipmeters under the same surface condition. Whenever there was a change in the slipmeters in the dry conditions, compressed air was used to remove debris on the test surfaces; surfaces were wiped with 50% ethanol; and surfaces were blown with the compressed air for drying. 2.4. Operation of the Brungraber Mark II Measurement started at a low friction coefficient which was increased by 0.05 as long as a non-slip persisted. There were at least three non-slips before a slip occurred if the friction coefficient was higher than 0.15. As soon as a slip occurred, the friction coefficient was reduced by 0.01 as long as the slip persisted. The measurement stopped when a non-slip occurred again. The friction coefficient of the last slip was recorded. 2.5. Operation of the English XL To activate the device, the actuation button for the pneumatic pressure was pressed and quickly released. Based on a recommendation from the manufacturer, the actuation button was pressed for 0.5 s, instead of the 1 s suggested by the standard. The measurement started with a low friction coefficient which was increased by turning the hand wheel a quarter turn as long as a non-slip persisted. A quarter turn of the hand wheel is equal to a change of approximately 0.008 in friction coefficient. There should be at least three non-slips before a slip occurred. As soon as a slip occurred, the measurement stopped and the friction coefficient value was recorded. 2.6. Criteria for a slip condition Two slip criteria which represented two extremes were used in this experiment. The first criterion was that any movement at the interface after impact was considered a slip. For the English XL, visual judgment was used for this criterion since no reference mark around the impact point was established. For the Brungraber Mark II, a line was drawn on the rotating support along the edge of the articulated strut at a non-slip position, as shown in Fig. 3. Based on this criterion, the alignment of the line and the strut was used to determine whether a slip occurred at the interface. For the Brungraber Mark II, the allowable dwell time was limited to 5 s after impact for the first criterion and any movement after 5 s was considered as a non-slip.
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The second criterion was that only a fast movement was considered a slip and, therefore, any slow movement, any finite dwell time at impact or any partial stroke were considered non-slips. For the second criterion, the judgment of whether a finite dwell time or a slow movement occurred was somewhat subjective. These slipmeters make two sounds during a measurement. The first sound comes from the footwear sample impacting the floor sample and the second sound comes from the slipmeter reaching a full stroke movement. When the operators have some experience with these slipmeters, they become very familiar with the duration between these two sounds. This helps establish the criterion of a fast movement. A slow movement has a noticeably different duration. Only one operator was used in this experiment. There could be arguments about how fast is fast and how precisely the operators could notice the differences. However, these differences should be very small. The difference in friction coefficient was usually smaller than 0.01 which was even smaller than the smallest adjustment that the operator made during the measurement. In practice, the differences in subjective judgement were smaller than the resolution in the operation of these slipmeters. Since the procedure used for the Brungraber Mark II proceeded from a slip to a non-slip by an increment of 0.01, the second criterion measurement was performed first with each sample followed by the measurement using the first criterion. This procedure was repeated consecutively three times for each sample under each test condition using each slipmeter.
Fig. 3. A line drawn on the Brungraber Mark II for slip criterion 1.
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2.7. Time factor Friction measurements were repeated at a different time for six different footwear materials, Nitrile Solid (NS), EVA (EVA), TPR (TPR), Polyurethane Elastomer (PE), Polyurethane Blown (PB) and Thermoplastic Butadiene (G) due to the limitation of resources. Only dry and wet surface conditions were repeated because of potential complexity involved with retesting oily surfaces over time. All three floor materials were used. Therefore, there were 36 different combinations of materials and surface conditions repeated. Three samples were used for each material combination and surface condition. The second slip criterion was used. In the second set of measurements, samples were prepared following the same procedures as the first set of measurements and each footwear pad was measured with the identical tile surface in the same direction as the first set. Two sets of measurements were taken several weeks to several months apart.
3. Results A three way ANOVA was applied to the friction coefficient results generated with the 106 different combinations of footwear and floor materials, and surface conditions to determine whether different slipmeters, slip criteria and samples resulted in a statistically significant difference. As expected, the results indicated that the differences in the friction coefficient obtained with different slipmeters were statistically significant. Out of 106, only 10 had non-significant differences with different slipmeters. There were 59 different combinations in which different slip criteria resulted in a statistically significant difference. Different samples resulted in significantly different friction coefficients in 15 different combinations out of 106. Since it is expected that different slipmeters will yield different friction coefficients, the differences due to different slipmeters might dilute the effect of the other two factors. Therefore, a two way ANOVA was applied to the friction coefficients generated with each slipmeter, material combination and surface condition to determine if different slip criteria and samples resulted in significantly different friction coefficients. The range of friction coefficient measured with the Brungraber Mark II is 0–1.1, while the English XL has a range of 0–1.0. Respective maximum friction coefficient values were used in the ANOVA analyses whenever the measurements exceeded the range of the slipmeters. The results indicated that some measurements in most of the material combinations for dry surfaces exceeded the range of the slipmeters used. Some of the statistically non-significant differences were caused by replacement with the maximum friction coefficient values when the ranges of the slipmeters were exceeded. A comparison of the results for dry surfaces solely based on the p values might not be appropriate and, therefore, the results of ANOVA analyses for dry surfaces are not presented here. Tables 2 and 3 contain the p values of the two way ANOVA analyses for wet surfaces for the Brungraber Mark II and the English XL, respectively. For oily and oily-wet surfaces, the p values of the two way ANOVA analyses are shown in
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Table 4. Although the results might be slightly different for different slipmeters, there appeared to be more significant differences statistically for different slip criteria than for different samples under wet, oily and oily-wet conditions as shown in Tables 2, 3 and 4. It was also observed that statistically there were more significant differences due to the different slip criteria for measurements taken on quarry tiles with the Brungraber Mark II than on the other two floor surfaces. For different sample variation, there appeared to be more significant differences statistically for measurements taken with the Brungraber Mark II on quarry and stainless steel surfaces than on vinyl surfaces. There were slightly more significant differences statistically due to different slip criteria for measurements taken with the English XL on quarry and vinyl tiles than on stainless steel surfaces. For different sample variation, there appeared to be more significant differences statistically for measurements taken with the English XL on vinyl and stainless steel surfaces than on quarry tiles. The results also showed that PB, PVC Blown (PVCB), SBRS, and TPU (TPU) were more likely to have statistically significant differences when different slip criteria were used on wet surfaces with both slipmeters than other footwear materials used in this experiment. In addition, EVA, TPU Toplift Laripur 5725 (L5725), NS, SBR Blown (SBRB), and TPR were more likely to have statistically significant differences using different slip criteria on wet surfaces with only the Brungraber Mark II on all three floor surfaces. The results also showed that L and L5725 were more
Table 2 The p values of the two way ANOVA analyses of friction coefficient for wet surfaces with the Brungraber Mark IIa Footwear material
PB EVA G2AE G L L5725 NL NR NS PE PVCB PVCS SBRB SBRS TPR TPU a b c
Slip criterion
Sample
Quarry
Steel
Vinyl
Quarry
Steel
Vinyl
0.0032 0.0015 —b 0.0079 0.0161 0.0032 0.0032 0.005 0.0122 0.0001 0.0001 0.0108 0.0128 0.0023 0.0065 0.0174
0.0252 0.0075 —b —b —b 0.0339 —b 0.0291 0.0108 0.0439 0.0017 —b 0.0005 0.0007 0.0359 0.0132
0.0035 0.0075 —b —b 0.0202 0.0226 —b ***c 0.0185 —b 0.0043 0.0005 0.0029 0.0057 0.0206 0.0104
—b 0.0381 —b —b —b 0.0074 —b —b —b 0.0036 0.0114 —b —b 0.0203 —b —b
—b —b —b —b —b —b —b —b 0.0283 —b 0.0308 —b 0.0071 0.0187 — —
— — — — — — — ***c — — — 0.0256 — — — —
p40.05 with some measurements exceeding the range of the slipmeter. —, p>0.05 without exceeding the range of the slipmeter. ***, p>0.05 with some measurements exceeding the range of the slipmeter.
