The fatigue life of thermite rail welds was increased by modifying standard thermite molds to improve their weld toe geometry and to avoid the formation of cold laps at the web-base fillet. The weld-toe flank angle was reduced by modifying the shape of the molds using a refractory molding compound.   Sealing the gap between the mold and the fillet with refractory paste prevented cold laps from forming. In this study, the rail gap width was increased form 1-in. to 1.4-in.  Increasing the gap width between the rail ends insured melt-back beyond the dimensions of the mold collar. The improved fatigue resistance of the thermite welds was demonstrated through full-scale 4-point bending fatigue tests.

The results of this study indicate that the fatigue resistance of the web and base of thermite rail welds can be improved by:

 ·        Eliminating cold laps in both the web-base fillet region and in the rail base by making the molds conform perfectly to the rail in these areas.  The mold dimensions can be altered so that the mold always fit this region snugly and possibly fit less well in areas of lesser importance like the wide part of the base or the central parts of the web of the sides of the head.  If that measure does not prove sufficient, small smears of mold sealant such as was used in this study can be applied to critical regions of the mold prior to being mounted on the rail.  Problems with insufficient melt-back which may contribute to the formation of cold laps can be reduced by increasing the gap between the rail ends from 1-inch to some larger value.  (1.4 inches was used in this study.

·        Reducing the flank angle to as small a practical value.  It may be possible to effect this change even in molds in which the weld collar is very pronounced because it is only the local stresses at the notch root which matter.

·        Increasing the weld toe radius to as large a value as possible.  This alteration is of equal importance to reducing the flank angle.

As a result of these findings, a follow-on project (HSR-41) was undertaken to further improve the weld geometry and conduct field tests of the welds in the TTCI FAST Track facility in Pueblo, Colorado. 

The final report for this IDEA project can be found at:
https://onlinepubs.trb.org/onlinepubs/archive/studies/idea/finalreports/highspeedrail/hsr-24final_report.pdf.