How TTCI is investigating the effects of heavy train loads on joint bar performance.
The effects of various track parameters and track maintenance on joint bar forces and strains were evaluated at TTCI in a series of inspections and tests sponsored by the Federal Railroad Administration Office of Research, Development and Technology. The results show that normal track surfacing procedures produce joint bar bending strains that are similar in magnitude to train loading. Given the limited number of load cycles resulting from tamping maintenance, as compared to traffic loading, the effect of tamping on joint bar failure is likely small.
Previous investigations by the FRA and the Association of American Railroads show that the predominant location of joint bar cracking is at the top (vertically)/middle (longitudinally) of the bar. However, this location should not have high tensile stresses under the assumed loading conditions. Therefore, an investigation of the effects of maintenance and track conditions, such as tamping and raising, rail end gap distance and rail height mismatches, was conducted.
Findings from TTCI’s in-depth study are as follows:
• Rail joint deflection under load is highly dependent on support conditions in the vicinity of the joint. Furthermore, rail joint deflections are highly correlated with failed or defective joint bars. As previous studies have shown, limiting joint deflection will extend joint bar service life significantly.
• Track surfacing is not likely the cause of crack development at the top longitudinal center of the joint bars. Measured joint bar strains from surfacing, including the raising and lowering of the rail joints, were low. Tensile strains on the top of the bars were lower than typically seen from train operations (5 ksi vs. 10–20 ksi). The top of the bars had largely compressive strains for most of the surfacing done to raise joints that were lower than 1 inch.
• The effects of rail end gaps at joints were measured under a heavy-axle-load test consist. Rail joints produce some dynamic load as compared to continuous welded rail. However, the effect of varying the rail end gap from 0 to 1 inch was small (i.e. 10% to 20% dynamic load above nominal wheel loads) over the range of speeds tested (i.e., 10-45 mph). The rail joint conditions were representative of a newly installed joint, with no end batter or foundation degradation.
• The effect of rail height mismatch at joints was measured under a heavy-axle-load test consist as well. Again, the effect was relatively small in terms of joint bar strains. However, the measured wheel and rail forces, which were relatively low, suggest that the test may not be representative of revenue service conditions.
• There was little correlation between joint bar surface hardness and metal flow depth at the top center of joint bars, which suggests that the use of a simple measure of flow depth cannot yet be used to reject joint bars from re-use in track. A larger sample, including more cracked bars, is needed to reach a definitive conclusion.
A review of the data collected from the field surveys and from rail joints at TTCI’s Facility for Accelerated Service Testing (FAST) reveals that vertical deflection at the rail joint has a significant statistical correlation with rail joint condition. Figure 1 (above) shows the distribution of vertical deflection measurements for 118 joint locations (50 intact locations and 68 failed locations). Vertical deflections of 1.5 inches or greater were measured at nine joint locations, all of which had defective (e.g. worn, loose or with broken bolts) or failed (i.e. broken or cracked in the longitudinal center) joint bars. Moreover, the figure shows more defective or failed joints than intact ones when the vertical deflections are ½ inch or greater. At deflections less than ½ inch, intact joints outnumber the failed ones.
This study has provided insight into why and how joint bars develop fatigue cracks by (1) examining the causes and locations of crack initiation and (2) quantifying the effects of various track parameters that may cause overstressing.
Ultimately TTCI believes that the results of this study will help develop guidelines for best practices and potential methods that reduce the occurrence of joint bar failures in revenue service.
References: Jeong, D.Y., R. Bruzek, A. Tajaddini. April 2014. “Engineering Studies on Joint Bar Integrity, Part I: Field Surveys and Observed Failure Modes,” JRC2014-3706. Proceedings of the 2014 Joint Rail Conference, Colorado Springs, Colo.
Davis, David, Darrell Collard, and Don Guillen, May 2004. “Bonded Insulated Joint Performance in Main Line Track.” Technology Digest TD04-006. Association of American Railroads, Transportation Technology Center, Inc., Pueblo, Colo.
Akhtar, Muhammad and David Davis, May 2006. “Development of an Improved Bonded Insulated Joint for HAL Service.” Technology Digest TD06-012. Association of American Railroads, Transportation Technology Center, Inc. Pueblo, Colo.