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likely to have statistically significant differences due to different samples with only the English XL on all three floor surfaces. The ANOVA results indicated that there were more combinations with statistically significant differences due to different slip criteria than due to different samples. Because of a large amount of data, it is more sensible to combine the friction coefficient values generated from different samples for the same material combinations, surface conditions and slipmeters used in order to compare the effect of different slip criteria. The average of three repeated measurements from three different samples from the same material combination, surface condition and slipmeter was calculated for each slip criterion. Since some measurements in most of the material combinations for dry surfaces exceeded the ranges of the slipmeters used, only the mean values of friction coefficient for contaminated surfaces are shown in Tables 5 and 6 for wet surfaces with the Brungraber Mark II and the English XL, respectively, and in Table 7 for oily and oily-wet surfaces. The respective maximum friction coefficient values were used in the mean value calculations whenever the measurements exceeded the ranges of the slipmeters. There were 36 different combinations of footwear and floor materials, and surface conditions for the repeatability evaluation. The results of a three way ANOVA indicated that the differences in the friction coefficient obtained with different slipmeters were statistically significant among the factors of time, slipmeter and sample. Out of 36, only seven had non-significant differences with different slipmeters. There
Table 3 The p values of the two way ANOVA analyses of friction coefficient for wet surfaces with the English XL Footwear material
PB EVA G2AE G L L5725 NL NR NS PE PVCB PVCS SBRB SBRS TPR TPU a b c
Slip criterion
Sample
Quarry
Steel
Vinyl
Quarry
Steel
Vinyl
0.0065 0.0327 0.0046 0.009 —a —a ***c 0.0022b —a 0.0134 0.0088b 0.0162 —a 0.0109 0.01 0.0147
0.0143 —a —a —a 0.0153 0.0261 0.0358 —a 0.0099 —a 0.0063 0.0413 —a 0.0371 —a 0.0215
0.0083 —a 0.0009 0.0119 0.0099 0.0315 0.0356 ***c —a 0.0108 0.0171 —a —a 0.0319 0.0002b 0.0193
—a —a —a —a 0.0203 0.0001 ***c 0.0455b —a —a ***c —a —a 0.0261 —a —a
—a —a —a 0.0403 0.0046 0.0469 —a 0.0024 0.0315 —a —a 0.0033 0.0152 0.0146 —a —a
—a —a 0.0041 0.0094 0.0008 0.025 —a ***c —a —a 0.0403 0.0133 —a —a ***c 0.0106
—, p>0.05 without exceeding the range of the slipmeter. p40.05 with some measurements exceeding the range of the slipmeter. ***, p>0.05 with some measurements exceeding the range of the slipmeter.
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were nine different combinations where measurements taken at different times resulted in a significant difference. Different samples resulted in significantly different friction coefficients in four different combinations. Similarly, a two way ANOVA was applied to the results generated with different slipmeters to determine if different times and samples resulted in different friction coefficients. Since some Table 4 The p values of the two way ANOVA analyses of friction coefficient for oily and oily-wet surfaces Footwear material
Surface conditions
Brungraber Mark II
English XL
Slip criterion
Sample
Slip criterion
Sample
a
L5725 NL SBRB SBRS TPU
Oily Oily Oily Oily Oily
— —a 0.0279 0.0022 —a
0.0122 —a —a 0.0357 —a
0.0187 0.0065 —a 0.0414 —a
0.006 —a 0.0421 —a —a
L5725 NL SBRB SBRS TPU
Oily-wet Oily-wet Oily-wet Oily-wet Oily-wet
0.0102 0.0131 0.0258 0.0015 0.003
—a —a —a 0.0227 —a
—a 0.0207 —a 0.0263 0.0145
—a —a —a —a 0.0148
a
—, p>0.05 without exceeding the range of the slipmeter.
Table 5 The mean values of friction coefficient for wet surfaces with the Brungraber Mark II Footwear material
PB EVA G2AE G L L5725 NL NR NS PE PVCB PVCS SBRB SBRS TPR TPU a
Quarry
Steel
Vinyl
Slip criterion 1
Slip criterion 2
Slip criterion 1
Slip criterion 2
Slip criterion 1
Slip criterion 2
0.274 0.216 0.061 0.176 0.063 0.212 0.176 0.206 0.154 0.202 0.173 0.197 0.283 0.173 0.220 0.184
0.601 0.453 0.134 0.600 0.093 0.321 0.621 0.552 0.463 0.529 0.581 0.472 0.680 0.358 0.550 0.471
0.086 0.071 0.019 0.066 0.021 0.138 0.068 0.062 0.056 0.109 0.079 0.102 0.093 0.058 0.118 0.109
0.169 0.097 0.020 0.097 0.030 0.184 0.166 0.118 0.092 0.224 0.228 0.146 0.146 0.131 0.334 0.270
0.117 0.093 0.028 0.102 0.033 0.113 0.078 0.097 0.084 0.122 0.122 0.124 0.116 0.087 0.176 0.119
0.281 0.149 0.046 0.243 0.047 0.183 0.269 0.573a 0.220 0.306 0.337 0.281 0.259 0.192 0.913 0.389
Some measurements exceeding the range of the slipmeter.
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measurements in most of the material combinations for dry surfaces exceeded the range of the slipmeters used, further discussions are limited to the friction coefficient of wet surfaces only. The p values of the ANOVA analyses for the factors of time and sample on wet surfaces are shown in Table 8. In addition, the combinations with friction coefficient exceeding the slipmeter ranges are also indicated. The friction coefficient measured at different times appeared to be more different statistically than the friction coefficient measured from different samples for wet surfaces. The friction coefficient values from the identical sample measured at different times were compared for each slipmeter. The ANOVA results indicated that there were slightly more statistically significant differences due to measurements taken at different times than due to different samples on wet surfaces. It is also more sensible to combine the friction coefficient values generated from different samples for the same material combinations, surface conditions and slipmeters used in order to compare the results of the measurements taken at different times. The mean values of friction coefficient for the repeatability evaluation on wet surfaces are listed in Table 9. Another interesting comparison was to evaluate the effect of slip criterion and time together. Since slip criterion 2 was used in the evaluation of time factor and it is more practical to use slip criterion 2 based on biomechanical observations, the results generated based on slip criterion 2 were used as a basis for comparison. For
Table 6 The mean values of friction coefficient for wet surfaces with the English XL Footwear material
PB EVA G2AE G L L5725 NL NR NS PE PVCB PVCS SBRB SBRS TPR TPU a
Quarry
Steel
Vinyl
Slip criterion 1
Slip criterion 2
Slip criterion 1
Slip criterion 2
Slip criterion 1
Slip criterion 2
0.391 0.401 0.458 0.620 0.168 0.447 0.889 0.710 0.667 0.461 0.641 0.680 0.780 0.537 0.596 0.690
0.671 0.530 0.671 0.816 0.213 0.447 1.000a 0.967a 0.844 0.717 0.947a 0.742 0.878 0.629 0.823 0.826
0.136 0.168 0.030 0.159 0.073 0.178 0.208 0.304 0.106 0.218 0.223 0.290 0.282 0.276 0.357 0.367
0.223 0.307 0.050 0.203 0.082 0.201 0.323 0.310 0.139 0.318 0.310 0.330 0.298 0.324 0.551 0.579
0.279 0.291 0.121 0.431 0.098 0.266 0.304 0.938a 0.314 0.301 0.427 0.589 0.451 0.321 0.519 0.564
0.400 0.594 0.197 0.537 0.109 0.278 0.453 1.000a 0.5222 0.430 0.490 0.622 0.560 0.400 1.000a 0.742
Some measurements exceeding the range of the slipmeter.
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Table 7 The mean values of friction coefficient for oily and oily-wet surfaces Footwear material
Surface conditions
Brungraber Mark II
English XL
Slip criterion 1
Slip criterion 2
Slip criterion 1
Slip criterion 2
L5725 NL SBRB SBRS TPU
Oily Oily Oily Oily Oily
0.276 0.073 0.051 0.071 0.107
0.284 0.190 0.086 0.119 0.210
0.237 0.139 0.096 0.097 0.136
0.273 0.242 0.110 0.133 0.192
L5725 NL SBRB SBRS TPU
Oily-wet Oily-wet Oily-wet Oily-wet Oily-wet
0.157 0.086 0.068 0.081 0.110
0.207 0.282 0.106 0.110 0.250
0.300 0.359 0.116 0.159 0.314
0.322 0.523 0.149 0.177 0.360
Table 8 Two way ANOVA results for time and sample factors for wet surfaces Footwear material Floor material Time
Sample
Brungraber Mark II English XL Brungraber Mark II English XL PB PB PB EVA EVA EVA G G G NS NS NS PE PE PE TPR TPR TPR a b c
Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl
—a 0.034 0.0275 —a —a —a —a —a —a —a —a —a —a —a 0.0493 —a 0.0209 ***c
0.0096 —a —a 0.0493 —a 0.044 —a —a —a 0.0477b —a ***c —a —a —a —a —a ***c
—a —a —a —a —a —a 0.0423 —a —a —a —a —a —a —a 0.0148 —a —a ***c
—, p>0.05 without exceeding the range of the slipmeter. p40.05 with some measurements exceeding the range of the slipmeter. ***, p>0.05 with some measurements exceeding the range of the slipmeter.
