In every scientific discipline, the cognoscenti devise a series of buzz-words, allegedly to facilitate technical discussion among themselves, but it is also observable that these terms of art tend to keep outsiders from understanding the discussion sufficiently to be able to avoid engagement of the scientists to solve their problems. Pedestrian slip resistance is no exception.
We do use some words in a specific way so they have precise meaning in our context, and we sometimes adapt words used in general vocabulary or other technical disciplines to denote some shorthand meanings we have agreed upon. In an effort to help my visitors get up to speed on the latest terminology (which goes hand-in-hand with the latest technology), a few terms are defined herewith. The following glossary was boiled down from my book Pedestrian Slip Resistance and some ASTM and ANSI standards.
Definitions of Terms Used by Slip Prevention Practitioners
Slip resistance is the property of a walking surface that tends to inhibit slipping of a pedestrian's shoes under the prevailing conditions. The property of a floor that makes it slip-resistant in the presence of a lubricating contaminant, such as water or oil, is its surface roughness. That is, the surface asperities must be tall enough and sharp enough to extend upward through the lubricating film sufficiently to engage the shoe bottom in a manner not unlike sandpaper. This phenomenon (of how the contaminant is manipulated on the interface of the two surfaces) is referred to in lubrication theory as the hydrodynamic squeezefilm or simply the wedge which forms under the leading edge of a sliding body.
According to the literature from the US National Bureau of Standards (The name has since been changed to National Institute of Standards and Technology, but so far nothing about slip resistance measurement has been published under the new name), there are two official definitions prescribed for traction measurement. They used the term coefficient of friction rather than slip resistance. In fact, both parameters set forth below are said to define static coefficient of friction (SCOF).
Static coefficient of friction is: (1) related to the force required to initiate relative motion between an object and a surface it is resting on. According to NBS terminology, SCOF is the ratio of the force required to move the object to its mass. That is, if it takes five pounds of horizontal force to move a ten-pound block resting on a floor, the SCOF is .50, or
(2) in variable incidence strut instruments the SCOF is the tangent of the angle from the vertical at which slipping begins to occur.
It is inherent in both of these definitions that the two surfaces be in direct contact with each other. If there is anything on the interface, you are not measuring SCOF. Therefore, by NBS definition, all static meters must be used only under clean, dry conditions. One cannot take a SCOF reading on a wet floor. It is for this reason that recent ASTM standards for SCOF measurement specify dry conditions only, and standards for the use of instruments under wet conditions refer to slip resistance rather than coefficient of friction. The output of the slipmeter is the slip index.
Dynamic coefficient of friction as defined in pedestrian tribometry is related to the force required to keep a sliding object in motion, once slipping has begun. The formula is sliding resistance divided by the vertical force. But since it is known that DCOF is sensitive to velocity, it is not possible to measure it until we know how fast the slip is supposed to be. In strobe photography of pedestrian heel slides (a photo is shown on p. 30 in Pedestrian Slip Resistance, Second Edition), it can be seen that the foot accelerates from the instant of slip initiation until the heel leaves the floor, so meters that move at a constant speed are not measuring anything relevant to pedestrian activity, if there is any lubricant on the interface.
NBS literature tends to hold to the SCOF as the relevant factor in the realm of pedestrian safety, as did European literature before the revelation of the SATRA Tester. European investigators now hold to the notion that DCOF is the relevant index, many even going so far as to say that SCOF doesn't actually exist in the real world. It has been only slightly facetiously reported that this latter finding emanated from the same university where it was discovered some years ago that careful study of aerodynamic principles has proven that bumble bees really can't fly.
Since most clean, dry surfaces are not slippery, it is almost always necessary to investigate some slippery contamination of the walking surface, in cases of pedestrian slips. We are looking at the ability of the floor asperities to protrude through the contaminant and engage the shoe bottom. That is a study of slip resistance, not coefficient of friction.
Slip resistance is defined in ASTM F1637.95, Practice for Safe Walking Surfaces as "The relative force which resists the tendency of the shoe or foot to slide along the walkway surface. Slip resistance is related to a combination of factors including the walkway surface, the footwear bottom, and the presence of foreign material between them."
There is another complicating phenomenon affecting slipmeters known as adhesion (usually applied to dry testing) or many investigators have termed it sticktion (under wet conditions). There are almost as many spellings of the sticktion term as there are investigators writing about it, though none of them will show up on your word processor's spell checker. When attempting to measure the slip resistance of a walking surface, it is more important to avoid it than it is to spell it right, however.
Adhesion, or sticktion, arises as a function of the slider pad's residence time on the surface. If there is any delay between the instant of surface contact and the application of the horizontal force, sticktion or adhesion will occur. With adhesion, the longer the slider pad is in contact with the test surface under the normal (vertical) force, the more the surface of the pad conforms to the topography of the test surface, thereby increasing the horizontal force required to initiate sliding. Water is a very tenuous film, and in wet testing any residence time of the shoe on the wet surface causes the water film to be squeezed out of the interface between the shoe bottom and the walking surface so that the tendency to slip is greatly reduced. This is called sticktion.
On dry surfaces, the longer the residence time, the greater adhesion will be. On surfaces wet with water, residence times shorter than .2 second (that's 200 ms in PhD-speak) are known to produce significant sticktion that can result in slipmeter readings that are higher under wet conditions than would be obtained on the same surface in a dry state.
All dragsled meters and all articulated strut instruments that do not apply the horizontal and normal forces simultaneously with initial surface contact are afflicted with disqualifying sticktion. That is, you can't take a valid reading on a wet surface with any dragsled instrument, regardless of what the testing standards say, even if your dragsled weighs 50 pounds! (You disciples of Napoleon can convert that to newtons for yourself, since the newton is not a unit of mass.)
Certain investigators have remarked about what they call a "suction cup effect" that sometimes occurs under shoe bottoms or slipmeters on wet surfaces, but this is not sticktion as defined herein. There is no evidence that the suction effect increases slip resistance. In fact it appears to promote greater slipperiness under some tennis shoes.
Tribometry is PhD-speak for the science of the measurement of friction; or in the case of pedestrian safety, it is the measurement of traction. Tribology is the science lubrication, such as may occur inside the crankcase of an engine. Although both terms are related to friction, they are not similar, and there is no transfer of knowledge from tribology to tribometry. A tribometrist is one who measures pedestrian traction on walking surfaces. A Tribologist studies lubrication efficiency in engines. The dynamics in the two distinct realms are entirely different.
For more thorough discussions of slip resistance as it applies to pedestrian safety, the technology of measuring it, and methods of making hazardous facilities safer, read my books on Pedestrian Slip Resistance: How to Measure it and How to Improve It.
To see how the above technical definitions compare with standard dictionary terminology, try an online dictionary such as http://www.m-w.com/
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