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Brittle Test For Steel Bridge Rail Tubes

Question
State WY
Description Text

When Wyoming's TL-3 and TL-4 Bridge Rail was approved to NCHRP 350, FHWA expressed concerns about addressing brittle failure for ASTM A 500 tubes, something they had seen in other bridge railings.  To address this concern, Wyoming adopted a requirement for testing in accordance with ASTM E 436 using criteria similar to the State of New York.  The specification calls for a Drop-Weight Tear Tests and reads, "the percentage shear area shall be determined by testing six specimens from the 6-inch side or sides of the structural tube not containing a weld.  If the average percent shear area falls below 50, the material represented by these tests shall be rejected."

Trinity is in the process of providing railing for a project and they observed the following results:

Of the six tests, three showed 100% shear and three tests showed 10 % shear.  The average of all six tests is 55% which according to the specification would pass.  We are concerned however about the spread of the results.  Should this be considered a pass, or should additional criteria be applied?  Any light you can shead on this mater would be helpful.  We are in the process of approving the material, so time is of the essence.

Thanks!



Keywords
  • Bridge Rails
Other Keywords brittle, drop weight tear test
Date May 18, 2015


Response
Response ASTM E436 appears to be a shear test for evaluation of the fracture propagation in the temperatures where the steel behavior transitions from ductile to brittle. In your question, you noted that three showed 100% shear, which would indicate ductile response, and three showed 10% shear, which would indicate a brittle response. However, there is no mention of the temperatures for these tests. According to  the ASTM spec, these tests should be run at varying temps to determine how the temperature is affecting ductility. 

As such, in order to answer this question, we would need to look at the temperatures that the tests were run at and compare them with your operational temps in order to decide if the ductility of the bridge rail was an issue. I don't believe that you can average the results as the samples were likely run at different temps. 

If you have some additional data from the testing that you can share, we can look into things further.

Thanks 
Date May 27, 2015


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