Material thickness determination standard and Testing

[cgwilson2]

I am currently involved in a rehabilitation project of a steel structure that suffered numerous localized coating failures based on the visual inspection. I am trying to come up with a reasonable specification to provide objective evidence to support the amount of steel to be replaced. I was considering Ultrasonic testing in the visualized damaged area and beyond to determine the extent of the material loss.I am having a problem find resources for specifing recommended grid patterns, and acceptance criteria. Any assistance is greatly appreciated

 

[metengr]

cgwilson2;
What I would do in your situation is to remove the coating starting at locations where visually the density of coating failure is the highest and prepare the surface for ultrasonic thickness testing.

You can establish a 2" x 2" grind pattern with chalk and fan out from the corroded steel surface. Depending on the thickness readings, you can adjust the grid pattern size to make it smaller or larger.

The more difficult task is to evaluate the combined effect of multiple LTA’s on the load-carrying capacity of a structural member or structure. One option is to use a fitness for service standard (like API 579, which deals specifically with LTA and general wastage). A second option may require some FEM modeling to look at how LTA affects the bulk strength of the member. You may be able to arrive at some value where the combined effect of LTA results in an increase in stress of x%, and compare to an allowable stress value.

 

[metengr]

Correction - grid not grind.
LTA - locally thinned area.