A reliable, low-cost, easy-to-use technology for measuring longitudinal stress in rail continues to be a high priority objective for both freight and passenger rail operators. The effects of thermal expansion and contraction and train forces can produce tremendous stress in rail; especially long strings of welded rail. Various rail-anchoring systems are used to prevent these stresses from pulling or pushing the rail out of alignment, or breaking it. These stresses can occasionally exceed the capabilities of the anchoring system and/or the strength of the rail, resulting in rail breaks or buckling, which continue to be significant causes of derailments and train delays (Figure 1).
This project and its predecessor project (HSR-30) evaluated “vibro-elastic” methods for measuring longitudinal stress in rails. This concept is based on the known sensitivity of bending vibrations to contained longitudinal stress. The rail is excited at a specified frequency using an electromagnetic shaker. A laser vibrometer that is scanned along the rail is used for dynamic displacement measurements to determine the frequency and wavelength of the resulting rail vibrations. The resulting wavelength, along with the rail rigidity (which can be determined by the rail dimensions and modulus) can be used to estimate contained longitudinal load.
The laboratory test results were encouraging, but did not meet the accuracy requirements established for the system. Field testswere conducted by installing the prototype system on rail at TTCI at a location that was also equipped with a strain gage system for measuring stress. Comparison of the strain gage data with the prototype data revealed large differences. Any further development of this concept will require an investigation of the causes of these differences and whether they can be corrected.
