Railway Track & Structures - December 2008 - (Page 40)

Optimizing winter speed restrictions head of the rail, SigmaR is the residual stress, SigmaT is the thermal stress, and KIC is the fracture toughness (from tensile testing). In the present study, a random number generator was then used to select values for various parameters from realistic distributions used in the above track strength equation. This numerical procedure generates a stochastic distribution of allowable impact force for all existing defects. After calibration using the defect and service records, the strength distribution obtained is used to establish allowable impact force vs. temperature relationship for each subdivision. maximum allowable impact force (horizontal red line). If a train has a WILD reading that exceeds the maximum allowable impact force it can be slowed down so that the wheel impact forces are all below the threshold. On the other hand, if a train has WILD readings that are all below the maximum allowable impact force, the train speed can be increased until either the maximum allowable impact force or the maximum allowable track speed is reached. THAW’s effect on train speeds Using the speed dependence of applied impact force for three WILD sites, the temperature dependence of track strength, To = -25ºC and the subdivision specific Fo values, optimized train speeds were calculated. The optimized train speeds were then compared with the speeds at the time of the WILD readings and the previous blanket cold slow orders. The maximum WILD readings from each train, along with the train speed and temperature were extracted from the CPR data. Only readings with temperatures below -25ºC were considered, resulting in nearly 840 trains worth of data for the three sites. The maximum WILD measurement, train speed, and temperature for each train were input into the THAW Meter to compute the maximum THAW speed. The result was that two-thirds of the trains were “low-stress” relative to the low temperature track strength and need not have been speed restricted while the remaining one third had at least one high stress wheel and would have to be slowed down even further from the current CPR practice. Calibration of the Strength Model With much uncertainty in many of the parameters used in the Strength Model, the model inputs were calibrated to match existing defect data provided by CPR. The calibration approach is illustrated in Figure 4. On the left is a representation of the distribution of train impacts (train force). On the right is the distribution of allowable impact forces determined by the statistical evaluation of track strength. The overlapping region between these two distributions should correspond to the number of failures. The poorly known residual stress value range was adjusted so that the area of the modeled overlap region equalled the total number of service failures reported over the past five years. An example is shown in Figure 5 for one subdivision. This calibration procedure was repeated for each subdivision. The THAW Meter The Stress versus Strength Model developed by NRCCSTT has been incorporated into an Excel-based tool for CPR called the “THAW” Meter. Trains Highball All Winter refers to the desired state of eliminating all cold weatherrelated slow orders. The THAW meter provides the maximum recommended speed for an individual train in cold weather, based on: • the subdivision (with its defect history, rail size distribution and target neutral temperature); • ambient temperature; • train wheel condition as measured by WILD values. Figure 6 illustrates the stress/strength concept on which the THAW Meter is based. The strength aspect of the track is illustrated in the left of Figure 6. The allowable impact force takes the maximum value (Fo) at higher temperatures, becomes temperature dependent as the temperature decreases beyond the transition temperature (T o ), and decreases thereafter with an approximately constant rate as temperature decreases. The transition temperature is suggested to be constant across all subdivisions, and a value of -25ºC has been chosen to coincide with CPR’s experience. Differences in the subdivisions are accounted for with the Transition Force (Fo), which for the nine subdivisions included in this study varies from 110-140 kips. The temperature dependence (rate) is obtained from the strength model calibration described in Section 3. The stress aspect is illustrated in the right hand side of Figure 6. The minimum speed (a) is provided by CPR and the maximum speed (b) comes from the railroad timetable. The speed dependence (c) of the impact force is established by WILD data analysis in Section 1. The Strength Model at a given temperature sets the Conclusions The THAW (Trains Highball All Winter) Meter is a software tool developed by the NRC-CSTT for the CPR to help optimize trains speeds in cold weather. It treats the problem of low temperature broken rails as one of stress versus strength. Stress is that applied by impacting wheels, while strength is derived from fracture mechanics models of rail failure from transverse defects. The model is calibrated against WILD data and recorded defects from the past five years. The algorithm produces a good guideline for subdivisionspecific speed restrictions in extreme cold weather on CPR. It offers the potential for low-stress trains to continue at normal timetable speeds, even as temperatures hit lows in the -35ºC (-31ºF) range. At the same time, this approach could decrease the risk of rail break under the extreme cold weather by slowing down “high-stress” trains. The operational challenge of train-specific cold weather speed restrictions is the effect that train speed variability has on operational fluidity. Further work at CPR will concentrate on the process for identifying inbound trains with cars having high impacts to ensure that outbound trains can traverse the territory at higher speeds and lower risk even as the thermometer plunges. References 1. Igwemezie, J.O., Kennedy, S.L., Feng, X., Cai, Z., “Defective Rail Fracture Under Residual, Thermal and Dynamic Stresses,” Transport Canada Report No. TP-11570 E, CIGGT Report 92-11, December 1992. 2. Jeong, D.Y., “Correlations between Rail Defect Growth Test Data and Engineering Analyses, Part 2: Field Tests,” Volpe Center Technical Report for the UIC/WEC Joint Research Project on Rail Defect Management, January 2003. 40 Railway Track & Structures December 2008 www.rtands.com 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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