Heavy-haul hammer test: increasing axle loads offers no benefits without a solid track foundation that can handle hammering by high-tonnage freight cars - Industry Overview

Railway Age, April, 2003 by Tom Judge

Heavy axle loads inspire a gleam in the eye of Class I operating officers and sales and marketing people. Moving more cargo with fewer cars adds up to big benefits for the bottom line. But out on the track, how will the m/w people cope with the stresses and strains those HAL cars pound into rail, ties, and special trackwork?

Researchers at the Transportation Technology Center, Inc., are working with railroaders to come up with some good answers to that question. TTCI's Eighth Annual Research Review last month in Pueblo high-lighted several areas of ongoing research.

The Association of American Railroads is undertaking an advanced rail steels program and rail performance evaluation at the Facility for Accelerated Service Testing (FAST) at TTCI. Objectives include developing and/or demonstrating advanced rail steels with lower life-cycle cost than current rail steels and evaluating the wear and surface fatigue performance of the most modern premium rail steels available by testing under HAL.

Previous rail tests showed a fair correlation between the hardness of the rail and wear rates. Current rail performance tests at FAST include high-hardness rail steel from Nippon Steel, Corus, Voest-Alpine, NKK, Pennsylvania Steel Technologies, and Rocky Mountain Steel Mills, and J6 bainitic steel rail.

After 145 MGT have passed over the rails, TTCI researchers estimate effective wear lives from 1,150 MGT to 1,600 MGT. The wear life estimate is based on wearing away 30% of the railhead area on 141-pound rail. TTCI is also monitoring rail performance in revenue service.

"Natural wear is not the only factor in rail life," says Joseph LoPresti of TTCI. "Rail that wears faster but needs less grinding can last longer. Improper or excessive grinding can over-whelm the benefits of better-wearing rail. This analysis is based on rail measurements from a railroad that grinds frequently. Preliminary results indicate that rails are providing improved wear performance compared to previous rail tested under Phase V FAST operations. "The surface condition of rails is generally good throughout the test zone," LoPresti says. "Statistical analysis is now being performed at the University of Illinois at Urbana-Champaign, but it is too early for conclusions on comparative wear." Newer high-hardness rails in the 420 to 430 Brinell range are currently being developed. "Steel hardness is an indicator of wear and fatigue performance, but not an absolute predictor," LoPresti says. "More study is needed."

In addition, work is now under way at the University of Illinois on material property characterization to develop a better understanding of wear and rolling contact fatigue performance to increase rail life. FRA is contributing matching R&D funds.

The goals of this research include developing an easy-to-use procedure for estimating the yield stress in shear (property-influenced wear and rolling contact fatigue). The research should develop a model that can be used to evaluate new rail steels to identify the role of hardness and other material variables on wear behavior. Another goal is to determine the influences of the microstructure of bainitic rail steels on performance, compared to pearlitic steel. This would facilitate additional development and optimization of rail steel microstructure. "Bainitic microstructure inhibits the flow and reduces the thickness of the deformed layer at the rail's surface," LoPresti says. "This characteristic is likely the reason for superior surface performance and should allow further development of rail steels. While harder rails in general wear better, the latest measurements show a poor correlation. The technology for making harder rails may be ahead of the science of understanding the effect. Also, there may be a p oint of diminishing returns. Does making hard rail even harder continue to improve performance? And if there is such a point, is current rail near it?"

TTCI is in close contact with other researchers at universities, research organizations, and rail manufacturers in the U.S., Europe, and Japan that are also studying rail life. TTCI is also working to improve performance of special trackwork under HAL. Efforts to reduce the impact of HAL include ramped tread-bearing crossing diamonds and frog running surface profiles. TTCI is also testing bainitic steel frogs in revenue service.

"Ramped tread-bearing crossing diamonds are a low-cost method for reducing dynamic loads on high-angle crossing diamonds," TTCI's David Davis says. "Modeling suggests that upward sloping ramps at flangeways could reduce impacts for a limited range of speeds and car types."

TTCI conducted successful tests of prototypes in 19992000. Cleveland Track Material, Inc., built a ramped diamond for FAST testing in 2001. The bainitic thick-web J6 rail has 40mph ramps. FAST load measurements were made on the main line after 28 MGT of traffic at 30-45 mph. TTCI measured 70 kips peak vertical load at 30 mph and 58 kips peak vertical load at 40 mph. TTCI completed a revenue service installation of this ramped crossing diamond in December 2002 on the Union Pacific at Ontario, Calif., in the Los Angeles area.