0.0199 —a —a —a —a 0.0273 —a —a —a ***c —a ***c —a —a —a —a —a ***c
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each material combination, surface condition and slipmeter, a relative difference due to slip criterion was calculated by dividing the absolute value of the difference between the friction coefficient values of two slip criteria by the friction coefficient value of slip criterion 2. Similarly, the measurements taken at time 2 were used as a basis for comparison for the time factor. A relative difference due to the time factor was calculated by dividing the absolute value of the difference between the friction coefficient values taken at different times by the friction coefficient value taken at time 2. For simplicity, the mean values of friction coefficient shown in Tables 5, 6 and 9 were used. The relative differences due to both slip criterion and time factors are shown in Table 10. Such a comparison of the relative differences were appropriate since the results generated with slip criterion 2 for slip criterion evaluation and at time 2 for repeatability evaluation for each of these 18 material combinations were in fact identical. Therefore, they had a common denominator.
4. Discussion During the experiment, it was observed that significant amounts of footwear material residue accumulated on the stainless steel and vinyl floor surfaces whenever NS and Natural Rubber (NR) were used. These excessive residues affected the formation of water on the floor surfaces during the measurements. It became very time
Table 9 The mean values of friction coefficient taken at different times on wet surfaces Footwear material
PB PB PB EVA EVA EVA G G G NS NS NS PE PE PE TPR TPR TPR a
Floor material
Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl
Brungraber Mark II
English XL
Time 1
Time 2
Time 1
Time 2
0.636 0.221 0.387 0.467 0.121 0.192 0.649 0.114 0.286 0.420 0.166 0.464 0.551 0.256 0.349 0.626 0.471 0.843a
0.601 0.169 0.281 0.453 0.097 0.149 0.600 0.097 0.243 0.463 0.092 0.220 0.529 0.224 0.306 0.550 0.334 0.913
0.774 0.299 0.510 0.656 0.354 0.693 0.897 0.257 0.643 0.989a 0.420 0.903a 0.747 0.367 0.490 0.844 0.608 0.981a
0.671 0.223 0.400 0.530 0.307 0.594 0.816 0.203 0.537 0.844 0.139 0.522 0.717 0.318 0.430 0.823 0.551 1.000a
Some measurements exceeding the range of the slipmeter.
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consuming and problematic to carry out friction measurements especially with the slipmeters that require repeated measurements. It certainly affected the consistency of surface conditions throughout the measurements and, therefore, affected the repeatability of the outputs. It was thus not surprising to see the mean value of friction coefficient for NR on wet vinyl floor surfaces to be 0.573 while some of the measurements exceeded the range of the Brungraber Mark II as shown in Table 5. It was also not surprising to see more significant differences among measurements taken at different times with NS on wet stainless steel and wet vinyl floor surfaces as shown in Table 9. A ranking method was used to help compare the differences due to different slip criteria among various materials. The relative differences due to different slip criteria for different footwear materials on identical floor surfaces with each slipmeter were ranked first. The material with the minimum relative difference was ranked the highest. Therefore, the material with the highest ranking had the minimum relative difference between the results obtained with two slip criteria. The rankings for each footwear material on three different floor surfaces were added for each slipmeter to compute a combined score for each footwear material. These combined scores for different footwear materials were ranked for each slipmeter to obtain a footwear material ranking.
Table 10 Time versus slip criterion comparison Shoe material
PB PB PB EVA EVA EVA G G G NS NS NS PE PE PE TPR TPR TPR a
Floor material
Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl
Brungraber Mark II
English XL
|T1–T2|/T2a
|SC1–SC2|/SC2b
|T1–T2|/T2a
|SC1–SC2|/SC2b
0.058 0.308 0.377 0.031 0.247 0.289 0.082 0.175 0.177 0.093 0.804 1.109 0.042 0.143 0.141 0.138 0.410 0.077
0.544 0.491 0.584 0.523 0.268 0.376 0.707 0.320 0.580 0.667 0.391 0.618 0.618 0.513 0.601 0.600 0.647 0.807
0.154 0.341 0.275 0.238 0.153 0.167 0.099 0.266 0.197 0.172 2.022 0.730 0.042 0.154 0.140 0.026 0.103 0.019
0.417 0.390 0.302 0.243 0.453 0.510 0.240 0.217 0.197 0.210 0.237 0.398 0.357 0.314 0.300 0.276 0.352 0.481
T1, mean of measurements taken at time 1; T2, mean of measurements taken at time 2. SC1, mean of measurements taken with slip criterion 1; SC2: mean of measurements taken with slip criterion 2. b
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The material with the highest ranking had the smallest relative differences due to different slip criteria across the three different floor surfaces used in this experiment. A similar ranking for the floor surfaces can also be obtained. The relative differences for different floor surfaces with the identical footwear material with each slipmeter were first ranked. The rankings for each floor surface on all footwear materials were added for each slipmeter to compute a combined score for each floor surface. These combined scores for different floor surfaces were ranked for each slipmeter to obtain a floor surface ranking. The floor surface with the highest ranking had the smallest relative differences across different footwear materials used in this experiment. For wet surfaces measured with the Brungraber Mark II, L, L5725, and EVA had the highest footwear material ranking and TPR, PVCB and Nitrile Low Density (NL) had the lowest ranking based on different slip criteria on three floor surfaces. The stainless steel had the highest floor surface ranking and the quarry tile had the lowest ranking for wet surfaces measured with the Brungraber Mark II. For wet surfaces measured with the English XL, TPU, L5725, PVC Solid (PVCS) and SBRB had the highest footwear material ranking and G2AE, PB and EVA had the lowest ranking. The quarry tile had the highest floor surface ranking and the stainless steel had the lowest ranking for wet surfaces measured with the English XL. For oily and oily-wet surfaces, L5725 had the highest footwear material ranking among five footwear materials tested and NL had the lowest ranking based on the combined rankings from both slipmeters. Similar analyses were applied to the measurements taken at different times shown in Table 9. For wet surfaces measured with the Brungraber Mark II, PE had the highest footwear material ranking among six footwear materials tested, and NS had the lowest ranking based on the measurements taken at different times on three floor surfaces. The quarry tile had the highest floor surface ranking and the vinyl tile had the lowest ranking for wet surfaces measured with the Brungraber Mark II. For wet surfaces measured with the English XL, TPR had the highest footwear material ranking among six footwear materials tested although some of the measurements on vinyl exceeded the limit of the slipmeter, and NS had the lowest ranking. The quarry tiles had the highest floor surface ranking and the stainless steel had the lowest ranking for wet surfaces measured with the English XL. The repeatability of NS on stainless steel and vinyl surfaces was poor due to residue accumulation of footwear material on floor surfaces, and the measurements of TPR on vinyl with both slipmeters and NS on quarry with the English XL exceeded the ranges of the slipmeters. Therefore, the results from these material combinations were excluded from the comparison of the effect of slip criterion and time factors shown in Table 10. Among the remaining material combinations, the relative differences due to different slip criteria were mostly higher than the relative differences due to different times. For the Brungraber Mark II, the relative differences between different slip criteria and times differed less than 10% for EVA on stainless steel and EVA on vinyl. Material combinations with higher differences in the relative differences included PE on quarry, NS on quarry and EVA on quarry for the Brungraber Mark II. For the English XL, the relative differences between different
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slip criteria and times differed less than 10% for PB on stainless steel, PB on vinyl, EVA on quarry, G on stainless steel and G on vinyl. Material combinations with higher differences in the relative differences included EVA on vinyl, PE on quarry and EVA on stainless steel for the English XL.
