Welded steel bridges consist of many fracture critical members (FCMs) that are require periodic non-destructive testing (NDT) for fatigue crack detection. Fatigue cracks develop for several reasons including [1]: - Welded details that are susceptible to fatigue cracks - distortion-induced fatigue cracking - Many bridges are over 90 years old - Corrosion fatigue cracks, and - Increases in the number of heavy vehicles leading and d allowable leading to significant accumulation