Skiing

This work has been supported by the Swedish Nuclear Power Inspectorate (SKI) and ABB TRC. References... 

Bostrom, A., UTDefect - a computer program modelling ultrasonic NDT of cracks and other defects, SKI report 95 53, Stockholm, 1995. 

Bostrom, A., Jansson, P-A., Developments of UTDefect rough cracks and probe arrays, SKI report 97 28, Stockholm, 1997. 

The constitutive law solid models. The classical Hooke law... 

In addition to the estimate (5.21) one can prove (see Khludnev, Sokolow-ski, 1997) that the estimate... 

W. Virgin ski. Paper presented at the 46th Annual Technical Conference of the Society of Plastics Engineers, May 1988, p. 205. 

There are many suppHers of urethane systems in the United States. The TSE Company suppHes the mbber form many companies supply the prepolymer forms. It is estimated that close to 68,000 t of these mbbers are used in the United States if aU forms are included. Uses include sport wheels (roUer blades, ski wheels) printing, paper, and steel processing roUs gears pump liners appHance components and soHd industrial tires. 

Applications. Epoxy resias constitute over 90% of the matrix resia material used ia advanced composites. In addition, epoxy resias are used ia all the various fabrication processes that convert resias and reinforcements iato composite articles. Liquid resias ia combiaation, mainly, with amines and anhydride are used for filament winding, resia transfer mol ding, and pultmsion. Parts for aircraft, rocket cases, pipes, rods, tennis rackets, ski poles, golf club shafts, and fishing poles are made by one of these processes with an epoxy resia system. 

Case study 2 materials for skis and sledge runners... 

Skis, both for people and for aircraft, used to be made of waxed wood. Down to about -10° C, the friction of waxed wood on snow is very low - [jl is about 0.02 - and if this were not so, planes equipped with skis could not take off from packed-snow runways, and the winter tourist traffic to Switzerland would drop sharply. Below -10° C, bad things start to happen (Fig. 26.6) (jl rises sharply to about 0.4. Polar explorers have observed this repeatedly. Wright, a member of the 1911-13 Scott expedition, writes Below 0°F (-18°C) the friction (on the sledge runners) seemed to increase progressively as the temperature fell it caused the expedition considerable hardship. What determines the friction of skis on snow ... 

Below -10°C, heat is conducted away too quickly to allow this melting - and because their thermal conductivity is high, skis with exposed metal (aluminium or steel edges) are slower at low temperatures than those without. At these low temperatures, the mechanism of friction is the same as that of metals ice asperities adhere to the ski and must be sheared when it slides. The value of jl (0.4) is close to that calculated from the shearing model in Chapter 25. This is a large value of the coefficient of friction - enough... 

This is a standard friction problem. A glance at Fig. 25.5 shows that, when polymers slide on metals and ceramics, x can be as low as 0.04. Among the polymers with the lowest coefficients are PTFE (Teflon ) and polyethylene. By coating the ski or sledge runners with these materials, the coefficient of friction stays low, even when the temperature is so low that frictional heating is unable to produce a boundary layer of water. Aircraft and sports skis now have polyethylene or Teflon undersurfaces the Olympic Committee has banned their use on bob-sleds, which already, some think, go fast enough. 

A man of weight 100 kg standing on skis 2 m long and 0.10 m wide slides on the 2° mountain slope, at 0°C. Calculate the work done against friction when the ski slides a distance equal to its own length. Hence calculate the average thickness of the water film beneath each ski. (The latent heat of fusion of ice is 330 MJ m °.)... 

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