5. Conclusions The effects of slip criterion, time and sample on the measurements of friction coefficient were evaluated for 16 commonly used footwear materials with the Brungraber Mark II and the English XL. The results indicated that the variations due to different samples were probably the least significant statistically among the three factors evaluated. The effect of slip criterion on friction coefficient could be quite significant compared with the effect of time for some material combinations and surface conditions. Since different results of friction measurements for identical material combinations, surface conditions and slipmeters were reported from different institutions, a more consistent slip criterion could potentially help reduce the differences significantly. A more consistent slip criterion should be developed by examining biomechanical data, more discussions and further studies. Excessive footwear material residues which accumulated on the stainless steel and vinyl floor surfaces were observed when NS or NR was used. It is critical to avoid using the slipmeters that require repeated contacts between footwear and floor surfaces for a single measurement such as the two used in this experiment when these two materials are used on stainless steel or vinyl surfaces.
Acknowledgements The author likes to thank Mr. Richard Holihan, Mr. Simon Matz and Ms. Mary Jane Woiszwillo for their assistance during the course of this study. Fruitful discussions with Mr. Mike Wilson at Shoe and Allied Trades Research Association (SATRA), Ms. Ruth Payne at Artech Corporation and Mr. Philip Suraci at American Apparel and Footwear Association (AAFA), formerly known as Footwear Industries of America (FIA), were greatly appreciated. This work was partially supported by a funding from AAFA.
References American Society for Testing and Materials F-1677-96, 1996. Standard Method of Test for Using a Portable Inclineable Articulated Strut Slip Tester (PIAST), Annual Book of ASTM Standards. American Society for Testing and Materials, Philadelphia. American Society for Testing and Materials F-1679-96, 1996. Standard Method of Test for Using a Variable Incidence Tribometer (VIT), Annual Book of ASTM Standards. American Society for Testing and Materials, Philadelphia.
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Chang, W.R., 1999. The effect of surface roughness on the measurements of slip resistance. International Journal of Industrial Ergonomics 24 (3), 299–313. Fendley, A., Marpet, M.I., Medoff, H., Schutter, D., 1999. Repeatability and reproducibility in walkwaysafety tribometry: abrasive-grit size in test-foot preparation. Journal of Testing and Evaluation 27 (1), 76–82. Gro¨nqvist, R., Hirvonen, M., Tohv, A., 1999. Evaluation of three portable floor slipperiness testers. International Journal of Industrial Ergonomics 25, 85–95. Leamon, T.B., Li, K.W., 1990. Microslip length and the perception of slipping. In: Proceedings of 23rd International Congress on Occupational Health, Montreal, Canada, 22–28 September. Leamon, T.B., Murphy, P.L., 1995. Occupational slips and falls: more than a trivial problem. Ergonomics 38, 487–498. Marpet, M.I., 1996. Comparison of walkway-safety tribometers. Journal of Testing and Evaluation 24 (4), 245–254. Marpet, M.I., Fleischer, D.H., 1997. Comparison of walkway-safety tribometers: part two. Journal of Testing and Evaluation 25 (1), 115–126. Powers, C.M., Kulig, K., Flynn, J., Brault, J.R., 1999. Repeatability and bias of two walkway safety tribometers. Journal of Testing and Evaluation 27 (6), 368–374. Strandberg, L., 1983. On accident analysis and slip-resistance measurement. Ergonomics 26 (1), 11–32.
The effects of slip criterion and time on friction measurements Wen-Ruey Chang* Liberty Mutual Research Center for Safety and Health, 71 Frankland Road, Hopkinton, MA 01748, USA
Abstract Different institutions reported different results for friction measurements of identical material combination and surface conditions using identical slipmeters. The objective of this study was to evaluate three factors, slip criterion, sample, and time, that could contribute to such differences with two commonly used slipmeters, the Brungraber Mark II and the English XL. Friction between 16 commonly used footwear materials and three floor materials was measured under four surface conditions with two slip criteria at the interface. Some of the measurements were repeated at a different time. The results indicated that variations due to different samples were probably the smallest statistically among the three factors evaluated. The effect of slip criterion on friction coefficient could be quite significant compared with the effect of time for some material combinations and surface conditions. A more consistent slip criterion could potentially reduce the differences significantly in the results reported among different institutions. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Slip criterion; Time; Sample variation; Friction
1. Introduction Slips and falls are a serious problem. The annual direct cost of occupational injuries due to slips and falls in the USA may be as high as seven billion dollars based on the information provided by an earlier study (Leamon and Murphy, 1995). Falls on the same level accounted for 65% of claim cases and, consequently, 55% of claim costs in the total direct workers’ compensation for occupational injuries due to slips and falls (Leamon and Murphy, 1995).
* Tel.: +1-508-435-9061, ext. 219; fax: +1-508-435-8136. E-mail address: [email protected] (W.-R. Chang). 0925-7535/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0925-7535(01)00061-3
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Many slipmeters have been used to assess the slipperiness between footwear and floor surfaces which is a major factor in slips and falls accidents on the same level. These slipmeters can be quite different in their measurement characteristics. Friction coefficient, also called slip resistance, is the typical output of a slipmeter measurement. The Brungraber Mark II and the English XL as shown in Figs. 1 and 2, respectively, are widely used in the USA and were used in this experiment. Therefore, the following literature review is limited to the relevant publications related to these slipmeters. The operating principle of these slipmeters is to simultaneously apply forces parallel and normal to a floor surface with an impact of a footwear sample on the floor at an inclined angle in order to eliminate the dwell time problem with the static friction measurement. The Brungraber Mark II and the English XL are also known as a portable inclinable articulated strut slip tester (PIAST) and a variable incidence tribometer (VIT), respectively. Standard test methods for using these slipmeters are published by the American Society for Testing and Materials (ASTM; ASTM F-1677-96, 1996; ASTM F1679-96, 1996). The Brungraber Mark II is an inclined-strut slipmeter driven by gravity. The footwear pad impacts floor surfaces at an inclined angle with the vertical direction. If a non-slip occurs at the interface, the inclined angle is increased. Conversely, the angle is decreased if a slip occurs. According to the standard (ASTM F-1677-96, 1996), the starting angle is supposed to be smaller than the angle that a slip is
Fig. 1. The Brungraber Mark II, a portable inclinable articulated strut slip tester (PIAST).
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anticipated and it should be increased until a slip occurs. The tangent of the angle is the friction coefficient shown on the slipmeter. As indicated in the standard, it might be necessary to use the average of the maximum friction coefficient that a non-slip occurs and the minimum friction coefficient that a slip happens as the result of the measurement. The English XL is an inclined-strut slipmeter driven by a pneumatic pressure of 172 kPa (25 psi). According to its standard (ASTM F-1679-96, 1996), the contact force should not be applied to the footwear pad for more than 1 s and a slip occurs when the strut kicks out in an arc with the pneumatic cylinder extending to its full stroke. The operating principle of the English XL is very similar to that of the Brungraber Mark II. A measurement should start from a small angle with a non-slip to a larger angle until a slip occurs and the friction coefficient obtained from the angle at which a non-slip is changed to a slip should be recorded. The friction coefficient obtained directly with the Brungraber Mark II was compared with that calculated from the ground reaction forces, obtained with a force plate, produced by this slipmeter (Marpet, 1996; Marpet and Fleischer, 1997; Gro¨nqvist et al., 1999; Powers et al., 1999). The results indicated that the friction coefficients obtained from these two methods were in good agreement over different floor surfaces with different surface contaminants for non-slip and barely slip conditions. The friction coefficient measured with the Brungraber Mark II was also
Fig. 2. The English XL, a variable incidence tribometer (VIT).
