The Lying Gusset
A county bridge inspector pulled a cracked plate off a pedestrian overpass in late May and shipped it to us in a five-gallon bucket with a torn shipping tag. The paperwork said likely fatigue. The plate said something else, and said it loud enough that we opened a new case file before the coffee was done.
Everything about the sample looked textbook at first. A quarter-inch gusset, A572-50, fillet welds on three sides, roughly nine years in service. The crack ran clean along the toe of the weld, bright on the fresh fracture face, scaled brown on the older portion. You'd call it fatigue from across a parking lot, and you'd write it up that way if you were billing by the hour.
We didn't. Here's why.
What the macro-etch showed
Pris pulled a transverse slice at the crack plane, mounted it in Bakelite, polished to 1µm, and hit it with 2% nital. On a clean repair the etch shows three zones — base, heat-affected, weld — and the boundaries are roughly parallel to the original fusion line. This one showed two HAZs. Overlapping. Offset by about 4mm.
Somebody had rewelded this gusset in the field, on top of the original fillet, and whoever did it never ground the old cap back. The second weld sat on top of a first one the paperwork didn't mention.
What the hardness map showed
Vickers traverse across the HAZ came back ugly. We expected a smooth climb from base metal into weld and back down — call it HV 160 up to HV 220, normal mild-structural behavior. What we got was a spike. HV 340 in a narrow band about 1.5mm off the fusion line, roughly 32 HRC equivalent. That number has no business appearing on a plain-carbon structural plate.
Martensite. Full stop. Somebody had rewelded this gusset cold, in the field, in what was probably a January wind, and walked away without preheat. The untempered martensite that formed in that narrow band had been cracking quietly for nine years before the live load finally found it and finished the job.
"Fatigue was the symptom. The cause was a repair nobody logged."
What to do about it on Monday
If you're the one running the gun on field repairs: preheat the plate. 150°F minimum for anything over 3/8" in A572-50, higher if the ambient is below 40°F or the member is restrained. A rosebud and a surface pyrometer cost less than one deposition.
If you're the one signing off: ask for the repair log and don't accept "it's fine" from a foreman who wasn't there when the arc struck. A field repair without a written WPS addendum is, legally speaking, an unknown.
If you're the one writing the spec: require post-weld hardness verification on any field repair over 3/8". One portable Equotip traverse, five points, ten minutes. It would have caught this in 2015.
The samples
Three gussets in total. One from the failed bent, two from adjacent members pulled for comparison during the same maintenance window. The adjacent pieces were clean — normal HAZ profile, normal grain, peak hardness HV 215. Only the failed plate carried the anomaly. That localization is what convinced us this wasn't a bad heat of steel. It was a bad day on a ladder nine years ago.
Full micrograph set — 50×, 200×, 500× — runs on the back cover of the print edition. Photos are worth more than our opinions.
— R.V., bench log entry 37-A · signed off by D.H.