dStresen Validation Test Report
This report details the recent dStresen equipment validation testing work and results. There is a summary of the development of the dStresen equipment and an example of an application of the dStresen process to determine rail stress free temperatures (SFT) as part of the Australian Rail Track Corporation’s’ Goulburn to Yass re-sleepering project.
The dStresen equipment was developed by Jury and Jury Technologies Ltd, New Zealand. The methodology for applying the process to determine rail stress free temperature on railway tracks was initially developed during work trials in the United States at the TTCI test centre in Pueblo, Colorado in 2005. Since 2008 the dStresen system has been used extensively by railway companies in Australia to determine rail SFT. During this time the equipment and application methodology have been enhanced accuracy and the productivity of the SFT measurement process.
Currently the system is capable of completing eight rail SFT measurements per hour. The system is able to take SFT measurements when the rail is in a compressive or tensile state and within a range of -20⁰C to +10⁰C of actual SFT. There is also no restriction with the design of the track. Tangent and curved track with steel, timber or concrete sleepers are able to be tested.
Figure 1 The dStresen system in operation
The equipment consists of three major components; shaker/tune bar, control unit and computer. The equipment is powered by 240 Volts AC. This is provided 12 Volt car battery through a 12/240 Volt inverter. Both the shaker and twin tune bars (TB) are clamped to the rail head. During a test cycle the shaker rotates at 3900-4800 rpm for a period of 60 seconds. The shaker induces a lateral rail head displacement of up to 0.2 mm at a frequency range of 65 to 80 Hz. Each TB is a cantilever beam with an accelerometer mounted on the end to capture the beams (rail) first bending resonance and vibration data. The data is sent to the computer for analysis where the largest peak to peak Hertz value is recorded. The recorded peak Hertz value is largest for a rail without longitudinal thermal stress and is reduced in a predictable manner as the longitudinal thermal rail stress increases. The peak Hertz value is influenced by several factors including track strength, track condition, rail temperature and the rail stress free temperature condition.
The equipment must be calibrated each time the track design changes. The equipment is calibrated by selecting an appropriate track site representative of the track design to be tested. Several measurements are taken over a period of 2 to 3 hours ensuring that the state of the rail changes from a tensile state to a compressive state during this time. The data is recorded and used to determine the correct background number to be applied when calculating the subsequent SFT measurements. It is common during routine testing to measure the SFT when the rail is at or near the zero stress state. This data is also used to verify that the calibration data determined prior to testing is correct.
Equipment Validation Testing
At the request of SERS, Railcorp and Australian Rail Track Corporation, two validation projects were undertaken recently to compare SFT measurement performance between the rail stress transducer (strain gauges) technique and dStresen. One project was undertaken at the Douglas Park site on the Main South line in NSW. The second project was undertaken at the Tahmoor site nearby. At these sites two monitoring systems have been installed, rail stress transducers and temperature gauges at set intervals along the track. At the Douglas Park site, the rail stress gauges had been zeroed by “clinical” restressing earlier in the project, and changes to stress levels have been monitored continuously since. The track expansion monitoring system calculates stress free temperatures in continuously welded track as follows:
1. At each stress gauge the stress-temperature coefficient is calculated to indicate how stress values vary with changes in rail temperature. The Microsoft Excel slope function is utilised for this purpose, and calculates this coefficient at each 5-minute data point using the previous 48 hours of data. The theoretical coefficient is -2.38 MPa per degree C (Young’s modulus for rail steel multiplied by the coefficient of thermal expansion).
2. The instantaneous SFT is then calculated at each 5 minute data point. The instantaneous SFT equals the current rail temperature less (the rail stress divided by the stress-temperature coefficient) to give the temperature at which there would be no rail stress;
3. Due to small variations over the course of a day, due to the non-uniform nature of railway track and subsequent dynamic response, a rolling average of SFT is also calculated for a 24-hour period as an estimate of the SFT at the location. This value is used in trend analysis and in planning for maintenance work if the SFT values exceed trigger values set by the project.
The SFT calculated from rail temperatures and rail stresses is an estimate, however, it has been found that it is reliable and suitable for the project.
At the Tahmoor site, the design stress free temperature was put in place during the track possession by the “clinical” restressing process. The dStresen SFT measurements were undertaken at the same time of these “clinical” restressing activities. Appendix A provides a tabulated summary of these measurements. The results show that the dStresen system is capable of measuring the SFT of rail in CWR track with a good degree of accuracy.
ARTC Goulburn to Yass Re-sleepering Project
In 2008 ARTC replaced the timber sleepers with concrete sleepers on the Main South line between Goulburn and Yass. Towards the end of the project the rail stressing work was 3 months behind schedule. The dStresen system was used to measure the SFT along approximately 30 km of track. The dStresen SFT data was then analysed to identify the sections of track that required to be restressed. The SFT measurement sites were 250 metres apart. This resulted in 120, 250 metre rail sections with the SFT value known. The analysis identified 30 of the 250 metre rail sections required rail stressing to adjust the SFT back to design neutral temperature. The resulting adjustment calculation result data was compared to the previous dStresen SFT measurement data. Only two results of the 30 differed by more than 3ºC.
The dStresen system has proven to be a reliable SFT measurement process. It provides an efficient alternative to other SFT measurement systems and offers the railway owner substantial savings to the rail stressing process to ensure the track is at design neutral temperature.