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shown to have a good correlation (r>0.954) with that measured with a dynamic apparatus to simulate a slip although the absolute values of friction coefficients from these two devices could be quite different (Gro¨nqvist et al., 1999). Fendley et al. (1999) investigated the effect of sanding on the repeatability of friction measurement with the Brungraber Mark II. Their results indicated that the repeatability was better with sand paper of 180 grit than with that of 400 grit over three test surfaces. The friction coefficient directly obtained with the English XL was shown to be in good agreement with that obtained with a force plate for non-slip conditions (Powers et al., 1999). Chang (1999) investigated the effect of floor tile surface roughness on the friction measured with five commonly used slipmeters. The friction coefficient measured with the English XL was shown to have the best correlation with certain surface roughness parameters under dry and wet surface conditions among the slipmeters evaluated, while the friction coefficient measured with the Brungraber Mark II was the second best. Although the guidelines for these slipmeters are specified in their respective standards, the details of how these slipmeters ought to be used are somewhat ambiguous. One of the most critical elements in using these slipmeters is to determine whether a slip happens at the interface between footwear sample and floor surfaces. The manufacturers’ user manuals for these slipmeters are not clear regarding the slip criterion and operators are left to develop their own criteria. Any movement at the interface can potentially be interpreted as a slip. However, experiments in biomechanics suggested that slips without any fall or injury occur quite often (Leamon and Li, 1990) and a slip will lead to a fall if the sliding speed exceeds 0.5 m/s (Strandberg, 1983). Therefore, some movements can be allowed at the footwear/floor interface without leading to an accident. This concept can be applied analogously to the slip criterion used in friction measurements between footwear and floor samples. Therefore, some movements at the interface can be considered as a non-slip from an accident prevention viewpoint. Different slip criteria could potentially lead to very different results. Different results of friction measurements from identical material combination and surface conditions using identical slipmeters were reported from different institutions. The possible sources of discrepancy could include different slip criteria, different samples, measurements taken at different times, sample preparation, humidity, and temperature. It is not clear which factors contribute the most. While some of the factors such as measurements taken at different times and different samples are somewhat uncontrollable, slip criterion for the English XL and the Brungraber Mark II can be controlled through a more detailed and consistent measurement protocol. In this experiment, two slip criteria were used to determine the friction coefficients obtained with these two slipmeters. These two slip criteria are two extreme conditions and could cover all possible criteria that the operators could use. Multiple samples were used for the identical material combinations and identical surface conditions to ensure consistent results across different samples. Some of the measurements with identical samples were taken at a different time. An analysis of
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variance (ANOVA) was performed to determine if the differences among different slip criteria, different samples and different times were statistically significant.
2. Test apparatus and design of experiment All the friction measurements were performed at a temperature of 21 1.7 C (70 3 F) and a relative humidity of 50 5%. The 16 footwear sole materials used in this experiment, listed in Table 1, represent typical materials used as shoe soles by shoe industries. The abbreviations in parentheses following the full names of the materials shown in Table 1 will be used throughout this paper. For example, SBR solid with diamond tread will be referred to as SBRS. The full names of the material abbreviations shown in Table 1 are ethylene vinyl acetate (EVA), polyvinyl chloride (PVC), styrene butadiene rubber (SBR), thermal plastic rubber (TPR) and thermal plastic urethane (TPU). All the footwear materials except leather (L) and European Standard G2AE (G2AE) had a diamond pattern on the surfaces of measurement. Most of the diamond patterns had a dimension of approximately 21.1 mm (0.83 inches) in the longitudinal direction and approximately 11.9 mm (0.47 inches) in the latitudinal direction. The width of the groove was approximately 1.55 mm (0.061 inches) measured in the direction perpendicular to the groove. The depth of the groove was approximately 0.80 mm (0.032 inches). The diamond pattern on the footwear samples varied slightly in dimensions on different materials due to thermal processes involved in the manufacturing processes. Three floor materials of unglazed
Table 1 A list of all footwear sole materials used Material name and abbreviation
Skin surface shore hardness
SBR Solid—Diamond Design (SBRS) SBR Blown—Diamond Design (SBRB) Nitrile Solid—Diamond Design (NS) Nitrile Low Density—Diamond Design (NL) EVA—Diamond Design (EVA) Leather—Smooth (L) Natural Rubber—Diamond Design (NR) PVC Solid—Diamond Design (PVCS) PVC Blown—Diamond Design (PVCB) TPR—Diamond Design (TPR) TPU—Diamond Design (TPU) Polyurethane Elastomer—Diamond Design (PE) Polyurethane Blown—Diamond Design (PB) Thermoplastic Butadiene—Diamond Design (G) European Standard G2AE—Smooth (G2AE) TPU Toplift Laripur 5725—Diamond Design (L5725)
873 A 555 A 703 A 553 A 603 A As Sold 653 A 783 A 555 A 703 A 685 A 705 A 705 A 655 A 953 A 563 D
EVA, ethylene vinyl acetate; PVC, polyvinyl chloride; SBR, styrene butadiene rubber; TPR, thermal plastic rubber; TPU, thermal plastic urethane.
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quarry tiles, smooth stainless steel and vinyl composition tiles were used. Dry and oil free compressed air was used for cleaning and drying. Due to a concern that hoses for the compressed air might contaminate the air, new hoses and nozzles were used beyond the filters for oil and moisture. 2.1. Sample preparation All the floor materials were cut into tiles of approximately 15.2415.24 cm (66 inches). Diluted mild liquid detergent, 1% by volume mixed with water, was applied to a clean paper towel to wipe the specimen surfaces. The surface was rinsed thoroughly with tap water and compressed air was blown across the surface for drying. Then, 50% ethanol mixed with de-ionized water was applied to a clean paper towel to wipe specimen surfaces. The surface was dried with the compressed air. These cleaning procedures were applied to all the specimens, including footwear and floor tile samples. In addition, the washed footwear pads were sanded by hand with a fresh section of 400 grit silicon carbide abrasive paper in the longitudinal direction of the diamond pattern for a total of 10 complete movements (five strokes). The specimen was rotated 90 and the sanding process was repeated on a new area on the sand paper. One stroke consisted of one forward and one backward movement. The length of the stroke in each direction was approximately 15 cm. Test surfaces were cleaned with compressed air to remove excess particles generated by sanding. 2.2. Test conditions Surface conditions of dry, wet, oily and oily-wet were used. However, the oily and oily-wet conditions were limited to the measurements with the quarry tiles. For dry conditions, the floor and footwear specimen preparation procedures described earlier were used. For wet conditions, de-ionized water was applied to cover the test area on the floor tile surface. The amount of water was sufficient to flood the test area. Water was replenished throughout repeated impacts on the floor surface during the measurements. Vegetable oil was applied (2 ml) and spread across the entire test surface with a brush for oily conditions. For oily-wet conditions, the vegetable oil was applied as in the oily conditions and then de-ionized water was applied as in the wet conditions. Dry and wet testing was performed on all footwear and floor materials. However, oily and oily-wet testing was performed on five different footwear materials and quarry tiles. 2.3. Friction measurements Friction was measured in the longitudinal direction of the diamond pattern on the footwear specimen and perpendicular to the grooves in the back of the quarry tiles. For the stainless steel and vinyl composition tiles, one direction was chosen on each sample for friction measurements. There were three repeated measurements in the given direction for each surface condition and each sample pair with each slipmeter.
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No cleaning of the samples was required during three repeated measurements. For each material combination, there were three tile samples for dry and wet conditions and another three tile samples for oily and oily-wet conditions. For each material combination and slipmeter, there were three footwear pads for dry and wet testing and another three footwear pads for oily and oily-wet testing. The tests were performed in the sequence of dry then wet on one tile, and oily then oily-wet on another tile. The same tiles were used for different slipmeters under the same surface condition. Whenever there was a change in the slipmeters in the dry conditions, compressed air was used to remove debris on the test surfaces; surfaces were wiped with 50% ethanol; and surfaces were blown with the compressed air for drying. 2.4. Operation of the Brungraber Mark II Measurement started at a low friction coefficient which was increased by 0.05 as long as a non-slip persisted. There were at least three non-slips before a slip occurred if the friction coefficient was higher than 0.15. As soon as a slip occurred, the friction coefficient was reduced by 0.01 as long as the slip persisted. The measurement stopped when a non-slip occurred again. The friction coefficient of the last slip was recorded. 2.5. Operation of the English XL To activate the device, the actuation button for the pneumatic pressure was pressed and quickly released. Based on a recommendation from the manufacturer, the actuation button was pressed for 0.5 s, instead of the 1 s suggested by the standard. The measurement started with a low friction coefficient which was increased by turning the hand wheel a quarter turn as long as a non-slip persisted. A quarter turn of the hand wheel is equal to a change of approximately 0.008 in friction coefficient. There should be at least three non-slips before a slip occurred. As soon as a slip occurred, the measurement stopped and the friction coefficient value was recorded. 2.6. Criteria for a slip condition Two slip criteria which represented two extremes were used in this experiment. The first criterion was that any movement at the interface after impact was considered a slip. For the English XL, visual judgment was used for this criterion since no reference mark around the impact point was established. For the Brungraber Mark II, a line was drawn on the rotating support along the edge of the articulated strut at a non-slip position, as shown in Fig. 3. Based on this criterion, the alignment of the line and the strut was used to determine whether a slip occurred at the interface. For the Brungraber Mark II, the allowable dwell time was limited to 5 s after impact for the first criterion and any movement after 5 s was considered as a non-slip.
