Railway Track & Structures - December 2008 - (Page 37)

Figure 3, above: WILD force-speed relationship for Site #119 with curve fit. increased severity and numbers of shelled wheels. “Strength” refers to the ability of the track and infrastructure to endure forces, withstand damage and avoid breakage. Track strength in winter decreases primarily due to reductions in fracture toughness with temperature and due to increased rates of growth of transverse defects that can eventually lead to broken rails. Most of the time the strength is much higher than the stress and failures are rare. However, as the stress rises and the strength declines, the gap diminishes and the risk of failure, such as a broken rail, increases. The stress model The stress model is based on the analysis of Wheel Impact Load Detector data supplied by CPR. NRC-CSTT received roughly 50 million WILD readings for 75,000 trains from three different WILD sites. These data were used to analyze nine subdivisions. Figure 1 shows the frequency distribution of WILD impact readings from all available wheel impacts at the three WILD sites. Two distinct peaks are seen, corresponding to empty (leftmost peak) and loaded trains (rightmost peak). The peak representing the loaded trains is centered near the static vertical load carried by each wheel (33-36 kips). Beyond this, the loads are largely dynamic and arise primarily from wheel tread defects. The focus of this study is mainly concerned with the extreme right tail of the distribution. If viewed using a logarithmic scale, it is found that an impact force of 100 kips or more occurs once in every 10,000 wheel readings and that 150 kips or more occurs in approximately one out of every 1,000,000 readings. Figure 2 shows the 0.1 percent highest impacts binned according to the temperature at which the measurement was made. The red diamond at -20°C, for example, shows that 0.1 percent of all the WILD readings taken at the Kaministiquia WILD site between -19.5°C and -20.5°C were at or above 114 kips. Below 0ºC (winter) and above 10ºC (summer) there is no obvious temperature dependence. Peak WILD values at various speeds were examined to establish the speed dependence of impact force. Figure 3 shows the curve-fit to the force-speed relationship for one site, where the force level is taken as the 0.1 percent value from all trains at that speed. Only small differences were www.rtands.com Railway Track & Structures December 2008 37 http://www.rtands.com

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Railway Track & Structures - December 2008 Contents On Track Industry Today Supplier News AREMA News NRC News TTCI R&D Hand-Held Tools Continue Big Role in Railroad Engineering AREMA C&S Moving Ahead Optimizing Winter Speed Restrictions RTA 2008 Conference Makes Splash in Savannah, Ga. Products and Literature People Calendar Sales Representatives Advertisers Index Website Directory Professional Directory Classified Advertising Chicago Perspective

Railway Track & Structures - December 2008

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