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The second criterion was that only a fast movement was considered a slip and, therefore, any slow movement, any finite dwell time at impact or any partial stroke were considered non-slips. For the second criterion, the judgment of whether a finite dwell time or a slow movement occurred was somewhat subjective. These slipmeters make two sounds during a measurement. The first sound comes from the footwear sample impacting the floor sample and the second sound comes from the slipmeter reaching a full stroke movement. When the operators have some experience with these slipmeters, they become very familiar with the duration between these two sounds. This helps establish the criterion of a fast movement. A slow movement has a noticeably different duration. Only one operator was used in this experiment. There could be arguments about how fast is fast and how precisely the operators could notice the differences. However, these differences should be very small. The difference in friction coefficient was usually smaller than 0.01 which was even smaller than the smallest adjustment that the operator made during the measurement. In practice, the differences in subjective judgement were smaller than the resolution in the operation of these slipmeters. Since the procedure used for the Brungraber Mark II proceeded from a slip to a non-slip by an increment of 0.01, the second criterion measurement was performed first with each sample followed by the measurement using the first criterion. This procedure was repeated consecutively three times for each sample under each test condition using each slipmeter.
Fig. 3. A line drawn on the Brungraber Mark II for slip criterion 1.
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2.7. Time factor Friction measurements were repeated at a different time for six different footwear materials, Nitrile Solid (NS), EVA (EVA), TPR (TPR), Polyurethane Elastomer (PE), Polyurethane Blown (PB) and Thermoplastic Butadiene (G) due to the limitation of resources. Only dry and wet surface conditions were repeated because of potential complexity involved with retesting oily surfaces over time. All three floor materials were used. Therefore, there were 36 different combinations of materials and surface conditions repeated. Three samples were used for each material combination and surface condition. The second slip criterion was used. In the second set of measurements, samples were prepared following the same procedures as the first set of measurements and each footwear pad was measured with the identical tile surface in the same direction as the first set. Two sets of measurements were taken several weeks to several months apart.
3. Results A three way ANOVA was applied to the friction coefficient results generated with the 106 different combinations of footwear and floor materials, and surface conditions to determine whether different slipmeters, slip criteria and samples resulted in a statistically significant difference. As expected, the results indicated that the differences in the friction coefficient obtained with different slipmeters were statistically significant. Out of 106, only 10 had non-significant differences with different slipmeters. There were 59 different combinations in which different slip criteria resulted in a statistically significant difference. Different samples resulted in significantly different friction coefficients in 15 different combinations out of 106. Since it is expected that different slipmeters will yield different friction coefficients, the differences due to different slipmeters might dilute the effect of the other two factors. Therefore, a two way ANOVA was applied to the friction coefficients generated with each slipmeter, material combination and surface condition to determine if different slip criteria and samples resulted in significantly different friction coefficients. The range of friction coefficient measured with the Brungraber Mark II is 0–1.1, while the English XL has a range of 0–1.0. Respective maximum friction coefficient values were used in the ANOVA analyses whenever the measurements exceeded the range of the slipmeters. The results indicated that some measurements in most of the material combinations for dry surfaces exceeded the range of the slipmeters used. Some of the statistically non-significant differences were caused by replacement with the maximum friction coefficient values when the ranges of the slipmeters were exceeded. A comparison of the results for dry surfaces solely based on the p values might not be appropriate and, therefore, the results of ANOVA analyses for dry surfaces are not presented here. Tables 2 and 3 contain the p values of the two way ANOVA analyses for wet surfaces for the Brungraber Mark II and the English XL, respectively. For oily and oily-wet surfaces, the p values of the two way ANOVA analyses are shown in
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Table 4. Although the results might be slightly different for different slipmeters, there appeared to be more significant differences statistically for different slip criteria than for different samples under wet, oily and oily-wet conditions as shown in Tables 2, 3 and 4. It was also observed that statistically there were more significant differences due to the different slip criteria for measurements taken on quarry tiles with the Brungraber Mark II than on the other two floor surfaces. For different sample variation, there appeared to be more significant differences statistically for measurements taken with the Brungraber Mark II on quarry and stainless steel surfaces than on vinyl surfaces. There were slightly more significant differences statistically due to different slip criteria for measurements taken with the English XL on quarry and vinyl tiles than on stainless steel surfaces. For different sample variation, there appeared to be more significant differences statistically for measurements taken with the English XL on vinyl and stainless steel surfaces than on quarry tiles. The results also showed that PB, PVC Blown (PVCB), SBRS, and TPU (TPU) were more likely to have statistically significant differences when different slip criteria were used on wet surfaces with both slipmeters than other footwear materials used in this experiment. In addition, EVA, TPU Toplift Laripur 5725 (L5725), NS, SBR Blown (SBRB), and TPR were more likely to have statistically significant differences using different slip criteria on wet surfaces with only the Brungraber Mark II on all three floor surfaces. The results also showed that L and L5725 were more
Table 2 The p values of the two way ANOVA analyses of friction coefficient for wet surfaces with the Brungraber Mark IIa Footwear material
PB EVA G2AE G L L5725 NL NR NS PE PVCB PVCS SBRB SBRS TPR TPU a b c
Slip criterion
Sample
Quarry
Steel
Vinyl
Quarry
Steel
Vinyl
0.0032 0.0015 —b 0.0079 0.0161 0.0032 0.0032 0.005 0.0122 0.0001 0.0001 0.0108 0.0128 0.0023 0.0065 0.0174
0.0252 0.0075 —b —b —b 0.0339 —b 0.0291 0.0108 0.0439 0.0017 —b 0.0005 0.0007 0.0359 0.0132
0.0035 0.0075 —b —b 0.0202 0.0226 —b ***c 0.0185 —b 0.0043 0.0005 0.0029 0.0057 0.0206 0.0104
—b 0.0381 —b —b —b 0.0074 —b —b —b 0.0036 0.0114 —b —b 0.0203 —b —b
—b —b —b —b —b —b —b —b 0.0283 —b 0.0308 —b 0.0071 0.0187 — —
— — — — — — — ***c — — — 0.0256 — — — —
p40.05 with some measurements exceeding the range of the slipmeter. —, p>0.05 without exceeding the range of the slipmeter. ***, p>0.05 with some measurements exceeding the range of the slipmeter.
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likely to have statistically significant differences due to different samples with only the English XL on all three floor surfaces. The ANOVA results indicated that there were more combinations with statistically significant differences due to different slip criteria than due to different samples. Because of a large amount of data, it is more sensible to combine the friction coefficient values generated from different samples for the same material combinations, surface conditions and slipmeters used in order to compare the effect of different slip criteria. The average of three repeated measurements from three different samples from the same material combination, surface condition and slipmeter was calculated for each slip criterion. Since some measurements in most of the material combinations for dry surfaces exceeded the ranges of the slipmeters used, only the mean values of friction coefficient for contaminated surfaces are shown in Tables 5 and 6 for wet surfaces with the Brungraber Mark II and the English XL, respectively, and in Table 7 for oily and oily-wet surfaces. The respective maximum friction coefficient values were used in the mean value calculations whenever the measurements exceeded the ranges of the slipmeters. There were 36 different combinations of footwear and floor materials, and surface conditions for the repeatability evaluation. The results of a three way ANOVA indicated that the differences in the friction coefficient obtained with different slipmeters were statistically significant among the factors of time, slipmeter and sample. Out of 36, only seven had non-significant differences with different slipmeters. There
Table 3 The p values of the two way ANOVA analyses of friction coefficient for wet surfaces with the English XL Footwear material
PB EVA G2AE G L L5725 NL NR NS PE PVCB PVCS SBRB SBRS TPR TPU a b c
Slip criterion
Sample
Quarry
Steel
Vinyl
Quarry
Steel
Vinyl
0.0065 0.0327 0.0046 0.009 —a —a ***c 0.0022b —a 0.0134 0.0088b 0.0162 —a 0.0109 0.01 0.0147
0.0143 —a —a —a 0.0153 0.0261 0.0358 —a 0.0099 —a 0.0063 0.0413 —a 0.0371 —a 0.0215
0.0083 —a 0.0009 0.0119 0.0099 0.0315 0.0356 ***c —a 0.0108 0.0171 —a —a 0.0319 0.0002b 0.0193
—a —a —a —a 0.0203 0.0001 ***c 0.0455b —a —a ***c —a —a 0.0261 —a —a
—a —a —a 0.0403 0.0046 0.0469 —a 0.0024 0.0315 —a —a 0.0033 0.0152 0.0146 —a —a
—a —a 0.0041 0.0094 0.0008 0.025 —a ***c —a —a 0.0403 0.0133 —a —a ***c 0.0106
—, p>0.05 without exceeding the range of the slipmeter. p40.05 with some measurements exceeding the range of the slipmeter. ***, p>0.05 with some measurements exceeding the range of the slipmeter.
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were nine different combinations where measurements taken at different times resulted in a significant difference. Different samples resulted in significantly different friction coefficients in four different combinations. Similarly, a two way ANOVA was applied to the results generated with different slipmeters to determine if different times and samples resulted in different friction coefficients. Since some Table 4 The p values of the two way ANOVA analyses of friction coefficient for oily and oily-wet surfaces Footwear material
Surface conditions
Brungraber Mark II
English XL
Slip criterion
Sample
Slip criterion
Sample
a
L5725 NL SBRB SBRS TPU
Oily Oily Oily Oily Oily
— —a 0.0279 0.0022 —a
0.0122 —a —a 0.0357 —a
0.0187 0.0065 —a 0.0414 —a
0.006 —a 0.0421 —a —a
L5725 NL SBRB SBRS TPU
Oily-wet Oily-wet Oily-wet Oily-wet Oily-wet
0.0102 0.0131 0.0258 0.0015 0.003
—a —a —a 0.0227 —a
—a 0.0207 —a 0.0263 0.0145
—a —a —a —a 0.0148
a
—, p>0.05 without exceeding the range of the slipmeter.
Table 5 The mean values of friction coefficient for wet surfaces with the Brungraber Mark II Footwear material
PB EVA G2AE G L L5725 NL NR NS PE PVCB PVCS SBRB SBRS TPR TPU a
Quarry
Steel
Vinyl
Slip criterion 1
Slip criterion 2
Slip criterion 1
Slip criterion 2
Slip criterion 1
Slip criterion 2
0.274 0.216 0.061 0.176 0.063 0.212 0.176 0.206 0.154 0.202 0.173 0.197 0.283 0.173 0.220 0.184
0.601 0.453 0.134 0.600 0.093 0.321 0.621 0.552 0.463 0.529 0.581 0.472 0.680 0.358 0.550 0.471
0.086 0.071 0.019 0.066 0.021 0.138 0.068 0.062 0.056 0.109 0.079 0.102 0.093 0.058 0.118 0.109
0.169 0.097 0.020 0.097 0.030 0.184 0.166 0.118 0.092 0.224 0.228 0.146 0.146 0.131 0.334 0.270
0.117 0.093 0.028 0.102 0.033 0.113 0.078 0.097 0.084 0.122 0.122 0.124 0.116 0.087 0.176 0.119
0.281 0.149 0.046 0.243 0.047 0.183 0.269 0.573a 0.220 0.306 0.337 0.281 0.259 0.192 0.913 0.389
Some measurements exceeding the range of the slipmeter.
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measurements in most of the material combinations for dry surfaces exceeded the range of the slipmeters used, further discussions are limited to the friction coefficient of wet surfaces only. The p values of the ANOVA analyses for the factors of time and sample on wet surfaces are shown in Table 8. In addition, the combinations with friction coefficient exceeding the slipmeter ranges are also indicated. The friction coefficient measured at different times appeared to be more different statistically than the friction coefficient measured from different samples for wet surfaces. The friction coefficient values from the identical sample measured at different times were compared for each slipmeter. The ANOVA results indicated that there were slightly more statistically significant differences due to measurements taken at different times than due to different samples on wet surfaces. It is also more sensible to combine the friction coefficient values generated from different samples for the same material combinations, surface conditions and slipmeters used in order to compare the results of the measurements taken at different times. The mean values of friction coefficient for the repeatability evaluation on wet surfaces are listed in Table 9. Another interesting comparison was to evaluate the effect of slip criterion and time together. Since slip criterion 2 was used in the evaluation of time factor and it is more practical to use slip criterion 2 based on biomechanical observations, the results generated based on slip criterion 2 were used as a basis for comparison. For
Table 6 The mean values of friction coefficient for wet surfaces with the English XL Footwear material
PB EVA G2AE G L L5725 NL NR NS PE PVCB PVCS SBRB SBRS TPR TPU a
Quarry
Steel
Vinyl
Slip criterion 1
Slip criterion 2
Slip criterion 1
Slip criterion 2
Slip criterion 1
Slip criterion 2
0.391 0.401 0.458 0.620 0.168 0.447 0.889 0.710 0.667 0.461 0.641 0.680 0.780 0.537 0.596 0.690
0.671 0.530 0.671 0.816 0.213 0.447 1.000a 0.967a 0.844 0.717 0.947a 0.742 0.878 0.629 0.823 0.826
0.136 0.168 0.030 0.159 0.073 0.178 0.208 0.304 0.106 0.218 0.223 0.290 0.282 0.276 0.357 0.367
0.223 0.307 0.050 0.203 0.082 0.201 0.323 0.310 0.139 0.318 0.310 0.330 0.298 0.324 0.551 0.579
0.279 0.291 0.121 0.431 0.098 0.266 0.304 0.938a 0.314 0.301 0.427 0.589 0.451 0.321 0.519 0.564
0.400 0.594 0.197 0.537 0.109 0.278 0.453 1.000a 0.5222 0.430 0.490 0.622 0.560 0.400 1.000a 0.742
Some measurements exceeding the range of the slipmeter.
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Table 7 The mean values of friction coefficient for oily and oily-wet surfaces Footwear material
Surface conditions
Brungraber Mark II
English XL
Slip criterion 1
Slip criterion 2
Slip criterion 1
Slip criterion 2
L5725 NL SBRB SBRS TPU
Oily Oily Oily Oily Oily
0.276 0.073 0.051 0.071 0.107
0.284 0.190 0.086 0.119 0.210
0.237 0.139 0.096 0.097 0.136
0.273 0.242 0.110 0.133 0.192
L5725 NL SBRB SBRS TPU
Oily-wet Oily-wet Oily-wet Oily-wet Oily-wet
0.157 0.086 0.068 0.081 0.110
0.207 0.282 0.106 0.110 0.250
0.300 0.359 0.116 0.159 0.314
0.322 0.523 0.149 0.177 0.360
Table 8 Two way ANOVA results for time and sample factors for wet surfaces Footwear material Floor material Time
Sample
Brungraber Mark II English XL Brungraber Mark II English XL PB PB PB EVA EVA EVA G G G NS NS NS PE PE PE TPR TPR TPR a b c
Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl
—a 0.034 0.0275 —a —a —a —a —a —a —a —a —a —a —a 0.0493 —a 0.0209 ***c
0.0096 —a —a 0.0493 —a 0.044 —a —a —a 0.0477b —a ***c —a —a —a —a —a ***c
—a —a —a —a —a —a 0.0423 —a —a —a —a —a —a —a 0.0148 —a —a ***c
—, p>0.05 without exceeding the range of the slipmeter. p40.05 with some measurements exceeding the range of the slipmeter. ***, p>0.05 with some measurements exceeding the range of the slipmeter.
0.0199 —a —a —a —a 0.0273 —a —a —a ***c —a ***c —a —a —a —a —a ***c
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each material combination, surface condition and slipmeter, a relative difference due to slip criterion was calculated by dividing the absolute value of the difference between the friction coefficient values of two slip criteria by the friction coefficient value of slip criterion 2. Similarly, the measurements taken at time 2 were used as a basis for comparison for the time factor. A relative difference due to the time factor was calculated by dividing the absolute value of the difference between the friction coefficient values taken at different times by the friction coefficient value taken at time 2. For simplicity, the mean values of friction coefficient shown in Tables 5, 6 and 9 were used. The relative differences due to both slip criterion and time factors are shown in Table 10. Such a comparison of the relative differences were appropriate since the results generated with slip criterion 2 for slip criterion evaluation and at time 2 for repeatability evaluation for each of these 18 material combinations were in fact identical. Therefore, they had a common denominator.
4. Discussion During the experiment, it was observed that significant amounts of footwear material residue accumulated on the stainless steel and vinyl floor surfaces whenever NS and Natural Rubber (NR) were used. These excessive residues affected the formation of water on the floor surfaces during the measurements. It became very time
Table 9 The mean values of friction coefficient taken at different times on wet surfaces Footwear material
PB PB PB EVA EVA EVA G G G NS NS NS PE PE PE TPR TPR TPR a
Floor material
Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl
Brungraber Mark II
English XL
Time 1
Time 2
Time 1
Time 2
0.636 0.221 0.387 0.467 0.121 0.192 0.649 0.114 0.286 0.420 0.166 0.464 0.551 0.256 0.349 0.626 0.471 0.843a
0.601 0.169 0.281 0.453 0.097 0.149 0.600 0.097 0.243 0.463 0.092 0.220 0.529 0.224 0.306 0.550 0.334 0.913
0.774 0.299 0.510 0.656 0.354 0.693 0.897 0.257 0.643 0.989a 0.420 0.903a 0.747 0.367 0.490 0.844 0.608 0.981a
0.671 0.223 0.400 0.530 0.307 0.594 0.816 0.203 0.537 0.844 0.139 0.522 0.717 0.318 0.430 0.823 0.551 1.000a
Some measurements exceeding the range of the slipmeter.
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consuming and problematic to carry out friction measurements especially with the slipmeters that require repeated measurements. It certainly affected the consistency of surface conditions throughout the measurements and, therefore, affected the repeatability of the outputs. It was thus not surprising to see the mean value of friction coefficient for NR on wet vinyl floor surfaces to be 0.573 while some of the measurements exceeded the range of the Brungraber Mark II as shown in Table 5. It was also not surprising to see more significant differences among measurements taken at different times with NS on wet stainless steel and wet vinyl floor surfaces as shown in Table 9. A ranking method was used to help compare the differences due to different slip criteria among various materials. The relative differences due to different slip criteria for different footwear materials on identical floor surfaces with each slipmeter were ranked first. The material with the minimum relative difference was ranked the highest. Therefore, the material with the highest ranking had the minimum relative difference between the results obtained with two slip criteria. The rankings for each footwear material on three different floor surfaces were added for each slipmeter to compute a combined score for each footwear material. These combined scores for different footwear materials were ranked for each slipmeter to obtain a footwear material ranking.
Table 10 Time versus slip criterion comparison Shoe material
PB PB PB EVA EVA EVA G G G NS NS NS PE PE PE TPR TPR TPR a
Floor material
Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl Quarry Steel Vinyl
Brungraber Mark II
English XL
|T1–T2|/T2a
|SC1–SC2|/SC2b
|T1–T2|/T2a
|SC1–SC2|/SC2b
0.058 0.308 0.377 0.031 0.247 0.289 0.082 0.175 0.177 0.093 0.804 1.109 0.042 0.143 0.141 0.138 0.410 0.077
0.544 0.491 0.584 0.523 0.268 0.376 0.707 0.320 0.580 0.667 0.391 0.618 0.618 0.513 0.601 0.600 0.647 0.807
0.154 0.341 0.275 0.238 0.153 0.167 0.099 0.266 0.197 0.172 2.022 0.730 0.042 0.154 0.140 0.026 0.103 0.019
0.417 0.390 0.302 0.243 0.453 0.510 0.240 0.217 0.197 0.210 0.237 0.398 0.357 0.314 0.300 0.276 0.352 0.481
T1, mean of measurements taken at time 1; T2, mean of measurements taken at time 2. SC1, mean of measurements taken with slip criterion 1; SC2: mean of measurements taken with slip criterion 2. b
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The material with the highest ranking had the smallest relative differences due to different slip criteria across the three different floor surfaces used in this experiment. A similar ranking for the floor surfaces can also be obtained. The relative differences for different floor surfaces with the identical footwear material with each slipmeter were first ranked. The rankings for each floor surface on all footwear materials were added for each slipmeter to compute a combined score for each floor surface. These combined scores for different floor surfaces were ranked for each slipmeter to obtain a floor surface ranking. The floor surface with the highest ranking had the smallest relative differences across different footwear materials used in this experiment. For wet surfaces measured with the Brungraber Mark II, L, L5725, and EVA had the highest footwear material ranking and TPR, PVCB and Nitrile Low Density (NL) had the lowest ranking based on different slip criteria on three floor surfaces. The stainless steel had the highest floor surface ranking and the quarry tile had the lowest ranking for wet surfaces measured with the Brungraber Mark II. For wet surfaces measured with the English XL, TPU, L5725, PVC Solid (PVCS) and SBRB had the highest footwear material ranking and G2AE, PB and EVA had the lowest ranking. The quarry tile had the highest floor surface ranking and the stainless steel had the lowest ranking for wet surfaces measured with the English XL. For oily and oily-wet surfaces, L5725 had the highest footwear material ranking among five footwear materials tested and NL had the lowest ranking based on the combined rankings from both slipmeters. Similar analyses were applied to the measurements taken at different times shown in Table 9. For wet surfaces measured with the Brungraber Mark II, PE had the highest footwear material ranking among six footwear materials tested, and NS had the lowest ranking based on the measurements taken at different times on three floor surfaces. The quarry tile had the highest floor surface ranking and the vinyl tile had the lowest ranking for wet surfaces measured with the Brungraber Mark II. For wet surfaces measured with the English XL, TPR had the highest footwear material ranking among six footwear materials tested although some of the measurements on vinyl exceeded the limit of the slipmeter, and NS had the lowest ranking. The quarry tiles had the highest floor surface ranking and the stainless steel had the lowest ranking for wet surfaces measured with the English XL. The repeatability of NS on stainless steel and vinyl surfaces was poor due to residue accumulation of footwear material on floor surfaces, and the measurements of TPR on vinyl with both slipmeters and NS on quarry with the English XL exceeded the ranges of the slipmeters. Therefore, the results from these material combinations were excluded from the comparison of the effect of slip criterion and time factors shown in Table 10. Among the remaining material combinations, the relative differences due to different slip criteria were mostly higher than the relative differences due to different times. For the Brungraber Mark II, the relative differences between different slip criteria and times differed less than 10% for EVA on stainless steel and EVA on vinyl. Material combinations with higher differences in the relative differences included PE on quarry, NS on quarry and EVA on quarry for the Brungraber Mark II. For the English XL, the relative differences between different
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slip criteria and times differed less than 10% for PB on stainless steel, PB on vinyl, EVA on quarry, G on stainless steel and G on vinyl. Material combinations with higher differences in the relative differences included EVA on vinyl, PE on quarry and EVA on stainless steel for the English XL.
5. Conclusions The effects of slip criterion, time and sample on the measurements of friction coefficient were evaluated for 16 commonly used footwear materials with the Brungraber Mark II and the English XL. The results indicated that the variations due to different samples were probably the least significant statistically among the three factors evaluated. The effect of slip criterion on friction coefficient could be quite significant compared with the effect of time for some material combinations and surface conditions. Since different results of friction measurements for identical material combinations, surface conditions and slipmeters were reported from different institutions, a more consistent slip criterion could potentially help reduce the differences significantly. A more consistent slip criterion should be developed by examining biomechanical data, more discussions and further studies. Excessive footwear material residues which accumulated on the stainless steel and vinyl floor surfaces were observed when NS or NR was used. It is critical to avoid using the slipmeters that require repeated contacts between footwear and floor surfaces for a single measurement such as the two used in this experiment when these two materials are used on stainless steel or vinyl surfaces.
Acknowledgements The author likes to thank Mr. Richard Holihan, Mr. Simon Matz and Ms. Mary Jane Woiszwillo for their assistance during the course of this study. Fruitful discussions with Mr. Mike Wilson at Shoe and Allied Trades Research Association (SATRA), Ms. Ruth Payne at Artech Corporation and Mr. Philip Suraci at American Apparel and Footwear Association (AAFA), formerly known as Footwear Industries of America (FIA), were greatly appreciated. This work was partially supported by a funding from AAFA.
References American Society for Testing and Materials F-1677-96, 1996. Standard Method of Test for Using a Portable Inclineable Articulated Strut Slip Tester (PIAST), Annual Book of ASTM Standards. American Society for Testing and Materials, Philadelphia. American Society for Testing and Materials F-1679-96, 1996. Standard Method of Test for Using a Variable Incidence Tribometer (VIT), Annual Book of ASTM Standards. American Society for Testing and Materials, Philadelphia.
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